5 Injection Moulding Defects & How to Prevent Them

There’s a lot of room for expensive errors when it comes to injection molding. Quality issues in injection-mold products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the molding process, material use, tooling design or a combination of all three.

But as with any quality problems, knowing how molding defects arise is half the battle (related: How Experienced Importers Limit Product Defects in 3 Stages [eBook]). As an importer or manufacturer of injection-molded products, some knowledge of common molding defects and how to avoid them can help you cut costs related to unsellable goods and product returns.Let’s look at the most common quality defects related to injection molding, what causes them and what you can do to prevent them.

Moulding Defects Often Caused by Process Problems

Some molding defects may be difficult or costly to address. Others can be prevented by adjusting the molding process, without the need to redesign the mold tooling or replace other production equipment. You can typically avoid these defects relatively easily simply by adjusting the flow rate, temperature or pressure of your mold.

1. Flow lines

Flow lines appear as a wavy pattern often of a slightly different color than the surrounding area and generally on narrower sections of the molded component. They may also appear as ring-shaped bands on a product’s surface near the entry points of the mold, or “gates”, which the molten material flows through. Flow marks won’t typically impact the integrity of the component. But they can be unsightly and may be unacceptable if found in certain consumer products, such as high-end sunglasses.

Causes and remedies for flow lines:

Flow lines are most often the result of variations in the cooling speed of the material as it flows in different directions throughout the mold. Differences in wall thickness can also cause the material to cool at different rates, leaving behind flow lines.injection molding defects For example, molten plastic, cools very quickly during the injection process and flow marks are evident when the injection speed is too slow. The plastic becomes partially solid and gummy while still filling the mold, causing the wave pattern to appear.

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Here are some common remedies for flow lines in injection-molded products:

• Increase the injection speed, pressure and material temperature to ensure the material fills the mold before cooling
• Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines
• Relocate mold gates to create more distance between them and the mold coolant to help prevent the material from cooling too early during flow
• Increase the nozzle diameter to raise flow speed and prevent early cooling

2. Burn marks

Burn marks typically appear as black or rust-colored discoloration on an edge or surface of a molded plastic part.injection molding defects Burn marks generally don’t affect part integrity, unless the plastic is burned to the extent of degradation.

Causes and prevention of burn marks

The usual cause for burn marks in injection-molded parts is trapped air, or the resin itself, overheating in the mold cavity during injection. Excessive injection speeds or heating of the material often lead to overheating that causes burns. Consider the following preventative measures to avoid burn marks in molded components:

• Lower the melt and mold temperature to prevent overheating
• Reduce the injection speed to limit the risk of trapping air inside the mold
• Enlarge gas vents and gates to allow trapped air to escape the mold
• Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat

3. Warping

Warping is deformation that can occur in injection molded products when different parts of a component shrink unevenly. Just as wood can warp when it dries unevenly, plastic and other materials can warp during the cooling process when uneven shrinkage puts undue stress on different areas of the molded part. This undue stress results in bending or twisting of the finished part as it cools. This is evident in a part that’s meant to lie flat but leaves a gap when laid on a flat surface.

Causes and prevention of warping in molded parts:

One of the main causes for warping in injection-molded plastic and similar materials is that cooling happens too quickly. injection molding defects Often excessive temperature or low thermal conductivity of the molten material can worsen the problem. Other times mold design can contribute to warping when the walls of the mold are not of uniform thickness—shrinkage increases with wall thickness. Here are some common ways to prevent warping in your molded parts:

• Ensure the cooling process is gradual and long enough to prevent uneven stresses on the material
• Lower the temperature of the material or mold
• Try switching to a material that shrinks less during cooling (e.g. particle-filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades)
• Redesign the mold with uniform wall thickness and part symmetry to ensure greater stability in the part during cooling

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4. 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 molding defects But 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

5. 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