Extruder screw compression ratio is one of the most important parameters in plastic extrusion screw design, but it is also one of the most frequently misunderstood.

When buyers ask us to manufacture a replacement screw, they sometimes provide only the screw diameter, length and compression ratio.

For example:

“Diameter 90 mm, L/D 30:1, compression ratio 3:1.”

This information is useful, but it is not enough to determine whether the screw will work correctly.

Injection molding screw barrel replacement

What Is Extruder Screw Compression Ratio?

In a conventional single-screw extruder, the screw is generally divided into three functional sections:

• Feed section
• Compression or transition section
• Metering section

The feed section receives and conveys solid plastic material.

As the material moves forward, the channel becomes shallower through the compression section. The polymer is compressed, heated and gradually melted.

The metering section then stabilizes the molten material before it reaches the die.

A simplified screw compression ratio is commonly expressed as:

Compression Ratio = Feed Channel Depth ÷ Metering Channel Depth

For example, if the feed channel depth is 9 mm and the metering channel depth is 3 mm:

Compression ratio = 9 ÷ 3 = 3:1

However, this number should never be evaluated independently from the rest of the screw geometry.

Is a Higher Compression Ratio Better?

No.

A higher compression ratio does not automatically mean better plasticizing or higher output.

Increasing compression can create more intensive melting and mixing, but excessive compression may also generate too much shear and heat.

Injection Molding Screw Barrel Replacement: Key Signs to Check

Possible consequences include:

• Excessive melt temperature
• Polymer degradation
• High motor load
• Unstable extrusion pressure
• Poor feeding
• Excessive screw and barrel wear

A lower compression ratio can reduce shear, but if the screw does not provide sufficient compression for the material, plasticizing may become incomplete.

This can result in unmelted particles, poor mixing and unstable finished-product quality.

The objective is therefore not to maximize the compression ratio.

The objective is to select a geometry that melts and conveys the particular polymer efficiently.

How Does the Plastic Material Affect Compression Ratio?

Different polymers behave differently during feeding, melting and compression.

PE Extrusion

PE is used in applications ranging from blown film and pipe extrusion to recycling and pelletizing.

A conventional PE screw may use a moderate compression ratio, but the correct design depends heavily on whether the processor is running HDPE, LDPE, LLDPE, virgin pellets, recycled flakes or a mixture.

For example, recycled film material can have a much lower bulk density than virgin pellets.

In this case, feed-section geometry may be just as important as compression ratio.

Simply increasing the compression ratio will not solve a feeding problem.

PP Extrusion

PP generally processes well on conventional single-screw designs, but the required geometry depends on the application.

A PP sheet extrusion line, recycling pelletizer and profile extrusion line may all require different screw characteristics.

If the customer’s main problem is insufficient output, we first evaluate whether the limitation comes from feeding, melting capacity, screw speed, motor power, gearbox capacity or die resistance.

Changing the compression ratio alone may not increase production.

PVC Extrusion

PVC requires particularly careful screw design because it is sensitive to excessive heat and shear.

For PVC applications, simply applying a screw geometry developed for PE or PP can create processing problems.

Temperature control, residence time, compression profile and screw geometry must work together.

This is one reason there is no truly universal screw that performs optimally with every plastic.

Why Two 90 mm Screws Can Perform Differently

Suppose two screws both have:

• Diameter: 90 mm
• L/D: 30:1
• Compression ratio: 3:1

Does this mean they are the same screw?

No.

They can still have very different:

• Feed channel depths
• Metering channel depths
• Section lengths
• Flight widths
• Pitch
• Mixing sections
• Barrier designs
• Clearance
• Drive-end structures

Even if the calculated compression ratio is identical, the melting behavior and output can be completely different.

This is why copying only the diameter, L/D and compression ratio from another machine is risky.

For a replacement extruder screw and barrel, the complete geometry should be evaluated.

What Happens When the Compression Ratio Is Too High?

An excessively aggressive compression profile may create high shear and unnecessary melt temperature.

Operators may notice that the material becomes too hot even when barrel heater settings are reduced.

Other possible symptoms include:

• Material discoloration
• Degradation
• Increased motor current
• High melt temperature
• Unstable pressure
• Excessive mechanical wear

However, these symptoms do not prove that compression ratio is the only cause.

A blocked screen, incorrect temperature profile, excessive screw speed or unsuitable mixing section can produce similar symptoms.

Good troubleshooting requires looking at the entire extrusion process.

What Happens When the Compression Ratio Is Too Low?

If compression is insufficient for the polymer and production rate, the screw may have difficulty completing the melting process.

Possible symptoms include:

• Unmelted resin
• Poor color dispersion
• Unstable melt
• Reduced mixing quality
• Inconsistent product surface
• Difficulty achieving the required output

Again, the solution is not automatically to increase compression ratio.

The length of the compression section, channel depth, screw speed and available heating capacity also affect melting.

Can You Increase Extruder Output by Changing Screw Design?

Yes, sometimes—but the entire extrusion system must be considered.

We often receive requests such as:

“My current output is 150 kg/h. Can you design a new screw to produce 250 kg/h?”

Before answering, we need more information.

The screw is only one component of the extrusion line.

Output can also be limited by:

• Main motor power
• Gearbox torque
• Maximum screw speed
• Feeding capacity
• Barrel heating and cooling
• Screen changer
• Die design
• Downstream equipment

A more aggressive screw installed in a machine with insufficient motor power or gearbox capacity can create new problems rather than higher stable output.

For this reason, when customers request a higher-output replacement screw, we normally ask for both the current production conditions and target output.

Our Practical Approach to Replacement Screw Design

When an existing screw has already produced good-quality products for many years, accurately reproducing the proven geometry is often the safest solution.

But sometimes the customer contacts us because the original screw has never performed satisfactorily.

For example:

• Output is consistently too low
• Feeding is unstable
• Material slips in the feed section
• Plasticizing is excessive
• Melt temperature is too high
• Recycled material is difficult to convey

In these cases, simply copying the original screw can reproduce the original problem.

Before manufacturing, we prefer to understand what the customer wants to improve.

Depending on the application, modifications may involve channel depth, section distribution, compression profile or mixing design.

The material specification of the screw and barrel should also be selected according to abrasion and corrosion conditions.

What Information Should You Provide for a New Extruder Screw?

For a customized extrusion screw, provide as much of the following information as possible:

Machine Information

• Extruder manufacturer and model
• Screw diameter
• L/D ratio
• Motor power
• Gearbox model
• Maximum screw speed

Material Information

• Polymer type
• Virgin or recycled material
• Filler type
• Filler percentage
• Glass-fiber percentage, if applicable
• Additives or masterbatch

Production Information

• Current output in kg/h
• Required output in kg/h
• Current screw speed
• Melt temperature
• Main production problem

Dimensional Information

A complete manufacturing drawing is preferred.

If the original drawing is unavailable, detailed measurements and photographs should include the screw total length, working length, drive connection, keyway or spline, barrel flange and other installation dimensions.

Accurate connection dimensions are just as important as the screw profile.

A well-designed screw is useless if it cannot be installed correctly in the machine.

Compression Ratio Is Only One Part of a Good Screw

When purchasing a replacement screw, it is tempting to compare quotations using a few simple specifications:

Diameter + L/D + compression ratio + material + price.

But extrusion performance depends on how all these parameters work together.

A good plastic extrusion screw design should balance:

• Feeding
• Melting
• Compression
• Mixing
• Melt temperature
• Pressure stability
• Output
• Wear resistance

This is why we do not recommend choosing an extruder screw solely from a generic compression-ratio table.

The table may provide a starting point, but the final geometry should reflect the actual polymer and production conditions.

FAQ

What is a typical extruder screw compression ratio?

There is no single compression ratio suitable for every extrusion process. The appropriate ratio depends on the polymer, screw geometry, output requirement, screw speed and application. A ratio that works well for one PE extrusion line may not be appropriate for PVC, recycled plastic or another process.

Is 3:1 a good screw compression ratio?

A 3:1 ratio can be suitable for some applications, but the number alone cannot determine whether the screw design is correct. Feed depth, metering depth, transition length, screw speed and material characteristics must also be considered.

Does a higher compression ratio increase extrusion output?

Not necessarily. Output depends on feeding, screw geometry, speed, motor power, gearbox capacity, die resistance and other factors. Excessive compression can increase shear and melt temperature without providing a useful increase in stable output.

Can one extrusion screw process PE, PP and PVC?

A screw may physically process more than one polymer under some conditions, but one geometry is rarely optimal for every material. PVC in particular requires different consideration from typical PE or PP extrusion because of its processing sensitivity.

Can you redesign my existing screw for higher output?

Possibly. Before changing the screw geometry, the existing machine capacity, motor power, gearbox, screw speed, raw material, current output and target output should be evaluated. Increasing screw performance without checking the rest of the machine can overload other components.

Do you need the original screw drawing to manufacture a replacement?

A complete drawing is the best option. If no drawing is available, accurate measurements, dimensional photographs and sometimes the physical sample can be used to determine the required geometry and connection dimensions.

Need Help Selecting an Extrusion Screw Design?

If you are replacing an existing extrusion screw or trying to improve feeding, plasticizing or production output, send us your screw and barrel drawing, raw material, current output, target output, screw speed and motor power.

Zhoushan Juyuan Plastic Machinery manufactures customized extruder screws and barrels for PE, PP, PVC, recycled plastics and other extrusion applications.

Instead of selecting a screw only by diameter and compression ratio, we can review the actual production conditions and manufacture the replacement according to the required dimensions and application.