5 Causes of Molded-Part Variation: Mold Design

5 Causes of Molded-Part Variation: Mold Design Ideally, mold designers should have to concern themselves only with the linear type of shrinkage But in reality, they must be concerned with numerous other shrinkage factors, especially when dealing with plastic materials subject to anisotropic shrinkage For example, restraint in one direction will lead to an increase in shrinkage in another Linear shrinkage will also vary due to differences in orientation, pressures, and cooling rates throughout the cavity Volume shrinkage can be predicted; however, the length, width, and thickness shrinkage components that lead to a specific volume change must be established experimentally or approximated when designing the mold This chapter examines mold-design considerations that affect shrinkage: the geometry and placement of gates and runners, cooling systems, materials, and other factors such as gasassist processing 5.1 Cavity Dimensions and Design Factors Linear mold-shrinkage values are determined experimentally by molding parts and evaluating the differences between part and cavity dimensions The values are calculated by subtracting the dimension of the molded specimen from the corresponding dimension of the mold cavity and dividing by the latter These linear mold-shrinkage values are essentially thermalstrain values, and are reported in units per unit of length When given the experimental linear values of mold shrinkage, a tool designer can determine the appropriate cavity dimensions The information that can be generated with this shrinkage test procedure is fairly limited, especially in the case of the cross-flow specimen where flow is never really fully developed It is also important to note that the test standard emphasizes the importance of conditioning the parts before the part dimensions are taken The “normal” shrinkage data are reported after fortyeight hours of conditioning at standard conditions of temperature and relative humidity This is particularly important for semicrystalline and hygroscopic polymers The dimensions of parts produced from a glassy amorphous polymer such as polystyrene can stabilize in as little as 20–30 minutes On the other hand, morphological changes for semicrystalline polymers can © Plastics Design Library go on for hours or even days after molding The dimensional changes that occur in a part after molding are also affected by moisture reabsorption for hygroscopic polymers.[6] Hygroscopic polymers such as polybutylene terephthalate (PBT) or nylon 6/6 are dried prior to molding, and as a result the molded part is “dry” as it is ejected from the tool Over time, the part will reabsorb moisture from the atmosphere, but it can be days or weeks before an equilibrium moisture level is reached (depending on part thickness) As moisture is reabsorbed, the parts tend to swell or grow.[6] Achievable tolerances for injection-molded parts are to a large extent determined by the ability to correctly predict shrinkage The dimensional tolerances that can be achieved in practice are influenced both by processing conditions and tool dimensions An acceptable tolerance for a mold dimension is generally less than 50% of the part tolerance A 50/50 tolerance split would give the mold builder and the molder equal tolerance to compensate for variations in tool construction, shrinkage predictions, and molding conditions The charts in Fig 5.1 can help in establishing reasonable tolerances and improving communication between user, molder, and mold maker The tighter the tolerances dictated by the user, the higher the cost because of the increased time and effort expended by the molder and mold builder to achieve those tolerances The charts list a number of typically encountered dimensions and indicate what the user can reasonably expect from the molder Sidewalls should always include some draft allowance to ease the removal of the part from the mold When a sidewall is textured, the draft requirements increase dramatically to prevent scuffing or scoring the plastic part as it is removed from the cavity High quality tools are built to tighter dimensional specifications and limit the degree of elastic distortion due to cavity pressure or clamp tonnages It is important to remember that significant tool deflections can occur under high cavity pressures It is also easier to hit the target dimensions when the plastic material shrinkage is both low and predictable (such as in the case of filled amorphous polymer) Unfortunately, a number of parameters make it difficult to predict shrinkage and result in anisotropic shrinkage behavior and the potential for warpage or internal stress.[6] Ch 5: Causes of Molded-Part Variation: Mold Design 52 Figure 5.1 SPE size and tolerance recommendations.[17] (Courtesy of DuPont.) Ch 5: Causes of Molded-Part Variation: Mold Design © Plastics Design Library 53 Proper mold design and selection of mold materials helps dissipate heat in a manner that reduces warpage For example, the use of copper or aluminum alloys for cores and areas forming inside corners of plastic parts helps conduct the greater concentration of heat in these areas away from the plastic, thus causing it to cool more nearly at the same rate as other areas of the mold Concentrating more cooling in these critical areas also helps combat the differential cooling rates that cause warpage Incidentally, if stacking rings or ribs are planned, it is best to add them to the mold after the shrinkage of all related parts is determined, usually by mold trial.[18] A carefully planned mold design is crucial to achieving a high level of dimensional stability in the finished part If any of the numerous factors involved in mold design (see Sec 2.1) are overlooked, even the most ideally designed part may have a tendency to warp The factors listed below are especially likely to affect the warpage in the finished part: • The type, positioning, and size of the gates can influence molecular or fiber orientation, weld lines, and adequate holding pressure • The gate and runner system design can cause inadequate or uneven cavity pressures • The mold cooling system can cause uneven cooling rates in the molded part • The type of material from which the mold is made can influence proper cooling 5.2 Gate Types Probably the single most important part of the mold design is the type of gate or orifice through which the molten plastic must flow to enter the mold cavity, although it is often given less attention than it deserves Gates are usually thinner and significantly narrower than the part to which they connect The type of gate has a significant affect on packing, shrinkage, warpage, anisotropy, and ultimately the stability of the part Figure 5.2 shows a number of gate types, some of which are discussed in this section Each has its benefits and drawbacks Figure 5.2 Typical gates used in injection molding © Plastics Design Library Ch 5: Causes of Molded-Part Variation: Mold Design 54 A sprue gate, shown in Fig 5.3, connects the mold cavity directly to the nozzle of the molding machine The only restriction is at the tip of the nozzle so that the shear stresses and flow hindrances are at a minimum Sprue gates are typically used for large parts in single-cavity molds where large gate blemishes are not objectionable Sprue gates are very difficult to remove cleanly On foamed parts, a hot iron can sometimes be used to seal and improve the appearance of a previously trimmed sprue gate Figure 5.3 A sprue gate 5.2.1 Pin, Pinpoint, Tunnel, and Submarine Gates Pin or pinpoint gates, shown in Fig 5.4, can enter at the edge of a part—at the parting line or on the face of the part—from runners located on a different level of the mold than the part For example, a heated sprue can keep the material melted right up to the cavity and have a pinpoint gate at the surface of the part A variation of this scheme is to design the mold so that the runner is on a separate level from the part, and introduce the material to the part through a secondary drop and pinpoint gate Figure 5.4 shows some details of a pinpoint gate The pinpoint gate shown can be tapered either way to control the gate break-point Often a spherical projection on the part is placed at this type of gate to increase the wall thickness locally This aids in material distribution and reduces shear stresses in the plastic Pin gates at the edge of a flat part are prone to jetting, a phenomenon where material squirts across the part before beginning to fill out the cavity If the pin gate can be located so that the jet of material immediately impinges on a wall or core pin, the jet of material will be immediately forced to puddle and flow in a more controlled manner Pin gates will usually break cleanly enough that no further finishing is necessary They are prone to cause high shear stress in the part and are smaller than optimum size for best packing of the cavity Tunnel or submarine gates, as shown in Fig 5.5, are a variation on pinpoint gates that requires the gate to shear as the part is removed from the cavity The gate and runner must be designed with appropriate ejectors to reliably remove the portion of the gate and runner that is located below the parting line Tunnel gates rarely require any secondary trimming when properly located Sometimes tunnel gates are used to feed material under the edge of a part into a shortened ejector pin so that no exterior gate exists The plastic between the ejector pin and the part is normally broken off after removal from the mold Tunnel gate size should be between 30% and 70% of the wall thickness of the part The diameter of the gate normally should not be greater than 0.10 inch Gates larger than 0.10 inch may be too difficult to shear cleanly If the gates are too small, they may freeze off too early, causing excessive shrink, and possibly excessive shear and degradation of the plastic Figure 5.4 These drawings show some details of pinpoint gates Ch 5: Causes of Molded-Part Variation: Mold Design © Plastics Design Library 55 Figure 5.5 Tunnel or submarine gates Figure 5.7 Examples of straight-edge or edge gates Figure 5.6 shows a tunnel gate that can only be used with flexible plastics However, this design allows the part to be gated away from the edge and for the part to be automatically separated from the gate and runner as the mold opens and the part is ejected This type of gate is made in two halves to allow for machining and polishing 5.2.3 Fan Gates Fan gates help control the flow of material into a cavity, aid consistent orientation of fibers and molecules, and reduce the possibility of jetting However, they are more difficult to remove than edge gates and offer little resistance to flow from the runner to the cavity As a result, they are of little help in balancing flow into several cavities in multiple-cavity molds Figure 5.8 shows the essentials of a fan gate The gate should extend a significant distance along an edge of the part Its thickness should be from half to three quarters of the part thickness The transition from runner to gate should be radiused The length of flow across the gate should be about mm or less The runner may join the distribution part of the fan gate at any point, but preferably near the center, and the runner should be flared, as shown in the right-hand drawing, to encourage good material distribution across the gate Figure 5.6 A cashew or winkle tunnel gate [20] (Courtesy of Dupont.) 5.2.2 Edge and Straight-Edge Gates Figure 5.7 shows an edge and a straight-edge gate These gates are the easiest for a mold maker to cut from a runner They offer relatively low shear as the material flows into the cavity They are also less prone to jetting than pinpoint gates However, they are difficult to trim cleanly They are usually the full width of the runner and may be at or near the full partthickness This type of gate is used when large cross-section gates are needed: for example, when the plastic is very shear-sensitive or when large gates are needed when semicrystalline plastics are used to minimize shrinkage © Plastics Design Library Figure 5.8 Two fan gates 5.2.4 Film, Diaphragm, and Ring Gates The fan gate is sometimes called a film gate if the gate thickness is less than half the thickness of the material Film gates are an extension of fan gates to further control flow direction and fiber orientation Film Ch 5: Causes of Molded-Part Variation: Mold Design 56 gates are usually the full width of the part being molded Figure 5.9 shows several film-gate designs Film, diaphragm, and ring gates are related in that they extend the full width or circumference of a part They are an extension of fan gates except wider and usually thinner Ring or diaphragm gates can be used when roundness of a part is essential They are often removed with a special punch and die to shear the gate from the part multiple-cavity molds The tab provides a place for the initial puddling of the flow of material into the mold A pin gate into the tab provides a flow restriction to balance the filling of multiple cavities, and generates the shear heating that can be beneficial in filling cavities (providing the shear is not sufficient to cause significant material degradation) Figure 5.11 shows the essentials of a tab gate Figure 5.9 Film gate designs Figure 5.11 The essentials of a tab gate Disk gates or cone gates, shown in Figure 5.10, are special types of film gates They provide essentially radial flow to fill a circular part They are fairly difficult to remove cleanly, and typically are too thin to provide adequate pack time for the molded part This type of gate should be used when orientation of the molecules or fibers is very important 5.2.6 Multiple Gates Parts may have more than one gate Multiple gates improve the ability of the molder to fill the plastic part with reasonable pressures and temperatures and help maintain uniform shrinkage by minimizing flow distance from the gate to the extremities of the part If the flow path in the part is great, the pressure drop from the gate to the remotest corner of the part may be such that the shrinkage is excessive in the remote regions of the part One rule-of-thumb is to double the number of gates that you think are necessary based on available flow-length data and experience It is generally unwise to expect to mold a good part if the flow length is near or exceeds the published data Testing has proven that shrinkage increases as distance from the gate increases because cavity pressure is reduced by viscous action as the distance from the gate increases Round parts will, more often than not, benefit from an odd number of multiple gates Two gates, as shown in Fig 5.12, will result in a bi-lobed part The part will be larger in one direction than at 90º to that direction Figure 5.10 Disk or diaphragm, and cone gates [ ] (Courtesy of DuPont.) 5.2.5 Tab Gates Tab gates offer some of the benefits of fan gates in reducing or eliminating jetting, and some of the benefits of pin gates in helping to balance the filling of Ch 5: Causes of Molded-Part Variation: Mold Design Figure 5.12 A double-gated round part © Plastics Design Library 57 Three or five gates will create an odd number of lobes with each high point opposite a low point so that the average diameter will be more constant See also Sec 5.3.2 Multiple gates also have the advantage of helping to control fiber orientation With only one gate in the mold, the shape and profiles that exist in the cavity determine the flow pattern The initial flow is radial, becoming more linear as the flow front moves away from the gate More gates distributed in or around the mold cavity reduce the flow length causing more of the part to be filled with a radial-flow pattern from each gate This creates a more random flow pattern in the cavity The random flow pattern that results helps maintain a more isotropic shrink Multiple gates cause flow patterns more like the cross-flow patterns that exists in the American Society for Testing Materials (ASTM) cross-flow mold-shrinkage tests.[5] On the other hand, multiple gates along one side of a long part can cause warpage because there is usually less shrinkage near a gate than in areas remote from the gate A long slender part with multiple gates along a long edge is likely to bow or warp with a convex curve on the gate side A drawback of multiple gates is that each gate produces one or more weld lines One method of avoiding the weld lines is to provide valve gates from a separate runner system that sequentially open as the flow front of the plastic passes the gate Such a system is shown in Fig 5.13.[6] The solid ovals represent closed gates and the open ovals represent open gates The filling process starts at one gate, then as the melt front passes the next gate, it opens The flow from the first gate virtually stops and the second gate provides material until the melt front passes the third gate, etc., until the mold is filled Then all gates stay open until the packing or holding phase is complete The gates are closed during the cooling phase 5.3 Gate Location The position of the gate greatly affects material flow, part shrinkage, and ultimately, the dimensional stability of the part The location of the gate affects the fiber orientation and the presence and location of weld lines The location can strongly affect part warpage This is a direct result of the influence gating variables have on the orientation of the polymer Every effort should be made to position the gates to enhance uniform polymer orientation as the molten material fills the mold This is of prime importance when molding with semicrystalline resins Its importance derives from the high level of differential shrinkage typical of these resins (the differences in the amount of shrinkage in the flow direction versus that in the transverse direction) Molecular and fiber orientation can contribute to warpage (see Secs 2.1, 6.1, 4.2) Fiber-filled materials shrink less along the length of the fiber than across the fiber Fibers tend to align themselves along the direction of flow Therefore, shrinkage in the direction of flow in fiber-filled materials is less than across the flow On the other hand, unfilled materials can have anisotropic shrinkage if there is a high degree of molecular Figure 5.13 Sequentially opened gates to minimize flow distance from the gate © Plastics Design Library Ch 5: Causes of Molded-Part Variation: Mold Design 58 orientation, because there tends to be more shrinkage along molecular chains than across the chains The placement of the gate affects the flow pattern and molecular and fiber orientation within the cavity 5.3.1 Side, End, and Center Gates Figures 5.14 and 5.15 show side-gated and endgated rectangular parts If a part is end-gated, then the fiber orientation is essentially along the long axis of the part The shrink should be reasonably uniform across the part so there will be little warpage If the part is side-gated, then the flow along the gate side is parallel to the long axis while the flow on the opposite side is more nearly perpendicular to the long axis The end result for a fiber-filled material is for the part to bow convex on the gate side Figure 5.14 These drawings show good and poor gate placement for fiber-filled parts.[20] (Courtesy of DuPont.) Figure 5.16 shows a long slender part with a cored hole near one long edge When the material flows into this mold, fiber orientation will be parallel to the long axis on one side, but the flow is disturbed on the other side causing more random fiber orientation on that side Therefore, there is likely to be more shrink on the side with the hole One possible solution to this problem would be to core a hole on the opposite side of the mold, one that doesn’t quite go all the way through the part This blind core would tend to balance the flow pattern on both sides of the part An unreinforced T-shaped part, as shown at the top in Fig 5.17, will shrink more in the heavier section than in the thin section, resulting in a concave curve on the heavier side If the material is fiber-filled, the fibers will tend to flow along the thicker region and align in that direction; then the material will flow at approximately 90° to the initial flow into the thinner section Therefore, the fiber orientation in the thinner section will be less aligned along the length of the part, causing the thin section to shrink more linearly than the thicker section This causes the T-section to bend concave toward the thin rib The part on the left in Fig 5.18 shows warping Making a series of short shots, each progressively smaller in size, can give the molder and mold builder a better picture of the flow pattern in a mold In this case, it was determined that adding blind cores opposite the existing cores would help make the flow pattern more symmetrical and reduce warpage The horizontal bar at the right side of the figure represents the flow-direction shrink, while the vertical bar represents the cross-flow shrink for one fiber-filled material Figure 5.15 Gate positions.[21] (Courtesy of DuPont.) Figure 5.16 An asymmetrical part.[21] (Courtesy of DuPont.) Ch 5: Causes of Molded-Part Variation: Mold Design © Plastics Design Library 59 Figure 5.17 Warpage of a T-section, end-gated part with and without fiber filler.[21] (Courtesy of DuPont.) Figure 5.18 Flow restrictors to aid in counteracting warpage.[21] (Courtesy of DuPont.) If a fiber-filled flat part is center-gated, there is likely to be fairly severe warpage due to significantly higher cross-flow shrinkage compared to flow-direction shrinkage In Fig 5.19, the radial flow is constrained by the fiber filler, while the shrink along the outer edges is higher.[13] Thus, the perimeter of the part, © Plastics Design Library with its higher shrinkage, tries to become smaller and is likely to twist as shown To avoid this type of warpage, the gate(s) must be placed to promote parallel fiber orientation A wide tab or wide flash-gate across one of the narrow ends of this part would vastly reduce the tendency to warp Ch 5: Causes of Molded-Part Variation: Mold Design 60 Figure 5.19 The effects of differential shrinkage on a flat fiber-filled part that is center-gated 5.3.2 Determining Gate Position Several factors determine the gate location Cosmetic and physical property considerations aside, the gate should normally be located to minimize total length of flow in the cavity and should be located in the thickest area of the part Possible exceptions to this rule are when it is necessary to gate into the center of a round part, the bottom of a deep pocket, or when the optimum location otherwise might cause problems Gas entrapment and burn can be caused if the gate location causes the plastic to surround a pocket of air Avoid gating into an area that is subject to flexure or impact Typically, the gate area has the highest residual stress and, as a result, is a weak area In round parts like gears where there is a center-cored hole, it is best to use an odd number of gates equally spaced around the center core This breaks up the orientation patterns that single or even numbers of gates would leave An even number of gates creates orientation patterns that result in lobes on the diameter that are additive An odd number of gates creates lobe patterns that tend to cancel one another out Fiber-filled materials are considerably stronger and stiffer along the fiber orientation than across the fiber orientation If either strength or stiffness is a prime consideration, the gate must be located to maximize fiber orientation in the desired direction To otherwise could produce an unacceptably weak or flexible part Gate location must be in a low-stress area in this type of situation Ch 5: Causes of Molded-Part Variation: Mold Design The next consideration is to minimize or eliminate any hesitation by the plastic as it moves from thick to thin sections The plastic always takes the path of least resistance as it flows into the mold cavity When the flow front reaches a thin section, it will hesitate momentarily if there is any additional thick area to fill Any hesitation will show up as if there were a slight crack at the hesitation line All other things being equal, the flow front will advance uniformly in all directions Consider Fig 5.20 As a radially advancing flow-front reaches a wall at right angles to the flow, it generates an excess of material at that wall The excess material flows along the wall and develops a secondary flow-front somewhat like that shown This secondary flow-front is readily seen in glass-fiber filled plastics, especially when molded in relatively cool molds The end result is sort of a weld line that follows the advancing intersection of the two (or three in Fig 5.20) flow fronts Since the molecular and fiber orientations are predominantly perpendicular to the flow front, this type of filling pattern produces a rather confused orientation near the gate As the plastic moves down the cavity, the flow front becomes more nearly straight across the cavity and the orientation more predictable A double gate in this type of mold, separated by about one-third of the width of the cavity, would generate not only the two secondary flow-fronts along the wall but also one in the center between the two gates If there is a hesitation while moving from a thick to a thin section, the last thick section to fill will be the beginning of the flow into the thin section In all likelihood, the first point to hesitate will be the last point to continue the frontal advance With these factors in mind, the designer can approximate the flow front by sketching it on paper Figure 5.20 Advancing flow-front in a rectangular cavity © Plastics Design Library 63 Figure 5.25 Contour plots of constant mold shrinkage as a function of gate-area and part thickness The material is 12 MFR Polypropylene Impact Copolymer in a in × in adjustable-depth plaque.[23] (Courtesy of Equistar Chemical.) of the gate was constant and the depth varied as the cavity depth was varied, or if the gate was square and a number of gate inserts were used It seems more likely that square gate inserts of different sizes were used Consider parts produced using top-center gating (for example, using a sprue gate, pin gate, hot drop, etc.) and filled by radial or disc-type flow In radial flow, the radius could be viewed as the flow direction, while the circumferential direction is essentially the cross-flow direction If there is differential shrinkage, then center-gated parts can react in several different ways: • Remain flat • Warp into a dome shape • Warp into a saddle shape If the molded parts not warp, it is an indication that either the shrinkage is uniform or that the stiffness of the part (due to the combined effects of modulus and geometry) is great enough to resist the shrinkage stresses caused by differential shrinkage However, in this case, the part remains internally stressed If differential shrinkage is present, and the part does not have sufficient stiffness to withstand the shrinkage stresses, warpage or dimension distortion will occur, as shown in Fig 5.26 Dome-like warpage is likely to occur when the circumferential shrinkage is greater than the radial shrinkage Saddle-type shrinkage (like a potato chip) can be caused if the inner region shrinkages are greater than those of the outer region, or when radial shrinkages are higher than those in the circumferential direction In any case, minimizing differential shrinkage due to packing, orientation, or differential cooling effects can minimize warpage.[6] © Plastics Design Library Figure 5.26 The dome and saddle shapes caused by uneven shrinkage 5.6 Runner Design In multiple-cavity molds, the runner system must be balanced and the gates absolutely uniform to encourage uniform filling and holding pressure within all cavities Gate design varies depending on the needs of the specific resin chosen; however, many of the requirements regarding gate design are consistent for most resins Figure 5.27 shows some balanced runner systems as well as an unbalanced runner system Typically, runner sizes range from to 10 mm (0.118 to 0.394 in.), the most common size being mm (0.236 in.) Runners, like sprues, are usually short in length and generous in diameter since this reduces pressure loss and permits the application of adequate follow-up pressure However, if they are made too large, excessively long cycles and large material losses result If they are made too small, the mold is incapable of being filled and the large amount of pressure, which is lost, is transferred into heat In no case should the runner diameter ever be smaller than the maximum part-wall thickness Heat generation in injection molding during the filling process is proportional to the pressure drop in the process This heat will show up in the regions where the material is being sheared the most, that is, in the gate regions Such local temperature rises can be very high and can lead to material degradation This is why maximum shear rates are sometimes quoted for plastic materials It is generally true that, in the plastics industry, runners are made too large and gates are made too small However, if the runners are smaller in cross section than the maximum thickness of the part, then the runner will freeze before the part does, causing excessive shrinkage or even voids in the part.[24] Ch 5: Causes of Molded-Part Variation: Mold Design 64 Figure 5.27 Good and poor runner designs for multiplecavity molds.[20] (Courtesy of DuPont.) Figure 5.29 Partially filled cavities in an eight-cavity balanced runner mold.[25] (Courtesy of SPE.) 5.6.1 Figure 5.30 shows the flow lengths in the runner segments at sizes too small to even reach the cavities The runner segments are identified in Fig 5.31 The flow lengths become more even when the flow reaches the gate restrictions The study[25] is based on a Zytel 70G33-HS1L molded part in which there was significant variation in the size of the parts produced on the eight-cavity mold, even though the mold cavities themselves were quite uniform and the runner system was geometrically balanced Zytel 70G33-HS1L is a heat-stabilized and internally lubricated nylon 6,6 resin containing 33% by weight short E-glass fibers The study concluded that the lowest cavity-to-cavity and shot-to-shot variations occurred with relatively high mold temperatures and holding pressures Multiple-Cavity Molds In multiple-cavity molds, it is almost impossible to maintain uniform shrink unless the runner system (and vents) is carefully balanced Figure 5.28 shows a balanced runner eight-cavity mold Unfortunately, even with fully balanced runners, there is a phenomenon that results in uneven flow to inside versus outside cavities A study[25] of runner design and fill patterns in multiple-cavity molds indicates that the warmer plastic in the runner tends to stay against the wall nearest the sprue, causing the plastic to flow into the inside cavities more easily than the outside cavities Figure 5.29 shows a short-shot into an eight cavity mold where the inside cavities tend to fill easier than the outside cavities in another type of balanced runner system Figure 5.28 A good layout for an eight-cavity mold.[17] (Courtesy of DuPont.) Ch 5: Causes of Molded-Part Variation: Mold Design Figure 5.30 Flow lengths in runner segments and cavities.[25] (Courtesy of SPE.) © Plastics Design Library 65 Figure 5.31 shows the cavity and runner-segment identification for analyzing the next few graphs These graphs support the contentions expressed elsewhere related to shrinkage versus mold temperature and holding/packing pressures In each graph, the T direction is cross-flow shrink, and the L dimension is the flow direction open time and higher pressure in the cavity overcame the greater crystallization due to the higher melt temperature caused by the shear heating at the gate Thus, there was an overall decrease in shrinkage The shrink values shown are for lengths of 27.9 mm To obtain the shrink rate in units per unit, the shrink on the graph, say 0.3, must be divided by the length of the side In other words, 0.3/27.9 = 0.011 units per unit length Figure 5.35 shows the same mold and part with the mold temperature at 200°F The higher mold temperature caused the part to shrink slightly more than when molded in the same mold at 100°F The higher mold temperature encouraged more crystallization and caused higher shrink at the lower pressure Some of the results obtained in the study[25] contradict data shown elsewhere, where flow-direction shrinkage when molding glass-filled resin is less than cross-flow shrinkage, due to the stabilizing effect of Figure 5.31 Cavity and runner-segment identification.[25] (Courtesy of SPE.) As stated elsewhere, higher holding pressures result in lower shrinkage Figure 5.32 shows the direction of measurements made on individual cavities Figure 5.33 shows the shrinkage results in a cavity size of 27.9 mm square for the Zytel 70G33-HS1L part Additional data are shown in Fig 5.34 Here, there is an increase in shrinkage as a result of increased pressure up to a critical point, then additional pressure reduces shrink again What is almost certainly happening here is that the lower pressure caused the cavity to fill with material just above the melting point so that there was little thermal shrink, little crystallization, and quick solidification The increase in pressure to 1000 psi caused shear heating, which required more time for the part to cool The longer cooling time allowed more time for crystallization, and more shrinkage At 1500 psi, shear heating was still present but the greater pressure forced the gate to stay open longer with more compression of the plastic in the cavity The additional gate- © Plastics Design Library Figure 5.32 The individual cavities and the direction of measurements in the molded parts.[25] (Courtesy of SPE.) Figure 5.33 Cross flow shrinkage at 100°F mold temperature for each cavity molding Zytel 70G33-HS1L.[25] (Courtesy of SPE.) Ch 5: Causes of Molded-Part Variation: Mold Design 66 Figure 5.34 Flow direction shrinkage at 100°F for each cavity molding Zytel 70G33-HS1L.[25] (Courtesy of SPE.) Figure 5.35 Flow-direction shrinkage at 200°F for each cavity molding Zytel 70G33-HS1L.[25] (Courtesy of SPE.) the glass-fiber orientation in the direction of flow About half of the data shown in Table 5.2 show cross-flow shrinkage in a 100°F mold to be less than the flow direction shrinkage This is an example of data that really frustrate a mold designer trying to determine usable shrinkage values The data may be flawed as seen below These data, though, can be viewed from other perspectives In the first place, a 100°F mold is quite cold for glass filled nylon A cold mold almost always inhibits normal shrinkage If these parts were annealed, the expected shrinkage between flow and cross flow conditions might appear Secondly, the socalled “cross-flow” measurement at the gate end is really flow-direction shrinkage as the flow of the plastic progresses radially from the gate to the corners nearest the gate and if the parts are measured at the gate end The so-called “flow-direction” shrinkage measured at the edge of the part is, more correctly, cross-flow shrink If these assumptions are true, then the data make more sense and agree more readily with other data Tables 5.1, 5.2, and 5.3 contain data in tabular form for this eight-cavity mold experiment.[25] The mold cavities were 27.87 ± 0.01 mm in the L direction Unfortunately, the cross-flow dimension is not well defined, but the molded parts were square For the purposes of this experiment the cross-flow cavity dimension was assumed to be 27.9 mm Table 5.1 Shrinkage in mm in the L and T Directions L-Direction Shrinkage (mm) T-Direction Shrinkage (mm) Mold Temperature (°F) 100 175 200 100 175 200 Number Average Shrinkage (Sm) 257 2949 3028 257 3205 3308 Weight Average Shrinkage (Sw) 258 2949 3037 2574 3207 3310 Ratio (Sw/Sm) 1.0039 1.0040 1.00297 1.0015 1.0006 1.0006 Standard Deviation 0170 0195 0166 0094 0088 0079 Signal/Noise Ratio 15.12 15.22 18.24 27.34 36.42 41.87 Lowest* 229 229 254 229 292 305 Highest* 305 330 343 279 343 356 Range* 076 101 089 050 051 051 Lowest** 229 279 279 241 305 318 Highest** 279 330 330 279 343 343 Range** 050 051 051 038 038 025 * 232 Measurements ** Neglecting the frequency of occurrences less than 10 Ch 5: Causes of Molded-Part Variation: Mold Design © Plastics Design Library 67 Table 5.2 Shrinkage in mm in the L and T Directions (Shrinkage at the mold temperatures indicated; molding conditions as in Table 5.1.) L-Direction Cavity No 100°F T-Direction 175°F 200°F 100°F 175°F 200°F 2795±.0082 3108±.0082 3156±.0089 2571±.0082 3184±.0040 3300±0.000 10 2479±.0119 2884±.0109 2933±.0080 2540±.0083 3221±.0058 3300±0.000 11 2748±.0067 3225±.0068 3220±.0082 2752±.0064 3296±.0022 3394±.0059 12 2740±.0075 2915±.0095 3027±.0007 2540±.0047 3117±.0075 3229±.0069 13 2500±.0069 2938±.0114 2992±.0096 2526±.0004 3099±.0064 3237±.0070 14 2655±.0142 3110±.0081 3189±.0113 2544±.0087 3171±.0033 3288±.0050 15 2319±.0052 2678±.0193 2791±.0067 2549±.0033 3304±.0024 3408±.0069 16 2514±.0062 2889±.0101 2915±.0107 2540±0.000 3246±.0069 3309±.0033 Table 5.3 Shrinkage in mm at Different Packing Pressures and Mold Temperatures Mold Temp (°F) Packing Pressure (psi) 100 500 100 1000 100 1500 200 500 200 1000 200 Direction Average Standard Deviation Highest * Lowest * Range * Highest ** Lowest ** Range ** L 2475 0136 279 229 050 229 267 038 T 2492 0111 279 229 050 254 241 013 L 2733 0244 318 229 089 305 241 064 T 2578 0112 279 229 050 279 254 025 L 2456 0219 279 203 076 279 254 076 1500 *Average of 80 specimens T 2113 0106 229 203 026 229 203 026 L 3236 0155 356 292 064 343 305 038 T 3476 0104 368 330 038 356 330 026 L 2994 0151 330 267 063 318 279 039 T 3223 0108 343 292 051 330 305 025 L 2646 0126 292 341 051 279 254 025 T 2733 0097 292 254 038 279 267 012 ** Neglecting the frequency of occurrences less than at the low and high ends Three different mold temperatures were used Packing pressure was 700 psi and shot size was 1.4 inches on the barrel of a 90-ton Toyo injection-molding machine The material was DuPont’s 70G33-HS1L, a heat-stabilized, internally lubricated nylon 6-6 with 33% glass-fiber reinforcement Recently, John P Beaumont of Penn State University, Erie, PA, discovered the cause of this uneven flow between inner and outer cavities in an otherwise bal- © Plastics Design Library anced runner mold.[57] The basis of the problem is that plastic neither slides through a runner nor maintains a parabolic flow-velocity profile Rather, the leading edge of the flow-front of plastic adheres to the wall of the runner while the center of the slug of plastic in the runner moves at a relatively constant velocity Figure 5.36 shows the shear rate across half of a runner with the centerline of the runner shown by a dash-dot line A relatively thin layer of plastic adheres to the wall at Ch 5: Causes of Molded-Part Variation: Mold Design 68 fect of rotating the hottest region of the runner, as shown in Fig 5.38, toward the fixed side of the mold The length of the sprue and the velocity of the plastic in the sprue dictate the amount of rotation Figure 5.36 Shear-rate distribution through a circular-flow channel.[27] (Courtesy of SPE.) the top of the figure Between this solidified layer of plastic and the slug in the center is a cylindrical zone that is subject to a high rate of shear The high shear rate aligns the molecules and fibers, which reduces the viscosity of the plastic The shearing action also heats the plastic near the wall, further reducing the viscosity of that layer of plastic The temperature profile across the runner is roughly the same as the shear curve except that the layer against the wall of the runner is much cooler than the rest of the plastic in the runner Because of the high viscosity of the plastic in the runner, there is little or no mixing of the hot and cool zones The flow is almost entirely laminar Beaumont has shown that as the plastic branches, the hot material near the runner wall takes the shortest path and remains against the inner wall as shown in Fig 5.37 A finite element analysis program computed the temperature in a cross section of the runner (secondary runner A in Fig 5.37) The results were approximately as shown in Fig 5.38, which shows the hottest region against the left wall of the round runner, and colder material against the opposite wall After a branch, if there is sufficient length of runner, the distribution of heated plastic around the circumference of the runner will start to reestablish itself There is rarely, if ever, enough length of runner to equalize the distribution of heat around the runner One problem Beaumont encountered was the influence of the sprue shear heating as it branched into the primary runner The shear-heated plastic in the sprue added a layer of heated plastic to the fixed side of the primary runner as it branched from the sprue This has the ef- Ch 5: Causes of Molded-Part Variation: Mold Design Figure 5.37 Melt properties at the intersections of the primary and secondary runners.[27] (Courtesy of SPE.) Figure 5.38 Temperature gradient in a round runner just after a runner intersection, showing the hottest material on the left © Plastics Design Library 69 Without some means of redistributing the hot layer of plastic on one side of the runner, the hot plastic will flow in a laminar fashion into the tertiary runners or into the parts If the plastic flows into a cavity with this type of temperature variation in the runner, the part will likely have a temperature gradient from one side to the other The part will cool at a different rate from the cold side to the hot side, which will cause differential shrinkage and warpage Figure 5.39 graphically shows the temperature/shear profile across two molded parts immediately downstream from a runner branch Reinforcing fibers are more likely to be broken in the high shear areas adversely affecting physical properties and, if segregated (as shown in Fig 5.39), can also contribute to shrinkage variations Figure 5.40 is slightly misleading because it implies that uniformly hot material flows into one branch, while uniformly cool material flows into the other In fact, there is a temperature gradient across both tertiary runners, with the hottest material near the lower left wall of the left tertiary runner and the coolest against the lower right wall of the right tertiary runner However, there is a significant difference in the average temperature between the left and right tertiary runners If there were another branching of the runner system (such as for a sixteen-cavity mold), there would be a further segregation of the plastic temperature in the runner system Beaumont found that each plastic has a different reaction to shear rate Some were affected only slightly, while others were strongly affected Under some conditions, the inside cavities could be 75% filled when the outside cavities were only 25% filled Sometimes the same plastic would react strongly to shear at high Figure 5.39 Shear differences within molded parts.[27] (Courtesy of SPE.) © Plastics Design Library shear rates, but only slightly at low shear rates He further discovered that he could rotate the location of the hot layer in successive runners so that when those runners branched, equal amounts of hot and cold material would flow into each branch runner Figure 5.41 shows how the flow rotation repositions the hot layer of plastic By rotating the hot layer to the top of the secondary runner, the tertiary runner receives equal amounts of hot plastic The trick is to rotate the flow so that the hot layer is precisely centered when it reaches the next runner branch Figure 5.40 Melt properties at the intersection of the secondary and tertiary runners.[27] (Courtesy of SPE.) Figure 5.41 The effect of elevation change on melt rotation.[27] (Courtesy of SPE.) Ch 5: Causes of Molded-Part Variation: Mold Design 70 Figure 5.42 ignores the effect of the sprue as the plastic flows into the primary runner The top half of the primary runner contains plastic that has been heated by shear in the sprue This means that a perfect 90° melt rotation puts hotter plastic in the outer runners The melt-rotation system is approximately equal to using trapezoid runners and alternating the runners between the moving and fixed halves of the mold at each runner intersection Beaumont’s patented configuration approximates moving the plastic from a round runner to a trapezoid profile just before the intersection, to perform the rotation A keynote speaker from Moldflow at the ANTEC 2001 Conference showed an analysis of the Beaumont melt-rotation system which indicated that it heated the plastic on the side opposite the sprue Beaumont indicated that his system can be “tuned” to cause a perfect 90° rotation The actual tuning may be related to the shear heating caused by the profile changes that the Moldflow people have discovered in their analysis To summarize, rotating the melt at the runner intersections causes a more even distribution of heated plastic to the various cavities in a naturally balanced mold This leads to more even shrink and warp in multiple-cavity applications 5.6.2 Poor ejection also can cause distortion in the finished part If the part sticks in the mold in one area where there is not a sufficient number of ejectors, the part will be bent or deformed as a result of the ejection action in another area Another type of distortion is caused when there are not enough ejector pins and the pins bend or indent the part in the area immediately around the ejectors Occasionally, a mold is conceived and built so that the part tends to stay on the fixed side rather than the ejection side of the mold If this occurs, heat differential across the parting line can assist in forcing the part to stay on the ejection side of the mold This causes other problems See Sec 5.7 and especially Fig 5.45 5.7 Ch 5: Causes of Molded-Part Variation: Mold Design Mold-Cooling Design The mold should be designed as a heat exchanger Its primary functions are to shape, contain, and cool the molten plastic It is important to note that even when the thermal settings of both the coolant flowing to the core and into the cavity are identical, there can still be a difference in cooling capacity Cores and inside corners of plastic parts require more cooling than flat or outside corners of the same part Thick areas also require more cooling because of the increased mass of plastic that requires cooling For all but the simplest molds, six, eight, or even more zones of cooling may be necessary for best results Far too many molding machines are equipped with inadequate cooling lines They may not have enough flow capacity to maintain turbulent flow in all the cooling channels The flow limitations may occur in the main coolant lines to the machine, or the connectors and hoses to the mold may be too small The machine may not have a sufficient number of connections to feed each cooling zone Semicrystalline materials require more cooling than amorphous materials See Table 13.5 in Ref 63, Polypropylene, by Maier and Calafut, for some enthalpy (heat content) values 5.7.1 Figure 5.42 Positions of hot and cold layers before and after melt rotation.[27] (Courtesy of SPE.) Ejection Cooling Channels Turbulent flow in coolant lines is much more effective than laminar or streamline flow in transferring heat from the mold to the coolant Turbulent flow continually stirs the coolant to maintain a relatively uni- © Plastics Design Library 71 form temperature from the surface of the cooling channel to the center of the channel Laminar flow, where the surface flow and the core flow not mix (think of sheets of paper sliding past one another) on the other hand, results in a warm layer of coolant along the walls of the coolant channel with much cooler coolant flowing through the center, or core, of the channel The stagnant layer of coolant along the walls of the cooling channel acts a little bit like an insulation layer Turbulent flow breaks up this insulation layer Turbulent flow begins to occur when the Reynolds number (R) is somewhere in the range of 2200 to 4000 in the following equation, where v is the fluid velocity in m/s, d is the channel diameter in meters, and K is the kinematic viscosity of the fluid in m2/s It is recommended that the system be designed to operate with a Reynolds number greater than 5000 Eq (5.1) R = v×d/K At the temperatures shown, the kinematic viscosity of water is as follows: 0°C 1.8 × 10-6 m 2/s 20°C 1.0 × 10-6 m 2/s 60°C 100°C 0.45 × 10-6 m 2/s 0.28 × 10-6 m 2/s When extremely cold coolant is required, antifreeze is usually necessary in the cooling water to prevent its freezing, but antifreeze acts as a lubricant and promotes laminar flow instead of turbulent flow as in water without antifreeze at the same flow rate Even though the coolant is much cooler, changing from turbulent flow to laminar flow may not improve cooling; it may even make cooling less effective Therefore, when using water with antifreeze, the flow rates must be raised significantly to maintain turbulent flow and cooling efficiency If lowering the coolant temperature below freezing and adding antifreeze does not improve cycles and cooling, larger hoses, connections, and supply lines, and higher pressure across the mold may be necessary to maintain turbulent flow Note that if three-plate or hot-runner molds are used, cooling requirements between the runner level and the cavity level, and between the runner level and the molding-machine platen, must both be considered Uniform cooling is important because warmer areas solidify last, thus they shrink more than adjacent areas The hotter surfaces of the part will continue to shrink © Plastics Design Library more than the cooler surfaces after gate seal-off and part ejection This sets up bending stresses in the molded part that may or may not be apparent when the part is ejected When the part is rigid enough to prevent buckling (for example, due to its modulus or geometrical stiffening features such as edge stiffeners, ribs, etc.) it will keep its shape, but it will be stressed internally This is important because internal stress levels can lead to reduced environmental stress crackresistance, reduced impact performance, and warpage, if the part is exposed to elevated temperatures (where modulus is reduced) at some point during assembly, decorating, or in service On the other hand internal stress levels may show up immediately as distortion or warpage The following is a general rule for avoiding thermally induced warping: The coolant-flow rate multiplied by the temperature rise of the male half of the mold should be equal to the coolant flowrate times the temperature rise of the female half of the mold If these values are not equal, the side with the lowest rate of extraction should be treated as follows: • Decrease the coolant inlet temperature • Increase the coolant flow-rate • Increase the diameter of the cooling channels • Increase the number of cooling channels • Position cooling channels nearer the mold surface • Introduce heat pipes into tight or corner regions Inadequate cooling in the corners of a box-shaped profile can result in the type of warpage shown in Fig 5.43 Skilled metal-workers have known for a long time that localized heating causes increased shrinkage in the heated area after it cools, and they use this knowledge to straighten shafts and pop out dents The same principle applies when molding semicrystalline materials: the warmer areas have more time for crystallization; the higher the percentage of crystallization, the greater the shrink Due to the complexity of many part and mold designs, it is difficult to achieve completely uniform cooling in practice Differential shrinkage through the thickness of the part can be caused by differences in the cooling rate between the cavity and core Ch 5: Causes of Molded-Part Variation: Mold Design 72 This technique is essential when molding flat components to close tolerances or large components that include long melt-flow lengths from the gating position The effect of differing temperatures on opposite sides of the mold is shown in Fig 5.45 Assuming adequate cooling capacity, the time required to cool a plastic part (in seconds) is A × t1.8 The values for A (in s/mm) for various materials are given in Table 5.4 and t is the maximum thickness of the molded part (in mm) Figure 5.43 Corner shrinkage in box profiles The size and location of the cooling/heating channels are extremely important, as these allow a rapid and uniform heat removal during the material-solidification stage A guideline for proper cooling channel location in flat areas is shown in Fig 5.44 Cooling/heating channel diameters of 12 mm (about 7/16 in.) are recommended so that the flow rate of the cooling/heating medium is high enough to maintain turbulent flow and the mold surface temperature to within ±1°C (± about 2°F) To obtain a constant mold temperature, it is recommended that the molder use insulating plates attached to the back of each mold half to minimize the heat loss to the molding-machine platens Such plates also help minimize the time period needed to bring the mold to the required temperature Figure 5.45 The effect of differing temperatures on opposite sides of the mold: it causes the part to be concave towards the hot side Table 5.4 Constants (A) Used to Calculate the Time Required to Cool Various Materials Plastic Figure 5.44 Proper size and location of cooling channels in flat areas D = distance; d = diameter; P = the distance between cooling channels Ch 5: Causes of Molded-Part Variation: Mold Design A ABS 2.84 Polystyrene 2.84 UPVC 3.00 LDPE 3.12 Nylon 6,6 3.24 PP 3.53 HDPE 3.53 © Plastics Design Library 73 Differential shrinkage through the thickness of the part can be caused by differences in the cavity and core geometry that occur in areas such as corners Compared to the cavity side of a tool, the core side has a reduced surface area and can be difficult to cool effectively in practice due, for example, to structural concerns The core side of the molding tends to stay hotter, and therefore shrinks more when the part is ejected As a result, a stress is created that causes the part to warp inward after it is ejected from the mold Cooling the injection molding uniformly may mean cooling the mold at different rates, in different areas, so as to get uniformity of component cooling The aim must be to cool the component as quickly as possible while preventing faults, such as poor surface appearance and changes in physical properties Each part of the molding should be cooled at the same rate This often means that nonuniform cooling must be applied to the mold (for example, routing the coolest water to the smallest and most difficult to cool cores) Some of these warpage problems can be corrected during production If the tool has been built in such a way that the different cavity and core sections of the tool have individual cooling circuits, this allows the process engineer to make local tool-temperature adjustments in order to control the cooling rate from each surface Part of the heat transfer problem is to conduct adequate heat out of difficult areas One such difficult area is a core pin Figure 5.46 represents a cross section of a core pin Core pins must conduct massive amounts of heat away from the molded part because they are totally surrounded by molten plastic The shading represents the heat that must be removed from the plastic part and conducted away Note that all the heat that is transmitted into the core pin must be conducted down the length of the core pin before it can be transferred to the cooling water in the mold Even in the event that cooling water can be introduced into the core, the units of heat are converging, which inhibits their removal On the opposite side of the part, the units of heat are diverging, and cooling water can be placed quite close to the surface of the plastic part The “bottom line” is that heat can be removed from cavities more easily than it can be removed from cores One way to improve heat transfer out of a core that is too small to contain a cooling channel is to make it out of a material that conducts heat rapidly Unfortunately, most high–heat-transfer materials are too weak and soft to be used in an injection mold Figure 5.47 shows one way to improve heat transfer A steel shell with a solid core of copper is considerably more efficient than solid steel, and a heat pipe inside a steel core is even better, provided the heat pipe has an adequate heat sink or exposure to coolant Figure 5.46 The concentration of heat that must be removed by a small core pin in a molded part Figure 5.47 One method of increasing heat transfer rate out of a small core Arburg, a manufacturer of injection-molding machines, uses a simpler formula for approximating the cooling time of a plastic part For mold temperatures less than 60°C, the cooling time is approximately t(1+2t) where t is the material thickness (in mm) This yields a cooling time of 10 seconds for a material thickness of mm For temperatures above 60°C, Arburg uses (1.3t)(1+2t) to calculate the cooling time For a warm mold, the cooling time would be 13 seconds for a 2-mm thick wall Some exceptions to these rules are shown below: 30% carbon-fiber-filled PEEK 2.7 × t (sec) 30% glass-fiber-filled PEEK 4.7 × t (sec) 20% glass-fiber-filled PEEK 7.3 × t (sec) For unfilled grades of PEEK 20 × t (sec) © Plastics Design Library Ch 5: Causes of Molded-Part Variation: Mold Design 74 So-called “heat pipes” use evaporation and condensation of a coolant inside a sealed tube to increase conductivity of a given diameter to over thirty times that of copper If a core is too small for baffles or bubblers, half the length of a heat pipe can be inserted into a core with the other half of the heat pipe in a cooling channel or even exposed to air if cooling fins are added to the heat pipe If the core pin is large enough to contain water lines, spiral baffles are usually more efficient in transferring heat because it is easier to maintain turbulent flow within the cooling channel in the core than it is with bubblers (cascades) Figure 5.48 shows a typical baffle and a typical bubbler The baffle shown is straight If the blade up the center of the hole were twisted into a spiral, then it would be a spiral baffle, which improves uniformity of cooling Frequently, cores need to be on separate cooling channels to allow the molder to use a colder coolant in these difficult-to-cool areas 5.7.2 Effects of Corners The inside corners of the molded parts should receive special consideration High conductivity metals and heat pipes, baffles, or bubblers can be used to draw heat out of these high–heat-stress areas Figure 5.49 indicates a typical inside and outside corner of a molded part The same problem is present here as with a core pin The internal corner must dissipate heat faster than the outside corner If it does not, then the inside corner solidifies later than the outside corner and, as a result, shrinks more and tends to pull the outside walls as shown in the lower part of the figure Even with the cooling channels close together in the inside corner of the part, there is still a great deal Figure 5.48 A typical baffle and bubbler Ch 5: Causes of Molded-Part Variation: Mold Design of difference in the heat that must be dissipated between the inside and outside corners of the part and, as a result, the temperature of the mold in these areas is significantly higher Some improvement in corner cooling may be obtained by placing a cooling channel as close to the corner as possible and/or inserting the corner with a high-conductivity material such as highstrength aluminum or brass, as shown in Figure 5.50 Gate areas may need their own cooling channel to allow extra cooling because more heat must be dissipated near the gate Warmer water or reduced coolant flow may be required in the cavity areas, especially near the outside corners of the molded part These areas tend to cool too rapidly because they are surrounded on two or three sides with metal, as opposed to the inside corners of the molded part where the mold is surrounded on two or three sides with molten plastic Excessive flow-length between the cooling source and the return to the cooling source is detrimental to effective cooling All too often, molders have as few as four cooling channels for an entire mold For large molds with many water lines, many more channels are necessary for good cooling Coolant should not flow more than one meter (40 in.) between coolant pressure and return Longer flow-lengths result in too great a temperature increase in the coolant between pressure and return lines Figure 5.49 The female inside corner of the mold has less thermal mass to absorb heat than the male outside corner; this results in a cooling rate differential for the plastic part The inside corner of the part is the last to cool, so it shrinks more and creates corners that are slightly less than 90° © Plastics Design Library 75 5.7.4 Runnerless Molds Runnerless molds can reduce the heat that must be removed from the mold because the material in the runner system need not be cooled before the part can be ejected from the mold Often the cycle can be shorter than conventional molds Sometimes conventional mold cycles are longer to allow the runner to become more rigid to aid in handling and part separation than would be necessary with a runnerless system 5.7.5 Slides Slides must be cooled as effectively as the rest of the mold Failure to cool the slide may cause it to expand enough to bind against the surrounding mold components In addition, if the slide is hotter than the rest of the mold, the plastic in contact with the slide will shrink more than the plastic in other areas Figure 5.50 Cooling in the corner of a mold 5.7.6 5.7.3 Thickness Variations The length of coolant flow must be much shorter for areas where a great deal of heat must be removed, compared to areas that are relatively thin and easy to cool Gates and thick areas require significantly more cooling than other areas When dimensions and warpage control are critical, it may be necessary to use several temperature-control units so that zone cooling can be used to maintain as uniform a coolant temperature as possible Sometimes several different temperatures are necessary for best results Another problem with thickness variations is that when plastic enters a thinner area, it tends to slow down and solidify somewhat, causing even greater resistance to flow The hotter plastic that comes along later and “breaks through” or finds a path around the partially solidified plastic will be warmer, and as a result will shrink more, than the plastic that hesitated and partially solidified This differential shrinkage is likely to cause warp Therefore, when large areas are being filled and there are possible causes for plastic hesitation, it is advisable to use gates with more cross-sectional area so that the melt front will be encouraged to flow smoothly across the part with as little hesitation as possible © Plastics Design Library Venting The mold must be vented to allow for gas escape; such vents must be placed near weld lines and also near the last areas to be filled Typical vents are slots 6.00–13.00 mm (0.25–0.50 in.) wide and 0.01–0.03 mm (0.0004–0.0015 in.) deep; such slots are located on the mating surface of one of the mold halves If a negative-pressure cooling device is available, it may be possible to vent blind or dead-end pockets of the mold into the water channels if no other venting option is possible This can speed up mold-filling, reduce component-burning, and reduce the cycle times Some experts advise surrounding the cavity with a runner for air escape A land of 0.75–1.00 mm (0.03– 0.04 in.) should separate this runner from the mold cavity They suggest that the trapped air can escape to the vent-runner across the short land, and that there is less likelihood of parting-line damage from trapped plastic between the faces of the mold The short land will act as a cutting surface and any trapped material will be forced into the cavity or the runner There are sintered metal devices that can be placed in blind pockets to allow air to escape from areas not near the parting line or water channels International Mold Steel manufactures a sintered tool-steel that allows air to escape through the very surface of the mold Inadequate venting traps air in the mold When the high pressure applied to the molten plastic forces the Ch 5: Causes of Molded-Part Variation: Mold Design 76 plastic to displace the air, the air is heated due to the compression The temperature of the air can reach levels far above that which will degrade the plastic and can cause momentary flames in the mold, burning the plastic until the oxygen in the air is consumed in the combustion This typically leaves burned spots on the plastic part and incomplete fill in the burn areas It also leaves behind a deposit of degraded material, the product of the combustion Sometimes molders slow down the cycle time to allow the trapped air more time to escape to avoid burning the material, instead of adding adequate vents Obviously this is not the most efficient way to produce plastic parts here Table 5.5 displays values for selected materials in units of BTU/ft.hr.°F The mold designer must use materials that will withstand pressure and wear requirements and still enable the molder to have adequate cooling capability to evenly and rapidly remove heat in a manner that does not lead to part warpage Table 5.5 Thermal Conductivity of Various Tooling Materials Material Copper Alloy 5.8 Mold Construction Materials Keep in mind that the mold has to fulfill three functions: (1) shape the part, (2) contain the molding pressure without distortion, and (3) act as a heat exchanger to remove heat from the molten plastic as quickly and uniformly as possible An integral part of shaping the part and containing the pressure is resisting wear Many of today’s resins contain abrasive materials that quickly abrade surfaces over which they move Also, the higher clamping pressures being used tend to accelerate parting-line damage These factors lead to the conclusion that harder, more wear-resistant materials must be used in higher quality, higher performance molds Harder materials usually have lower heat conductivity than softer materials (especially copper or aluminum) Therefore, more sophisticated cooling techniques may be employed So-called “heat pipes,” which, in extreme cases, can have up to a thousand times the heat conductivity of copper, are often used to cool difficult areas Softer materials can sometimes be used for relatively low-volume production Softer materials can also be covered with hard coatings such as chrome, titanium nitride, or other materials Designing cooling channels that conform to the shape of the part is the emerging state of the art for making cores and cavities At least one supplier (DME) offers rough-profiled blanks with built-in water channels that conform to the shape of the finished part The blanks are formed by a printing process that builds up layers of powdered metal that are bonded together with a polymer Later the polymer is baked out and the powdered metal bonded together to form an impervious mass The thermal conductivity values of various materials commonly used in mold construction are listed Ch 5: Causes of Molded-Part Variation: Mold Design Thermal Conductivity (BTU/ft.hr.°F) 187 Aluminum 2017 95 Brass 69 Beryllium Copper 64 Steel (1% Carbon) 26 Tool Steel P20 21 Tool Steel H13 12 Stainless 316 10 Epoxy-Glass Tooling 0.3 Molten Plastic 0.1 5.9 Prototype Molding with SLA or Similar Type Molds One valuable tool for perfecting a plastic-part design, but one that is often overlooked, is to prototype the molded part using a mold created by stereo-lithography (SLA) techniques Often the part design must be simplified somewhat to allow the use of an SLA mold An SLA mold must have generous draft and cannot be used for small details or cores These types of detail must be omitted or added via aluminum or steel inserts Neglecting SLA prototype testing can be a significant pitfall for mold designers In one study,[30] tests were made to compare shrinkage, strength, and flexural modulus in polycarbonate parts made in SLA and in steel molds Shrinkage results from these tests are given in Table 5.6 Shrinkage is shown as a percent change in length according to the following calculation © Plastics Design Library 77 native conclusion is that because SLA is such a poor conductor of heat, a very thin skin is formed as the mold fills, leaving the bulk of the plastic part molten for a very long time This reduces the tendency to create and maintain fiber orientation High fiber orientation is essential for high physical values Not surprisingly, the SLA-molded parts had lower physical values than the samples molded in steel molds The slower cooling rate in the SLA mold increases shrinkage The slow heat-transfer rate of SLA molds has an effect that is similar to molding in a hot mold All other things being equal, hot molds increase shrinkage and reduce fiber orientation  part dimension   × 100 Shrinkage (%) = −    cavity dimension  Unfilled polycarbonate (Lexan 141); 10% glass-filled polycarbonate (Lexan SP7602); 20% glass-filled polycarbonate (Lexan 7604); and 20% glass-filled polycarbonate with mold release (Lexan 3412 R), were used in this experiment The primary aim of the study was to discover if prototype parts molded in SLA molds had physical characteristics identical to or near enough to those parts molded in steel molds, to make tests on SLA short run prototype parts valid Their conclusion was that SLA molds could be used successfully to evaluate a plasticpart design The authors of that study may have misinterpreted the data when concluding that the slightly lower physical characteristics and higher shrinkage they observed in the parts molded in the SLA mold were due to a longer cycle time required in SLA molds, causing material degradation in the barrel An equally valid alter- 5.10 Pitfalls to Avoid The mold builder’s major pitfall is overlooking one or more of the items mentioned in this book, for example, determining gate size without considering gate location and runner design, or designing cooling channels without provision to zone cool Table 5.6 Shrinkage of a Variety of Polycarbonate Grades in Steel and SLA-Mold Cavities Unfilled Flow SP7602 SP7604 3412 R Steel SLA Steel SLA Steel SLA Steel SLA Cross 0.6 0.7 0.2 0.3 0.5 0.4 0.4 0.4 Along 0.6 0.6 0.3 0.3 0.2 0.2 0.2 0.3 © Plastics Design Library Ch 5: Causes of Molded-Part Variation: Mold Design