How to Avoid Common Plastic Injection Molding Pitfalls

Plastic injection molding process technicians have many things to consider when avoiding the most common injection molding pitfalls. They have to understand the entire injection molding process and analyze the quality of each process input to end up with a quality finished product.

To understand how to avoid the most common pitfalls, consider the injection molding process and the role of each input.

Avoid Common Plastic Injection Molding Errors With These Tips

The Role of Plastic Injection Molding Process Inputs

Plastic injection molding is a process using raw plastic resin in pellet form, which is pushed through an injection molding press into a hopper, where it is fed into a barrel and heated to plasticize the resin. This is done at the optimum melt flow temperature for the material, which is injected by force of a barrel and screw assembly into a steel mold.

The steel mold contains two halves, an A side and B side, which close together to mold the plastic into the shape of the cavity, or cavities carved out on the A side of the mold. The part(s) are ejected upon solidifying, with the entire cycle typically measured in seconds. This is a very simple explanation for the injection molding process, which combines many elements with many variables. 

Critical factors include part design, mold design, plastic material and injection molding press that must come together with perfect execution. If one component or process input is not within exact parameters, the entire process will encounter pitfalls and defective product will result.

With that being said, here are some of the most common pitfalls in plastic injection molding and how to avoid them.

The Most Common Pitfalls in Plastic Injection Molding

The following are some of the most common processing errors that occur in plastic injection molding.

Sink Marks and Warping – Warping and sink marks may occur when the material cools at various rates within the part and may cause stress between thin walls and thicker areas. Part design is a critical factor in preventing sink marks. Designing parts with consistent wall sections helps to minimize sink marks and warping.

Knit Lines – As the molten resin is injected into the mold, it begins cooling immediately with the leading edge solidifying quickest. If the material encounters an obstruction around a part feature and comes back together on the other side, a knit line may occur. The type of material chosen is an important factor, as ABS is known to be prone to knit lines. A mold flow analysis can help to identify a part design prone to this error, but a good part and tool design can significantly reduce or eliminate knit lines. 

Shrink – All raw materials have a certain degree of shrink rate although they vary considerably. Careful consideration should be taken on material shrink rate, especially in critical dimensions. Part design with consistent wall thickness or gradual introduction into thicker areas with angles to aid material flow help to reduce out-of-spec shrinkage. 

Flash – Flash is a common defect in plastic injection molding and appears as excess material in a protrusion or thin lip at the edge of a part. This occurs when material flows outside of the runner path, in between the mold or upon injection. Flash may be subtle, although it is a major defect in precision molded parts. Increasing the clamping force or adjusting the mold temperature may improve material flow. Flash causes additional labor to rework the product by trimming, although can usually be corrected with tooling rework, engineering changes or new mold construction. 

Experienced plastic injection molding technicians know how to avoid these common pitfalls and prevent product rework, internal scrap, and the worst case – returned goods.  Prevent these defects from impacting your bottom line, and conduct the proper product and quality planning during part and mold design stage. 

QT Manufacturing Offers Plastic Injection Molded Part and Tool Design

QT Manufacturing is an ISO9001:2015 and AS9100 rev D registered plastic injection molding supplier. We offer part and tool design services, with experienced injection mold construction and injection molding of quality, finished product. We meet the stringent requirements of demanding industries such as aerospace, medical, fiber optics and telecommunications.

QT Manufacturing offers turnkey domestic manufacturing services with plastic injection mold construction and injection molding, as well as precision CNC machining and prototype solutions. We follow a strict quality policy to avoid common pitfalls and eliminate defects. Contact us to learn more about our turnkey manufacturing services with experienced plastic injection molding.