Common Problems, Causes, and Solutions in POM Injection Molding

POM-commonly known as "Saigang" or "Teling"-is characterized by high rigidity, excellent wear resistance, and self-lubricating properties. However, it exhibits extremely high crystallinity and exceptionally large shrinkage; furthermore, it decomposes very easily to generate gas, is highly corrosive, and is flammable. Its processing strictly prohibits high temperatures and requires the complete avoidance of acidic gases. Consequently, it ranks as one of the most dangerous materials among all general-purpose engineering plastics, imposing the most stringent processing requirements.

I. Decomposition: Yellowing, Smoking, Pungent Odor, and Scorching (The #1 Critical Issue for POM)
Phenomena
Product yellowing, browning, or scorch marks; smoke emitting from the barrel; pungent formaldehyde odor; foaming caused by decomposition.
Causes

• Excessively high melt temperature: POM is highly susceptible to thermal decomposition, which releases formaldehyde gas.
• Excessively long residence time of the material within the barrel.
• Excessive shear stress: High screw rotation speeds generate significant shear heat.
• Material accumulation in "dead zones" (stagnant areas) within the barrel, leading to prolonged exposure to high temperatures and subsequent carbonization.
• Contamination by acidic impurities (such as PVC or PC), which catalyze and induce decomposition.

Solutions

• Strict temperature control: Maintain barrel temperatures between 170°C and 190°C; strictly prohibit exceeding 200°C.
• Reduce screw rotation speed to minimize shear stress.
• Shorten the molding cycle; strictly avoid prolonged machine shutdowns where material remains stagnant in the barrel.
• Before shutting down the machine, the barrel must be thoroughly purged using PE or PP material; strictly ensure that no POM residue remains.
• Strictly prohibit mixing POM with PVC, or using a barrel previously used for PVC without proper cleaning; completely eliminate the presence of acidic impurities.

II. Silver Streaks, Flow Marks, Water Marks, and Surface Roughness
Phenomenon
Silver-white streaks, hazy patterns, flow lines, and pitting on the surface.
Causes

• Slight decomposition of POM material, resulting in the emission of formaldehyde gas.
• Moisture contamination of the raw material.
• Air entrapment caused by an excessively high injection speed.
• Poor venting within the mold.
• Insufficient back pressure, leading to uneven plasticization.

Solutions

• Dry the raw material at 70–80°C for 2–3 hours.
• Appropriately lower the material temperature to inhibit decomposition and gas generation.
• Reduce the injection speed during the initial phase.
• Deepen the mold's vent channels to rapidly expel decomposition gases.
• Appropriately increase the back pressure to stabilize plasticization.

III. Shrinkage Depression (One of the most significant shrinkage issues in virgin POM material)
Phenomenon
Severe depression and distinct sink marks appear on rib sections, boss posts, and the backsides of thick-walled areas.
Causes

• POM exhibits extremely high crystallinity, resulting in very significant volumetric shrinkage.
• Insufficient holding pressure or excessively short holding time.
• The gate is undersized and freezes prematurely, preventing effective shrinkage compensation.
• Material and mold temperatures are too high, leading to more intense crystallization.
• Non-uniform wall thickness in the product design.

Solutions

• Significantly increase holding pressure and extend the holding time.
• Increase the dimensions of the gates and runners.
• Appropriately lower material and mold temperatures to reduce crystallization shrinkage.
• Optimize the product structure to ensure uniform wall thickness whenever possible.
• Implement a secondary, multi-stage holding pressure process.

IV. Warpage and Dimensional Instability
Phenomenon
Product bending, twisting, significant dimensional deviations, and deformation due to elastic recovery upon demolding.
Causes

• Excessive crystallization shrinkage; uneven cooling between the inner and outer layers.
• Mold temperature is too high, resulting in more complete crystallization and consequently greater shrinkage.
• Significant variations in wall thickness, leading to inconsistent shrinkage across different sections.
• High injection speed, resulting in high molecular orientation stress.
• Insufficient cooling time.

Solutions

• Lower the mold temperature and extend the cooling time.
• Reduce the injection speed to minimize orientation stress.
• Balance the mold's cooling channels to ensure a consistent temperature difference between the front and rear mold halves.
• Design the product structure to be as symmetrical as possible, with uniform wall thickness.
• For glass-fiber-reinforced POM, the mold temperature may be appropriately increased to reduce deformation.

V. Short Shots, Incomplete Filling, and Underfilling
Phenomena
Corners and thin-walled sections fail to fill completely; material shortage at the flow front.
Causes

• Material temperature is too low, resulting in poor flowability.
• Insufficient injection pressure and injection speed.
• Gate and runner dimensions are too small.
• Poor venting leads to trapped air and flow blockage.

Solutions

• Slightly increase the material temperature (without exceeding the upper limit of 190°C).
• Increase injection pressure and injection speed.
• Enlarge the gate and runner dimensions.
• Improve mold venting.

VI. Prominent Weld Lines and Poor Strength
Phenomenon
Weld lines are deep and exhibit discoloration (whitening); parts are prone to fracture under stress.
Causes

• Material temperature and mold temperature are too low.
• Injection speed is too slow, causing the flow front to cool rapidly.
• Air is trapped at the flow convergence point due to a lack of venting.
• Gate placement is inappropriate, resulting in an excessively long flow path.

Solutions

• Appropriately increase material temperature and mold temperature.
• Increase injection speed.
• Add vent grooves at the locations where weld lines occur.
• Optimize gate placement to shorten the material flow path.

VII. Bubbles and Internal Voids
Phenomena
Surface pinholes; internal air bubbles/voids.
Causes

• Moisture content in the raw material.
• Gas generation due to high-temperature decomposition.
• Air entrapment caused by excessive injection speed.
• Formation of vacuum voids due to insufficient holding pressure and shrinkage in thick-walled sections.

Solutions

• Thoroughly dry the raw material.
• Strictly control material temperature to prevent decomposition.
• Employ a low injection speed during the initial phase to minimize air entrapment.
• Apply high holding pressure for an extended duration to compact the molded part.

VIII. Cracking, Brittle Fracture, and Stress Cracking
Phenomena
Cracking during demolding, chipping during assembly, fracture of screw bosses
Causes

• High internal stress; mold temperature too low, leading to excessively rapid cooling
• Unbalanced ejection; concentrated stress on ejector pins
• Surface tearing due to insufficient draft angle
• Material temperature too low, resulting in poor plasticization

Solutions

• Appropriately increase mold temperature to reduce internal stress
• Increase the diameter of ejector pins and/or increase the number of pins to ensure balanced ejection
• Increase the draft angle
• Ensure an appropriate material temperature to achieve complete plasticization

IX. Flash and Burrs
Phenomenon
Material overflow at the parting line, inserts, or ejector pin locations.
Causes

• Material temperature is too high, resulting in increased fluidity.
• Injection pressure and injection speed are excessive.
• Clamping force is insufficient.
• Excessive clearance in the mold assembly.

Solutions

• Lower the material temperature.
• Reduce injection pressure and injection speed.
• Increase the clamping force.
• Modify the mold to reduce clearance.

X. Surface Luster Issues: Roughness and Haziness
Phenomenon
Matte finish, haziness, lack of luster, roughness
Causes

• Mold temperature is too low
• Material temperature is insufficient
• Mold cavity surface is rough
• Poor venting

Solutions

• Appropriately increase mold temperature
• Raise material temperature to ensure proper plasticization
• Polish the mold cavity
• Improve mold venting

XI. Ejector Whitening, Damage, and Protrusion
Phenomena
Whitening, protrusion, or drag marks at the ejector pin locations.
Causes

• The molded part possesses high rigidity and is somewhat brittle; the mold temperature is too low.
• Insufficient cooling, resulting in premature ejection.
• The ejection contact area is too small, leading to concentrated stress.
• The draft angle is insufficient.

Solutions

• Appropriately increase the mold temperature.
• Extend the cooling time.
• Increase the size of the ejector pins and/or distribute the ejection force more broadly.
• Increase the draft angle.

1. High-temperature decomposition of POM releases toxic formaldehyde gas; ensure the workshop is well-ventilated at all times.
2. Under no circumstances should POM be processed in the same machine or mixed with PVC; contact between acids and POM will result in violent decomposition.
3. Do not allow material to sit idle in the barrel for extended periods; the barrel must be thoroughly purged whenever the machine is shut down.
4. POM exhibits extremely high shrinkage; therefore, sufficient holding pressure is essential. Due to its strong crystallinity, warping is a common issue that is very difficult to completely eliminate.