Tài liệu Mold Selection P1 pdf

113 4 Mold Selection 4.1 Selection of an Appropriate Mold Once a good product design has been achieved and it is decided where the product will be made and how many cavities are required, we must consider the available alternatives for the molds. 4.1.1 Dedicated Mold, Universal Mold Shoe “Dedicated mold” means a complete mold that is used for one purpose only. After use, the mold is put into storage until it is used again. This is the most common type of mold. Occasionally, especially with molds with 2–8 cavities, the same mold shoe can be and often is used for more than one set of cavities and cores. In principle, there is nothing wrong with this concept, provided the molding shop is well organized (good record keeping and proper storage facilities for the loose stack parts) and the personnel is capable of making the switch from one product to another without the need for high-priced mold makers. It may take a few hours to switch from one set of stacks to another and there is always the risk of damage to the mold components in handling and during assembly. The question is whether it is worthwhile to switch molds, especially if it is done frequently. If the mold shoe is quite simple, it would be better (safer and more economical) to have a dedicated mold. But there are cases where the mold shoe is large, complicated, and relatively expensive; if the stacks for the various (preferably similar) products are designed from the beginning so that they can be easily interchanged, this is a very good and economical solution. Typical examples are 4- or 6-cavity molds for a series of round containers, with none or only small differences in diameters, but with large differences in height, as would be the case with small tubs for dairy products, e.g., in sizes from 0.25 liter to 1 liter capacity. Such molds can be designed and built with all the advantages of a dedicated mold, but saving the cost of several mold shoes. “Universal mold shoes” are used mainly for low production runs, for which only small numbers of cavities are required. They are based on the principle that stack inserts can be easily and quickly interchanged by the molding technicians or setup personnel, often even without removing the mold shoe from the machine. The stacks do not necessarily have to be for the same or even similar products. They are usually designed for one cavity per insert. If there is space, two or more cavities and cores could well be placed within one insert. The disadvantage is that, because the stacks are designed for easy interchangeability in the mold shoe, it may not be possible to provide them with the best cooling layouts (facilitating faster cycles) of a dedicated mold. In addition, the product requiring the longest cooling time governs the cycle Figure 4.1 A 4-level mold designated to quickly switch to different sets of inserts (Courtesy: Stackteck) Dedicated molds are usually preferred. However, the use of a common mold shoe with different sets of stacks can often be more economical For small products and low quantities, universal molds can often be the most economical solution 1281han04.pmd 28.11.2005, 11:14113 114 4 Mold Selection time; however, for low production, short cycle times are not as significant for the unit cost as is the lower mold cost. 4.1.2 “One-Product” Molds or “Family” Molds? “One-product mold” is a mold built for one specific product. The best layout for minimum mold size, space (stack location), cooling, ejection, etc., can be achieved with a dedicated (one-product) mold. A “family mold” is a dedicated mold, in which more than one shape of product is made during the same injection, which will be of the same material and color. A very serious disadvantage of all family molds is that the cycle time of the mold is governed by the product (and the mold stack) that is most difficult to cool. This difference can be substantial, and should be seriously considered, particularly with products as described in Section 4.1.2.1 and 4.1.2.2. For all family molds producing pieces of different size, we must make sure that the mold is laid out so that the clamp forces are balanced as well as possible, i.e., that the sum of all projected areas is about equal in each of the 4 mold quadrants. In other words, the projected areas of the cavities above and below the horizontal center line of the mold must be nearly equal and so must be the sum of the projected areas to the right and the left of the vertical center line of the mold (see Fig. 4.2). 4.1.2.1 Family Molds for Composite Products For composite products, such as toys and games, it may be desirable to make all the components of the toy in one shot. Often, the various pieces are kept on the runner system of a 2-plate mold and are packed and shipped together with the runner; it is left to the user to take the pieces off the runner during assembly of the toy. The production runs are usually relatively small; therefore, this is a most effective method of producing with low cost molds (don’t forget to include the cost of the runners in the cost of the product). Occasionally, a product, e.g., a toy car, may have two or more colors. It could be a car with a blue body, red wheels, and yellow bumpers, etc. By molding equal production runs of first blue, then red, then yellow parts, 3 sets of cars can be produced, in the three combinations of colors. In this case, the runners are not shipped with the product. This method is also used occasionally for technical products. 4.1.2.2 Family Molds for Small or Medium-Sized Technical Products Family molds for small or medium-sized technical products are used when a number of different sizes of similar, rather small products, such as washers or seals, are molded in one mold. But such molds can also be used for larger products, which are required as a “set” in production, as they are used, e.g., for home appliances, among others. Any type of mold can be used (hot runner Figure 4.2 Schematic of symmetrically balanced cavities in relation to the centerline of the clamp Figure 4.3 Stack family mold for container and lid (Courtesy: Husky) Figure 4.4 Cavity view of 72-cavity cutlery mold; 24 forks, 24 spoons, and 25 knives are molded every shot (ca. 8–10 s) 1281han04.pmd 28.11.2005, 11:14114 115 4.1 Selection of an Appropriate Mold or cold runner, 2-plate or 3-plate). There are two main disadvantages of this type of mold: (1) Except for edge-gated 2-plate molds, the products fall out of the mold all mixed together and must be separated before storage or use. (2) Stock and production control can have serious problems when some of the products are used up (e.g., wear) faster than others, and must be available as spare parts. It may then be necessary to run the mold to produce the full shots while only some of the items are required. This problem can be overcome by blocking off the runner system ahead of the unwanted cavities and running the mold only for the products required; this means to run the mold less efficiently. 4.1.2.3 Family Molds for Perfect Color Matching Any plastic, and especially colored plastic, whether colored in-house or bought already colored from the supplier, comes in batches. Within each batch, the plastic can be considered uniformly mixed and colored. These batches are supplied in bags, or in large carboys, or in truckloads, etc. Even though the specifications to make the batches were identical, there are mixing tolerances in manufacturing and small variations from batch to batch are unavoidable. It is better to work with large batches, which will yield large numbers of matching-colored pieces, but this is not always practical or economical. If a product pair (or assembly) must have a perfect color match, the answer is to make the matching pieces in one shot, which is of course supplied by the same injection unit, at the same time. A typical application for this is a “lady’s compact”, consisting of a base (for the face powder) and a matching lid (for the mirror). But there are other applications, some of them in the technical field. It is quite common to build molds that have the same number of each of the products that require the perfect color match. If the pieces are required in pairs and their projected areas are about the same, a mold layout is rather easy and the stacks can be laid out symmetrically. A problem is that the pieces are ejected together and must be separated after molding; also, they must also be stored so that the matching colors are kept together and are not mixed with products from another color batch (this can also add costs to the product). 4.1.2.4 Family Molds for In-Mold Assembly Family molds for in-mold assembly are more sophisticated molds, usually for very high production volumes and are only rarely used. A multiple of two different but matching pieces is molded in the same mold; they are assembled during the ejection time, using special motions, which are part of the mold or during the mechanical removal (with synchronized take-offs or robots), so that already assembled pieces are ejected to a conveyor or carried away under controlled conditions. Such assembly methods may require longer ejection times but can save subsequent assembly equipment, and time. Perfect color matching can easily be achieved with family molds Figure 4.6 Color matched parts for personal care products Figure 4.5 View of ejected array in robot end of arm tooling 1281han04.pmd 28.11.2005, 11:14115 116 4 Mold Selection 4.1.2.5 Family Molds Using Controlled Ejection for Subsequent Assembly This method is almost exclusively used for products where the required annual quantities are very large and virtually no changes are expected for years. In these cases, a number of pairs of matching pieces, usually of the same projected area or with only small difference in area, are molded in one (single level or stack) mold and then removed either by take-off or by other methods, which maintain the orientation of the matching pieces so that they can be easily assembled in a specially designed machine or mechanism, usually adjacent to the molding machine. Typical examples are Petri dishes, video and audio- cassettes, CD “jewel boxes,” and so forth. Figure 4.7 shows a Petri dish system taken from the rear of the clamp, which is protected by guards (A). The bottoms and the tops of the Petri dish are molded on each face of a 2 × 4, 2 × 6, or 2 × 8 stack mold. Guide rails transport the molded parts by conveyor (B) to an assembly station (C); from there the assembled dishes move to a stacker (D) and the stacks of assembled Petri dishes are then moved to an (open) “sleeving” station (E) where plastic sleeves are manually pulled over the stacks for boxing and shipping to a sterilizer; cycle time: 3.5 s, productivity (with 2 × 8 mold): 8,200 assembled dishes/hour. 4.1.3 Where to Gate The next issue to consider is the location of the gate. The gate is the point where the plastic enters the cavity space. In some cases, the product designers will indicate where they believe the gate should be. They may select this location because of the function and strength of the product and in some A B C D E Figure 4.7 Petri dish system (Courtesy: Husky) Figure 4.8 Petri dishes and CD jewel boxes are typically molded using family molds Figure 4.9 Cutlery is also often molded in family molds 1281han04.pmd 28.11.2005, 11:14116 117 cases, because any projecting gate vestige may be bad for appearance or even harmful to the user. However, such suggested location may not always be the best for filling the cavity space or for the best strength properties of the product. At this point of the development, the input by a molder or the mold designers could be very valuable and a dialogue between the product and mold designers should be encouraged to find the best location for the gate. These days, computer aided mold filling simulation packages can accurately predict the fill patterns of any part. This allows for quick simulations of gate placements and helps finding the optimal location. 4.1.3.1 Cup- or Box-Shaped Products In general, for cup- or box-shaped products, outside center gating is most desirable, because it ensures more evenly distributed flow from the gate towards the rim or edge. However, center gating (except for single-cavity molds) implies the use of either 3-plate or hot runner molds, both of which are more expensive than 2-plate molds. Note that the gate area is always an area of inherent weakness; molding conditions such as higher melt tempera- tures, longer molding cycles, and higher cooling temperatures can improve the strength there and this must be considered as a factor affecting the cycle time and cost of the product. It should also be noted here that hot runner valve gating reduces the stresses in the gate area. The foregoing does not imply that 2-plate molds cannot be used for cup- or box-shaped products; in fact, 2-plate molds are used for many such products, but usually only those with larger wall thickness. 4.1.3.2 Flat Products “Flat” in this context means relatively flat, as opposed to “cup-shaped.” It includes really flat pieces (in one geometric plane) but also curved products, such as automotive panels, trays, etc. of all shapes. Flat products are preferably gated from the edge of the product, because the flow away from the gate (or gates) will result in a stronger product; it also ensures that there are no unsightly gate marks in the middle of the product. Here also, it is much better if the incoming stream of plastic will be directed against a solid portion of the core or at least a projection of the core and not to flow into an open space, such as a rib or an open surface. Thin-walled, round products, such as lids for containers and trays, should be center-gated for faster filling and to reduce possible distortion when ejected early to gain cycle speed; however, they can also be edge-gated when the center of the lid must not show a gate mark. Figure 4.12 shows a selection of typical automotive products. The quantities are usually small compared with the huge numbers molded for packaging and medical products and the molds are usually small cavitations (1 or 2). But even so, most of these products are molded with hot runners, because it is easier and more effective to control the quality of the products and there is often less labor required than with cold runner molds. Also, the use of regrind is sometimes not possible, which makes the justification of a hot runner easier. Figure 4.12 Selection of typical automotive products 4.1 Selection of an Appropriate Mold Figure 4.10 Mold filling analysis is very useful for finding the best gate location Figure 4.11 Typical bottom center-gated parts 1281han04.pmd 28.11.2005, 11:14117 118 4 Mold Selection Because in most molding machines the injection unit is in line with the center of the machine platens, it is not possible to edge-gate a single-cavity mold, unless either there is a large enough opening near the center of the product, from where a cold or hot runner system can feed one or more edge gates (see Fig. 4.13), or a hot runner system is used with drops outside of the profile of the product, feeding into cold runners (see Fig. 4.15). An alternative is to have the cavity located completely to one side of the centerline of the machine, which could be possible for any product small enough to fit there. This however could leave to a severe unbalance in the clamp. This is not recommended. There are ways of balancing the clamp forces, e.g., by doubling the size of the mold and providing a second, similar cavity if the cavity is not too complicated and expensive, or by adding a pressure pad in a location on the platen symmetrically opposed to the cavity (see Fig. 4.14). For more details on this subject, refer to [5] Chapter 6. If the product is very large, edge-gating into a single cavity can be achieved with a 3-plate mold (now rarely used for this purpose) or by using a hot runner system, which enters one or several cold runner systems outside the edge of the product. From there, cold branch runners can lead to edge or tunnel gates into the side of the product, just like in a regular 2-plate mold. This method is used for large, mostly flat products, such as automotive panels, and so forth (see Fig. 4.15). Figure 4.7 shows a flow model of a large automotive panel with three gates (A) from a hot runner system (B), but without the use of cold runners as in the schematic of Fig. 4.6. The runners are shown schematically, superimposed over the photo. Note that here again, the gates are near the edge of the panel for greater strength. Figure 4.13 Two examples of gating into the center of an open product. The sprue could be a cold sprue or a hot (runner) sprue Figure 4.14 Balancing of mold clamp- ing forces; (left) added second cavity; (right) added balancing pressure pad Figure 4.15 Schematic of large, single-cavity mold with hot runners feeding cold runners; (a) product (a large panel); (b) sprue; (g) hot runner channel; (h)”drop” to cold runner; (j) cold runner; (i) gate Fill time = 0.9408 [s] 0.9408 0.7056 0.4704 0.2352 0.0000 [s] A A A B Figure 4.16 Flow model of an automotive panel 1281han04.pmd 28.11.2005, 11:14118 119 4.1.3.3 Deep, Large Cup-Shaped Products Products in this category are large pails, boxes, garbage containers, large crates, children’s bathtubs, and so forth. It is always desirable to use one center gate, if the L/t ratio is low enough (200 or less.) Today’s machines with high injection pressures have made it even possible to mold large industrial pails with an L/t ratio of up to 500 with only one gate. However, most large products (tubs, boxes, etc.) have two or more gates in the bottom for faster, lower- stress filling and to reduce the L/t ratio for each gate. Large industrial containers, crates, pallets, etc. may have four or more gates. It is important to provide venting where the streams coming from the gates are expected to meet to avoid the risk of air enclosures or even holes at the predicted weld lines. Such molds with one gate can use a cold sprue (simplest mold) or they can have a hot sprue. If two or more gates are required, a hot runner system must be used. 3-plate molds, although theoretically possible, are almost never used in this arrangement, because of the large size and mass of the cavity block that would have to move (float) between the moving and stationary platens to allow ejection of the runner. Figure 4.17 shows a typical heavy crate (A) for bottles with separators (B) for individual bottles. This design requires a mold with side cores for the deep engravings (C) and the openings (D) in the sides. There are also two baskets (E) with openings (F) in all 4 sides. Because the sides are angled, the openings can be produced by so-called “shut-offs” between core and cavity contacting in each opening, thus not requiring side cores. Such a mold is much less expensive and can cycle much faster than the mold with side cores. The other picture illustrates a large box (G) with matching, flat snap-on lid (H). Figure 4.18 shows 10 and 20 Liter industrial pails. Depending on the ratio of flow length to wall thickness (L/t ratio), they use either a single gate in the center or three gates near the rim to facilitate filling. 4.1.3.4 Elongated Products For maximum strength it is always better to gate near the end of the product (cold runners) or on the top surface (A) near the end of the product (3-plate or hot runners). Gating into the top may be undesirable for appearance, but proper function of the product should always be the first consideration. A gate mark at the top surface can often be hidden, for example, inside a letter or and ornament on such surface, or by creating a “fake vestige” in a location symmetrically opposite the gate (see also Section 2.8.3, Witness Lines). Figure 4.19 shows typical elongated products (tooth brush, safety razor handle), which must be gated near the end for maximum strength. Similarly, other products (not shown), such as cutlery (disposable or not), must also be gated at the end. If these parts were to be gated in the middle they would break at the gate. H G F E A B C D Figure 4.17 Typical crates and baskets Figure 4.18 Very large industrial pails AAAA Figure 4.19 Typical elongated products 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14119 120 4 Mold Selection 4.1.3.5 Inside Center Gated Parts Cold Runner 3-Plate and Hot Runner Molds In all gate locations mentioned so far, the gate is always located on the outside (top or side) of the product, i.e., in the hollow (concave) portion of the cavity. This is good practice because  It is the shortest path for the plastic between the machine nozzle and the gates  The product will stay with the core from where it can be easily ejected by any conventional method  The best cooling is on the core where the product shrinks on, to ensure proper ejection However, there are cases where a gate on the outside of the product is not desirable, mostly because of required esthetic appearance. Typically, this is the case with high-quality closures (for perfume bottle caps, some in-mold labeled products, etc.) or some spray bottle or over-caps, where a gate vestige on top would “cheapen” the appearance of the package. But there are also some technical products and enclosures for which inside gating is preferred. Figure 4.20 shows over-caps (A) with inside center gating. It requires long nozzles (B) and, as can be seen, there is not much space for core cooling. These molds cycle 2–3 times longer than outside center-gated molds, but have no gate vestige on the outside. This example shows clearly how little space there is to provide good cooling, a gate insert, and good heat insulation from the nozzle tip. There are some basic drawbacks with inside center gating (ISCG) (see Fig. 4.21):  The sprue (or drop of a hot runner system) is much longer than with outside gating to reach the bottom of the product.  Because the cavity is on the moving mold half, the product cannot be ejected easily. The product will most likely shrink onto and stay with the core from where it is injected; therefore, an ejection system must be incorporated into the injection side of the mold and, if necessary, into the stationary platen of the machine. Ejection by air would be best, but is often not possible because of the shape of the product and/or the plastic processed. Therefore, mechanical ejectors (strippers or ejector pins) must be on the injection side of the mold, which also carries the cores and the runner system. This ejector system is either air actuated or driven by mechanical links connected to the moving platen. Most likely, this ejection mechanism adds still more length to the sprues or drops. These difficulties are even greater with unscrewing molds, with the cores on the injection side and the sprue inside the cores. Figure 4.21 Outside center gated (top), and inside center gated mold (bottom) A B B Figure 4.20 Over-caps (A) with inside center gating (Courtesy: Husky) 1281han04.pmd 28.11.2005, 11:14120 121  The cooling on the inside of any core contributes always more than 60% of the cooling efficiency of a mold. But with ISCG, inside the core is also the sprue of a 3-plate system or the drop of a hot runner system. For shortest molding cycles, we need to cool the core efficiently to remove the heat both from the product and the sprue in 3-plate molds; however, in hot runner molds, we must remove the heat from the product while keeping the hot drop well insulated from the cooled core so that the plastic in the drop will not freeze. Both these conditions mean that there is very little space to provide good cooling for the product and much slower cycles will be unavoidable compared to a similar product gated from the outside.  The need to provide an ejection system on the injection side makes it difficult to position the runners and cooling lines in either 3-plate or hot runner mold. While the moving mold half with the cavities becomes very simple and relatively small, the injection side with the cores will be very complicated and large. With these problems, an ISCG mold is always considerably more complicated and about 25% more expensive to design and build and will cycle two to three times slower than a comparable mold with outside gated cavities. 4.1.3.6 Slender Products Round, thin-walled products such as vials, syringes, etc. are best (outside) center-gated, using either 3-plate molds or, preferably, hot runner systems, Fig. 4.22. Core Shift Slender products (length over diameter ratio of more than 2.5 : 1) have the problem of “core shift”; in fact, the core is not shifting but a bending of the core is caused by differences in the plastic injection pressure. It is practically impossible to gate exactly concentric between cavity and core; even with the closest practical tolerances, there will always be some minute misalignment between the center of the gate (at the closed end) and the center of the cavity space between the cavity and the core, Fig. 4.23. Such misalignment will allow more plastic to flow into one side of the core than into the opposite side and as the cavity space fills, the core will deflect because of a pressure differential in the plastic between the sides and remain deflected to some extent until the product is ejected. After ejection, the core returns (elastically) to its original straightness. The effect of such core deflection can be measured in the wall thickness but can also be seen easily by rolling the molded piece on a flat surface; its easily recognizable “banana shape” is caused by the different shrinkage conditions of the thicker and the thinner side of the vial (the thicker side takes longer to cool and thus bends the product after ejection). The thinner the walls are, the worse is the problem and the more precision in mold making will be required, adding to the cost Figure 4.22 Gating for vials; (top): center gating (hot runner or 3-plate); (bottom): cold runner 2-plate gating. A and B show top view of gating at 180° and at 120° 4.1 Selection of an Appropriate Mold 1281han04.pmd 28.11.2005, 11:14121 122 4 Mold Selection of the mold. On the other hand, thicker walls require more plastic (cost!) and longer cooling (more cost!). When center-gating, there is no problem with venting because the plastic flows toward the parting line where good venting is easy to achieve. To overcome core deflection, there are various methods (some of them patented) of stabilizing the core inside the cavity and/or selecting a stiffer core material with a greater modulus of elasticity (E) than mold steel to reduce the deflection of the core. Some tungsten-carbide alloys exhibit a modulus of elasticity 2.5 times greater than steel; however, they have little shock resistance and are expensive to manufacture. There are mold makers specializing in these products (vials, syringes, pen barrels, etc.) who have the experience and skills to overcome the problems and provide good molds. An older method for making these products is to use cold runner gates (either self-degating or not) into the side of the barrel (see Fig. 4.22) at or near the open end, and to use two gates located at 180°, or three gates at 120° around the circumference of the barrel, or to provide a continuous ring gate all around the opening. The ring gate will then be machined off. The advantage of any of these methods is that the plastic enters the cavity space from two or more symmetrically opposed gates and flows in parallel streams, which tend to hold the slender core in center. One serious problem with this method is that the cold runner from the sprue is located in the same plane as the stripper. Using floating stripper rings is not possible because of the gap required for floating between the rings and the stripper plate and positioning the stripper rings exactly in line with the core is very difficult to achieve and very costly. Still, there are many multi-cavity molds built this way. The second serious problem with this method is the venting of the air as it is pushed ahead of the inrushing plastic. With vials, there is no opening at the closed end and vent pins at the top (dome) are absolutely necessary. A composite cavity, with a separate part forming the dome will permit vent gaps between the dome and the cavity portion forming the sides of the vial. Products such as syringes have a hole in the dome and can be vented there. Note that uneven filling because the center of the gate is not in line with the centers of the cavity and the core cannot only happen with slender products as schematically shown in Figs. 4.22 and 4.23. Figure 4.25 shows a fairly stubby container with ribs and outside center gating. The lower part of the photo shows a complete shot. Because the gate is (unintentionally) off-center, the cavity space fills unevenly, as shown clearly with the 6 short shots, from right to left. By progressively increasing the shot size it can be clearly seen how the plastic gradually fills the cavity space and produces an air entrapment that causes much difficulty when molding. Figure 4.26 shows the short shot of a thin-walled cap with a “corrugated” sidewall. The corrugations have slightly different radii in the cavity and on the core so that the outer tips of the corrugations are somewhat thicker than the sidewalls, permitting the plastic to flow easier through the thicker tips Figure 4.24 Typical long slender products Figure 4.23 Schematic of effect of misalignment of gate and center of the core 1281han04.pmd 28.11.2005, 11:14122 [...]...123 4.1 Selection of an Appropriate Mold Figure 4.25 Stubby container with ribs and outside center gating while filling the cavity space This can be easily seen by the plastic being father advanced at the tips in the short shot Note that the mold is almost perfect, with the advance practically equal all around This is the result... 4.27 Over-cap with shallow, flat ribs Next Page 124 4 Mold Selection The various shapes of gate designs are discussed in detail in [5] Here, we will discuss the advantages of certain gates and their influence on productivity 4.1.4.1 Edge and Fan Gates (Cold Runners) (a) (b) Both edge and fan gates have been used from the beginning of the injection molding technology and are still used today Both gates... plastic advances faster through heavier sections in the flow path To demonstrate and check the filling pattern, the mold was first injected with clear plastic until it ran at optimal conditions Then, some yellow colorant was added to the extruder When the colored plastic reached the mold, the front of the incoming melt was still “clear” but the following melt was already yellow The plastic advances... be added to the cost of production Also, the cost of any jigs or fixtures required for this purpose must be added to the mold cost There are a few occasions where the products should stay connected with the runners: The products will be shipped with the runners, e.g., with family molds, when it is of advantage to have the end user remove the products from the runner when needed Figure 4.28 Schematics... case, it may be easier to handle the whole array of runners and products from molding to inspection This handling and the inspection can also be automated Products are degated after inspection Oriented packaging into boxes is easier from complete arrays, where the products (e.g., cutlery) are still in the attitude as they were molded, rather than being randomly ejected 4.1.4.2 Self-Degating Cold Runner... gate The most frequently used self-degating method is tunnel gating (Fig 4.29) If the runners and the gates are properly designed and sized, the products are severed from the runner as the mold starts opening The molded pieces and the runners fall out together and must then be separated There are automatic separating machines on the market The gate vestige is small, usually a round or oval, slightly... rather shallow, the steel remaining between the gate and the parting line can be small and fragile and easily be damaged; in this case, steel selection is very important, but even so, this is an area requiring frequent repairs Placing an insert in this area when the mold is built will save much cost and downtime when the gate is damaged or breaks ... but the following melt was already yellow The plastic advances much faster in the sidewall with the shallow ribs than in the wall sections between the ribs These different wall sections could cause some molding difficulties, if the plastic in the thicker sections fills so fast that it causes an air entrapment Figure 4.26 Short shot of a thin-walled cap with a “corrugated” sidewall 4.1.4 Gate Size and... shape of the cross section of the gate Fan gates leave a very narrow, long vestige that can be almost invisible Large edge gates could be required for very large products or for products that must be molded with a long low-pressure hold cycle to ensure that the cavity will be fully filled without visible sinks or voids The large gates are milled or sawed off, if clipping with pliers is not acceptable . for the molds. 4.1.1 Dedicated Mold, Universal Mold Shoe “Dedicated mold means a complete mold that is used for one purpose only. After use, the mold is. cost. 4.1.2 “One-Product” Molds or “Family” Molds? “One-product mold is a mold built for one specific product. The best layout for minimum mold size, space (stack