What Causes Splay on Plastic Injection Molding Parts?

Just like in all manufacturing, plastic injection molding brings with it their own kinds of defects that can mar or damage parts. It’s critical to work with parts manufacturers who both understand and have the quality control and troubleshooting skills needed to avoid them hurting your bottom line. Not all defects are created equal, and it’s important for you as a client to understand what they are and their causes. Today we’re looking at “splay,” a typically cosmetic defect that might point at issues with your resin or manufacturing process.

In Plastic Injection Molding, What is Splay?

Splay is a term in the injection molding industry to refer to streaking that occurs along the surface of a molded part. Often silvery in color (from which it gets its nickname of “silver streaks”), this reaction is due to moisture getting caught in the resin during the heating process inside the injection molding machine. This is due to moisture in the plastic feed, which turns to steam when heated in the barrel of the machine. This, in turn, turns to tiny gas bubbles that streak across the plastic as it is injected, leaving behind a silvery pattern on the surface of the plastic.

What are the Root Causes of Splay?

The base issue of this injection molding defect is moisture, but how does the moisture get into the process? While it’s easy to think of the plastic resin pellets that are melted to form parts as being waterproof, it’s important to remember that many of these plastics can absorb or be coated by moisture at various points in the process. They could come from the manufacturer with moisture content, or gain that moisture when being transported, stored, or being prepped for manufacturing. Plastic needs only a little moisture to cause this streaking.

Troubleshooting Splay in Plastic Parts

Whether you’re dealing with splay or avoiding it in the first place, it’s important to keep moisture out of the plastics and machines. These are the most common issues.

• Select Non-Hygroscopic Plastics: Translating as roughly “tending to retain moisture,” hygroscopic plastics (such as Nylon or Polycarbonate) are prone to absorbing moisture.
• Make Sure Your Plastics are Dried: The use of plastic dehumidifiers and dryers significantly reduce the risk of splaying and other water-related issues. Not all are created equal.
• Avoiding Degrading Plastics: If plastics are left heating too long or too high a temperature, they can start to degrade, breaking down and venting gas and moisture that causes splay.
• Avoiding Using Too Much Regrind: Too high a regrind ratio, the use of reusable thermoplastics that have been reground after being used before, can also become contaminated with moisture.
• Splaying Due to Screw or Gate Shear: If the screw that melts the plastic is running at too high a speed, or the gate it improperly sized, these can both also create splay effects.

It’s important to make sure your parts manufacture is up to the test of producing defect-free plastic injection molding parts. Learn about all the common molding defects and issues in part one and part two of that series, as well as our featured article on short shot molding defects. With the possibility of splay and other defects, it’s important to work with a manufacturer with high standards. Midstate Mold & Engineering is such a company. Contact us to consult on your current project, or you can directly request a quote on any or all of the services you need.