PMMA Characteristics: High transparency, high hardness, high brittleness, extremely prone to moisture absorption, temperature-sensitive, prone to developing internal stress, moderate flowability, susceptible to cracking and scratching; it places very high demands on processing techniques and mold design.PMMA Characteristics: High transparency, high hardness, high brittleness, extremely prone to moisture absorption, temperature-sensitive, prone to developing internal stress, moderate flowability, susceptible to cracking and scratching; it places very high demands on processing techniques and mold design.
I. Silver Streaks, Water Marks, and Air Voids (Most Common)
Phenomena: Silver-white streaks, hazy patches, water-like patterns, or blurred textures on the surface.
Causes:
- PMMA has high hygroscopicity; the raw material was either not dried or insufficiently dried.
- Material temperature is excessively high, causing 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:
- Ensure thorough drying: Dry at 80–90°C for 3–4 hours; the use of a dehumidifying dryer is recommended.
- Appropriately lower the barrel temperature.
- Reduce the injection speed during the initial filling stage to ensure smooth mold filling.
- Improve the mold's venting channels.
- Appropriately increase back pressure to ensure more uniform plasticization.
II. Cracking, Brittle Fracture, Chipping, and Stress Cracking
Phenomena:** Cracking during demolding, cracking during assembly, chipping at corners/edges, spontaneous cracking after storage.
Causes:
- Excessive internal stress (due to high injection speed, high shear stress, or low mold temperature).
- Mold temperature is too low, resulting in excessively rapid cooling.
- Uneven ejection force, causing "ejector whitening" or deformation; ejector pins are too small.
- Insufficient draft angle, leading to drag marks or surface tearing during demolding.
- Cracking induced by contact with organic solvents (e.g., alcohol, cleaning agents).
Solutions:
- Increase mold temperature to 60–80°C to significantly reduce internal stress.
- Reduce injection speed and minimize shear stress.
- Adjust the layout of ejector pins to increase the total ejection area.
- Increase the draft angle.
- Avoid contact with organic solvents; critical components may undergo annealing treatment.
III. Short Shots, Uneven Flow, and Insufficient Filling
Phenomena: Failure to fully fill thin-walled sections, distal areas, sharp corners, or deep ribs.
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 materials have absorbed moisture, resulting in reduced flowability.
Solutions:
- Increase melt temperature (to 230–260°C).
- Increase injection pressure and injection speed.
- Enlarge the gates and runners.
- Improve venting.
- Ensure raw materials are thoroughly dried.
IV. Prominent Weld Lines and Poor Transparency
Phenomenon:The weld line is deep, appears whitish, and forms a distinct visible seam that impairs light transmission.
Causes:
- Low melt temperature and/or mold temperature, resulting in poor fusion.
- Slow injection speed, causing the melt flow front to cool prematurely.
- Poor venting, leading to air entrapment at the point of melt convergence.
- Improper gate placement, resulting in an excessively long flow path.
Solutions:
- Increase the melt temperature and mold temperature.
- Appropriately increase the injection speed.
- Add vents at the location of the weld line.
- Optimize the gate design to shorten the melt flow path.
V. Shrinkage and Depression
Phenomenon: Surface depressions occurring at rib locations, bosses, 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.
- 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.
VI. Bubbles, Pores, and Internal Voids
Phenomenon:** Bubbles, pinholes, or hollows 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 parts 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.
VII. Flash / Burrs
Phenomenon: Material overflow at the parting line or inserts.
Causes:
- Material temperature is too high.
- Injection pressure / injection speed is excessive.
- Clamping force is insufficient.
- Excessive clearance in the mold fit.
- Injection volume is excessive (overfilling).
Solutions:
- Reduce material temperature.
- Decrease injection pressure and speed.
- Increase clamping force.
- Modify the mold to reduce clearance.
- Reduce injection volume.
VIII. Scorching, Black Spots, Black Streaks, and Yellowing
Phenomena:Black spots, burnt patches, yellowing, scorched corners/tips.
Causes:
Material temperature is excessively high; PMMA undergoes thermal decomposition.
Residence time of the material in the screw is too long.
Poor venting leads to trapped air being compressed and scorched.
Carbon buildup in the barrel results in black spots.
Injection speed is too fast, causing shear overheating.
Solutions:
- Lower the material temperature; shorten the molding cycle.
- Purge the barrel before shutting down the machine.
- Improve venting.
- Clean the barrel and the screw.
- Reduce the injection speed.
IX. Warpage and Deformation
Phenomena: Bending, twisting, dimensional instability
Causes:
- Uneven cooling; high internal stress
- Mold temperature too low, or significant temperature difference between the front and rear mold halves
- Uneven wall thickness; inconsistent shrinkage
- Excessively high injection speed; severe molecular orientation
- Unbalanced ejection
Solutions:
- Increase and equalize mold temperature; extend cooling time
- Reduce injection speed to minimize molecular orientation
- Optimize wall thickness design
- Adjust ejection balance
- Perform annealing treatment if necessary
X. Poor Transparency, Matte Finish, Dull Surface
Phenomenon: Opaque, hazy, poor gloss
Causes:
- Mold temperature is too low
- Material temperature is insufficient
- Mold surface is rough or contaminated with oil
- Poor venting
- Raw material contains moisture
Solutions:
- Increase mold temperature to 60–80°C
- Appropriately increase material temperature
- Polish the mold; clean the cavity
- Improve venting
- Thoroughly dry the raw material
XI. Ejector Whitening, Protrusion, and Cracking
Phenomenon: Whitening, protrusion, or cracking observed at the ejector pin locations.
Causes:
- Low mold temperature, resulting in a brittle part.
- Unbalanced ejection; undersized ejector pins.
- Ejection initiated before sufficient cooling has occurred.
- Insufficient draft angle; excessive gripping force on the core.
- Excessive internal stress within the part.
Solutions:
- Increase the mold temperature.
- Increase the diameter of the ejector pins and/or increase the number of pins.
- Extend the cooling time.
- Increase the draft angle.
- Reduce the injection speed to minimize internal stress.
