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Gate Types
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The style of gate used in a mold design is integral to the appearance and robustness of the molded part. The geometry of a part, material being used and cosmetic requirements will decide what type of gate will work best. The gating scheme (style, number and location) will influence the filling, packing and holding phases of the molding process and will have a large impact on a part's final dimensions, as well as its performance and esthetics.
A gate creates an opening for plastic to enter the mold cavity. When plastic crosses the gate, the friction created by flow through the small opening re-heats the plastic. When traveling long distances, the re-heated material with flow more evenly and will be able to fill the complete mold cavity. The style of gate used will determine the finished look of the part – vestige left by the gate and to what degree the gate may have to be trimmed (secondary operation).
Some of the more common styles of gates which need to be trimmed from the cavity include;
Edge Gate or Side Gate – the most common type of gate, is suitable for medium and thick sections and can be used on multicavity two plate molds. The gate is located on the parting line and the part fills from the side, top and bottom.
Figure 1. Edge Gate or Side Gate
Fan Gate – is a wide edge gate which tapers in both width and thickness, thereby keeping the cross sectional area constant. This ensures a constant and more even plastic flow into the cavity into a wide area at a slow injection speed, minimizing backfilling and reducing imperfections and stresses in the part. Typical application would be in thin parts with a large area eg lenses.
Figure 2. Fan Gate
Tab Gate – is an adaptation of an edge gate. The tab gate is usually located at right angles to the flow of the resin into the molded part, so that the material is not directly injected into the cavity. Thus, jetting, splay, gate blush and molding strain is minimized in the molded part as such molding issues are now confined to the tab.
Figure 3. Tab Gate
Film or Flash Gate - is a more extreme version of the fan gate. It consists of a length of straight runner and a gate land across either the entire length or a portion of the cavity. It is used for long flat thin walled parts and provides even filling resulting in even shrinkage along the length and minimizes the generation of bubbles and flow lines in the part as when the part would be a lens.
Figure 4. Film or Flash Gate
Sprue Gate – is often used on larger molded parts in a single cavity mold and is suitable for thick sections. It is recommended for circular parts requiring symmetrical filling.
Figure 5. Sprue Gate
Diaphragm or Disc Gate – is often used for gating cylindrical or round parts that have an open inside diameter such as a tube or ring. It is used when concentricity is important and the presence of a weld line is not acceptable. The diaphragm gate delivers material simultaneously to all points on the circumference.
Figure 6. Diaphragm Gate
Some of the other more common styles of gates which are trimmed automatically when part is ejected from the cavity include;
Sub(marine) or Tunnel Gate – is used in two plate mold construction. An angled, tapered tunnel is machined from the end of the runner to the cavity, just below the parting line. As the parts and runners are ejected, the gate is sheared at the part and the tunnel bends as the runner is being ejected. The tunnel can be located in either the moving half or the fixed half of the mold.
Figure 7. Sub or Tunnel Gate
Pin Gate – is only feasible with a three plate mold because it must be ejected separately from the part in the opposite direction. The gate must be weak enough to break off without damaging the part. This type of gate is most suitable for use with thin sections. The design is particularly useful when multiple gates per part are needed as in larger parts.
Figure 8. Pin Gate |
