Injection Molding Holding Pressure Adjustment Guide

Holding pressure is a critical parameter in the injection molding process. Its primary function is to apply continuous pressure to the molten plastic after the mold cavity is filled to compensate for volumetric shrinkage during cooling, thereby preventing sink marks and voids while ensuring dimensional accuracy and uniform density. The adjustment of holding pressure must consider material characteristics, part geometry, and mold design, following the industry-recognized principle of "starting low and optimizing incrementally."

1. Prerequisites for Adjusting Holding Pressure

Adjustments to holding pressure must be based on stable injection parameters. Two key steps must first be completed. First, establish reasonable injection pressure and speed to ensure the melt completely fills the cavity. The switch to holding pressure should occur when the cavity is 95% to 98% full, a state where the part is free from short shots and excessive flash. Second, define the transfer point. While both position and time transfer are common, position transfer is preferred. Switching too early causes short shots, while switching too late means the melt is already compacted, rendering holding pressure ineffective for shrinkage compensation.

2. Initial Setting Principles for Holding Pressure

Holding pressure is not an independent parameter but is correlated to injection pressure. The industry-standard initial setting range is 50% to 80% of the injection pressure, adjusted based on material flowability and wall thickness. For high-flow materials such as PE, PP, and PS, which have low filling resistance and moderate shrinkage compensation needs, the initial holding pressure should be set at 50% to 60% of the injection pressure to avoid flash or excessive internal stress. For medium-flow materials like ABS and PC/ABS alloys, the initial setting is typically 60% to 70% of the injection pressure to balance compensation and stability. For low-flow materials such as PC, PMMA, and glass-filled plastics, which exhibit high viscosity and significant shrinkage, the initial holding pressure should be set higher, at 70% to 80% of the injection pressure, to ensure sufficient packing of thick sections and prevent voids.

3. Practical Adjustment Steps for Holding Pressure

(1) Initial Sampling and Defect Observation

Inject 3 to 5 samples using the initial settings and observe for key defects. Sink marks or voids on the part surface indicate insufficient holding pressure, meaning the melt lacked pressure to compensate during cooling. Flash or burrs at the part edges indicate excessive holding pressure, causing the melt to squeeze out of the mold parting line. If the part dimensions are consistently small and unstable, this usually indicates a need to adjust holding time rather than simply increasing pressure.

(2) Incremental Parameter Fine-Tuning

The adjustment process must follow the principle of "small adjustments, verify shot-by-shot," with each adjustment limited to 5% to 10%. If sink marks are present, increase the holding pressure by 5% increments until the defect is eliminated. If flash occurs, decrease the holding pressure by 5% increments until the flash disappears. It is crucial to note that the upper limit of holding pressure must not exceed the mold's clamping force capacity to prevent mold deformation or damage.

(3) Matching Holding Time and Pressure Gradient

Holding pressure must work in conjunction with holding time. Insufficient time leads to poor compensation, while excessive time increases internal stress, causing warpage or cracking. The standard holding time is generally 10 to 15 times the part's wall thickness (in seconds). For example, a part with a 3mm wall thickness requires a holding time of approximately 30 to 45 seconds. For structurally complex parts with varying wall thicknesses, a multi-stage holding strategy is recommended. This involves using a higher pressure initially for rapid shrinkage compensation, followed by a lower pressure to maintain packing, thereby reducing internal stress.

(4) Final Validation and Stability Confirmation

After determining the optimal parameters, run a consecutive 10 to 20 shots. Inspect the dimensional accuracy and surface quality for consistency. Additionally, test the mechanical properties, such as impact strength and tensile strength, to ensure the holding parameters satisfy both aesthetic requirements and the part's functional performance.

4. Common Defects and Corresponding Adjustment Strategies

When sink marks or voids appear, the core issue is insufficient holding pressure. The solution is to increase the holding pressure by 5% to 10%, and optionally extend the holding time or slightly raise the melt temperature to enhance compensation. When flash or burrs occur, the cause is excessive holding pressure leading to melt overflow. The remedy is to decrease the holding pressure by 5% to 10%, while also checking the clamping force and optimizing the transfer timing to prevent over-packing. When parts exhibit warpage, the primary cause is usually excessive internal stress from high holding pressure. The adjustment involves reducing the holding pressure and shortening the holding time, supplemented by optimizing the cooling channels and lowering the mold temperature to address uneven cooling. When part dimensions are unstable, the issue typically lies in fluctuating holding pressure or an inaccurate transfer position. The fix requires stabilizing the hydraulic system to ensure consistent pressure output and calibrating the transfer position accurately.

5. Special Considerations for Specific Materials and Parts

For crystalline materials such as PA, POM, and PE, which have high cooling shrinkage rates, the holding pressure should be set at the upper limit (70% to 80% of injection pressure), and the holding time should be extended to ensure complete crystal formation and minimize shrinkage defects. For amorphous materials like PC, PMMA, and ABS, which have low and uniform shrinkage, the holding pressure can be set at the middle to lower range. The focus should be on controlling the holding time to avoid internal stress-induced cracking. For thin-walled parts (wall thickness < 1.5mm), where melt filling is rapid, the holding pressure should be reduced to 40% to 50% of the injection pressure, and the holding time shortened to prevent mold damage or flash caused by high pressure. For thick-walled parts (wall thickness > 4mm), a "high pressure for a short time + low pressure for a long time" strategy is essential. This uses high pressure initially to quickly compensate for shrinkage in thick areas, followed by low pressure to maintain packing, thus reducing internal stress and warpage.