Southern California Section of the Society of Plastics Engineers
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How Proper Gating Help Improve Molded Parts - By Tuan Dao
In this article we discuss the non – automatic gates. Many parts can be designed for gating with automatic runner separation. Others, however cannot. Or, for any of several reasons, the molders may choose not to separate the runner automatically. In fact, it may be very desirable to keep the parts on the runner as an aid to part separation and collection by individual cavity. Isolation of parts from a faulty cavity can thus be made easier. There are automatic means of separating parts by cavity presently on the market, but the volume of parts may not justify the extra cost of this kind of equipment.
In cases where parts are to remain on the runner for degating outside the molding machine, choice of a round, half round or some form of rectangular parting line gate should be made. This choice is usually based on the different way that a gate can affect part properties, part dimensions and ease of molding. Unfortunately, a gate is often chosen because it’s easier to machine than another type of gate, even though the other gate might offer better molding performance. With modern forms of machining available, such as electro-discharge machining (EDM), this is penny-pinching in the wrong place.
FACTORS AFFECTING GATE FREEZE TIME
• Round and half round gates have a built in disadvantage, in that, if gate size must be increased to cavity filling, the gate thickness must also be increased. This results in an increase in the time required to freeze the gate which in turn may lengthen the cycle if the screw cannot be retracted before the gate is frozen. On the other hand, most rectangular gates can be enlarged for improved flow simply by widening the gate. If the gate thickness is kept the same, the freeze-off time will remain essentially the same.
• Gate thickness is not the only factor that controls gate freeze time. How fast the heat is taken from the resin located within the gate land also determines freeze time. Anything that speeds or slows this heat removal will change gate freeze time. For example, as long as resin continues to flow through the gate, bringing heat with it, the gate does not freeze. With some resins, such as acrylic resin, the slow fill may be used to keep the gate open and promote the packing of thick parts. With other resins, however – glass reinforced nylons and crystalline thermoplastics – the slow fill can seldom be used. This is because the resin in the cavity freezes quickly and if cavity pressure is not applied quickly enough, surface blemishes such as wrinkles, pits, orange peels, etc. will occur. Slightly slower fill may be helpful in these resins if venting is a problem. In some cases, the slower fill may help improve toughness.
• Length of the gate land also has an e ect on gate freeze time. If the land is short as usually recommended), the heat from the part and runner on either side of the gate may keep it open much longer than a gate of the same thickness but with a longer land length. A shorter land is desirable, because it is necessary to keep the gate open to
permit pressure to pack – out a thick part. Sometimes adding a “bulge” or enlargement of the runner just before the gate will maintain a little extra heat in the gate area and keep the gate open long enough for adequate pack – out.
• Mold temperature is another factor in controlling gate freezing time. Thus, if the cooling water temperatures or the overall cycle is changed, the screw forward time may need to be adjusted to insure that the gate is frozen before screw retraction.
SEVERAL TYPES OF GATES
As shown in Figure 1, rectangular or “slit” gates can take many forms, from simple square gates to ash and ring gates. Of these, the impinging type is preferable, since the resin ow strikes the cavity wall or a core before it can jet across the part and cause a blemish.
Some of the wide rectangular gates can be deceiving. One might think that the full width of the gate would be active during part lling. But the gate is beyond a certain thickness, ow will be higher right at the runner and less at the outer edges of the gate. This can cause a local overpacking in the part at the center of the gate.
By using a thin gate in a ring gate system such as shown in Figure 1, the length of ll down the part may be nearly equal all around the tube. If the gate thickness must be increased, nonuniform ll may cause local packing in a line down the part near the runner entrance. This is likely to produce a banana – shaped part because the shrinkage is less nearest the gate where the highest pressure is.
Tuan Dao: Formerly with DuPont Polymers. He is a technical consultant with Polymer Engineering Group and currently teaching Plastics Engineering at the University of California-San Diego, Extension.
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