Injection Molding - Kehui Mold Co., Limited

1. Flash in Injection Molding

Problem Phenomenon

Flake-like excess material appears around the parting surface or ejector pin hole of the product, which is common in transparent parts or precision structural parts, directly affecting the assembly accuracy.

Cause Analysis

Insufficient Clamping Force: Poor mold rigidity or too small an injection molding machine tonnage deforms the template during clamping (using old equipment, a 15% drop in clamping force leads to a surge in flash rate).
Process Parameter Out of Control: Injection pressure is too high (>120MPa), melt temperature is too high (ABS exceeds 240℃), and material fluidity is too strong.
Mold Wear: Scratches on the parting surface, slider wear exceeds 0.02mm, and sealing is reduced.

Solution

Mold Optimization:

Use nitriding treatment to improve the hardness of the parting surface (HRC 58-62).
Increase the number of locking modules to ensure uniform distribution of clamping force (reference formula: clamping force ≥ projected area × material pressure × safety factor 1.2).

Process Adjustment:

Step Injection: 60% pressure fills the cavity in the initial stage, and then drops to 40% to maintain pressure in the later stage.
Reduced the melt temperature by 5-10℃, and monitored the material MFI value (melt index).

Equipment Upgrade:

It is recommended to use a two-plate injection molding machine, with a clamping accuracy of ±0.03mm.

2. Sink Mark in Injection Molding

Problem Phenomenon

Insufficient volume compensation when the material cools and shrinks, which often occurs in thick-walled areas such as ribs and screw columns. In severe cases, the structural strength is reduced by more than 50%.

Influencing Factors

Defects in the holding Stage: Holding pressure is lower than 80% of the injection pressure or the holding time is shorter than 30% of the cooling time.
Cooling System Design: The water channel is >15mm from the product surface or the flow is uneven (CFD simulation shows that the risk increases when the temperature difference is >5℃).
Wrong Material Selection: For example, the shrinkage rate of PP (1.5-2.5%) is much higher than that of ABS (0.4-0.7%).

Solution

Process Parameter Optimization:

Holding Pressure = injection pressure × 0.8, segmented holding (high pressure in the front section to compensate for shrinkage, low pressure in the back section to prevent overflow).
Extend the cooling time to 1.5 times the square of the wall thickness (unit: mm²/s).

Mold Modification:

Add conformal water channels in the thick wall area to ensure a consistent cooling rate.
Use gas-assisted molding technology to form hollow air channels inside to compensate for shrinkage.

Material Alternatives:

Glass fiber-reinforced materials (shrinkage rate reduced by 30-50%) or micro-foamed materials are recommended.

3. Bubble in Injection Molding

Bubble Type Diagnosis

Water Vaporization Bubbles: Irregular distribution, mostly due to material moisture content > 0.03%.
Air-involved Bubbles: Arranged along the flow direction, high risk when injection speed > 90%.
Degradation Bubbles: Accompanied by burn marks, melt temperature exceeds the limit, or residence time is too long.

Solution

Material Pretreatment:

Dehumidification and Drying: PC materials need to be dried at 120℃ for 4 hours, dew point ≤-40℃.
Vacuum Feeding System: Avoid secondary moisture absorption.

Mold Exhaust Design:

Exhaust groove depth = material viscosity coefficient × 0.03 (for example, PP takes 0.03mm, PC takes 0.02mm).
Add exhaust steel inserts, and the air permeability is increased by 10 times.

Process Innovation:

Use “slow-fast-slow” injection curve: initially 20% slow to remove air, high-speed filling in the middle, and slow down at the end to prevent overflow.

4. Warpage in Injection Molding

Root Cause

Warping = uneven cooling (accounting for 60%) + orientation stress (25%) + shrinkage difference (15%).

Actual Case

A certain car lampshade customer had water leakage due to warping deformation exceeding 0.5mm.

Problem Tracing

The mold cooling water channel is only arranged on one side, and the temperature difference is 12℃.
The product rib thickness is 4mm, the main wall thickness is 2mm, and the shrinkage rate difference is large.

Solution

Redesign the Mold Temperature System: Use 3D printing conformal water channel, and the temperature difference is controlled within ±2℃.
Modify Product Design: Reduce rib thickness to 2.5mm, and increase transition radius.
Process Adjustment: Increase mold temperature from 40℃ to 80℃ to reduce orientation stress.

Comprehensive Countermeasures

Mold Flow Analysis First: Use Moldflow simulation to predict deformation, and optimize gate position and cooling scheme.
Material Modification: Add 0.5-1% nano clay to reduce anisotropic shrinkage.
Post-processing Process: Annealing Treatment: PC parts are kept in an oven at 110℃ for 2 hours to eliminate residual stress.