While gate type and location may seem like something you can decide randomly at the last minute, they can actually have a big impact on the overall effectiveness of your mold. In fact, gate type and placement can affect lead times, tooling costs, cycle times, the location of witness marks, and many other common injection molding defects.
Let’s talk about the different types of gates and where you should place them for the best results.
Every cavity in a mold must have a small opening called a gate, through which molten plastic is injected into the mold after traveling through sprues and runners. The gate is also the boundary between the part itself and scrap plastic.
As gates force molten plastic under pressure into the cavity, it heats and accelerates the plastic, which can create a variety of negative effects such as jetting and overheating. You can read more about cosmetic defects here. The last thing you want in your part is cosmetic – or, worse, structural – defects.
Here are some tips for choosing which type of gates to use and where to place them in the mold.
Two types of gates are used in plastic injection molding: manually trimmed and automatically trimmed.
Runner systems can be either hot or cold.
There are many different types of injection molding gates, each with its own advantages and disadvantages.
Edge gates are one of the most common gate designs due to their simplicity and effectiveness in the injection molding process. They are easy to make and modify. As the name suggests, edge gates are located on the edge of the part.
Best suited for flat parts, they work well with thick and medium sections and can be used on multi-cavity two plate tools. An edge gate can have a larger cross-sectional area compared to other gates, which allows more plastic flow and the ability to have longer hold times because of the longer gate freeze. Edge gates leave a scar at the parting line.
Another popular option, tab gates contain an auxiliary tab section where shear stress can safely be absorbed without affecting the quality of the part. They resemble edge gates but are a mostly consistent thickness for a short distance leading into the molded part and are typically used on thin or flat parts to reduce shear stress.
Fan gates are an alternative type of edge gate. The fan maintains a consistent thickness but spreads out to increase the volume over a larger area. Fan gates have a wider width than the runner, which can help with dimensional stability and plastic flow while avoiding some cosmetic issues.
A manually trimmed gate, a direct or sprue gate is used to produce symmetrical filling of single cavity molds for large cylindrical parts. A direct gate can almost be thought of as the absence of a gate: the sprue carries material directly into the mold cavity. One disadvantage to this type of gate is that it leaves a significant witness mark.
Only possible on two-plate mold construction, submarine or tunnel gates have a number of variations and require ejector pins. The runner directs plastic to the edge of the cavity at the parting line, but then the gate drops below the parting line and tunnels upward to fill the part from below, allowing you to place the gate away from the parting line. Tunnel gates leave a very small scar.
More difficult to implement than other gate types, pin gates are located on the B-side of the mold, where the ejector pins are located. When the mold cavity opens at the parting line, the pin gate is torn from the part. Pin gates are ideal for parts that can’t have gate vestiges on either the cosmetic side of the part (where hot tip gates are located) or the parting line (as with tab gates).
Pin gates are both geometry- and material-dependent. Glass-filled and “slow-flow” resins can be problematic, but pin gates are common on gears to avoid a tab gate that cannot be used on the outside perimeter of the part.
When you want even plastic flow on a part with a large open diameter in the middle, diaphragm gates are ideal. Diaphragm gates are generally used on open-ended cylindrical parts.
Similar to tunnel gates, the cashew gate is machined below the parting line and is automatically sheared during ejection. Cashew gates can get to injection locations below or behind show surfaces; however, removing the gate during ejection requires the plastic to pull out around an arc. Usually, removable gate inserts are added so a human operator can quickly remove the broken plastic to prevent the gate from breaking off.
Ideal for round or conical shapes that need uniform flow, hot tip gates are usually located at the top of a part. They require hot runners and improve flow. Hot tip gates leave a small raised spot on the part’s surface.
If you need another level of control on a hot runner system beyond what a hot tip gate offers, a valve gate may be ideal. With valve gates, a moveable pin inside the hot runner tip assembly moves back and forth to allow and shut off the flow of plastic. With the valve shut, you can begin the crew return process without having to wait for the gate to freeze which can improve cycle times.
Universal Plastic Mold (UPM) has been the large part injection molding manufacturer of choice for companies across the United States for more than 50 years. We have the experience and equipment to handle all your injection molding needs while speeding up your time to market, reducing overhead, and increasing your product quality.
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