By Jack Rulander, Senior R&D Engineer, Protolabs
Injection molding is responsible for bringing us many of the products we rely on daily, from children’s toys to household items and smartphones. Working for a digital manufacturer that serves nearly 50,000 product developers annually, we see countless CAD files for injection molded parts big and small, representing nearly every industry. But just because the service is so widely used doesn’t make it the easiest to master.
Especially in the product iteration stage, time is money. That’s why it is vital to identify design-for-manufacturability issues before a mold is machined to avoid all potential pitfalls. Polling our applications engineers who work directly with customers, the following are the most common issues we see that lead to less-than-desirable aesthetics and overall part performance:
Sink: Appearing as a dimple or shallow depression in your part, sink is a common aesthetic blemish that points to poor part design. The most common culprits include thicker than normal cross sections, non-uniform wall thickness, or an improper gate placement, which is the doorway where hot plastic first enters the mold cavity.
Solution: Follow injection molding wall thickness guidelines from your manufacturer, including the guideline that the minimum wall thickness on your part should be no less than 40 to 60% of its thickest section. Re-positioning the gate so material flow travels from the thick to thin sections of your part will also help avoid sink.
Swirling: Swirling of the material on the surface of a part occurs due to the process of mixing colorant pellets, which add color to your part, with the base resin pellets. “Hot” colorants — red, orange, and yellow — tend to exhibit a higher swirling risk than cool colors like blue and green.
Solution: While the molding process can be adjusted to help address any swirling, the only sure-fire way to avoid part inconsistency due to swirling is providing a pre-compounded resin that your manufacturer can then use during the molding process.
Drag marks: Before we get to the implications, let’s start with the importance of draft, or the taper applied to the faces of a part that prevents them from being parallel to the motion of the mold. Following basic draft guidelines will prevent your part from being damaged when it is ejected from the mold. Drag marks result from insufficient draft due to a part dragging along the mold wall upon ejection.
Solution: When designing your CAD file, add a minimum draft angle of 0.5°, and 2° is even better; heavily textured surfaces may require 5° or more.
Knit Lines: A knit line in a plastic injection-molded part is created when two or more separate plastic flows meet within the mold. Depending on the resin and other manufacturing factors, knit lines can either be virtually invisible or look like cracks in the plastic. Worse yet, knit lines can lead to reduced mechanical properties and eventually breakage.
Solution: First, limiting obstructions in the part design, such as holes and slots that create separate flow fronts, reduces the occurrence of knit lines. Second, the resin you select may make a difference. For example, unfilled materials tend to have stronger knit lines than filled materials. In fact, knit line strength will decrease with higher filler content and with longer fibers.
Flash: All plastic injection molds contain parting lines, which create the potential for small amounts of molten plastic to escape the mold cavity’s boundaries. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of the mold meet can also be a source of flash. The result is an overhang of material that needs to be removed.
Solution: Moving the parting line to a less cosmetic area of the part could be a solution if the design allows. If your product has critical flash requirements, we recommend chatting with your manufacturer. At Protolabs, our applications engineers identify where there is potential for flash and work with the customer to present options in the design of the CAD file or the manufacturing process.
Burns: Air trapped in dead-end sections of the mold can rapidly increase in temperature as it is compressed by the high-pressure incoming resin. This can cause unsightly scorching of the resin. Some resins, especially nylon, burn more easily than others.
Solution: Vents may need to be added to relieve areas of potential trapped gas. We work with customers to recommend vents where needed but warn they may leave small blemishes similar to ejector pin marks. Changing material to one less susceptible to burns is another option, or even changing the design to remove a dead end may be necessary.
Designing for injection molding may take some trial and error, but fortunately, there are more resources than ever to help you along the way. The rise of automated, AI-powered DFM tools, for example, allows customers to upload a part and get DFM feedback within hours. Furthermore, utilize the experts at your manufacturer to help you talk through your CAD file and identify solutions.
Once your CAD files are perfected, the consistency and quality of injection molding cannot be beat.