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Design Considerations in Complex Injection-Molded Parts

January 26th, 2022

Unique features of different types of injection-molded parts can make them complex to manufacture successfully. However, those same features also tend to be the ones that make parts more functional which can reduce production costs when they’re done right, so it isn’t always possible to avoid dealing with them.  Knowing the types of details that typically cause problems and how to design parts to get them right is part of what we do at Southwest Plastics. Here are a few examples of common useful features on plastic parts that take a little extra care in production:

Living Hinges: One of the biggest considerations in designing a living hinge is the material used. This thin area of a plastic part has to be able to flex well enough to allow the two halves of a molded container to fold together, but it has to be able to able to open and close over and over again without losing strength or flexibility. If the part also includes a clip of some sort, you’ll need to choose a material that will work for both purposes. In addition, when the hinge is designed, the section will need to be thin enough to flex but also thick enough to withstand repeated bending. A radius or groove at the midpoint may be necessary to accommodate the required folding.

Clips and Snap Fits: A clip or a snap fit, in which a flexible, hook-like piece fits into a slot on a mating part to create a catch, may need one of a few advanced molding techniques to be accomplished successfully. These include sliding shutoffs, side-actions, or hand-loaded inserts. The material spec’d should also be flexible enough for the purpose, although some material constraints can be alleviated through adjusting a clip’s length or geometry.

Bosses, Stand Offs, and Other Tall Features: Tall ribs, thick standoffs, and bosses are potential problem areas in injection-molded parts. If the part features are too thick, sink becomes an issue. Tall features equal deep molds, which require longer end mills and slower feed rates to cut them. Careful venting is also needed to avoid shorts, burning, or incomplete parts. Possible design solutions include using vertical ribs or gussets for support to allow thinner walls on features like bosses and including strategically placed vent holes.

Text on Parts: It’s very common to include a company logo or text on an injection-molded part. To avoid having this kind of detail turn into a problem, there are several factors to consider. Raised text is easier to create and comes out more legibly than sunken text. Unless it’s very large, text should also be no more than 0.015 inches high (i.e., no deeper than that in the mold). If you’re using a very tiny font, it must be a sans serif font like Helvetica or Arial, and the smallest stroke length should be no shorter than 0.020 inches long. Text placement should be considered, as a location down inside a pocket can make it difficult to reach with an end mill. Finally, in general text should face the direction of the mold pull to avoid difficulties with part ejection.

Overmolding: Rapid overmolding allows you to add a feature like an ergonomic grip to a tool without having to glue or screw it to your part. Potential issues to be aware of in making this process live up to its potential include the choice of materials. The materials you choose should be compatible, and the substrate must be able to handle the molding temperature of the overmold material. While some combinations of materials produce a secure chemical bond in overmolding, not all do. If the pair needed for your part do not, then they will need to be mechanically bonded to assure long-term stability.

Manufacturing Expertise in the U.S.

At Southwest Plastics, our 55 years of experience and expertise is put to work solving challenging design problems for our clients every day. In our service to the automotive, aerospace, healthcare, electronics, telecom, and defense industries, we stay abreast of the latest techniques and technologies to provide superior quality, competitive pricing, and fast turnaround times on production runs both large and small. To find out more about how Southwest Plastics can meet your injection molding needs with value and reliability, contact us here.

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