1. Vacuum voids / air pockets

Vacuum voids, or air pockets, are trapped air bubbles that appear in a finished molded component. Quality control professionals typically consider voids to be a “minor” defect (related: 3 Types of Quality Defects in Different Products). injection Plastic Pail Moulding defectsBut larger or more numerous voids can weaken the molded part in some cases, as there’s air below the surface of the part where there should be molded material.

Common causes of and steps to prevent vacuum voids in molded parts

One of the chief causes of voids is inadequate molding pressure to force trapped air out of the mold cavity. Other times, the material closest to the mold wall cools too quickly, causing the material to harden and pull the material toward the outside, creating an abscess. The material itself may be especially vulnerable to voids if its density changes significantly from the molten to hardened state. Voids are more difficult to avoid in molded parts which are thicker than 6 mm. Common ways to prevent voids include:

Raise the injection pressure to force out trapped air pockets

Choose a grade of material with lower viscosity to limit the risk of air bubbles forming

Place gates close to the thickest parts of the mold to prevent premature cooling where the material is most vulnerable to voids.

2. Sink marks

Sink marks are small recesses or depressions in an otherwise flat and consistent surface of a molded part. These can occur when the inner part of a molded component shrinks, pulling material from the outside inward.

Causes and prevention of sink marks

Sink marks are similar to vacuum voids but are reversed in cause and effect.injection molding defects Rather than the material cooling too rapidly near the exterior of the part, the material cools too slowly. The resulting shrinkage pulls the outside material inward before it’s had a chance to adequately cool, leading to a depression. As with voids, sink marks are more likely to occur in thicker parts of a component. Here are some steps you can take to prevent this defect:

Increase holding pressure and time to allow the material near the part’s surface to cool

Increase cooling time to limit shrinkage

Design your mold with thinner component walls to allow for faster cooling near the surface

3. Weld lines

Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off into two or more directions in a mold. The hair-like weld line is the result of weak material bonding, which lowers the strength of the part.

Common causes of weld lines and how to prevent them

Two or more fronts of polymer or other molten material need to maintain a certain temperature when colliding. injection molding defectOtherwise, they become partially solidified and won’t sufficiently bond where they meet, resulting in weld lines. Common remedies for weld lines in molded parts include:

Increase material temperature to prevent partial solidification

Raise injection speed and pressure to limit cooling before the material has filled the mold

Redesign the mold to eliminate partitions

Switch to a material with a lower melting temperature or viscosity to allow faster flow and prevent early cooling

4. Jetting

Jetting refers to a kind of deformation in a molded component that can occur when there’s an initial “jet” of molten material injected into the mold cavity that starts to solidify before the cavity is filled. Jetting often appears as a squiggly line in the surface of the finished component, typically leading from the initial gate of injection. This visible flow pattern can result in part weakness.

Causes and remedies of jetting in molded parts

The chief cause for jetting is excessive injection pressure. injection molding defectsWhen molten polymer or other material is injected through a small gate at high pressure it often squirts rapidly through the gate, rather than filling the mold cavity gradually. As this initial line of material cools against the mold walls and starts to harden, the remaining mold material pushes it, leaving impressions in the surface of the finished part. Avoid jetting in molded parts by:

Reducing injection pressure to prevent rapid squirting of the material into the mold cavity

Increasing material and mold temperature to keep the initial jet of material from solidifying early

Designing the mold with the injection gate located such that the material is directed across the mold, rather than lengthwise.

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