The Importance of Gates
Gates can be a standard part of the injection molding process. They are normally formed as molten plastic flows from the machine’s nozzle tip into a sprue and runner system, then into the part’s cavity. Gates are the last section of the runner system and attach it to the part. Therefore, everything from the gate up (gate, runner, and sprue) becomes scrap. (**note: with a hot runner system and special tool design, there is no resulting sprue, runner or gate scrap)
Gates are a critical aspect of mold design. Their type, location and number can impact everything from the cycle time, to the cosmetics of a part, and even possibly its structural integrity.
There are many types of gates, but you can think of them as falling into two main categories: manually trimmed gates and automatically trimmed gates. We said earlier that everything from the gate up is scrap, and doesn’t belong to the actual part. That scrap needs be separated from the part before it can actually be, well, scrapped. There are two ways to do this: automatically by the injection molding machine when the mold separates and the part is ejected, or manually by an operator who uses either a tool or fixture to cut off the gate and runner system.
Automatically trimmed gates have the obvious advantages of lower cost (since there’s no secondary operation), a shorter cycle time and greater consistency. Despite these benefits, there are times when manual trimming must be used: when the part can’t withstand the forces of automatic shearing, or when the gate is too big to be automatically removed.
There are far too many gate types to list here, so we’ll discuss only a few of the most common, and why/when they are used.
Let’s start small. Sub gates (short for submarine gates) are very small and thus always automatically trimmed by shearing when the mold opens at the end of the cycle. Sub gates offer mold designers much more flexibility than other gate types when it comes to gate location, since sub gates don’t have to be at the parting line (the seam on the part where the two halves of the mold join together). Their other major advantage is the fact that the mark left when the gate is trimmed off, is minimal and thus easier to hide. Submarine gates got their name due to gates being located just below the parting line, and are often used in two-plate mold designs.
Like sub gates, pinpoint gates (also called pin gates) are small and are automatically trimmed. Unlike sub gates, however, pinpoint gates are used in 3-plate molds since the runner exists in its own plate. To completely pack the cavity, multiple pin gates are used in different locations and the resin that flows through the mold is divided among them. This “divide and conquer” approach of pinpoint gates can eliminate long resin flow paths and ensure that thecavity is symmetrically filled. Pinpoint gates do have a drawback though: their runners tend to be big, which creates a large amount of scrap that can be costly.
Now let’s turn to two manually trimmed gate types. Edge gates are probably the most common gate type used, and as their name suggests, are located on the edge of the part. Edge gates leave their trim mark at the parting line and are easy to remove because the cross-sectional area of these gates is thin. If you’re using a viscous resin however, that small area can be a problem since the cavity will take longer to fill. (This type of drawback can also affect pin and sub gated parts.)
Lastly, fan gates create a stable, uniform flow into wide parts and therefore reduce deformation and flow lines. The molten resin “fans out” much like how a river spreads into a delta. Since the cross-sectional area of a fan gate is large, the resulting mark after it is trimmed is also large.
The Importance of Gate Location
Here are some important tips on gate location, regardless of what type of gate is used:
- Put gates on a non-functional and/or non-cosmetic area of the part
- Put gates at the thickest cross section of the part, as this will minimize sinks and voids
- Place gates in areas that will make them easy to remove, whether manually or automatically
- Don’t put gates near features that will act like flow obstructions, like cores and pins
Careful selection of the location and number of your gates can improve resin flow, which in turn can prevent flow marks and weld lines. Gate location also impacts not only cosmetics, but the strength of your finished product.
Design Tradeoffs and Gate Selection
From the above discussion of the four gate types, you may have noticed a pattern: small gates leave small trim marks, while bigger gates leave larger trim marks. Here’s another: larger gates allow higher resin flow rates with a given amount of pressure than smaller gates. To increase flow rates (and thus decrease cycle times) when using small gates, you’ll have to increase the injection pressure if possible. Remember, your contract manufacturer may be limited by the machines they have, so increasing the pressure may not always be an option.
Gate design is a multifaceted engineering challenge; part appearance, cycle time and tooling constraints must be balanced against your design requirements and estimated annual usage (EAU). This challenge however, can be made easier when you have access to expert input from a knowledgeable contract manufacturer like Providence Enterprise.