What is Parting Line in Injection Molding Plastic Industry Parts?

What is Parting Line in Injection Molding Plastic Industry Parts?

Injection molding parting lines can be seen on all injection-molded plastic parts. The two halves of the mold form a parting line between them when they close together. While the mold needs to be completely sealed during this process, there is always a fine line visible on the final product. The mold design and the type of plastic used determine the size and appearance of the lines.

How to determine the parting line of plastic

Plastic part mold showing parting line in injection molding.

The location of the plastic parting line is determined by both mold function and part function. The parting line is where the mold opens, so it needs to be easy to release from that point. It may also be visible and affect the functionality of the final part.

At Rex Plastics, we work hard to determine the optimal parting line for easy mold operation and a final product that will please you. In the left image, the parting line follows the edge of a part designed and molded by Rex Plastics. This is where the mold seams are located. Placing and designing parting lines around part edges like this is often desirable.

Why is there a parting line in injection molding?

Due to the function of the plastic part mold, there is an injection molding parting line. The mold has two halves that are brought together to allow the plastic to be injected into the center. Although the mold seal is very tight, when the mold is opened to reveal the part, there will be a small parting line in the area around the seam. It can be very small depending on how tight the mold fits, but parting lines are something that must be considered and planned for with every mold.

What is the Parting Line Challenge?

As with anything in the injection molding process, plastic parting lines present some challenges. Below are a few common challenges that injection molding professionals face when it comes to parting lines.

In addition to the silicone replacement, there's also a pair of earbuds. Parting lines for complex shapes

For complex shapes, determining the plastic parting line becomes more difficult. Being in the middle of a part may not always work. Mold designers will find a location that meets the needs of part designers and facilitates production. Parting lines can take on different shapes and need not be straight. It can also be placed around some edges of the part, or in some subtle places that are mostly hidden in the final product.

Preventing Parting Lines on Liquid Silicone Parts

Liquid silicone rubber parts (LSR) are more complicated to work with. Melted LSR fits into smaller gaps and can create more flash or thin excess plastic around the parting line. It's hard to get right, but if designed properly, the parting line on an LSR part will be smaller and less noticeable. These ear sleeves (right) are made of LSR.