TOP 10 molding problems

1: MOISTURE IN POLYMER GRANULES

Many plastic resins have the tendency to absorb moisture from the surrounding air, with the extent of absorption varying based on the type of polymer. Even minor surface condensation on granules can lead to significant issues during the moulding of engineering plastics. These moisture-related problems may manifest as processing difficulties, surface imperfections, or diminished mechanical properties in finished components. Because visual inspection is generally insufficient to detect moisture content, this guide offers practical insights for moulders working with moisture-sensitive plastics.

Drying Requirements for Plastics

Engineering thermoplastics typically demand low moisture levels prior to processing. The necessity and extent of drying depend primarily on the resin’s sensitivity to moisture. Other influential factors include the material’s initial moisture content upon delivery, the type of packaging, and storage duration.

For example, polyamides are commonly supplied in moisture-resistant, foil-lined packaging, allowing for immediate use. Nevertheless, many processors still opt to dry polyamide resins, especially if they are not used promptly. In contrast, PET and PBT are significantly more moisture-sensitive and must always be thoroughly dried. Insufficient drying can negatively impact the impact resistance of moulded parts.

Furthermore, PET and PBT quickly reabsorb atmospheric moisture post-drying. Processors should handle open containers with care, particularly during transfer and storage in hoppers or conveyor systems. Under humid conditions, PET can exceed the allowable 0.02% moisture limit within just ten minutes. When drying fully saturated granules or regrind—such as material left exposed in open containers—extended drying times are essential. For instance, saturated polyamide may require over 12 hours to dry completely, with some yellowing likely to occur. To mitigate moisture-related risks:

Store sprues and regrind in sealed containers.
Reseal partially used bags or containers.
Keep hoppers covered at all times.

Proper Drying Techniques

Achieving high-quality moulded parts depends heavily on proper drying procedures. Basic hot-air dryers are inadequate for sensitive polymers like polyesters. Instead, desiccant dryers or systems utilizing dehumidified air are necessary, as they maintain stable drying performance regardless of environmental conditions. Besides maintaining the appropriate drying temperature, it’s crucial that the drying air’s dew point remains at or below –20°C. For multi-container systems, ensure sufficient airflow through each vessel, taking into account variations in fill level and bulk density.

Moisture Measurement

Accurate measurement of moisture content in granules is vital and can be achieved using commercial devices, such as manometric or Karl Fischer titration systems. To minimize error, samples should be collected from deep within the hopper and sealed immediately in moisture-proof containers. Heat-sealable PE/aluminium sachets or laboratory-grade airtight glass vessels are both suitable for this purpose.

2: PART WARPAGE

Warpage is common with semi-crystalline materials due to their shrinkage behavior and, for reinforced types, fiber orientation.

Prevention Tips

Ensure uniform wall thickness.
Avoid melt accumulation.
Use balanced gating and mould heating.
Analyze melt flow to predict fiber orientation.
Modify mould design as a last resort.

3: INCORRECT MELT TEMPERATURE

Precision in melt temperature is vital when moulding semi-crystalline polymers. Errors on either side—too high or too low—lead to structural problems, surface defects, and mechanical failure.

Indicators and Consequences

Overheating causes degradation, bubbles, odour, discoloration, and breakdown of additives.
Underheating leads to poor homogeneity and reduced toughness.

Monitoring Melt Temperature

Rely on actual melt temperature readings, not just heater settings. Use pre-heated, thin-diameter probes in insulated containers. Set barrel temperature profiles based on melt point and adjust based on measured output.

4: INSUFFICIENT HOLDING TIME

Processors often underestimate the hold pressure time needed for semi-crystalline resins, applying practices suitable for amorphous polymers. This can severely impact quality.

What Happens During Hold Time?

After the cavity fills, polymers begin to crystallize from the outside in, causing volume shrinkage. This must be offset by continued melt injection. Too-short hold time results in voids, sink marks, warping, and reduced strength.

Recognizing and Correcting It

Short hold time manifests as visible shrinkage, dimensional variability, or voids in thick sections. Weighing successive mouldings helps determine the ideal hold time. For reinforced materials, inspection of fractures or polished sections under a microscope reveals internal flaws.

Hold pressure should range from 60 to 100 MPa. Cooling time should be minimized—just above plasticizing time.

5: MOULD SURFACE DEPOSITS

Additives like flame retardants or impact modifiers can lead to deposits. Common causes include thermal degradation, excessive shear, and poor venting.

Types and Solutions

Localized deposits often trace to nozzle or runner issues.
Broad, uniform deposits suggest material breakdown.
Early cleaning is critical—daily maintenance prevents buildup.

Preventive Actions

Optimize venting.
Use coatings or temperature control to reduce buildup.
Test solvents for cleaning based on deposit composition.

Regular checks and informed troubleshooting can keep moulding operations efficient and consistent.

6: INCORRECT TOOL TEMPERATURE

Tool surface temperature heavily influences shrinkage, surface finish, and dimensional stability in semi-crystalline polymers. Low or uneven temperatures can lead to excessive shrinkage or post-moulding distortion.

Best Practices

Design tools for consistent thermal distribution.
Use pressurized water systems and robust temperature control equipment.
Include separate circuits for tool halves and side actions.
Connect circuits in series to avoid flow imbalances.
Measure temperature directly at the tool surface using thermocouples or fast-acting pyrometers.

7: INCORRECT GATE LOCATION

Gate location significantly influences flow behavior, holding pressure, and final part strength. Designers and mould makers must involve moulders early to ensure gate placement supports quality production.

Consequences of Poor Gate Position

Incorrect gate location can lead to weld lines, trapped air, sink marks, voids, warping, and uneven shrinkage. If gates are few or poorly placed, excessive flow distances and high injection pressure may occur, limiting process adjustability.

Placement Recommendations

Gate into the thickest section.
Avoid gates near high-stress areas.
Gate longitudinally for long parts.
Use symmetrical arrangements for multi-cavity tools.
Centrally gate symmetrical parts.
Ensure weld lines are positioned away from hinges.
Gate near the base for cup-like parts to avoid air entrapment.
Allow even melt flow in tubular or insert-moulded parts.

8: POOR SURFACE QUALITY

Aesthetic flaws can stem from design, processing, or material issues. Start by identifying the defect’s location, frequency, and link to specific cavities or batches.

Common Causes

Material contamination or poor drying.
Incorrect injection speed or temperature.
Defective hot runner design or venting.
Additive degradation.

Prevention

Avoid hot runner systems if surface quality is critical. Use cold slug catchers and ensure proper venting. Regular maintenance and timely defect identification are key.

9: Undersized Feed System

Even with modern design tools like CAD and mould-flow simulations, feed systems can be inadequately dimensioned, especially for semi-crystalline polymers. Proper feed design must be paired with correct gate positioning and adequate hold time, which will be covered in upcoming sections.

Behavior of Semi-Crystalline Polymers

These resins shrink as they cool from molten to crystalline state, often by up to 14%. This shrinkage requires a continuous melt supply during the hold phase—only possible if the gate cross-section allows for sustained pressure.

Identifying the Problem

A feed system that is too small results in ineffective hold pressure, leading to voids, pinholes, and sink marks. Under the microscope, these defects are easily observed. Reinforced materials are especially susceptible, as narrow gates damage fiber integrity, reducing strength. Additional signs include high injection pressure, long fill times, jetting, dull spots, marbling, and build-up near the gate.

Design Guidelines

Key considerations:

Runner diameters should always exceed wall thickness.
Cold slugs must be intercepted via extensions at the gate.
Gate thickness should be at least 50% of the wall thickness for unreinforced and up to 75% for reinforced polymers.
Gate length should not exceed 1 mm to prevent premature solidification.

10: HOT RUNNER ISSUES

Hot runner systems for semi-crystalline polymers require precise thermal control. Inappropriate setups can degrade materials and create pressure losses.

System Requirements

Use externally heated, full-cross-section nozzles.
Avoid narrow runners and annular designs.
Install PID controllers for accurate temperature regulation.
Use interchangeable tips for abrasive materials.