To Control Common Injection Molding Defects

In mass plastic injection molding production, minor surface or structural

defects will directly raise scrap rates, delay delivery and increase

manufacturing costs. Most manufacturers only adjust machine parameters

blindly after defective parts come out, while ignoring the three core control

dimensions: product DFM design, mold structure optimization, and

standardized injection process tuning. This article systematically sorts out

mainstream defects and actionable control solutions, combining two-plate

mold structure design (as shown in the exploded mold drawing) to explain root

causes and preventive measures.

1. Typical Defects Caused by Unreasonable Mold Structure

The A-side cavity plate, B-side core plate, cooling channels, runner-gate

system and vent slots jointly determine melt filling and cooling performance,

which trigger over 60% of molding defects.

Sink Marks & Internal Voids

Visible depressions on thick bosses and ribs, or hidden internal air bubbles,

mainly stem from unbalanced cooling and insufficient packing material supply.

The cooling channel layout shown in the mold drawing is the key control point:

uneven coolant flow leads to inconsistent cooling speed between thin and

thick walls. Designers must arrange conformal cooling channels close to thick

wall areas, and add auxiliary overflow runners near thick bosses. In

production, extend holding pressure time to feed extra molten plastic before

gate solidification.

Burn Marks & Bubbles (Splay)

Trapped air cannot escape narrow vent slots at cavity filling ends, and high

speed compressed air generates local high temperature to degrade plastic.

The standard solution is to open 0.01–0.02mm deep vents at weld line

positions and terminal flow paths of the cavity plate. For hygroscopic materials

like PC and PA66, drying raw materials below 0.02% moisture can eliminate

moisture vapor bubbles.

Flash (Parting Line Burrs)

Excessive injection pressure or insufficient mold clamping force pushes

molten plastic out of the parting line. From the mold structure perspective,

insufficient support pillars under the core plate will cause mold plate deformation under high pressure. Optimize the layout of support pillars to

reinforce the B-side core plate, and repair worn parting line surfaces during

mold maintenance.

2. Process Parameter Tuning for Real-Time Defect Control

Even well-designed molds produce defective parts with improper machine

settings. Four core parameters form a stable process window: melt

temperature, injection speed, packing pressure and cooling cycle.

Weld lines are unavoidable where two melt fronts converge, but their strength

and visibility can be controlled. Increase melt and mold temperature to keep

melt fluidity, adopt multi-stage injection speed: slow filling at the gate to avoid

jetting, then medium speed to ensure tight fusion of melt fronts. For high

strength plastic parts, add auxiliary gates to shift weld lines to non-stress

areas.

Warpage is the most difficult dimensional defect to fix, caused by uneven

shrinkage of core and cavity sides. Balanced cooling channels on both A/B

plates must keep temperature difference within ±5℃. After ejection, use

cooling fixtures to constrain hot parts until full solidification to release residual

stress.

3. Upfront DFM Prevention: The Most Cost-Effective Control Method

Defect control should start at product design rather than post-production

repair. Uniform wall thickness is the primary rule; avoid sudden wall thickness

changes over 2:1 ratio. Add proper draft angles on vertical core surfaces to

eliminate ejection drag marks caused by friction between ejector pins and

plastic parts. When designing ribs and bosses, use fillet transitions to reduce

melt flow resistance and shrinkage differences.

Conclusion

Controlling molding defects is a closed-loop management covering design,

mold manufacturing and on-site injection process. Many factories only focus

on temporary parameter adjustment, while long-term yield improvement relies

on standardized mold structure design (reasonable cooling, venting, runner

layout) and early DFM review. Combining mold flow simulation before mold

trial can predict warpage, air traps and weld line positions in advance, cutting

defect rates by more than 40% and greatly reducing mold rework and

production scrap loss