A decrease in extruder screw output is a common problem in plastic extrusion. A production line that previously produced 300 kg/h may gradually fall to 250 kg/h or even lower under similar operating conditions.

The first reaction is often to replace the screw and barrel. However, low output does not always mean the screw is worn.

Before ordering replacement parts, check these seven common causes.

1. Screw and Barrel Clearance Is Too Large

Screw and barrel wear is one of the most common reasons for decreasing extrusion output.

During long-term production, the screw flight OD gradually becomes smaller while the barrel ID becomes larger. This increases the clearance between the screw and barrel.

When the clearance becomes excessive, more molten plastic can flow backward over the screw flights instead of moving toward the die.

Possible symptoms include:

• Lower output
• Unstable melt pressure
• Higher screw speed required
• Poor plasticizing
• Increased temperature

Do not judge wear only by appearance. Measure the screw OD and barrel ID at several positions and compare them with the original dimensions.

2. Raw Material Has Changed

If production output suddenly changes, ask a simple question:

Are you still processing the same material?

A screw designed for virgin HDPE pellets may perform very differently when processing recycled HDPE flakes.

Material properties affecting output include:

• Bulk density
• Melt flow rate
• Moisture
• Particle size
• Filler content
• Recycled-material contamination

Low-density flakes require more volume to produce the same weight output as dense pellets.

Therefore, the screw may appear to have insufficient capacity even though it is not mechanically damaged.

3. Feeding Is Unstable

The screw cannot produce stable output if it does not receive a stable supply of material.

Check the hopper and feed throat.

Powders, flakes and irregular recycled materials can bridge inside the hopper and prevent material from falling continuously into the screw.

For lightweight recycled film or fluffy material, gravity feeding may not be sufficient.

A force feeder or compactor may be required to maintain consistent feeding.

Before changing the screw design, make sure the feeding system is working correctly.

4. The Feed Zone Is Too Hot

The feed section should allow solid plastic to enter the screw channel and move forward efficiently.

If the feed throat becomes too hot, pellets or flakes can soften prematurely and stick together.

This reduces conveying efficiency and can make extruder screw output unstable.

Check:

• Feed-throat cooling
• Rear barrel temperature
• Cooling-water circulation
• Material sticking near the hopper

Sometimes restoring proper cooling can improve output without replacing any mechanical components.

5. The Screen or Die Creates Excessive Pressure

The screw may be working correctly while another component restricts material flow.

A blocked screen pack, contaminated filter or restrictive die can increase melt pressure and reduce production capacity.

Compare current pressure with previous normal production data.

If pressure before the screen changer is unusually high, inspect the filtration system before blaming the screw.

This is particularly important when processing recycled plastics because contamination can block screens more quickly.

6. The Screw Design Does Not Match the Application

Screw diameter alone does not determine extrusion capacity.

Two 90 mm screws can produce very different output because their designs are different.

Important parameters include:

• L/D ratio
• Feed depth
• Compression ratio
• Screw pitch
• Mixing section
• Barrier design
• Screw speed

The polymer must also be considered.

A screw designed for PVC should not automatically be used for PE, PP or PA under the same operating conditions.

If you change the raw material or significantly increase the required output, the original screw design may no longer be suitable.

7. Motor or Gearbox Capacity Is Limiting Output

Customers sometimes request a new screw to increase production from 300 kg/h to 500 kg/h.

A redesigned screw may improve efficiency, but it cannot overcome every machine limitation.

Before increasing output, check:

• Main motor power
• Gearbox torque
• Maximum screw speed
• Heating capacity
• Cooling capacity
• Die capacity
• Downstream equipment

For example, increasing screw depth and speed can increase material throughput, but it can also increase torque demand.

The complete extrusion system must be considered.

Should You Replace the Screw and Barrel?

Replacement is reasonable when dimensional measurements confirm excessive wear or when the existing screw design is unsuitable for the current material.

However, replacing only the screw may not solve the problem if the barrel is also severely worn.

Always inspect both components.

A new screw running inside a worn barrel can still have excessive clearance and poor conveying efficiency.

What Information Should You Send to the Manufacturer?

If your extrusion output has decreased, provide:

• Extruder manufacturer and model
• Screw diameter
• L/D ratio
• Screw drawing
• Raw material
• Filler percentage
• Current output
• Previous or required output
• Screw rpm
• Motor power
• Melt pressure
• Photos of the screw and barrel

The more operating information available, the easier it is to determine whether the problem comes from wear, feeding, screw design or another part of the extrusion line.

Conclusion

Low extruder screw output does not automatically mean you need a new screw.

Check the raw material, feeding system, temperature, screen pressure and machine capacity first. If these conditions are normal, measure the screw and barrel to determine their actual wear.

If you need a replacement screw and barrel, send us your machine information, drawings, material and production requirements.

Zhoushan Juyuan Plastic Machinery manufactures customized screws and barrels for extrusion, recycling and plastic processing applications.