The process of designing a mold for injection molding is complicated. It requires coming up with the most cost-effective way to mold a part that doesn’t compromise the complexity and function of its original design. It first starts with thoroughly understanding the injection molding process.

In short, injection molding is the process of injecting hot, molten plastic into a mold. As the plastic cools, it forms itself to the shape of the metal mold. This process involves a number of factors and tools, but here we’ll specifically be looking at ejector pins, parting lines, and gates.

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Ejector Pins

How do you get the plastic out of the mold with relative ease and minimal disfiguring of the part? You use ejector pins.

Ejector pins are usually located on the ‘B’ side of the mold - the side in which your plastic part will remain once the mold is opened. When it is opened, those pins push the plastic out. The pins then retract back so that the process can begin again. This process can leave small marks or dents in the plastic where it meets the pins. Part of designing the mold involves making sure these marks are unobtrusive.

Depending on manufacturing needs, different pins are used.

  • Through-hard pins: heat-treated pins to make sure their hardness is consistent. Able to withstand temperatures up to 200 degrees.
  • Case hardened pins: even harder than through-hard pins and able to withstand temperatures above 200 degrees.

Parting Lines

Where the two parts of the mold come together around the part is called the parting line. As the mold is clamped down in the machine, the parting line should make a good seal. If it’s loose the plastic will leak, creating ‘flash’, or extra plastic.

Parting lines are an inevitable part of the molding process, so planning for where they’ll occur on the plastic is important. Most of the time it will just be a visible line running down the middle of the part. But since the parting line determines the direction of the mold opening, the complexity of the part’s design may require something more complicated.


How does the plastic get into the mold in the first place? That’s where the gate comes in. It’s a small opening in each cavity of a mold where the plastic can be injected. The shape of the item is a big factor in determining which gate to use.

Since the gate is where the plastic enters the mold, it affects how the molten plastic flows and thus, how successful the process is. A gate that’s too small can cause a buildup of pressure inside the mold. The gate inevitably leaves a small mark, so ideally it’s placed somewhere out of sight.

Once the plastic is injected, it’s trimmed off by either a manually or automatically trimmed gate. As you can imagine, automatic is cheaper, but manual might be necessary if automatic trimming would damage the part or if the gate is too thick for automatic cutting.

Designing a mold for injection is complicated, but that’s why you want it handled by professionals who know all the potential pitfalls that can come with the molding process. Contact us and let us solve the hard design problems.

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