Common varieties of PA include PA6, PA66, and glass-fiber-reinforced PA. Its characteristics include extremely high moisture absorption, rapid crystallization, high shrinkage, susceptibility to warping and sink marks, extreme sensitivity to moisture, and a tendency to degrade at high temperatures; consequently, it is an engineering material that demands highly precise control during the molding process.
I. Silver Streaks, Water Marks, and Air Voids (Most Common)
Phenomena: Silver-white streaks, hazy patches, water-like patterns, or bubble-like textures on the surface.
Causes:
- PA (Polyamide) has extremely high moisture absorption; the material was either not dried or insufficiently dried.
- Material temperature is excessively high, leading to thermal decomposition and gas generation.
- Injection speed is too fast, causing air entrapment.
- Poor venting in the mold.
- Insufficient back pressure, resulting in uneven plasticization.
Solutions:
- Ensure thorough drying of the material.
PA6: 80–90°C for 4–6 hours.
PA66: 90–110°C for 6–8 hours.
- Ideally, use a dehumidifying dryer.
- Appropriately lower the barrel temperature.
- Reduce the injection speed during the initial filling phase to ensure smooth mold filling.
- Improve mold venting.
- Appropriately increase the back pressure.
II. Short Shots, Uneven Flow, and Insufficient Filling
Phenomenon: Thin-walled sections, ribs, distal areas, and sharp corners fail to fill completely.
Causes:
- Melt temperature and/or mold temperature are too low.
- Injection pressure and/or injection speed are insufficient.
- Runners and/or gates are too small.
- Poor venting leads to trapped air and flow blockage.
- Raw material moisture content is too high, resulting in reduced flowability.
Solutions:
- Increase melt temperature (PA6: 230–260°C; PA66: 260–290°C).
- Increase injection pressure and injection speed.
- Enlarge the gates and runners.
- Improve venting.
- Ensure the raw material is thoroughly dried.
III. Prominent Weld Lines and Poor Strength
Phenomenon: Weld lines appear deep and discolored (whitish), and are prone to fracture.
Causes:
- Low melt temperature / mold temperature.
- Slow injection speed, causing the melt flow front to cool prematurely.
- Poor venting, resulting in air entrapment at the weld interface.
- Improper gate placement, resulting in an excessively long flow path.
Solutions:
- Increase melt temperature and mold temperature.
- Appropriately increase injection speed.
- Add vents at the weld line location.
- Optimize gate placement to shorten the flow path.
IV. Shrinkage and Sink Marks
Phenomenon: Visible depressions in pillars, ribs, and thick-walled sections.
Causes:
- Insufficient holding pressure; short holding time.
- Injection pressure is too low.
- Gate is too small, leading to premature freezing.
- Melt temperature and/or mold temperature are too high, resulting in significant crystallization shrinkage.
- Uneven wall thickness.
Solutions:
- Increase holding pressure; extend holding time.
- Appropriately increase injection pressure.
- Enlarge the gate; delay gate freezing.
- Appropriately lower the melt temperature.
- Design the product with as uniform a wall thickness as possible.
V. Warpage and Deformation (Particularly Severe in PA)
Phenomenon: Bending, twisting, dimensional instability
Causes:
- PA exhibits high crystallization shrinkage and uneven cooling.
- Uneven mold temperature; significant temperature difference between the front and rear mold halves.
- Significant variations in wall thickness, leading to inconsistent shrinkage.
- Excessively high injection speed, resulting in severe molecular orientation.
- In fiber-reinforced PA, fiber orientation leads to anisotropic deformation.
Solutions:
- Balance mold temperatures and extend cooling time.
- Reduce injection speed to minimize molecular orientation.
- Optimize wall thickness design.
- For fiber-reinforced PA, appropriately increase mold temperature to reduce warpage.
- If necessary, perform moisture conditioning to stabilize dimensions.
VI. Flash / Burrs
Phenomenon: Material overflow at the parting line or inserts.
Causes:
- Material temperature is too high, resulting in excessive fluidity.
- Injection pressure and/or injection speed are too high.
- Clamping force is insufficient.
- Mold clearance is excessive.
- Overfilling.
Solutions:
- Lower the material temperature.
- Reduce injection pressure and injection speed.
- Increase the clamping force.
- Modify the mold to reduce fitting clearance.
- Reduce the injection volume.
VII. Cracking, Brittle Fracture, and Stress Cracking
Phenomena:Cracking during demolding, cracking during assembly, bursting of screw bosses
Causes:
- Excessive internal stress.
- Mold temperature too low; cooling rate too rapid.
- Uneven ejection; "ejection whitening" (stress marks).
- Degradation of material properties due to moisture absorption.
- (For Glass-Filled PA) Exposed glass fibers; stress concentration.
Solutions:
- Increase mold temperature (40–80°C).
- Reduce injection speed to minimize shear stress.
- Adjust ejector pins; increase ejection contact area.
- Thoroughly dry raw materials.
- (For Glass-Filled PA) Slightly increase mold temperature; optimize gate design.
VIII. Bubbles and Voids
Phenomenon: Bubbles or voids present on the surface or within the interior of the part.
Causes:
- Raw material contains moisture, which vaporizes at high temperatures.
- Material temperature is excessively high, leading to thermal decomposition and gas generation.
- Injection speed is too fast, resulting in air entrapment.
- Holding pressure is insufficient, preventing proper compaction.
- Shrinkage in thick-walled sections creates internal vacuum voids.
Solutions:
- Thoroughly dry the raw material.
- Reduce the material temperature.
- Use a slower injection speed during the initial phase to minimize air entrapment.
- Increase the holding pressure and extend the holding time.
- Extend the cooling time for thick-walled parts.
IX. Ejector Whitening, Protrusion, and Cracking
Phenomena: Whitening, protrusion, or cracking at the ejector pin locations.
Causes:
- Low mold temperature; the molded part is brittle.
- Unbalanced ejection; ejector pins are too small.
- Ejection initiated before sufficient cooling.
- Insufficient draft angle; high mold-clamping force (part adhesion).
- Excessive internal stress.
Solutions:
- Increase mold temperature.
- Use larger ejector pins or increase the number of pins.
- Extend cooling time.
- Increase the draft angle.
- Reduce injection speed to minimize internal stress.
X. Exposed Glass Fibers / Rough Surface (Glass Fiber-Reinforced PA)
Phenomena: Surface whitening, protruding glass fibers, rough/pitted surface texture.
Causes:
- Melt temperature and/or mold temperature are too low.
- Injection speed is too slow, failing to adequately encapsulate the glass fibers.
- Mold temperature is excessively low.
- Insufficient back pressure leads to uneven dispersion.
- The mold surface is rough.
Solutions:
- Increase the melt temperature and mold temperature.
- Appropriately increase the injection speed.
- Increase back pressure to improve glass fiber dispersion.
- Polish the mold surface.
- Select a specialized coupling agent masterbatch.
XI. Poor Gloss / Matte Finish
Phenomenon: Surface appears dull, lacks luster, or looks hazy.
Causes:
- Mold temperature is too low.
- Material temperature is insufficient.
- Raw material contains moisture.
- Poor venting.
- Mold surface is rough.
Solutions:
- Increase mold temperature.
- Increase material temperature appropriately.
- Ensure thorough drying of the material.
- Improve venting.
- Polish the mold.
