PBT (polybutylene terephthalate) is an engineering plastic characterized by rapid crystallization, high shrinkage, high moisture absorption, high fiber reinforcement, easy warping, easy whitening, obvious weld lines, good temperature resistance, and excellent electrical properties. It is widely used in electronic connectors, sockets, appliance frames, coil frames, and automotive components. The vast majority of PBT is glass fiber modified PBT.
I. Silver Streaks, Flow Marks, and Splash Marks (Most Common)
Phenomena: Surface silver streaks, misty patterns, flow lines, and striations.
Causes:
- PBT is highly hygroscopic; raw materials have not been sufficiently dried.
- Melt temperature is too high, causing slight material decomposition and gas generation.
- Injection speed is too fast, resulting in air entrapment.
- Poor mold venting.
- Insufficient back pressure, leading to uneven plasticization.
Solutions:
- Strict drying: 120–140°C for 2–4 hours; use of a dehumidifying dryer is recommended.
- Appropriately lower the barrel temperature to prevent thermal decomposition.
- Reduce the injection speed during the initial phase to ensure smooth mold filling.
- Deepen mold vent channels to improve the release of trapped air.
- Appropriately increase back pressure to stabilize plasticization.
II. Warpage and Deformation (The primary defect in PBT-exacerbated by glass fiber reinforcement)
Phenomena: Bending, twisting, significant dimensional deviations, and surface non-flatness.
Causes:
- Extremely rapid crystallization rate, leading to uneven internal and external shrinkage.
- Glass fiber orientation, resulting in significant anisotropic shrinkage differences.
- Excessively high mold temperature, promoting more complete crystallization and consequently greater shrinkage.
- Uneven product wall thickness.
- Uneven cooling or an ill-conceived mold cooling channel layout.
Solutions:
- Appropriately lower the mold temperature and extend the cooling time.
- Optimize gate placement to minimize the impact of glass fiber orientation.
- Balance the mold cooling channels to ensure consistent temperatures between the front and rear mold halves.
- Design the product structure to be as symmetrical as possible, with uniform wall thickness.
- Select modified PBT raw materials specifically formulated for low warpage.
III. Shrinkage and Depression
Phenomenon: Visible sink marks on ribs, boss posts, and the backsides of thick-walled sections.
Causes:
- High crystallization shrinkage combined with insufficient holding pressure.
- Holding time is too short, causing the gate to freeze off prematurely.
- Melt temperature and mold temperature are excessively high, exacerbating shrinkage.
- The gate is too small to effectively compensate for shrinkage.
- Significant variations in product wall thickness.
Solutions:
- Increase holding pressure and extend holding time.
- Appropriately enlarge the dimensions of the gate and runners.
- Lower melt and mold temperatures to reduce crystallization shrinkage.
- Implement multi-stage holding pressure.
- Optimize product structure to avoid localized thick-walled sections.
IV. Deep, Whitened, and Weak Weld Lines
Phenomenon:The lines where material flows converge are distinct and appear whitened; the part is prone to fracture under stress.
Causes:
- Material and mold temperatures are too low, causing the flow front to cool rapidly.
- Poor venting leads to air entrapment at the convergence point.
- Injection speed is too slow.
- Gate placement is suboptimal, resulting in an excessively long flow path.
Solutions:
- Increase the barrel and mold temperatures.
- Appropriately increase the injection speed.
- Create dedicated vent channels at the locations where weld lines occur.
- Optimize gate placement to shorten the mold filling path.
V. Exposed Glass Fibers, Surface Whitening, Rough and Lusterless Finish (Issues Specific to Glass-Filled PBT)
Phenomena: Surface whitening, pitting, protruding glass fibers, rough and lusterless finish.
Causes:
- Melt temperature is too low; the resin fails to fully encapsulate the glass fibers.
- Mold temperature is too low.
- Injection speed is too slow, resulting in exposed glass fibers on the surface.
- Insufficient back pressure leads to poor dispersion of glass fibers.
- Poor surface finish (roughness) of the mold.
Solutions:
- Appropriately increase the melt temperature to ensure the resin is fully molten.
- Increase the mold temperature to improve surface encapsulation.
- Appropriately increase the injection speed to better encapsulate the glass fibers.
- Increase back pressure to optimize glass fiber dispersion.
- Thoroughly polish the mold surface.
VI. Short Shots / Incomplete Filling
Phenomena:Thin-walled sections, sharp corners, or distal areas remain unfilled.
Causes:
- Material temperature is too low, resulting in poor flowability.
- Injection pressure or injection speed is insufficient.
- Gate or runner dimensions are too small.
- Poor venting leads to trapped air blocking the material flow.
- Raw material has not been dried sufficiently.
Solutions:
- Appropriately increase the material temperature.
- Increase the injection pressure and injection speed.
- Enlarge the dimensions of the gates and runners.
- Improve mold venting.
- Ensure the raw material is thoroughly dried.
VII. Bubbles and Internal Voids
Phenomena: Surface pinholes, internal hollowness, bubbles.
Causes:
- Raw material contains moisture, which vaporizes at high temperatures.
- Material temperature is excessively high, leading to decomposition and gas generation.
- Injection speed is too fast, resulting in air entrapment.
- Insufficient holding pressure causes shrinkage in thick-walled sections, forming vacuum voids.
Solutions:
- Thoroughly dry the raw materials.
- Strictly control material temperature to prevent overheating and decomposition.
- Use a low injection speed during the initial phase to minimize air entrapment.
- Increase holding pressure and extend the holding time.
VIII. Cracking, Brittle Fracture, and Screw Boss Breakage
Phenomena:Cracking during demolding, cracking during assembly, chipping/splitting at boss locations.
Causes:
- High internal stress; mold temperature is too low, or cooling is too rapid.
- Uneven ejection; stress is concentrated on specific ejector pins.
- Insufficient draft angle; severe drag marks/scoring during demolding.
- Material temperature is too low, resulting in poor plasticization.
- Sharp glass fibers create stress concentration points.
Solutions:
- Appropriately increase mold temperature to relieve internal stress.
- Increase the size/number of ejector pins to distribute ejection force.
- Increase the draft angle.
- Ensure adequate material temperature for complete plasticization.
- Incorporate a rounded fillet transition at the base of the screw boss.
IX. Flash / Burrs
Phenomenon:Material overflow at the parting line, inserts, or ejector pins.
Causes:
- Material temperature is too high, resulting in increased fluidity.
- Injection pressure and injection speed are excessive.
- Clamping force is insufficient.
- Mold mating clearance is too large.
Solutions:
- Lower the material temperature.
- Reduce the injection pressure and injection speed.
- Increase the clamping force.
- Repair the mold to reduce the mating clearance.
X. Ejector Whitening and Damage
Phenomenon: Whitening or scuffing observed at the ejector pin locations.
Causes:
- The molded part is excessively hard or brittle; mold temperature is too low.
- Insufficient cooling; ejection occurs prematurely.
- Small ejection surface area, leading to stress concentration.
- Draft angle is too small.
Solutions:
- Appropriately increase the mold temperature.
- Extend the cooling time.
- Increase the diameter of the ejector pins or increase the number of pins.
- Increase the draft angle.
