Molding Defects | Mold Flash

Molding Defects and How to Fix Them: Understanding Mold Flash


Molding defects reduce the surface quality of rubber and plastic parts. The causes include problems with flow lines, sink marks, vacuum voids, and surface delamination. Molding defects are also caused by short shots, burn marks, jetting, and warping. Flash or flashing, the formation of excess plastic or rubber on the surface of molded parts, is also common. Cryogenic deflashing, a batch processing technique, provides molders with a cost-effective way to improve surface quality while protecting dimensional tolerances.

The Problem with Mold Flash

Mold flash affects a part’s surface finish but can also interfere with sealing. Flash is usually a cosmetic defect, but molders want to avoid the appearance of unwanted lines that suggest poor part quality. During injection molding, plastic or rubber can leak between the mold’s parting line. With overmolding, there may be leakage between the base material and the mold instead. Mold flash can also occur during the filling phase or the pack/hold phase. Both conventional parts and thin-walled parts are susceptible.

The causes of flashing can be divided into five main categories.

  • Parting line mismatches
  • Improper venting
  • Inadequate clamping pressure 
  • Poor sprue bushing support
  • Excessively low viscosity

The following sections examine these causes and explain when to use cryogenic deflashing as a solution. Parting Line Mismatches

A parting line is a dividing line that separate the two halves of a mold. Mismatches occur when dust, dirt, contaminants or residues prevent these two sections, the core and the cavity, from mating tightly and closing properly. Mold flash is also caused by old, worn tooling. During the molding process, the tons of pressure that are exerted on mold surfaces can deform the metal. Mold flash is also a challenge with rubber and plastic parts that have complex part geometries. 

Improper Venting

During the rubber or plastic molding process, the air contained in the tool needs a way to escape. Otherwise, the melt will compress and trap the air inside the cavity. Mold vents support the passage of air, but some vent designs are inadequate. For example, if the rubber or plastic material is stiff, a mold’s vents may not be deep enough. If the material is fluid, the vents may be too thin. Worn vents that no longer meet required tolerances can also cause the formation of flashing.     

Inadequate Clamping Pressure

Clamping pressure refers to the force that’s applied to the mold by a molding machine. Known as tonnage, this force must be strong enough to oppose the force that’s caused by injecting rubber or plastic into the mold. With plastic and rubber injection molding, the pressure that’s produced during the pack/hold phase can be strong enough to push the mold’s parting line apart. If the core and the cavity of the mold separate, flashing will occur.

Poor Sprue Bushing Support

Sprue bushings are mold components that provide an opening for transferring molten rubber or plastic into the cavity. They’re made of hardened steel and designed to accept an extrusion nozzle. These bushings help to produce a more rigid sprue, the passage through which the liquid material is introduced into the mold. Sometimes, however, inadequate support for the sprue bushing transfers pressure to the parting line. Thermal expansion in the bushing can also open the mold and cause flash to develop.

Excessively Low Viscosity

Finally, mold flash may be caused by excessively low viscosity – a measure of a fluid’s resistance to flow. When molten rubber or plastic have a low viscosity, they flow quickly and thinly. Conversely, materials with a high viscosity flow slowly and thickly. Viscosity can be an effective tool for process optimization, but flash can form if there are excessive melt temperatures, overly long residence times, or residual moisture from inadequate drying.

Cryogenic Deflashing for Molding Defects

Cryogenic deflashing for molded rubber and plastic parts removes mold flash from hard-to-reach areas such as cross-holes, blind holes, and other difficult geometries. Gaseous nitrogen is used to freeze your parts while they are tumbled and blasted with plastic media. This cryogenic treatment embrittles the flash for ease-of-removal but does not change the physical or mechanical properties of your parts. Plus, unlike manual deburring, cryogenic deflashing is a semi-automatic process that reduces labor costs.

Watch this video to see how the Nitrofreeze® cryogenic deflashing process works and then contact us to review your requirements. If your part is a viable candidate, Nitrofreeze® can perform sampling to demonstrate the process. The consultation is free of charge. To learn more, contact us at the phone number and email listed below.

(508) 459-7447 x109 |