Design Considerations for Injection Molding

The mold is often the biggest investment in the manufacturing process for plastic parts.


When you were young, you likely played with a G.I. Joe action figure, a Barbie doll or a flashy toy car. Today your cellphone may have a plastic case and your car can’t operate without all of its plastic components. Each of those objects were made using injection molding, a process that was specifically engineered to produce precision parts at a low cost.

Designing plastic parts and their manufacturing process is a combination of art and engineering. A good design means parts are consistently high quality, and can be produced at high volume. A bad part or process design means costly rework, poor quality and disappointed consumers. How can you set yourself up for success? Follow a few relatively simple rules.

1. Design with the Manufacturing Process in Mind

Plastic parts are produced by injecting plastic pellets into a mold that is custom-designed for each part. A mold can cost tens of thousands dollars to even a few million dollars—so, you want that mold design to be right the first time, and certainly before you start cutting metal. To do this, you want to optimize the part and the mold at the same time.

One example of this is to try to keep a consistent wall thickness. Plastic likes to flow along the path of least resistance, and a complex thick/thin setup will cause the plastic to flow to the thick areas first, causing gaps and uneven cooling. If your design doesn’t allow for a consistent thickness, play with gate placement or consider a post-molding coring or thinning step that still lets you keep the thickness uniform during injection.

The best way to design the part and manufacturing process in parallel is through simulation. Build a CAD model of the part, simulate the injection process, adjust, try again and so on. Iterations like these cost you the license for the simulation software and a bit of time, but can vastly improve outcomes.

2. Design the Outside of the Part

The outside is what your buyer sees. You want it to be free of seams and welds, sinks, warping, shrinkage or other visible defects. Many designers work on the inside of the part because that is where ribs or bosses and other structural and functional elements reside. Those are important because they enable the part to fulfill its mission, but they’re not sufficient to ensure a happy customer. Again, simulation can help answer questions like: Is it possible change a gate location to make it less visible? Can the boss be a bit smaller, so that it’s not visible one the lip of the part? Simulation enables you test out many possible scenarios before building a prototype mold so that you reach the best combination of function and form.

3. Play with Materials

Digitally experiment with the materials available to you to come up with a balance between design aesthetic, cost and manufacturability. Amorphous resins, for example, are more viscous and tend to shrink less than crystalline or semi-crystalline plastics, which have better flow characteristics but also shrink more. Your part and mold design must factor in shrinkage, delamination, color streaks, blisters and flow marks—all of which will affect the customers’ perception of the part. Also consider cooling: might another material have a different profile, so you can produce more parts in a given time period? All of these affect the economics of part production and are important factors in the overall design.

4. Measure, Measure, Measure

Even the best simulations can only estimate the manufacturing process. Often, a sample mold is used to produce a test run of the part. This step helps determine if adjustments need to be made to the mold, the resin, or its temperature and flow rate. Simulation can speed the front of the design process, allowing time for more creative iterations, but can’t completely replace the need for prototyping.

The mold is often the biggest investment in the manufacturing process for plastic parts, so getting it right is critical to the success of the project. You want accuracy and consistency in production, and to get to market at the right time. A mold with the correct gate locations will yield high precision parts, with optimal structural properties and appearance. Getting the mold wrong can be costly—not only in cash terms, as rework increases the cost of the mold and creates production delays, but also in damaged reputations.

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About the Author

Monica Schnitger's avatar
Monica Schnitger

Monica Schnitger is the founder, president and principal analyst of Schnitger Corporation. She has developed industry forecasts, market models and market statistics for the CAD/CAM,CAE, PLM, GIS, infrastructure and architectural / engineering / construction and plant design software markets since 1999.

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