Mold Design Fundamentals P2 pot

1 6 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Insulated Runner System Insulated Runner System Molten state melt Solidified resin shell Cooling Lines Emergency parting line Parting line q Oversized the runner diameter (15~30mm) q Insulation effect of frozen skin shell q Works for most olefinic resins(PE,PP ) and PS In the insulated runner system, the runner diameter is oversized (say, 15~30mm) in order to maintain the molten state of the material. The large diameter runner allows an inner molten melt to pass through during the molding cycle because of the insulation effect of frozen skin shell surrounding the melt core. The insulation runner system has the advantage of extremely simple construction, low cost tooling, and high efficiency, provided the system can be left running undisturbed for long periods. This design is suitable for most olefinic plastics (such as polyethylene (PE), polypropylene (PP) ) and polystryene (PS). The disadvantages of the insulated runner system includes: - it requires fast cycle to maintain molten state within runner (at least 5 shots/min). - it requires long start-up periods (15-25min) to stabilize the runner temperature (up to 150 o C) - it needs a long color change time - it needs very accurate gate temperature control in order to have a satisfactory production rate. - Additional emergency parting line is required to facilitate the removal of the frozen runner in the case of prolonged delay in the cycle time. 1 7 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Internally Heated Hot Runner Internally Heated Hot Runner System System q Material is heated by the heating element in the center of the runner q Annular gap for melt flow Heater Cartridge Heated Probe (Torpedoe) Part Melt Tempertature Profile Vlocity Profile In the internally heated hot runner system, the material is heated and kept at a molten state by the heated probe (torpedoe) in the center of the runner. The melt is allowed to flow in the cross section of the annular gap of the runner. The advantages of the internally heated hot runner systems include: -Less heat loss and lower heating power required since the thermal insulation of polymer melt -Less mold components mis-matching problem arising from thermal expansion -Inexpensive (as compared with the external heated runner system) -Little space required. The disadvantages of this design include: -Higher shear rate and pressure drop since the restricted flow area -Sophicated heat control required (temperature profile exists in the cross section of the annular gap of the runner). 18 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Externally Heated Hot Runner Externally Heated Hot Runner System System q Material is heated by the cartridge-heating manifold in the housing of the runner q Circular cross section for melt flow Cooling Lines Heater Cartridge Heated Manifold Part Air gap insulation Insulation Blocks Hot Runner Vlocity Profile: plug-like flow Temperature Profile: constant temperature profile In the externally heated hot runner system the material is heated by the cartridge-heating manifold in the housing of the runner. Thus a plug-like flow profile and an approximately constant temperature profile across over the circular flow area is developed. Thus the flow resistance is smaller than that of the internally heated system. The advantages of this design are: -More uniform temperature distribution. -Better temperature control -Lower melt stresses and pressure drop -Color/material changes easily The disadvantages of the externally heated hot runner system include: -More complicated design -More Expensive -Significant thermal-expansion-induced mis-match problems for various mold components. 19 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Design of Gate Design of Gate Generalities •ease of demolding •ease of degating •weld lines •distortion •molding defects •cost Part Design •geometry •wall thickness •direction of mechanical loading •quality demands (dimensions,cosmetics, mechanics ) •Flow length Plastic Material •viscosity (MFI) •processing temperature •flow characteristic •fillers •shrinkage behavior Then gate provides the connection between the runner and the mold cavity. It must permit enough material to flow into the mold to fill out the cavity, raises melt temperature by viscous (frictional) heating, and freezes-off when the holding stage is over. It should be smaller in the cross section so that it can be easily separated from the molded part (degated). The type of the gate and its size and location in the mold strongly affect the molding property and the quality of the molded part. The factors which determine the gate design is summarized here briefly. General speaking, the gate should be small, simple to demold and easily separated from the part. The gate should be connected to the molding in such a manner that the latter is not distorted (the molding tends to deform concave to the feed ) and does not exhibit blemishes. Cost of tooling is also a consideration factor. The location of the gate must be such that weld lines are avoided or shifted to a less critical position. Molding defects such as jetting, burning, thermal degradation, short shot, etc. should be avoided in the production. Gating scheme and location of gates are crucial to the quality of the molding. Filling pattern and cavity pressure profile are closely related to the final properties of molded parts, such an mechanical properties, cosmetics (surface properties), dimensional accuracy. A gate should provide appropriate filling pattern and viscous heating effect, permit effective packing and holding of the material within the mold. These criteria depend on both part design as well as physical properties of the plastic material. 20 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Gating Scheme Gating Scheme Direct/Sprue Gate Side/Edge Gate ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ Pin Gate There are several gate type can be adopted in the mold design, and each has its own advantage for application. The direct gate or sprue gate feeds material directly into the cavity. It is used for temperature-sensitive or high viscosity materials, and is suitable for producing part with heavy sections. The direct gate can be applied in high quality part because it allows effective holding (minimum pressure loss) and exact dimensions can be obtained. However, it is suitable only for single-cavity molds. Visible gate mark and the high stress concentration around the gate area are the disadvantges. The side gate or edge gate is the standard gate for injection molding. It is used wherever the product can be or must be gated from the parting line and where self-degating is not required or practical. It is carried out at the side of the part and is easy to construct and degate. The pin gate or pinpoint gate is a kind of restricted gates that are usually circular in cross section and for most thermoplastics do not exceed 1.5mm (0.06 in.) in diameter. It is generally used in three-plate molds (with automatic gate removal) and hot runner construction. It provides rapid freeze-off and easy degating of the runner from the gate. Flexibility in gate location is another advantage of the pin gate. It can easily provide multiple gating to a cavity for thin-walled parts. Viscous heating as the melt passing through the restricted pinpoint gate raises melt temperature and improves the filling process since the melt viscosity is lowered. Higher pressure drop is a drawback. 21 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Gating Scheme Gating Scheme Fan Gate Film Gate Tab Gate Disc Gate The fan or fin gate is a fanned out variation of the edge gate. It is used for large flat parts (say,over 8cm x 8cm or 3 in x 3 in) or when there is a special reason such as elimination of weld lines. when the danger of part warpage and dimensional change exists, the fan gate is often adopted. The film gate or flash gate involves extending the fan gate over the full length of the part but keeping it very thin. It is used for flat molded part in the situation that the orientation of flow pattern in one direction is required, this is important in the applications of optical parts. It has the advantages that there is no weld line, reduced warpage and improved part dimensional stability. However, postoperation for gate removal is required for this type of gate. The tab gate is used in cases where it is desirable to transfer the stress generated in the gate to an auxiliary tab, which is removed in a postmolding operation. The tab gate is capable of preventing the jetting problem during the filling stage. Flat and thin parts require this type of gate. The disc gate or its variation, the diaphragm gate, has a conical manifold. It is used for rotationally symmetrical parts (hollow tubes) with core mounted at just one half of the mold. The advantage of using this gate system is that there are no weld lines, and concentricity of the molded part is ensured. This is a important dimensional requirement for pipe fittings. The cone or diagram region eliminates stress concentration around the gate since the whole area is removed, but the postoperation is necessary and more difficulty. 22 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Gating Scheme Gating Scheme Ring Gate Submarine/Tunnel Gate The ring gate accomplishes the same purpose as gating internally in a hollow tube, but from the outside. In the ring gate the melt reaches an annular channel manifold next to the sprue. The gate has a small cross section and acts as a throttle. Therefore the annular channel fills before melt begins to fill the cavity. It is adopted in the case that the core cannot be mounted on just one side of the mold such as in the case of disc gating. The ring gate is used to produce sleeve- like parts with core mounted at both sides of the mold.The advantages of this gating scheme include: uniform wall thickness around circumference can be obtained, applicable for long cylindrical part, as well as easy production. However, final finishing of molded part is necessary and sometimes slight weld line may appear. The submarine or tunnel gate is used mainly for small parts in multicavity mold where it is possible to locate the gate laterally. This gate is automatically degated as soon as the mold opens, this is the primary advantage of this gate system. However, it is used for simple part only because of high pressure loss as the melt passing through the small gate cross section and the runner length. The tunnel gate can be used only for tough, elastic materials, since the material in the tunnel has to withstand deformation during mold opening; the tunnel could break and plug the runner system if brittle materials are used. 23 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Effect of Gating Scheme Effect of Gating Scheme Side gate: possibility of jetting Tab gate: uniform filling, no jetting The filling pattern of melt flow is largely governed by the location and size of the gate(s). For example, jetting of the plastic into the mold cavity may occur if a fairly large cavity id filled through a narrow gate (such as a side gate) is used, especially in the case of low-viscosity plastic melt. Jetting gives rise a random filling pattern: the melt no longer fills the mold by an advancing front way but snakes it away into the cavity without wetting the walls near the gate. Surface defects, flow lines, variations in structure, and air entrapment are related to the jetting phenomena. Jetting can be prevented by enlarging the gate or locating the gate in such a way that the flow is directed against a cavity wall. For example, tab gates (or fan gates) can minimize the potential of jetting by reducing the inertia of the inlet melt flow. 24 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Effect of Gating Scheme Effect of Gating Scheme Time Cavity Pressure Sprue Gate ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ ‧ Pinpoint Gate Film Gate )Different influence on holding stage and effective holding time The gating scheme has a significant influence on the holding pressure profile during the cooling stage. For exmple, the size of a sprue gate is large so that the holding pressure can be transmitted without difficulty. The gate freezing-off time is longer due to the larger gate size, leads to a slower droping in the cavity pressure and a longer effective holding time. Hence in general a sprue gate is used for part that the dimensional accuracy is important. On the other hand, the pinpoint gate freezes early and leads to a shorter effective holding time. This may cause sink marks and voids in the final part. The cavity pressure curve of part with film gate is located between that of sprue gate and pinpoint gate. In the mold design phase, one have to consider if the gate can provide suitable filling pattern, viscous heating, as well as its influence on effective holding time. 2 5 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Weld line and Gate Location Weld line and Gate Location q Hot Weld (Streaming Weld, Meldline) ) Weld lines arising from obstructions (core,insert,pin ) in the flow the melt is split by the obsraction into two fronts the two streams are brought back together the temperature at the weld line does not differ much Weld lines or knit lines are formed during the mold filling process where two melt fronts meet each other. Microscopically, in the weld lines (or weld planes) the two fronts are made of molecules that are aligned with the front shape and will meet tangentially. The incomplete molecular entanglement and diffusion, unfavorable frozen-in molecular (or fiber) orientation, as well as the crack-like V-notches at the weld surface lead to structural weaknesses in the weld line area. The presence of weld lines causes reduced mechanical strength for structural applications and surface visual imperfections in the part. The allowable working stress would be reduced by at least 15% in the weld line area. In general, the colder the merging flows of melt, the more these weld lines become visible and the poor is their strength. Hot weld lines (or streaming weld line, meldline) is formed in the molds with obstructions such as core, insert, or pin. In this case the melt front is separated by cores or obstructions and recombines at some downstream location. Experimental results indicate that the strength of the weld would decrease as the distance between the obstruction and the gate increases, since the average flow front temperature has been reduced. [...]... mobility and their coupling Increase the mold temperature is another strategy to improve the welding strength Besides, welding strength can be improved by good molding venting (avoid air entrapment), high injection speed (decrease the temperature drop) Gate design play an important role in the removal or elimination of weld lines 27 Mold. ppt Weld line and Gate Design Edge gating will lead to a weld line... melt temperature (that is, chain mobility and coupling) - Increase mold temperature (that is, chain mobility and coupling) - Increase injection pressure (that is, lower the temperature difference) - Avoid use of external release mold lubricant (avoid the presence of substances at the weld interface) foreign 29 Mold. ppt Weld Line and Gate Design Allow cavity filling with a minimum no weld lines more significant... weld strength due to shorter flow lengths Sprue gating at the cup bottom will eliminate weld line,gate mark problem CORETECH SYSTEM Consider the gate design in an injection-molded cup This part can be produced using a single edge gate in a two-plate mold This gating scheme would result in a cold weld line opposite the gate As the diameter of the cup is increased, the weld line becomes more visible... depend on the product specification and quality requirement Computer analysis is capable of predicting the possible location of weld line According to the analysis result we can modify the gate design, part design (modify the part thickness), or process condition, to relocate the weld lines to visually or structurally less sensitive areas Consider a scanner cover that is produced by three submarine... unfavorable From the CAE analysis result we can predict the possible weld line locations and check if they occur in critical regions This precautions from CAE analysis in the design phase will minimize the risk of part failure We can modify the design conditions to see if the weld lines can be relocated to noncritical regions When they are unavoidable, venting plays an important role in improving the weld strength... σ 3 Thin sections are particular prone to weak welds because of rapid melt solidification and less chance for chain diffusion 26 Mold. ppt Weld line and Gate Location q Cold Weld (Butt Weld) arise from the impingment of advancing fronts from different gates in multi-gating molds Worst welding manner the temperature of the fronts has dropped somewhat at the welding zone Melt fronts traveling in opposite.. .Mold. ppt Example of Hot Weld lines Average Temperature Melt Front b.weld @ 185 oC a.weld @ 188 oC c.weld @ 184 oC CORETECH SYSTEM Consider a part has one rectangular and two circular inserts obstructing... fronts traveling in opposite directions meet, and are almost immediately stoped after meeting CORETECH SYSTEM On the other hand, the so-called cold weld lines or butt weld lines present in multiple gating molds where the impingment of advancing fronts from different gates may occur Cold weld lines are generally considered to be the worst welding manner because they are formed from melt fronts traveling... sprue gate at the cup bottom is used in this case No weld line would be produced in the final part However, the significant gate mark is a problem and an postoperation is require to finish the product 28 Mold. ppt Location of Weld Line Top Cover of Scanner Possible weld line location Possible weld line location PLAY447 CORETECH SYSTEM In general, weld lines would be visually unacceptable, or, since they... too long and/or too high the injection pressure is required to fill the cavity In some cases a multiple gating scheme is required to avoid short-shot (incomplete filling) problems Consider the injection-molding of the motorcycle side cover by ABS If two gates per cavity is adopted, one weld line is produced in each cavity However, the injection pressure required is high and short-shot problem will present . complicated design -More Expensive -Significant thermal-expansion-induced mis-match problems for various mold components. 19 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold. ppt Design of Gate Design. easily separated from the molded part (degated). The type of the gate and its size and location in the mold strongly affect the molding property and the quality of the molded part. The factors. drop). Gate design play an important role in the removal or elimination of weld lines. 28 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold. ppt Weld line and Gate Design Weld line and Gate Design Edge