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3 Keys to Tight Tolerance Injection Molding Design Engineers Must Know

Posted by Dennis McGuigan on Jun 2, 2021 8:20:00 AM
Dennis McGuigan
tight tolerance desing for plastic parts

When it comes to designing and launching a new injection-molded part, design engineers can always count on:

  • The design drives part performance
  • If something goes wrong with the part, the design is the first place to look for answers
  • The importance of part design increases considerably with tight tolerance injection molding

No pressure, right?

There’s a lot riding on design engineers to develop tight injection molding tolerances, especially when margins are as slim as +/- .001 inches in medical, automotive, industrial, and consumer applications.

Managing tight tolerance injection molding — and, by extension, taking some pressure off of designers — is done most effectively when you follow these three expert tips:

1. start early in the design phase

Identifying tight tolerances early in the design phase allows design engineers to factor in requirements that influence injection molding tolerances such as part geometry, overall size, and wall thickness.

Thick-walled designs, for example, may have variable shrink rates that "move" within sections, making it difficult to hold tight tolerances. Likewise, part size plays a role in tight tolerance injection molding. Simply put, the larger the part's dimension, the harder it is to control shrinkage and manage tight tolerances.

2. Pay Attention to Complexity

When done correctly, a complex design can aid in maintaining an injection-molded part's tight tolerances. However, shrinking and warpage commonly occur as part complexity increases, meaning tight tolerances — and the molding process — may not be repeatable.

Engineering teams need to be dialed into the possibility of shrinking and warpage in complex designs so they can head off costly delays or rework. To do so requires deep dives into plastics selection, tooling design, and material flow using moldfill analysis. This sophisticated Design for Manufacturability (DfM) approach helps engineers accurately analyze and dictate three essentials of tight tolerance injection molding: cavity fill rates, proper cooling temperatures, and the overall cooling process.

3. understand the Environment

When designing injection-molded parts for complex applications, designers must clearly understand the environment wherein the part will function.

Why is this so important? The environment influences the behavior of plastic. If plastic responds unpredictably to environmental conditions, tight tolerances are at risk.

To better illustrate the point, consider that plastics typically have large thermal expansion coefficients. As a result, parts may have to be measured at a consistent temperature to accurately determine expansion, contraction, and ultimately the part’s ability to maintain tight tolerances.

Only the BEGINNING

While the design phase is the obvious place to start the conversation about tight tolerances, there's much more that contributes to successfully manufacturing injection-molded parts for complex applications. To get up to speed on material selection, tooling, process design/control, and other key factors read our guide How to Achieve Tight Tolerances in Custom Plastic Injection Molding

Achieve Tight Tolerances in Your Custom Plastic Injection Molded Applications

Topics: Plastic Part Design

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