PU is broadly classified into two categories: castable PU (CPU) and thermoplastic PU (TPU). In everyday parlance, "injection-molded PU" typically refers to TPU. Additionally, there is Reaction Injection Molding PU (RIM-PU), which encompasses rigid foams and self-skinning materials.
Characteristics: High moisture absorption, susceptibility to hydrolysis, tendency to stick to molds, prone to shrinkage and warping, susceptibility to yellowing at high temperatures, and a wide range of hardness options.
I. Silver Streaks, Flow Marks, and Air Voids
Phenomena: Silver streaks, hazy stripes, or bubble marks on the surface.
Causes:
- TPU has absorbed moisture, which vaporizes at high temperatures.
- Melt temperature is excessively high, leading to slight material degradation.
- Injection speed is too fast, causing air entrapment.
- Poor venting in the mold.
- Insufficient back pressure, resulting in uneven plasticization.
Solutions:
- Mandatory drying: Dry at 80–90°C for 2–4 hours; dehumidifying drying is recommended for best results.
- 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. Bubbles / Pores / Voids
Phenomenon: Bubbles, hollows, or pinholes present on the surface or within the interior.
Causes:
- Primary Cause: Moisture in the raw material.
- Excessive material temperature, leading to decomposition and gas generation.
- Excessively fast injection speed, resulting in air entrapment.
- Insufficient holding pressure, preventing proper compaction.
- Shrinkage in thick-walled sections, creating vacuum voids.
Solutions:
- Thoroughly dry the raw material.
- Lower 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.
III. Shrinkage and Depression
Phenomenon: Visible depressions occur at rib locations, boss locations, and thick-walled sections.
Causes:
- Insufficient holding pressure; holding time is too short.
- Injection pressure is too low.
- Gate is too small, leading to premature freezing.
- Melt temperature and/or mold temperature are too high.
- 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 and mold temperature.
- Design the product with as uniform a wall thickness as possible.
IV. Mold Sticking and Ejection Difficulties
Symptoms:Sticking to the front mold, sticking to the rear mold, ejection deformation, surface scuffing.
Causes:
- Mold temperature is too high, causing the TPU to soften and become tacky.
- Insufficient draft angle; rough mold surface finish.
- Insufficient cooling time.
- Melt temperature is too high.
- Mold cavity contains oil residue or solidified cold material.
Solutions:
- Lower the mold temperature and extend the cooling time.
- Increase the draft angle and polish the mold surface.
- Appropriately lower the melt temperature.
- Clean the mold to remove solidified cold material and oil residue.
- Apply a small amount of mold release agent.
V. Prominent Weld Lines and Poor Strength
Phenomenon: Weld lines are deep, discolored (whitish), and prone to fracture.
Causes:
- Material temperature and/or mold temperature are too low.
- Injection speed is too slow, causing the flow front to cool prematurely.
- Poor venting leads to air entrapment at the weld interface.
- Gate placement is inappropriate.
Solutions:
- Increase material temperature and mold temperature.
- Increase injection speed as appropriate.
- Add vents at the weld line location.
- Optimize gate placement to shorten the flow path.
VI. Warpage and Deformation
Phenomena: Bending, curling, dimensional instability
Causes:
- Uneven crystallization shrinkage (TPU exhibits strong crystallinity)
- Inconsistent mold temperature (front vs. rear)
- Significant variations in wall thickness
- Excessively high injection speed, resulting in high internal stress
- Unbalanced ejection
Solutions:
- Equalize mold temperature and extend cooling time
- Reduce injection speed to minimize molecular orientation
- Optimize wall thickness design
- Adjust ejection balance
- Lower mold temperatures help to reduce deformation
VII. 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 clearance.
- Reduce the injection volume.
VIII. Cracking, Brittleness, and Stress Cracking
Phenomena: Cracking upon demolding, cracking during bending, susceptibility to cracking at low temperatures.
Causes:
- Excessive internal stress
- Mold temperature is too low
- Material degradation due to moisture absorption
- Uneven ejection or "ejector whitening"
- Hardness grade selected is too high
Solutions:
- Increase mold temperature; reduce injection speed
- Thoroughly dry the raw material
- Adjust ejector pin layout; increase ejection surface area
- Select a TPU grade with appropriate hardness
IX. Surface Roughness, Pitting, and Lack of Gloss
Phenomenon: Haziness, granular texture, lack of luster
Causes:
- Mold temperature is too low
- Material temperature is insufficient; poor plasticization
- Mold surface is rough
- Poor venting
- Raw material contains moisture
Solutions:
- Appropriately increase mold temperature
- Increase material temperature to enhance plasticization
- Polish the mold
- Improve venting
- Thoroughly dry the raw material
X. Yellowing and Discoloration
Phenomenon: The molded product turns yellow or brown, and its transparency decreases.
Causes:
- Material temperature is excessively high, leading to thermal oxidative yellowing.
- Material residence time in the screw is too long.
- Overheating caused by shear forces from the screw.
- Yellowing occurs under light exposure due to the absence of a UV-resistant formulation.
Solutions:
- Strictly control the material temperature, ensuring it does not exceed 220°C.
- Shorten the molding cycle; purge the barrel to clear out material during shutdowns.
- Reduce the screw rotation speed and back pressure.
- Select a yellowing-resistant, aromatic- or aliphatic-modified TPU grade.
XI. Overmolding Delamination (TPU over ABS/PC/PA)
Phenomenon: Separation of the soft material from the hard material; poor adhesion.
Causes:
- Contamination on the hard material surface; excessive use of mold release agent.
- TPU melt temperature and/or mold temperature are too low.
- Slow injection speed, leading to premature cooling at the interface.
- Material incompatibility; lack of bonding polarity.
- Hard material was not preheated.
Solutions:
- Clean the surface of the hard material; minimize the use of mold release agent.
- Increase the TPU melt temperature and mold temperature.
- Perform high-speed injection to ensure effective thermal fusion.
- Select a bonding-grade TPU material.
- Preheat the hard material prior to overmolding.
