Injection molding is an excellent tool for manufacturing high-quality complex plastic parts at an affordable cost. However, it is essential to design the mold correctly from the start. Mold flow analysis (MFA) software is an excellent tool for mitigating potential problems before the mold is built. It can predict filling and cooling patterns, minimize voids, improve the flow consistency of melted resin, and eliminate errors.
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Knit Lines
Injection molding is a simple and efficient process that produces high-quality plastic parts. However, product designers relying on this molding method encounter various challenges during tool design and development. One of these is knit lines, also called weld lines. They occur when two resin flows meet at different temperatures. Knit lines can appear anywhere on a part, but they’re most common in areas with holes, bosses, or other features that block resin flow. Thankfully, you can reduce the likelihood of problematic knit lines by avoiding these features as much as possible. You can also choose resins that are less susceptible to knit line formation. Changing your injection profile parameters – increasing the injection speed, for instance – may help eliminate these problems. You can also change the location of gates or bosses.
Short Shots
Short shots are one of the most common defects to appear in parts manufactured through injection molding. They happen when molten plastic does not fill the mold cavity with the material before it cools, leaving holes or thin sections in part. These problems can be prevented or troubleshot through a variety of methods. For example, designers can redesign parts or molds before production to enhance the molding window. They can also use sliding side-action cores and other in-mold mechanisms that can help reduce the cost of tooling. Another common cause of short-shot defects is inadequate flow balance in multi-gated or cavity tools. This can be prevented by using mold-filling simulation software to test the part before it is built. In addition, manufacturers can improve the quality of their final product by increasing the melting temperature. This will decrease the viscosity of the molten plastic and make it easier for the material to flow through the part.
Incomplete Parts
Even when a tool is designed perfectly, accounting for material shrink rates, the mold machine operator must mitigate or eliminate pitfalls inherent to the injection molding process. Tolerances, draft angles, undercuts, and other factors must be addressed in the design stage. It is also beneficial to embed pressure and temperature sensors in the mold tooling to provide real-time feedback and control during manufacturing. A higher draft angle on tall features like bosses and ribs is essential to ensure the part leaves the mold without drag lines or other ejection issues. This is especially true for pieces with high feature complexity and complex geometries. During ejection, the walls of a molded plastic part can slide or scrape against the mold cavity, causing drag marks on the surface. These can be cosmetic or functional and can decrease the part’s strength.
Weak Parts
Injection molding is a process where thermoplastic material (liquid) is heated and injected into a mold tool at high pressure. It sets to its final shape once the device has cooled. If the injection molding process’s hold pressure or cooling time is incorrect, this can lead to distortion or warpage within the part. This can be especially common in thinner parts as the plastic shrinks while it cools in the mold. It’s essential to monitor hold pressures and cooling times during the development of your design so that the components produced by the mold tool are consistent. This will help ensure your part can be successfully molded into its final form. Vulnerable parts are also a common source of defects in the injection molding process and can be avoided by ensuring the strength and stiffness of your part are addressed early on in the design process. This can reduce costly changes later on.
