Solutions to Warpage Issues in Injection-molded Products

I. Core Causes of Warpage in Injection-molded Products

1. Differences in Raw Material Properties

   Different polymer materials have inherent variations in shrinkage rate, crystallinity and melt flowability, which are fundamental factors causing warpage. For instance, crystalline plastics (e.g., PP, PA) typically have a molding shrinkage rate of 1.0%–2.5%, much higher than that of amorphous plastics (e.g., PS, ABS) at 0.5%–1.0%. Uneven volume changes during crystallization easily lead to internal stress accumulation. Improper additive ratios or impurities in raw materials will further exacerbate uneven melt flow, resulting in warpage after cooling.

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3. Unreasonable Mold Design

   The mold is the core component of injection molding. Its gate design, cooling system layout and cavity structure directly affect melt filling and cooling processes. Gate positions deviating from the product center of gravity will cause uneven melt flow paths and shear stress differences. Excessively large cooling channel spacing (over 50mm) or asymmetric distribution will lead to over 20% difference in cooling rates across product areas, triggering uneven shrinkage. Insufficient mold release draft angle (less than 1°) will cause forced deformation during demolding, leaving residual internal stress.
4. Deviations in Molding Process Parameters

   The matching degree of molding process parameters directly determines melt filling, cooling and solidification quality. Excessively high melt temperature will intensify thermal degradation, reduce intermolecular bonding force and increase the risk of shrinkage warpage. Insufficient holding pressure or short holding time cannot effectively compensate for melt shrinkage during cooling, easily causing sink marks and warpage. Inadequate cooling time will result in excessively high product temperature (over 50% of the material’s heat deflection temperature) when demolded, leading to further shrinkage and warpage during subsequent natural cooling.
5. Unstable Equipment Conditions

   The stability of injection pressure, mold clamping precision and screw speed uniformity of the injection molding machine will affect the molding process. Injection pressure fluctuations exceeding ±5MPa will cause uneven melt filling rates, easily leading to local stress concentration. Mold clamping clearance exceeding 0.05mm will result in uneven cavity stress, excessive wall thickness deviation of molded products and subsequent shrinkage warpage. Screw speed fluctuations will cause uneven melt plasticization, leading to local performance differences and eventual product warpage.

II. Targeted Solutions to Warpage Issues

1. Raw Material Optimization and Pre-treatment

   For crystalline plastics, adding an appropriate amount of nucleating agents can increase the crystallization rate by 10%–15%, making the crystallization process more uniform and reducing internal stress. Raw materials must be fully dried before use; for example, the moisture content of PA materials should be controlled below 0.2% to avoid melt foaming caused by vaporization during molding and local product warpage. Meanwhile, raw materials with uniform molecular weight distribution should be prioritized to improve melt flow stability.
2. Optimization of Mold Structure Design

   Optimize gate positions and quantity. For large flat products, multi-point gating or film gates are adopted to achieve synchronous melt filling of the cavity and reduce flow stress. The cooling system adopts conformal cooling channel design, with channel spacing controlled at 30–40mm to ensure that the cooling rate deviation across product areas is less than 10%. Increase the mold release draft angle to 1.5°–3°, and optimize the ejection mechanism with multi-point synchronous ejection to avoid excessive local stress during demolding. For easily warped slender products, anti-warpage inserts can be installed in the mold to restrict shape changes during product cooling.

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4. Adjustment of Molding Process Parameters

   Adopt the process strategy of "high temperature, slow injection, high holding pressure and sufficient cooling". Control the melt temperature at 20–30℃ above the material melting point to improve melt fluidity and reduce shear stress. Set the holding pressure at 60%–80% of the injection pressure, and extend the holding time to over 80% of the complete melt solidification time to effectively compensate for shrinkage. Determine the cooling time based on the product demolding temperature being 30% lower than the heat deflection temperature to avoid subsequent shrinkage warpage. Meanwhile, reduce injection speed fluctuations and control screw speed fluctuations within ±3r/min to ensure uniform melt plasticization.
5. Supplementary Post-treatment Processes

   For products warped due to internal stress, annealing treatment can be applied: heat the product to 10–20℃ below the heat deflection temperature, keep it warm for 1–2 hours and then cool it slowly to release internal stress. For products with high dimensional accuracy requirements, shaping treatment can be carried out using a special shaping mold; apply a certain pressure before the product cools to room temperature to force it back to the designed shape. This method can reduce warpage by 30%–50%.

III. Prevention and Control Measures for Warpage Issues