ABS (Acrylonitrile Butadiene Styrene) plastic holds a pivotal position in industries ranging from electronics and machinery to automotive and toy manufacturing. Known for its high mechanical strength¹ and excellent comprehensive properties, it is the go-to material for large box structures, stress-bearing components, and decorative parts requiring electroplating.

Based on years of practical experience, this guide analyzes the critical process parameters for injection molding² ABS to ensure high-quality production.

💡 Pro Tip: ABS is highly sensitive to moisture and temperature. Precise control of these factors is the difference between a high-gloss finish and a rejected part.

Drying ABS Plastic: The Critical First Step

ABS has high hygroscopicity (water absorption) and is sensitive to moisture. Insufficient drying is the primary cause of defects such as silver streaks, bubbles, and cloud patterns on the surface of the part.

To ensure quality, the moisture content of the raw material must be controlled to below 0.03%.

Recommended Drying Conditions

Equipment Advice: It is highly recommended to retrofit the machine hopper with a hot air hopper dryer. This prevents the dried ABS from re-absorbing moisture from the air while waiting to be processed. However, humidity must be monitored closely to prevent overheating if production is interrupted.

Injection Temperature Control

Unlike other amorphous plastics, the relationship between temperature and melt viscosity in ABS is unique. Once the plastic reaches its plasticization temperature (approx. 220～250 °C),blindly increasing the heat does not significantly lower viscosity. Instead, it leads to thermal degradation.

Why Temperature Control Matters

• Degradation: ABS contains butadiene. If the temperature is too high, the material degrades and carbonizes, leading to yellow or brown scorched particles embedded in the part.
• Viscosity: Overheating can actually increase melt viscosity, making injection more difficult.
• Sensitivity: The front end of the barrel and the nozzle are highly sensitive. Even minor fluctuations here can cause weld lines, poor gloss, or flashing.

Barrel Temperature Settings

[Image Suggestion 2]: A diagram showing the temperature zones of an injection molding screw and barrel, highlighting the nozzle area. Alt Text: Injection molding barrel temperature zones for ABS.

Injection Pressure

The viscosity of molten ABS is higher than that of Polystyrene (PS) or modified PS. Therefore, higher injection pressure³ is generally required.

• Low Pressure Risks: If pressure is too low, the plastic shrinks significantly and may detach from the mold surface, causing a foggy or matte finish (poor replication).
• High Pressure Risks: Excessive pressure causes strong friction between the plastic and the mold cavity, leading to sticking (mold adhesion) or difficulty in ejection.

Note: Small, simple, and thick parts can be processed with lower pressure, but complex parts require higher pressure to fill.

Injection Speed

For ABS, a medium injection speed⁴ usually yields the best results.

• Too Fast: Can cause burn marks (dieseling), gas decomposition, and red discoloration near the gate.
• Too Slow: Thin-walled or complex parts may not fill completely (short shots).

Mold Temperature

ABS requires a relatively high forming temperature, and consequently, a higher mold temperature⁵.

• General Setting: 75～85 °C.
• Large Projection Area: Fixed mold (70～80 °C) / Moving mold (50～60 °C).

For large, complex, or thin-walled parts, active heating of the mold is often necessary. To shorten the production cycle and maintain stability, manufacturers often use water baths (hot or cold) or mechanical fixtures immediately after ejection to compensate for cooling time.

Shot Size Control

To ensure part quality, dimensional stability, and uniform color/gloss, the shot size should be carefully managed relative to the machine's capacity.

• Standard Practice: Usually, the shot volume is about 75% of the machine's standard capacity.
• Optimal Quality: For the best surface finish and stability, it is ideal to use a machine where the part weight is roughly 50% of the machine's rated shot volume.

Summary Checklist for ABS Molding