Common Plastics Characteristics and Molding Process Features 

PE Plastic, Chemical Name: Polyethylene

• English Name: Polyethylene (abbreviated as PE)

• Density: 0.94-0.96 g/cm³

• Molding Shrinkage Rate: 1.5-3.6%

• Molding Temperature: 140-220℃

• Characteristics:

  • Excellent corrosion resistance and electrical insulation (especially high-frequency insulation).

  • Can be chlorinated, chemically cross-linked, and modified by irradiation cross-linking.

  • Can be reinforced with glass fibers.

  • Low-pressure polyethylene has higher melting point, rigidity, hardness, and strength, low water absorption, good electrical properties, and radiation resistance.

  • High-pressure polyethylene has better flexibility, elongation, impact strength, and permeability.

  • Ultra-high molecular weight polyethylene has high impact strength, fatigue resistance, and wear resistance.

  • Low-pressure polyethylene is suitable for making corrosion-resistant and insulating parts.

  • High-pressure polyethylene is suitable for making films.

  • Ultra-high molecular weight polyethylene is suitable for making shock-absorbing, wear-resistant, and transmission parts.

Molding Characteristics:

1. Crystalline material, low moisture absorption, no need for thorough drying, excellent flowability sensitive to pressure. High-pressure injection is recommended, with uniform material temperature, fast filling speed, and sufficient holding pressure. Direct gates are not suitable to prevent uneven shrinkage and increased internal stress. Gate position should be chosen to prevent sink marks and deformation.

2. Large shrinkage range and value with significant directionality, easy to warp. Slow cooling is recommended, with a mold cooling system.

3. Heating time should not be too long to avoid decomposition.

4. Soft plastic parts with shallow side grooves can be forcibly demolded.

5. May experience melt fracture, should not contact organic solvents to prevent cracking.

UHMWPE (Ultra-High Molecular Weight Polyethylene)

• Chemical Name: Ultra-High Molecular Weight Polyethylene

• English Abbreviation: UHMWPE

• Characteristics:

  • Exceptional impact, friction, corrosion, and chemical resistance.

  • Ultra-high molecular weight ensures unique properties with 9 to 10 million molecular weight compared to 300,000 for HDPE.

  • Cannot melt and flow like liquid, processed using powder metallurgy techniques.

  • Most common processing method is compression molding.

  • Excellent impact resistance, even at low temperatures.

  • Lightweight, only 0.94 specific gravity, lighter than water, and significantly lighter than steel.

  • Outstanding wear resistance, best among all thermoplastic polymers.

  • Low sliding friction coefficient, superior to steel.

  • Superior noise dampening properties due to its self-lubricating and durable nature.

  • Excellent electrical insulation with high resistance.

  • Unique corrosion resistance, especially in abrasive environments.

  • Zero water absorption, preventing size changes in water.

PP Plastic, Chemical Name: Polypropylene

• English Name: Polypropylene (abbreviated as PP)

• Density: 0.9-0.91 g/cm³

• Molding Shrinkage Rate: 1.0-2.5%

• Molding Temperature: 160-220℃

• Characteristics:

  • Low density, better strength, rigidity, hardness, and heat resistance than low-pressure polyethylene, can be used at around 100℃.

  • Good electrical properties and high-frequency insulation, unaffected by humidity.

  • Brittle at low temperatures, not wear-resistant, easy to age.

  • Suitable for making general mechanical parts, corrosion-resistant parts, and insulating parts.

Molding Characteristics:

1. Crystalline material, low moisture absorption, prone to melt fracture, long-term contact with hot metals can cause decomposition.

2. Good flowability but large shrinkage range and value, prone to shrink holes, dents, and deformation.

3. Fast cooling speed, gating and cooling systems should dissipate heat slowly, and molding temperature should be controlled.

4. Uniform wall thickness to avoid underfilling, sharp angles to prevent stress concentration.

PVC Plastic, Chemical Name: Polyvinyl Chloride

• English Name: Poly(Vinyl Chloride)

• Density: 1.38 g/cm³

• Molding Shrinkage Rate: 0.6-1.5%

• Molding Temperature: 160-190℃

• Characteristics:

  • Excellent mechanical properties, electrical properties, strong acid and alkali resistance, and good chemical stability.

  • Low softening point, suitable for making thin plates, wire and cable insulation layers, and seals.

Molding Characteristics:

1. Amorphous material, low moisture absorption, poor flowability. Pre-drying plastic can improve flowability and prevent bubbles. Short and thick gating system, large gate cross-section, no dead angles, and mold cooling with a chrome-plated surface.

2. Special equipment and molds are best due to its corrosiveness and flow characteristics. Products should include different additives as needed.

3. Prone to decomposition at 200℃, especially in contact with steel and copper, releasing corrosive gases. Narrow molding temperature range.

4. Screw injection nozzle with a large diameter to prevent dead angles and stagnation. Avoid inserts if possible, and preheat if necessary.

PS Plastic, Chemical Name: Polystyrene

• English Name: Polystyrene

• Density: 1.05 g/cm³

• Molding Shrinkage Rate: 0.6-0.8%

• Molding Temperature: 170-250℃

• Characteristics:

  • Excellent electrical insulation (especially high-frequency insulation), colorless and transparent, high light transmittance second only to plexiglass, good coloring, water resistance, and chemical stability.

  • Moderate strength, but brittle and prone to stress cracking, not resistant to benzene and gasoline solvents.

  • Suitable for making insulating transparent parts, decorative parts, and chemical and optical instrument parts.

Molding Characteristics:

1. Amorphous material, low moisture absorption, no need for thorough drying, not prone to decomposition, but high thermal expansion coefficient and easy internal stress. Good flowability, can be molded using screw or plunger injection molding machines.

2. High material and mold temperatures with low injection pressure and extended injection time can reduce internal stress and prevent sink marks and deformation.

3. Various gate types can be used, gates should connect to parts with a circular arc to avoid damage when removing gates. Large demolding angle and uniform ejection, with uniform wall thickness, preferably without inserts. Preheat inserts if necessary.

ABS Plastic, Chemical Name: Acrylonitrile-Butadiene-Styrene Copolymer

• English Name: Acrylonitrile Butadiene Styrene (abbreviated as ABS)

• Density: 1.05 g/cm³

• Molding Shrinkage Rate: 0.4-0.7%

• Molding Temperature: 200-240℃

• Drying Conditions: 80-90℃ for 2 hours

• Characteristics:

  • Good overall performance with high impact strength, chemical stability, and electrical properties.

  • Good adhesion with 372 plexiglass for making two-color parts, can be chrome-plated or spray-painted.

  • Available in high-impact, heat-resistant, flame-retardant, reinforced, and transparent grades.

  • Flowability slightly worse than HIPS, but better than PMMA and PC, with good flexibility.

  • Suitable for making general mechanical parts, wear-resistant parts, transmission parts, and telecommunication parts.

Molding Characteristics:

1. Amorphous material with medium flowability, high moisture absorption, requiring thorough drying. For high gloss surface parts, preheat dry at 80-90℃ for 3 hours.

2. High material and mold temperatures are recommended, but excessive temperatures can cause decomposition (>270℃). For high precision parts, mold temperature should be 50-60℃, for high gloss and heat-resistant parts, mold temperature should be 60-80℃.

3. To resolve water streaks, increase material flowability by using high material and mold temperatures or changing the gating position.

4. For heat-resistant or flame-retardant grades, molds may need cleaning after 3-7 days of production due to plastic decomposition residue, and additional mold venting may be required.