Avoiding Problems With Complex Parts

When designing for plastic injection molding, it’s important to know that some parts are much more complicated than others. Complex parts can raise any number of challenges that must be tackled from the beginning of the design process. 

Knowing how to address these issues right from the start can save you time and money while designing the mold. Let’s look at some of the most common complex parts and things you need to keep in mind when designing them.

Living Hinges

Living hinges are a great way to have a lid built right into something, since they keep two halves of a molded product together. Usually, a product will have a living hinge on one side and a clip on the other.

You have to consider certain accommodations when designing a product with a living hinge. After all, the hinge must be thick enough to survive repeating bending while still being thin enough to flex and function. 

To accommodate the range of motion you want, you might need a groove or radius at the midpoint of the hinge. Also, you may experience flash and fill problems, since the material flow will usually be thick-to-thin-to-thick.

Snap Fits, Clips, Undercuts, and Recessed Parts

Recessed parts, undercuts, clips, and snap fits all have similar challenges. A sliding shutoff is often the best way to create slots, pockets, and any other recessed features. This part slides into the mold, preventing material from flowing into the desired area. With this design, you also need a hole in the part beneath the snap to create a snap fit.

To accomplish this, the part feature must be aligned with the direction of the mold opening, and a relief hole at the base of the clip must be allowable in the design. This may require additional draft. You may be able to use a side action, but it must be oriented perpendicular to the pull direction and aligned with the mold’s parting line. Another option is to use hand-loaded inserts, which must be manually put into and taken out of the mold during each cycle.

One final thing to keep in mind when designing clips is the material you want to use. Thanks to its flexible nature, ABS is usually better than PEEK or acrylic. However, there are some design tweaks that can help you use a different material.


If you want to use two different materials on your product, you don’t need to glue or screw those parts on. Rapid overmolding lets you bond different materials in a two-step process that creates better adhesion than other methods. This allows you to have things like ergonomic or nonslip grips on your product.

With overmolding, you place a previously molded part into a second mold, then shoot it with the new material. You need to ensure the two materials are compatible – thermoplastic polyurethane (TPU) over polycarbonate or ABS works well. And while LSR is a desirable overmold material, it has a high molding temperature and needs to be paired with a material that can handle that heat, like glass-filled nylon.

While many materials can be chemically bonded, some need to be mechanically bonded. Using undercuts on your injection-molded parts can assure a no-fail mechanical interlock on overmolded parts. Additionally, many products should get both chemical and mechanical bonding.

Standoffs and Bosses

Thick standoffs, tall ribs, and bosses can pose unique challenges. To avoid ejection problems, you may need additional draft up to 3 degrees or more. If any of these parts are too thick, it can cause sink issues. And tall features need deep molds, which need slower feed rates and longer end mills.

Using gussets or vertical ribs around the periphery of the boss to support it can allow you to use thinner walls. However, you may need to use vent holes in standoffs, deep (tall) ribs, and bosses, and hand inserts may be required.

Text on Parts

While words and designs on parts are common, they can cause big problems if they aren’t handled properly. Small writing should be a non-serif font (like Arial or Century Gothic), and the smallest stroke length, like the legs on a K or the crossbar on a T or A, must be at least 0.020 inches across.

Raised rather than sunken text is easier to read and create. It should be no more than 0.015 inches high (deep, as far as the mold is concerned). The location of the text is also important. The text should always face the direction of mold pull and away from tall standing features in the mold.

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We have a wealth of experience across multiple industries in handling complex part design. Contact us today to get a quote and find out how we can help you design your mold.

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