Matching Criteria for Injection Holding Pressure and Injection Speed

Injection speed and holding pressure are the two most critical parameters in injection molding. Their matching quality directly determines dimensional stability, weight consistency, surface quality, and internal stress of plastic parts. Reasonable matching ensures complete filling, effective packing, uniform shrinkage, and stable molding. Poor matching leads to shrinkage, flash, warpage, weld lines, and inconsistent dimensions. This article explains the interaction mechanism between injection speed and holding pressure and provides practical matching criteria for different materials, structures, and mold conditions.

Basic Principles of Speed and Pressure Matching

Injection speed controls melt flow behavior during filling. Holding pressure compensates for volumetric shrinkage during cooling. The two parameters must be smoothly connected at the V/P switching point. High speed requires stable and reasonably high holding pressure. Low speed requires lower but sustained holding pressure. The goal is to achieve full filling without excessive pressure or residual stress.

V/P Switching Position and Its Influence

V/P switching is the transition point from speed control to pressure control. It is usually set at 95%–98% cavity filling. Switching too early leads to shrinkage, short shots, and dimensional instability. Switching too late causes flash, overpacking, high stress, and mold burden. The ideal switching point is when the cavity is nearly full but not yet pressurized.

Matching Criteria for Different Plastic Materials

Crystalline Plastics

Materials such as PP, PE, PA66, and POM have high shrinkage and require sufficient holding pressure. Medium‑to‑high injection speed is used for filling, followed by high holding pressure and long holding time.

Amorphous Plastics

Materials such as ABS, PC, PMMA, and PS have low shrinkage and are less sensitive to holding pressure. Moderate injection speed is used with medium holding pressure to avoid excessive internal stress.

Glass-Fiber-Reinforced Plastics

These materials are prone to fiber exposure and warpage. Moderate injection speed is used with stable holding pressure to reduce orientation and deformation.

Heat-Sensitive and Low-Flow Materials

Materials such as PVC, flame‑retardant ABS, and PMMA require low‑to‑moderate speed to avoid shearing and burning, with moderate holding pressure to ensure dimensional stability.

Matching Criteria by Product Wall Thickness

Thin-Wall Products

Thin‑wall products cool quickly and require high injection speed to ensure complete filling. Holding pressure must be applied early and maintained at a high level to compensate for rapid solidification.

Normal Wall Thickness

Standard products use medium‑to‑high injection speed and medium holding pressure for balanced quality and stability.

Thick-Wall Products

Thick‑wall products have serious shrinkage and require moderate injection speed to avoid turbulence. High holding pressure and long holding time are needed to suppress sink marks.

Matching Criteria by Product Structure

Large, flat, or long‑flow products require high injection speed with graded pressure control. Complex, ribbed, or deep‑cavity products require multi‑stage speed and moderate holding pressure to avoid trapped gas and uneven filling. Precision components require stable speed and precise pressure control for high dimensional consistency.

Matching Criteria by Mold Conditions

Molds with small gates require higher injection speed and higher holding pressure to ensure effective packing. Molds with good venting can use moderately high speed. Molds with poor venting require reduced final speed and stable pressure. High mold temperature allows lower speed and pressure; low mold temperature requires higher speed and proper pressure compensation.

Common Defects and Parameter Optimization

Shrinkage and sinking are caused by insufficient holding pressure or late switching. Flash is caused by excessive speed, high pressure, or early switching. Weld lines are caused by low speed or insufficient pressure. Warpage is caused by uneven pressure or inconsistent cooling. High internal stress is caused by overpacking or excessively high speed.

Practical Debugging Procedure

Start with standard parameters based on material and wall thickness. Adjust V/P switching position to ensure stable part weight. Optimize injection speed to improve surface quality. Fine‑tune holding pressure and time to eliminate shrinkage and flash. Record and lock stable parameters for mass production.

Reasonable matching of injection speed and holding pressure is the key to stable injection molding. It requires comprehensive consideration of material characteristics, product structure, mold design, and processing conditions. Systematic parameter matching improves product quality, reduces rejection rate, and increases production efficiency.