Common Defects of Medical Injection Parts: Flash, Fabric Shifting & Bubbles Analysis & Solutions from Injection Mold Perspective

For fabric insert molding for disposable medical consumables like NPWT wound dressings, many defects are rooted in mold structure, machining tolerance, venting and locating design, instead of only molding machine parameters.

1. Flash

Mold-related causes

1. Fabric is placed on the parting line. Fabric thickness & pores prevent full closure of parting surface and create tiny gaps.

2. Insufficient mold rigidity. Template slightly bends under injection pressure and causes gaps.

3. Poor flatness of ground parting surface, bad mold assembly parallelism, indentation after repeated trials.

4. No relief structure for fabric edges. Fabric burrs or wrinkles get trapped on sealing area.

Mold solutions

1. Design partition pressure-bearing ribs on parting line. Make relief slot for fabric area. Clamping force is carried by outer ribs instead of squeezing fabric.

2. Add support pillars to strengthen mold base and cavity plate, reduce template deflection.

3. Precision grinding of parting surface, check parallelism after mold assembly, repair indentation timely.

4. Add shallow locating pocket to prevent fabric overlapping & wrinkles.

2. Fabric Shifting

Mold-related causes

1. Weak fabric locating structure: only simple pins without vacuum suction; large clearance between pins and fabric holes.

2. Poor gate location: melt directly hits fabric and pushes it away.

3. Smooth cavity surface with no anti-slip feature.

4. Sharp depth changes in cavity cause turbulent melt flow and drag fabric.

Mold solutions

1. Integrate vacuum suction channels inside mold with tiny suction holes on cavity surface, together with multi-position locating pins to lock fabric in X/Y axes.

2. Adopt edge fan gate away from main fabric area, make melt flow smoothly along fabric surface, avoid direct impact.

3. Apply fine medical-grade anti-slip texture on cavity surface.

4. Optimize part wall thickness at DFM stage to reduce cavity height difference and turbulent flow.

3. Bubbles & Voids

Mold-related causes

1. Insufficient vents. Air trapped inside fabric fiber gaps. Improper vent depth: too deep leads to residue contamination; too shallow blocks air release.

2. No dedicated vents at weld lines.

3. Unbalanced cooling circuit. Local fast cooling freezes melt early and traps air; uneven cooling creates shrinkage voids.

4. No vacuum assist to evacuate air before injection.

Mold solutions

1. Set multiple vent slots along fabric perimeter and weld line ends, control vent depth strictly for medical application.

2. Optional built-in vacuum system to evacuate air trapped inside fabric pores before filling.

3. Optimize cooling channels for balanced mold temperature, prevent early melt freezing.

4. Adjust gate position to shorten melt flow path and minimize weld line.

Conclusion

Fabric insert medical molds follow different design principles from standard plastic molds. Standard molds only consider plastic filling, while fabric insert molds need to handle parting sealing, fabric fixation and air evacuation at the same time. These factors should be reviewed in early DFM stage rather than fixing problems after T1 trial, to cut mold iteration times and stabilize mass yield.