There’s a lot of room for expensive errors when it comes to injection molding. Quality issues in injection-molded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product.
But as with all quality problems, knowing how molding defects arise goes a long way toward preventing them (related: How Experienced Importers Limit Product Defects in 3 Stages [eBook]). Brands and retailers of injection-molded products should have basic knowledge of common molding defects and how to avoid them in order to prevent losses 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.
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.
Here are some common defects that can arise due to errors in the manufacturing process:
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.
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.
Here are some common remedies for flow lines in injection-molded products:
Burn marks typically appear as black or rust-colored discoloration on an edge or surface of a molded plastic part.
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:
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.
One of the main causes for warping in injection-molded plastic and similar materials is that cooling happens too quickly.
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).
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:
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.
Sink marks are similar to vacuum voids but are reversed in cause and effect.
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.
Two or more fronts of polymer or other molten material need to maintain a certain temperature when colliding.
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.
The chief cause for jetting is excessive injection pressure.
Injection molding defects can often originate from the material itself or how the manufacturer stores and handles the material prior to the production process. These defects can range from minor aesthetic issues to compromised strength of the finished component. Serious safety concerns may also result, depending on the intended application of the product affected.
Discoloration, or “color streaking”, occurs when a molded part is a different color than intended. Often the discoloration is limited to a localized area or a few streaks of abnormal color on a molded part. This defect typically affects the appearance of the part without reducing its strength.
A common cause of discoloration is leftover pellets in the hopper or residual resin in the nozzle or mold from a previous production run.
If you find thin layers on the surface of a molded part are easily separating or peeling off the underlying material, you’re seeing a molding defect called delamination. Delamination is a defect characterized by a flaking surface layer, similar to what you’d commonly find on flake mica. This is generally regarded as a relatively serious defect because it reduces the strength of the component.
The most common cause of delamination is contamination of the resin pellets or other base material with a foreign material. Flaky separation results when the two materials cannot properly bond to each other. For example, you might combine a common base plastic like acrylonitrile butadiene styrene (ABS) with an incompatible plastic, such as polypropylene (PP). The resulting loss of strength of the material would be very dangerous if your part is intended for a safety-critical use.
Aside from material fed into the hopper, the contaminant could also be any excess release agents coating the mold for easier component separation. Excess moisture on the material, due to improper drying prior to use, can also cause delamination. Consider the following corrective actions to prevent recurrence if you discover delamination affecting your molded parts:
Defects can be introduced into molded products by issues with the mold tooling itself. Certain defects are likely to occur when the mold is poorly maintained or designed. Especially in the latter case, these defects can be difficult or costly to address in future production runs when it’s necessary to completely overhaul the mold.
A short shot occurs when the flow of molten material doesn’t completely fill the cavities in a mold. The result is that the molded component is incomplete after cooling. Short shot might appear as incomplete compartments in plastic shelves of a display or missing prongs on a plastic fork, for example. Short shots are typically classified as a major defect that can inhibit the function or appearance of the molded part.
The most common cause of short shots is flow restriction resulting from narrow or blocked gates. Sometimes the material is too viscous or the mold is too cold to allow the molten material to completely fill the mold before cooling. And other times trapped air pockets may be hindering proper flow or injection pressure may be inadequate. Consider the following steps to prevent short shot:
Flash, also called “spew” or “burrs”, is an excess of molding material that appears as a thin lip or protrusion at the edge of a component. Flash appears because material has flowed outside of the intended flow channels and into the space between the tooling plates or at the injector pin.
A poorly-designed or worn and degraded mold is the most common contributor to flash. Excessively high mold temperature or injection pressure can also cause flash. Material flowing through the mold cavities can force its way between plates when plate clamping force is inadequate. Common methods for addressing flash in molded products include:
Manufacturing using injection molding typically requires significant upfront investment in tooling. That’s why it’s especially important to design your mold right the first time, rather than having to start again after finding serious defects. Defects related to the molding process or material tend to be easier and less costly to solve. But no matter the cause, defects in molded products can hurt your bottom line tremendously.
Establishing quality control measures such as raw material inspection and in-process inspection can help to minimize injection molding defects and lead to improved product quality, reduced scrap rates, and increased customer satisfaction.
Now that you’re familiar with common injection molding defects to look out for, you can take steps to prevent them in the future and ensure your products meet your customers’ quality standards.
AQF is a leading Quality Control service provider, serving global brands, importers, and retailers of Consumer Products since 2007. We provide a range of quality control inspections to help brands and retailers secure their supply chains. This includes raw material inspection as well as in-process inspections such as defect sorting.
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Editor’s Note: This article was originally published in DECEMBER 2015 by OLIVER KNACK. It has since been updated to reflect current best practices, services, and capabilities.