Injection molding produces hundreds of millions of plastic parts every year for industries including automotive, HVAC, energy, and agriculture. The process allows repeatable, high-volume plastic production. Many injection molding defects come from poor design choices. Defects reduce part quality, lower yield, and increase costs. By following design best practices, manufacturers maximize efficiency, reduce risks, and maintain consistent quality.
Most Common Injection Molding Defects
Injection molding defects are common but preventable. Below are the most frequent issues in plastic manufacturing.
Warping
Warping occurs when parts cool unevenly, pulling material in different directions.
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Causes: Inconsistent wall thickness, poorly designed cooling channels, or incorrect resin use
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Consequence: Parts do not fit, causing assembly delays and complaints
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Remedy: Maintain balanced wall thickness and apply optimized cooling
Sink Marks
Sink marks are small depressions on the surface of a part.
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Causes: Slow cooling of thick walls, insufficient holding pressure, or poor mold temperature control
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Effect: Reduces strength and lowers cosmetic quality
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Remedy: Replace thick walls with ribs, apply holding pressure, and ensure balanced cooling
Flow Lines
Flow lines are streaks formed by molten plastic flowing at different speeds.
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Causes: Poor gate location, uneven wall thickness, or resin freezing too quickly
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Effect: Lowers appearance quality and indicates weak weld lines
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Solution: Adjust gate placement, maintain even thickness, and control injection speed
Short Shots
Short shots occur when a mold cavity is not completely filled.
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Causes: Low injection pressure, poor gate design, or inadequate venting
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Effect: Incomplete parts that cannot be used
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Solution: Improve venting, optimize gate design, and increase injection pressure
Flash
Flash happens when excess plastic seeps out of the mold cavity.
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Causes: Low clamping force, tool wear, or excessive pressure
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Impact: Requires trimming, adding cost and extra time
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Solution: Update tooling, correct clamping, and refine injection settings
For additional troubleshooting guidance, see Plastics Technology’s molding defect chart.
Why Design Impacts Injection Molding Defects
Most molding defects originate in design. Once tooling is built, correcting flaws is costly and time-consuming. Key design considerations include:
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Uniform wall thickness: Prevents warping and sink marks
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Rib design: Strengthens parts without creating thick defect-prone areas
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Gate placement: Controls resin flow; poor placement leads to flow lines and short shots
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Draft angles: Enable easy ejection of parts
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Material selection: Proper resin choice prevents shrinkage and warpage defects
Learn more about injection molding services by Cary Products.
Best Practices to Prevent Defects in Injection Molding
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Early manufacturer involvement: Work with your molder during the design stage to reduce rework
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Mold flow simulation: Predicts resin behavior and prevents costly trial-and-error
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Design for manufacturability (DFM): Improves production efficiency and lowers risk
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Cooling optimization: Reduces cycle times and stabilizes quality
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Mold maintenance: Clean, accurate tooling prevents flash and extends tool life
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Prototyping and testing: 3D printing and pilot runs reveal design flaws before scaling up
The Cost of Defects in Plastic Manufacturing
Defects create both direct and indirect costs:
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Scrap and rework: Faulty or incomplete parts must be reproduced
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Downtime: Mold repair stops production and delays schedules
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Customer dissatisfaction: Defects erode confidence and harm reputation
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Supply chain interruption: Defective parts delay automotive and HVAC assembly
Preventing design defects preserves margins and supports on-time delivery.
Case Example: Automotive Louvers
A Tier 1 automotive supplier needed louvers for a new platform. Cary Products engineers identified warping risks from non-uniform wall thickness. By redesigning with ribs and consistent walls, they eliminated the problem. The customer reduced cycle time by 15 percent.
Case Example: HVAC Components
An HVAC manufacturer required strong, cosmetic fan housings. Gate analysis revealed flow line risks. Cary Products repositioned gates, optimized cooling, and validated results with simulation. The finished part met both strength and cosmetic requirements.
How Cary Products Minimizes Injection Molding Defects
Cary Products ensures reliable production through:
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ISO 9001:2015-certified processes
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In-house design review and simulation
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Injection molding and thermoforming capabilities
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High-volume, high-precision tooling
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Expertise in automotive, HVAC, energy, and agriculture
See our full range of plastic manufacturing capabilities.
Superior Design, Reliable Results
Injection molding defects are preventable with the right design principles. From resin selection to tooling care, every step improves quality. Cary Products provides engineering expertise, advanced manufacturing technology, and consistent results. Contact us today to reduce risk, eliminate defects, and achieve dependable plastic part production.
