Solutions for Modified PP Screw Wear

1. Why should we pay attention to screw wear in modified polypropylene?

Polypropylene (PP) is a common material for automobile bumpers and appliance housings, but during processing, fillers such as glass fiber and mica are added to enhance performance. However, these fillers can wear out the screw, leading to fluctuations in extrusion output (up to ±5%), part shortfalls, and uneven wall thickness. In severe cases, the screw may need to be replaced after only 4,000 hours, directly increasing production costs—a common problem in the industry.

2. Two Core Causes of Screw Wear (Based on PP Characteristics)

1. Improper Volume Compression Ratio Adjustment: The Screw Root Suffers First

PP Melting Characteristics: A melt index range of 1-40g/10min, a narrow melting temperature range (160-180°C), and good solid particle flowability.

If the low compression ratio (1.8-2.5) of a pure PP screw is used, the glass-fibered PP will not be compressed sufficiently within the screw channel, resulting in unstable melt pressure. Solid PP hasn't softened enough to form a lubricating layer, so the glass fiber directly "scrapes" the root of the screw channel. This causes the channel to deepen later, making parts (such as bumpers) prone to material shortages.

2. Filler "Abrasive Effect": Most severe in the feed and transition sections.

Feed section (third and fourth turns): At temperatures of 140-160°C, PP has just softened but hasn't melted, leaving the glass fiber (3-6mm long) and mica (5-20μm flakes) as "hard particles."

Due to PP's high fluidity, these two turns fill the channel with material, and the sharp end of the glass fiber "cuts" the screw channel. Data from a power plant: When processing 30% glass-fiber PP, the screw channel roughness (Ra) soared from 0.8μm to 3.2μm after 1000 hours, double that of pure PP.

Transition Zone (3rd, 4th, and 5th Turns): Temperature 170-190°C, PP melts (viscosity 1000-5000 Pa·s), but the glass fiber remains rigid.

The groove depth gradually decreases from 15-20mm to 5-8mm. The material flow generates vortices, causing the glass fiber to "swirl" and impact the groove walls. For example, when processing 20% ​​mica-filled PP, after 2000 hours, the flight width of the fifth turn in the transition zone decreases by 0.3mm, making it three times that of pure PP.

3. Three-Step Solution: Design, Materials, and Process

1. Design Optimization: Adjust Parameters Based on Filler
Glass-Fiber-Reinforced PP: Increase the volume compression ratio to 2.8-3.5, allowing the PP to melt quickly and wrap around the glass fiber, reducing direct friction.

Mica-Filled PP: The groove depth in the transition zone decreases by 1.0mm per turn (originally 1.5mm) to reduce the impact of vortices on the groove.

2. Choosing the Right Material: Longer Screw Durability

PP processing temperatures are below 200°C, so there's no need to worry about high-temperature damage to the coating. Two types of screws are recommended:

Plasma nitrided screws: Surface hardness HV1000+, a common choice.

Tungsten carbide-coated screws (50-100μm thick): Tests at an automotive factory showed that processing 30% glass-fiber PP extended the screw life from 4,000 hours to 8,000 hours, halving the cost of screw replacement.

3. Process Fine-Tuning: Reduce Wear Risk

Infeed temperature: Increase by 5-10°C (e.g., 150°C to 155-160°C) to allow the PP to soften and form a lubricating layer.

Screw speed: Keep it between 300-400 rpm, and avoid excessive speeds—higher speeds increase the frequency of friction between the glass fiber and the screw groove.

Temperature control in the transition section: 170°C in the first half and 180°C in the second half to prevent localized melt viscosity variations that could lead to filler agglomeration and wear.

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