The Basics of Injection Mold Design

Author: Brandon S. Pfitzenmeier

When it comes to plastic part manufacturing, a solid mold design is the key to success. Whether you're producing automotive components, industrial parts, or consumer products, designing a mold that ensures quality, consistency, and efficiency is essential. There are many design features to consider, and in this article, I'll walk you through some of the basics of injection mold design, along with a few important rules that, if followed, can save you time, money, and a lot of headaches down the line.

Key Considerations for Injection Mold Design

Maintain Uniform Wall Thickness

The first rule of plastic part design is simple but crucial: maintain uniform wall thickness across the entire part. This might sound straightforward, but as designs evolve and assembly requirements change, it's a rule that often gets broken. Yet, it's a critical one for a successful production part.

Generally, wall thickness should be between 0.050” and 0.120” depending on the material being used. Deviating from this can result in a variety of problems (e.g., differential shrinkage, warp, voids, sink marks, and splay) all stemming from the fact that plastics are poor conductors of heat and flow. When you have uneven thicknesses, thin sections cool faster than thicker, causing these issues.

Thoughtful Rib Design

Ribs can add strength to a part without increasing wall thickness, but they must be carefully designed to avoid cosmetic and structural problems. Make sure your ribs are not the same thickness, or thicker, than the walls they support. If they are, you’re likely to see sinks and voids on adjacent surfaces.

A good rule of thumb is to keep ribs approximately half the thickness of the wall they’re attached to. In addition, you’ll want to design in some draft – approximately 2-3 degrees depending on the texture of the part – to ensure ejection from the mold. Keep in mind, more draft is often better, as it can help the part cleanly come out of the mold and reduce stress on the part and the mold itself.

Core Out Thick Areas

Another common tactic in injection mold design is to core out thick sections. Doing so helps to keep wall thickness uniform, reduces material usage, and lowers part weight—ultimately leading to cost savings in both manufacturing and material. However, be careful not to core too deep, as it can create hot spots in the mold, making those areas difficult to cool. If the steel becomes too thin, cooling channels (e.g., bubblers or baffles) might not be able to reach necessary areas to regulate temperature.

Always balance the need to core out sections with keeping the strength of the part and the mold. Removing too much material can weaken the mold, causing production problems down the road.

Avoid These Common Pitfalls

Minimize Undercuts

Undercuts can add unnecessary complexity, and cost, to your mold design. An undercut is any feature that can't be formed in the primary mold opening direction. Avoiding undercuts as much as possible will keep your mold design simpler and less expensive. If undercuts are necessary, they’ll often require additional features like side actions or lifters, which increase tooling costs and can complicate cooling in those regions of the mold.

Eliminate Sharp Corners

Plastics like to flow smoothly, so sharp corners in your design can cause problems. When molten plastic hits a sharp corner, it disrupts the flow, causing shear, flow lines, and potential short shots. Rounded corners, on the other hand, allow the resin to flow more easily through the mold, preventing these defects. Additionally, sharp corners can lead to wear and tear on the mold itself, eventually causing flash (i.e., where plastic seeps out of the mold along parting lines).

It’s always better to round off edges and corners to ensure smooth resin flow and prevent long-term mold maintenance issues.

Understanding the Injection Molding Process

Now that we’ve covered some of the key design rules, let’s briefly dive into how the injection molding process works. Understanding this will help you appreciate why these design principles are so important.

Injection molding uses a reciprocating screw to heat and inject molten plastic resin into a mold cavity. The hydraulic pressure used to inject the resin creates plastic pressure within the mold, which can be roughly 10 times the hydraulic pressure used. For example, if the injection machine shows 1,600 psi, the plastic pressure inside the mold is around 16,000 psi.

Once the resin fills the mold, it’s held under pressure until the gates freeze. At that point, the screw rotates to prepare the next shot of material, while the mold is still closed until it’s safe to eject the part. Timing here is key. Leave the mold closed too long, and you waste cycle time. Open it too soon, and the part may not have set up properly.

Gate Placement is Important

Gates, which control the entry of molten plastic into the part, should always be located at the thickest section of the part to promote a balanced flow pattern and help improve fiber orientation. Be sure to gate away from any thin or fragile features to avoid creating weld lines, which form when the flow of resin splits and comes back together around obstacles such as pins or cutouts in the part.

Additionally, gate size should be 70-90% of the wall thickness to ensure proper flow and avoid part defects.

Collaborate Early with Engineers and Molding Partners

One of the best ways to ensure a successful mold design is to get tool engineers involved early in the process. Their insights can help you avoid costly design changes down the road and speed up the design-to-production cycle. Tool engineers can also help customers with improving tolerances and finding potential complications with cooling, undercuts, or wall thickness, all before the mold is even built.

By collaborating early, you can save time and money while ensuring your part will meet performance and manufacturing requirements.

Bringing It All Together: Keys to Successful Injection Mold Design

Injection mold design is an intricate process that directly affects the quality, efficiency, and cost of plastic part production. By following key principles—such as maintaining uniform wall thickness, minimizing undercuts, and rounding sharp corners—you can avoid common design flaws and ensure a more efficient manufacturing process. Engaging with engineers early on is another critical step in streamlining the design process and avoiding these potential pitfalls.

At E-S Plastics, we believe in the power of collaboration and thoughtful design to deliver parts that not only meet, but exceed, expectations. Whether you're working on a new project or troubleshooting an existing design, keeping these basics in mind will help guide you toward a successful outcome.