Designs We Love: Injection Molding

Posted On February 11, 2022 By Gus Breiland

There are a lot of design elements we love seeing in models destined to become injection-molded parts. Sometimes, we get models that just nail it. You can tell the designer/engineer has an excellent sense of what you can and can’t do with the process, and it all starts with solid knowledge of how injection molding works.

Get Your Design for Molding Check-up!

If you want a molded part, your designs should speak to that process. We can review your CAD and determine early-on if the part is designed for moldability, or we may suggest using another option, such as 3D printing or CNC machining. We may even suggest small changes that can make a world of difference once production of parts begins. Getting that initial feedback can save you a lot of headaches, time, and money later.

Cooling Down: How Thickness Affects Parts

Do you design your molded parts thinking about wall thickness? We love to see uniform wall thicknesses in models. It lets us know you were visualizing how molded parts cool and harden. Having uniform wall thicknesses between .060 and .120 in. (1.5-3.0mm) on parts is crucial to ensure that the parts cool evenly. Looked at another way, be wary of saving too much weight at the risk of making parts too brittle or difficult to fill.

Beyond using uniform thicknesses, if you design thin walls, you could end up with:

  • cracking
  • suppressed cooling and possibly warp if they are surrounded by thick walls
  • incomplete filling or shorts
  • weak knit lines and fracture points
  • excessive / premature wear at the parting line

Make your walls overly thick and you’ll get other functional messes:

  • sink
  • porosity
  • warp
  • flow lines

#1 Draft Picks OR Adding 1 Degree for Separation

Draft is that (often) slight angle included on parts to ensure that they can easily pop out of a mold without damage. Hopefully, you add draft automatically. If you create a part without draft in the design, you may have difficulty adding it at the end. We also recommend not expecting your supplier decide where draft should and shouldn’t be. This is risky when they are suppling a part to your assembly. Communicating draft locations in your design can tell the correct story to the mold maker. What we love about draft is how easy it is to include it in your models, so always add some to your designs.

Draft allows the parts to be bumped or pushed off the mold easily without added strain. We often use cooking examples like waffle irons or Bundt pans. Each has a healthy amount of draft to allow for the baked good to fall freely from its mold. 

Cosmetic parts need more draft, especially when applying textures and polish.  Functional parts may not need as much draft, but 1 degree should be the minimum draft considered in injection molding, especially when using aluminum tooling. Also, the number “1” is easier to type than “0.5” for one-half degree.  Just think of the number 1 (at minimum) when adding draft.

Draft, in combination with uniform wall thicknesses, allows the part to cool within the mold without binding or twisting, which would add internal stress to the part and mold. If your part potato chips inside of the mold, the added locking force of the part binding adds stress to the ejection system and can bend ejector pins and other components risking damage to molds and down time. In the worst-case scenario, a lack of draft can break aluminum and steel features in the mold and make for costly repair both in dollars and in time.

Why We Love Radii (and Why You Should, Too)

With injection molding, we are flowing molten resin through a cavity. Resin hates to be forced into a sharp corner—especially as it cools and relaxes. Radii help reduce shear and turbulence created by sharp corners and abrupt changes in flow. A sharp edge on your parts naturally wants to relax and form a small radius, so give it something it wants to mirror—a curve, rather than a sharp corner. Adding radii to internal corners of your part geometry is critical in helping a part eject cleanly from the mold.

Get ready! Put your visualization hats and goggles on—your part is the positive form. The mold is the negative form split in half.  So, an inside corner on your part is an outside corner on the mold. As the part cools, it shrinks. When it does, the inside corner of the part is pressed harder against the outside corner of the mold. If that outside corner of the mold is sharp, the plastic part will end up hugging a sharp edge. That leads to grabbing or pinching on that sharp corner, creating grab or bind. Ejector systems must push this grab off, and that creates stress in the mold and on the part. You end up with broken parts, possible broken molds, and poorly processed parts as the molder tries to reduce risk for both the mold and customer parts.

If you use pass-through cores to eliminate expensive tooling such as lifters, cams, and other undercut forming technology, these need radii on the corners, too.  Far too often we create solutions to reduce cost by projecting the undercut through the part to form the pass-through core without creating corner reliefs or adding room for radii to help the part eject from the mold. Bottom line: Don’t forget your radii.

The Resin Goes Round and Round and It Comes Out Here

It seems funny to say this to seasoned parts designers but remember: Injection molding models require an orifice (gate) to push resin into the cavity and ejector pins to push the part off the mold. You probably knew that, and that makes us happy. Here are a few more tips to ensure that your designs are ready for molding.

Standard single cavity molding and rapid turn-time molding will default to the tab gate. It is simple, effective, and doesn’t require special hardware to make it happen. Consumers are used to seeing high-volume production parts with hidden or camouflaged gating. Generally, companies pay a significant up-front cost to have specialty gating added to hide that. It is costly and requires extended timelines to hide your gate. A tab gate doesn’t require time, it just needs to be cut at the parting line and—boom—you can start molding parts.

Ejector pins are required. They are simple, and effective. The good news is typically there is a show side and a non-show side to a part. The non-show side is inside of the assembly—the inside of the shell or housing. Where you may struggle is if you are making something like a soap dish where the inside is the show side. Remember, the part is shrinking onto the mold as it cools. That could cause it to stick to the mold, requiring the molder to place the inside of the part on the ejector side and it would also require ejector pins to push the now smaller part off the mold core. This leaves little round features on the inside of our soap dish, the bane of the soap dish industry.

We love to see CAD files where it’s obvious that the designer has considered the injection molding process. What we like even more is when you upload your CAD file early to receive feedback on your geometry and our manufacturing process. If you don’t quite have all the rules figured out, start a dialogue with us early in the design so that we can guide you to the best part in the shortest amount of time. Waiting until your design is complete may surprise you when the molder asks for changes. It could also affect additional parts in your assembly right at the moment when you are ready to buy.

So, those are some of the things we love to see in CAD models for injection molding. Read about the design elements we love in our other service lines: CNC machining, 3D printing, and sheet metal fabrication.