How to Control the Dimensional Accuracy of Plastic Products
2026-04-14 11:21:22
Plastic Molds
Dimensional accuracy is a core indicator of plastic product quality. Products with unstable dimensions, out-of-tolerance, or severe deformation will lead to assembly failure, functional failure, and high rejection rates. Dimensional deviation is affected by material shrinkage, mold design, injection process, equipment stability, and environmental changes. To achieve high-precision and consistent dimensions, a full-process control system must be established from material selection to final inspection. This article summarizes practical and effective methods for controlling the dimensional accuracy of plastic products.
I. Material Selection and Shrinkage Rate ControlShrinkage is the root cause of dimensional changes in plastic products. Crystalline plastics such as PP, PA, POM, and PBT have large shrinkage rates and are greatly affected by cooling rate and mold temperature, resulting in obvious dimensional fluctuations. Amorphous materials such as ABS, PC, and PS have small and stable shrinkage, making them more suitable for high-precision parts. Glass fiber or mineral filling can significantly reduce shrinkage and improve dimensional stability.
In mold design, the shrinkage rate must be accurately calculated according to material characteristics. Different wall thicknesses, rib structures, and flow directions may lead to differential shrinkage, so local compensation is required. In production, the material grade and recycled material ratio must be fixed. Changes in material batch or regrind ratio will directly change shrinkage and cause dimensional drift. Strict material management helps maintain consistent molding conditions.
II. Mold Design and Machining Accuracy
Mold accuracy determines the basic dimensional level of plastic products. Cavities and cores must be machined with high precision, and the tolerance should be controlled within 0.01 mm for precision parts. The cooling system should be evenly arranged to avoid local temperature differences, which cause uneven shrinkage and warpage. Unbalanced cooling is one of the main reasons for unstable dimensions.
Gating design directly affects holding pressure distribution. Improper gate position or size leads to insufficient holding in some areas, resulting in shrinkage and dimensional deviation. The ejection system must be balanced to prevent deformation during ejection. Sliders, lifters, and inserts must have high matching accuracy to avoid displacement and flash. For high-precision products, mold steel with high stability and good thermal conductivity should be selected.
III. Injection Molding Process OptimizationProcess parameters are the most direct means to control dimensions on site. Mold temperature must be kept constant using a mold temperature controller, with a temperature fluctuation within ±2°C. High mold temperature increases crystallinity and shrinkage, while low mold temperature reduces shrinkage but may cause internal stress.
Holding pressure and time are the core of dimensional control. Insufficient holding pressure leads to large shrinkage and small dimensions; excessive holding pressure causes internal stress, oversized dimensions, and warpage. The holding time should be maintained until the gate is completely frozen to prevent melt backflow.
Injection speed, backpressure, and screw speed must be stable to ensure uniform melt and consistent density. Unstable plasticization will lead to changes in melt volume and shrinkage. Cooling time and molding cycle must be fixed; products must be fully cooled before ejection to avoid springback deformation.
IV. Equipment and Environmental Stability
Injection molding machine performance directly affects dimensional repeatability. Wear of the check ring, screw, and barrel will cause inconsistent injection volume and pressure. Regular maintenance and replacement of wearing parts are essential. Clamping force must be stable; insufficient clamping force leads to parting surface opening and increased product thickness.
Workshop temperature and humidity should be kept stable. Large environmental temperature changes affect mold cooling efficiency and melt fluidity. Raw material drying must be stable, as moisture changes will affect melt viscosity and shrinkage. Frequent opening of safety doors and random parameter changes should be avoided during production.
V. Process Inspection and Trend MonitoringDimensional control requires preventive management rather than post-inspection. First article inspection must be conducted before mass production to confirm all key dimensions. During production, regular sampling and inspection are carried out to record dimensional trends. Continuous deviation indicates process drift and requires timely adjustment. Statistical process control can effectively predict out-of-tolerance risks and reduce batch defects.
VI. SummaryControlling the dimensional accuracy of plastic products is a systematic project involving material, mold, process, equipment, and inspection. Stable material shrinkage, high-precision mold design, constant molding parameters, and effective process monitoring are the key factors. Through full-process standardized management, dimensional deviation and deformation can be effectively reduced, assembly performance improved, and high-quality and high-efficiency injection production realized.
