To design a part well, you must first understand the injection molding process. A part is created by injecting molten plastic into a tool cavity at very high pressures, where it then hardens to its final shape. The mold opens, the hardened part is released, and the process is repeated. This process may repeat up to 100,000 times with each part taking only a few seconds to form.
When it comes to evaluating a design for injection molding, many people believe that elaborate and expensive FEA simulation is the go-to tool for molding manufacturers. Although this software is useful, the vast majority of design optimization is much simpler and can be located is most standard CAD toolbars. We like to call it the “Molding Trinity.”
The Molding Trinity: Undercuts, Uniformity, and Drafts
Undercuts, wall uniformity, and draft angles are the first three things any provider of injection molding services will check when you provide a 3D model. These are the core indicators for molding design for manufacturability. Knowing how to check for each, and why you need to check these features, can lead to better parts, lower costs, and fewer headaches when launching your product.
Imagine holding a flashlight to your part design, everything the light touches would glow and the rest would be in shadow. Now imagine having another flashlight shining exactly on the opposite side of the part. Features like holes on the side may still have areas in a shadow, this would be an undercut.
It is important to identify undercuts in designs because they will require a slide, lifter, or hand-loaded core to produce. These side actions can create the feature and either move out of the way or be removed with the part to prevent die lock, where the part cannot be removed from the mold without damage. Undercut analysis is also a great tool for helping understand where parting line and shutoff locations may be on a part.
Best practices when designing for injection molding is to mitigate as many undercuts as possible. This reduces the complexity of the tool and saves potentially thousands of dollars on upfront costs. A useful trick for features like snap tabs is to design slots underneath the clip portion, creating a feature called a pass-thru core and removing the need for additional slides.
Wall thickness uniformity predicts the outcome of the part after molding. When a part is molded, the melted plastic fills the cavity at an even rate, as it cools it will pull the part features inward as the material shrinks. If a part has even walls throughout, then the overall shrinkage as the part cools will be even. However, with large differences between the thickest and thinnest sections of a part, there will be uneven shrink as the part cools leading to physical divots in the part called a sink.
Sink typically occurs where a rib or boss feature meets an outer wall but is also found in thick sections. The sink can often interfere with the cosmetic intent of the part, and in bad cases, it can affect the geometry of neighboring features, holes, and faces. The good news is it is simple to detect and mitigate using a thickness analysis tool. Using this tool you can see a heatmap of wall thicknesses, if you set the nominal outer wall as the baseline, the thick areas will quickly be highlighted for CAD adjustments.
Best practices for uniform wall thicknesses is to use ample coring and ribs instead of adding thickness for part strength. Internal ribs and walls can be 40-60% the thickness of the outer wall. Most part walls should not be over 0.100-0.125” depending on the material viscosity. Cores and ribs should be designed in the direction of pull, which can be checked with–you guessed it–the undercut analysis.
The draft is a direction-dependent feature applied to a part which allows for its release from a tool without dragging in the mold. A good analogy is how paper cups stack up and can easily be separated while paper tubes would interfere with each other. The paper cups stack easily because of their angled walls, the moment one moves from the other they have an air-gap across the entire surface without scraping. Draft angles are applied to any surfaces that are in-line to the direction of pull, whether it’s from the main tool direction or respective to side actions that generate undercut features.
Of the three checks in the Molding Trinity, draft analysis is usually the final check because many adjustments are often made with the feedback from undercut and wall thickness analysis. As a general rule, at least one-degree draft angle should be added for every inch of travel. So if a cavity is 2” deep, then 2 degrees draft may be best practice.
Another best practice for applying draft angles is to be mindful of cosmetic surfaces, such as matte mold texturing, which usually requires 2-3 degrees to prevent cosmetic drag marks on the molded part. If a smaller draft angle is required to hit a particular design goal, then the surface may require polishing to allow for better part release.
Injection molding can make plastic parts that scale in high quantities. Although many low volume injection molds may serve a life of 500-10,000 pieces, the process can be used to take parts tens, to hundreds of thousands of iterations with high reliability. Injection-molded parts can be plastic or thermoset, and can even be multiple staged over-molding to use more than one material or insert molded to incorporate features like machined components. For rapid prototyping injection molded parts, many engineers use 3D printing services hand-in-hand with CAD modeling for fit checks and market evaluation. Knowing the Molding Trinity analysis tools, most designers can quickly evaluate their part model and make adjustments before approaching a manufacturer. These tools can help you communicate about your molding project better with your toolmaker and other engineers. It’s a win-win.