Plastic injection molding is one of the most reliable and widely used manufacturing processes across industries. Nevertheless, despite very optimized manufacturing conditions, issues can arise that compromise part quality, dimensional integrity, or productivity as a whole.
Combined with some experienced plastic manufacturers in China, we identify 13 common plastic injection molding problems, explain why they occur, and offer practical solutions so you can maintain consistent, high-quality output in this guide.
1. Short Shot
An incompletely filled mold cavity is one of the most visible and immediate injection molding faults. Let’s go through its causes and solutions.
Problem: The mold cavity is not fully filled with molten plastic, resulting in incompletely formed parts or missing parts.
Solution:
- Raise injection speed and pressure to push material entirely into detailed areas.
- Raise material or mold temperature to improve flowability and reduce viscosity.
- Design runner and gate systems to achieve maximum flow with lower resistance and hesitation.
- Or, as an alternative, think about regrading the material to a flowing grade if redesigns are not feasible.
2. Flash (Overflow)
Flash is ubiquitous both in new and aged molds. If one understands why flash occurs, remedial solutions can be quickly implemented.
Problem: Plastic escapes too much via parting lines, vents, or via spaces between inserts and lays down thin, unwanted sheets.
Solution:
- Enhance clamping pressure to ensure a good seal of the mold during injection.
- Check for mold alignment and replace worn-out or damaged components.
- Modify injection pressure to prevent excessive force application that can cause the opening of the parting line.
- Clean and condition mold surfaces from time to time to ensure effective sealing.
3. Burn Marks
Burn marks indicate air entrapment and localized overheat. Their early detection will prevent functional failures and aesthetic rejections.
Problem: Black streaks or brown streaks along part surface, commonly within the flow front or vent area.
Solution:
- Boost or adjust mold venting in the correct positions.
- Reduce injection speed to minimize the opportunity for compressed trapped air.
- Regulate barrel and nozzle temperature and check for material degradation prevention.
- Reduce residence time in the barrel by adjusting cycle parameters.
4. Splay Marks (Silver Streaks)
Splay marks deteriorate cosmetic quality and potential mechanical integrity. They are more likely to reflect hidden material handling problems.
Problem: Shiny, silver-like streaks on the surface of molded parts, most often because of moisture vapor or volatile contamination.
Solution:
- Dry hygroscopic materials like nylon, PET, or ABS well before molding.
- Maintain a clean production environment in order to prevent oil, water, or solvent contamination.
- Lower barrel temperatures in order to decrease thermal degradation.
- Examine for over-worn check rings that encourage improper backflow mixing.
5. Weld Lines
Weld lines are inevitable where two flow fronts meet, but their effect must be minimized to maintain part integrity.
Problem: Lines where separate plastic flows come together and fail to bond correctly, creating regions of weakened structure.
- Solution:
Increase melt and mold temperatures to enhance flow front welding. - Relocate gate positions to change flow streams and remove weakened convergence areas.
- Increase injection speed to provide better bonding for hotter flow conditions.
- Select resins for good weld line strength when critical zones are involved.
6. Vacuum Voids
Internal voids can greatly reduce part strength and surface finish. These hidden imperfections are usually the result of poor packing and cooling.
Problem: Air pockets form in thick sections as the outside layers solidify before the inside, creating hollow cores.
Solution:
- Optimize pack pressure and hold time to deliver molten material into shrinking areas.
- Design walls of equal thickness to avoid differential cooling rates.
- Use sequential valve gating or multi-stage packing on large parts.
- Adjust processing conditions to maintain the flow of material until solidification becomes uniform.
7. Warping (Part Distortion)
Warping is perhaps the most frustrating molding flaw because it normally involves a combination of process, material, and design variables.
Problem: Warped, twisted, or deformed molded parts after cooling cause fit or assembly issues.
Solution:
- Provide equal wall thickness to minimize uneven shrinkage.
- Design cooling channels for even temperature distribution.
- Balance packing pressure and solidification cooling time to minimize stresses.
- Bear in mind material selection; fiber-reinforced resins will warp differently from amorphous polymers.
8. Too Much Shrinkage
Unanticipated shrinkage will ruin close-tolerance parts and cause downstream assembly failure. Understanding its causes is crucial.
Problem: Parts are significantly smaller than their designated sizes upon molding.
Solution:
- Compensate for shrinkage in mold design using material-specific shrinkage values.
- Apply greater packing pressure and longer hold time to minimize internal shrinkage.
- Apply resins with lower shrinkage to high-accuracy parts.
- Implement better process control during cooling to manage uniform contraction.
9. Mold Sweat (Condensation Defects)
Environmental conditions within the molding environment have the ability to silently introduce defects in the shape of mold surface condensation.
Problem: Water droplets condense on the mold surface due to temperature gradients, resulting in surface discolorations or flow interruptions.
Solution:
- Keep mold surface temperatures above ambient dew point temperatures.
- Improve workshop climate control by air conditioning or dehumidification systems.
- Adjust mold coolant controls to prevent overcooling and subsequent condensation.
- Use insulated mold components where possible to minimize temperature gradients.
10. Part Breakage During Ejection
Even properly molding parts can be a failure if the ejection process is not controlled correctly.
Problem: Parts chip, crack, or deform while ejecting, especially in thin-walled or complex designs.
Solution:
- Add sufficient draft angles on vertical walls to permit free ejection.
- Smooth mold surfaces to reduce friction forces on the release of parts.
- Redesign the ejection system using stripper plates, air ejection, or sleeve pins for delicate parts.
- Decrease ejection speeds to prevent mechanical shock.
11. Flow Lines
Observed flow patterns may not always impair performance but can detract from visual quality, especially in consumer applications.
Problem: Wavy or streaked flow patterns are on the surface over the path of flow of molten plastic.
Solution:
- Raise injection speed and material temperature to prevent early cooling of the front of the flow.
- Optimize runner design and gate placement to enhance more even, smoother flow.
- Round off sharp corners inside the cavity to reduce the sudden change of flow direction.
12. Cold Slugs
Cold slugs are a consequence of material lingering in the runner or sprue and cooling before its time, resulting in imperfections.
Problem: Small particles of hardened plastic break away and find their way into the cavity, forming stains or short shots.
Solution:
- Use cold slug wells in the runner design to trap cooled material.
- Keep the nozzle and runner at ideal molding temperatures.
- Flush machines extensively for color changes or material changes.
13. Color Streaks
Color consistency is basic to aesthetics and brand. Swirls or streaks of color are commonly caused by improper mixing or processing.
Problem: Inhomogeneous distribution of color on the surface of the formed part.
Solution:
- Pre-mix pigments or masterbatches several days prior to loading material into the hopper.
- Use screw designs that allow homogeneous mixing.
- Regulate processing conditions to avoid localized overheating or material degradation.
- Clean the screw and barrel between color or material change to avoid cross-contamination.
14. Conclusion
Plastic injection molding is a very accurate and efficient process, but it is not without its challenges. Familiarity with the most prevalent molding problems—and implementing focused solutions—can significantly enhance production efficiency, part quality, and overall profitability.
By paying close attention to design, material handling, process parameters, mold maintenance, and environmental conditions, manufacturers can avoid most defects before they become expensive production problems.
An anticipatory and solutions-oriented approach for plastic injection molding guarantees high-level output of even the most stringent production circumstances on a consistent basis.