1. 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

2. 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

DEFECTS TYPICALLY RELATED TO MATERIAL USE OR STORAGE

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.

3. Discoloration

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.

Causes of discoloration in molded products and how to prevent it:

A common cause of discoloration is leftover pellets in the hopper or residual resin in the nozzle or mold from a previous production run. injection molding defectsPoor thermal stability of the coloring agent or improper mixing of the masterbatch are other potential causes. Take the following precautions to limit the risk of discoloration in your injection-molded products:

• Ensure that workers properly clean the hopper, nozzle and mold between production runs to eliminate any residual pellets or base material
• Consider using a purging compound to remove excess color from the machine
• Ensure you or your supplier is using a color agent with adequate thermal stability
• Ensure that the masterbatch is evenly mixed for consistent color output

4. Delamination

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.

Causes and prevention of delamination in injection molding:

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.injection molding defects

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:

• Increase the mold temperature or pre-dry the material properly if excess moisture is an issue
• Ensure workers are properly storing and handling the resin pellets or base material to prevent contamination
• Consider redesigning the mold with a focus on the injection nozzle to limit your dependence on release agents

INJECTION MOLDING DEFECTS CAUSED BY POOR MOLD DESIGN OR MAINTENANCE

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.

5. Short shot

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.injection molding defects

Causes and remedies of short shot in molded products:

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:

• Redesign the mold with wider channels or gates for better flow
• Increase injection speed or pressure or choose a thinner base material to improve flow
• Increase mold temperature to prevent material from cooling too rapidly
• Add additional air vents or enlarge existing vents in the mold to allow trapped air to escape

6. Flash

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. injection molding defectsFlash is usually subtle but might be considered a major defect if particularly obvious on a product. The process for reworking a molded product with flash often includes trimming the excess material.

Common causes of flash and prevention in molded products:

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:

• Retool or redesign the mold if plates don’t fit together properly or allow material to flow outside the channel
• Increase plate clamping force to confine material flow to the channel
• Adjust mold temperature, injection pressure and ventilation to improve material flow

CONCLUSION

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.

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.