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5 Things to Consider When Designing Plastic Injection Molded Parts

Trevor English
July 25, 2018

To aid in this never-ending conquest of designing for manufacturability, we’ve put together 5 of the top design and mold aspects that you need to consider when you’re working with injection molded parts.

5. Uniform Wall Thickness

Whenever you design a part to be injection molded, you want to try to make your part’s wall thickness as consistent as you possibly can. There’s obviously some variability that can be tolerated on this design criteria, but without proper planning, uneven wall thickness can increase the probability of defects. Ultimately, it can lead to warpage, voids, longer cooling time, sink marks, and even molded-in stress. The effects of uneven wall thickness will vary for each part design, but understanding the potential downsides to this design element is crucial to designing a part that can be successfully produced through injection molding.

​​​4. Avoid Undercuts and Consider the Parting Line

Undercuts are often necessary design elements and they won’t inherently affect the manufacturability of your part. However, they can influence the ability to “demold” a part, increasing cycle times and the possibility for defects. The undercut part of your overall design can get trapped inside the mold once the part cools, making it nearly impossible to eject. There are ways to mitigate the ejection of undercut parts, but expect your tooling costs to increase, thus affecting the bottom line of your part cost. It’s for this reason that you should try to avoid undercuts where possible.

Coupled with undercuts, you’ll need to imagine how your part will be released from the mold. More specifically, you need to visualize the parting line of the two molds that would best release your part. Failing to do so could result in your part getting trapped in the mold, sparking necessary redesigns and added time.

3. Take Into Account Structural Support

Decreasing the overall amount of material used in your design ultimately makes your part cheaper to produce. Yet, you don’t want to decrease its necessary structural integrity in the process. This constant battle of constraints is what can make or break a good engineer. One consideration is that thin walls will need support so they don’t collapse or warp in the cooling process. When designed correctly, adding ribs, bosses, or other projections to the part can increase your overall part strength, while only minimally increasing material usage.

2. Evaluate Shrinkage and Warpage

Each kind of plastic will shrink and cool at different rates and to different amounts. The two main problems that arise here and will need to be mitigated are shrinkage and warpage. Shrinkage is the side effect of a difference in the linear dimensions of the part and the mold. Warpage is a distortion in the plastic caused by stress of the plastic part after the melting, pressurization, and cooling it undergoes while being injection molded. These two issues ultimately arise when we fail to take into account material properties, tricky part geometry, tooling types, and final processing. Shrinkage and warpage are common problems that can be minimized by closely considering all aspects of the molding process.

1. Consider Gating and Ejection

Finally, considering the gate location is crucial to designing a good injection molded part. This location is where all the material will enter into the mold cavity, so proper provisions will need to be made. You’ll need to consider whether it’s possible to place a gate where you’re envisioning. Ask yourself whether material will flow from thick walls to thin walls inside the mold cavity, otherwise you may not have enough pressure to fill the part. The gating location also influences defect locations, such as where the weld line will be. You’ll also need to consider how the part will be ejected – whether ejector pins can be accommodated in your design or not. Gating and ejection considerations can drastically affect how you design your part, so it’s better to consider them on the front end rather than when you think you’ve completed the design.

Final Considerations

As you can likely tell, designing parts for injection molding can require increased design for manufacturability workflows compared to other techniques like subtractive processes, and this article only touches the surface of these considerations. With one of the major advantages of injection molding being uniform part duplication, one of the biggest ways that you can be aided in these considerations before moving on from the design process is through simulation software. By utilizing Autodesk Moldflow in your design process, you can validate and improve your designs before ever going to production. Ultimately, this will spare you increased development and revision time, which is crucial to creating the best parts economically.

To see what’s new in Moldflow 2019, you can watch the video below or explore more videos on the Moldflow YouTube page here.


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Trevor English

Trevor is a civil engineer (B.S.) by trade and an accomplished author with a passion for inspiring everyone with new and exciting technologies. As the former editor of one of the world's top engineering websites, you can find his work covering technical topics across the web. In his free time he loves improving his design skills, reading about new technological advances, and exploring the realm of making things.