Choose a screw compression ratio based on the material grade, processing method and complete screw design. A higher ratio may help some materials melt and compact, but it can also increase heat and processing load. A lower ratio may suit some sensitive materials, but it does not automatically provide better processing.
Before selecting a ratio, check whether the machine is used for injection molding or extrusion. Then review the material supplier’s guidance, feedstock condition, screw geometry and production requirements.
This guide explains how to calculate screw compression ratio, what affects selection, and what to investigate when the existing screw does not perform as expected.
What Is Screw Compression Ratio?
Screw compression ratio describes the change in screw channel volume between the feed section and the metering section.
For a conventional screw with constant pitch and comparable channel widths, it is commonly estimated using:
Compression ratio ≈ feed channel depth ÷ metering channel depth
For example, if the feed channel is 9 mm deep and the metering channel is 3 mm deep:
9 ÷ 3 = 3:1
This describes the screw’s geometry. It does not mean that the plastic’s density becomes three times greater or that the machine produces three times more pressure.
For designs with changing pitch, channel width or more complex geometry, ask the screw designer how the compression ratio is defined.
Choose a screw compression ratio based on the material grade, processing method and complete screw design. A higher ratio may help some materials melt and compact, but it can also increase heat and processing load. A lower ratio may suit some sensitive materials, but it does not automatically provide better processing.
Before selecting a ratio, check whether the machine is used for injection molding or extrusion. Then review the material supplier’s guidance, feedstock condition, screw geometry and production requirements.
This guide explains how to calculate screw compression ratio, what affects selection, and what to investigate when the existing screw does not perform as expected.
What Is Screw Compression Ratio?
Screw compression ratio describes the change in screw channel volume between the feed section and the metering section.
For a conventional screw with constant pitch and comparable channel widths, it is commonly estimated using:
Compression ratio ≈ feed channel depth ÷ metering channel depth
For example, if the feed channel is 9 mm deep and the metering channel is 3 mm deep:
9 ÷ 3 = 3:1
This describes the screw’s geometry. It does not mean that the plastic’s density becomes three times greater or that the machine produces three times more pressure.
For designs with changing pitch, channel width or more complex geometry, ask the screw designer how the compression ratio is defined.
Choose a screw compression ratio based on the material grade, processing method and complete screw design. A higher ratio may help some materials melt and compact, but it can also increase heat and processing load. A lower ratio may suit some sensitive materials, but it does not automatically provide better processing.
Before selecting a ratio, check whether the machine is used for injection molding or extrusion. Then review the material supplier’s guidance, feedstock condition, screw geometry and production requirements.
This guide explains how to calculate screw compression ratio, what affects selection, and what to investigate when the existing screw does not perform as expected.
What Is Screw Compression Ratio?
Screw compression ratio describes the change in screw channel volume between the feed section and the metering section.
For a conventional screw with constant pitch and comparable channel widths, it is commonly estimated using:
Compression ratio ≈ feed channel depth ÷ metering channel depth
For example, if the feed channel is 9 mm deep and the metering channel is 3 mm deep:
9 ÷ 3 = 3:1
This describes the screw’s geometry. It does not mean that the plastic’s density becomes three times greater or that the machine produces three times more pressure.
For designs with changing pitch, channel width or more complex geometry, ask the screw designer how the compression ratio is defined.
Choose a screw compression ratio based on the material grade, processing method and complete screw design. A higher ratio may help some materials melt and compact, but it can also increase heat and processing load. A lower ratio may suit some sensitive materials, but it does not automatically provide better processing.
Before selecting a ratio, check whether the machine is used for injection molding or extrusion. Then review the material supplier’s guidance, feedstock condition, screw geometry and production requirements.
This guide explains how to calculate screw compression ratio, what affects selection, and what to investigate when the existing screw does not perform as expected.
What Is Screw Compression Ratio?
Screw compression ratio describes the change in screw channel volume between the feed section and the metering section.
For a conventional screw with constant pitch and comparable channel widths, it is commonly estimated using:
Compression ratio ≈ feed channel depth ÷ metering channel depth
For example, if the feed channel is 9 mm deep and the metering channel is 3 mm deep:
9 ÷ 3 = 3:1
This describes the screw’s geometry. It does not mean that the plastic’s density becomes three times greater or that the machine produces three times more pressure.
For designs with changing pitch, channel width or more complex geometry, ask the screw designer how the compression ratio is defined.
Figure 1. Channel depth approximation for a conventional constant-pitch screw.
Does a Higher Compression Ratio Mean Better Performance?
A higher ratio does not guarantee more output or better melt quality.
The result depends on how the ratio is achieved. A deeper feed channel and a shallower metering channel can both increase the ratio, but they affect the process differently.
For example, these two screws have the same calculated ratio:
| Feed channel depth | Metering channel depth | Approximate ratio |
|---|---|---|
| 9 mm | 3 mm | 3:1 |
| 12 mm | 4 mm | 3:1 |
Their absolute channel depths are different. You therefore need more than the ratio alone to compare them.
Also review the screw diameter, transition length, mixing sections, speed and operating conditions.
Figure 2. High compression ratio concept. The example range is not a universal material recommendation.
Figure 3. Low compression ratio concept. Actual behavior depends on the complete design and operating conditions.
What Is a Typical Screw Compression Ratio?
General-purpose injection molding screws commonly use compression ratios around 2.5:1 to 3.0:1, according to Reiloy’s screw and barrel handbook. This is a reference range, not a recommendation for every material. Reiloy Screw & Barrel Handbook
Extrusion screws can have different requirements. Continuous output, die resistance, feedstock form and venting arrangements all need consideration.
Use a typical ratio as a starting point for discussion. Confirm the complete design before ordering.
Figure 4. Conventional-ratio illustration. Example values need to be checked for the material and process.
How Does the Material Affect Selection?
PP and PE
PP and PE applications can use different screw designs depending on the grade and process.
A screw used for injection molding a PP component may have different requirements from one used for PE film extrusion. Melt flow, additives, output and melt temperature all matter.
Avoid choosing a high compression ratio simply because the material is PP or PE.
PVC
PVC processing requires careful control of heat, residence time and shear.
Discuss the formulation, stabilizers, processing method and equipment with the material supplier and screw manufacturer. A recommendation for a conventional single screw cannot automatically be transferred to a conical twin-screw system.
PC and PA
PC and PA should not be placed into one universal “low compression” group.
The required design depends on the grade, fillers and application. For example, BASF recommends a compression ratio of 3:1 to 3.5:1 for certain Ultramid PA film extrusion applications.
That recommendation applies to the specified extrusion context. It should not be copied directly into an injection molding specification. BASF processing guidance
Glass-Fiber-Filled Materials
For glass-fiber-filled materials, review the entire processing path.
The screw should provide adequate melting and mixing while limiting unnecessary fiber damage. Compression ratio is only part of that decision. Mixing features, screw speed and other restrictions also need attention.
Wear resistance matters too. Discuss both geometry and material selection when specifying an injection molding screw and barrel.
PLA
For PLA, start with the processing guidance for the exact grade.
Drying, residence time, actual melt temperature and screw design can all affect material quality. A single compression ratio cannot replace these checks.
For example, TotalEnergies Corbion recommends proper drying and a mild screw design for its discussed PLA compounding applications. This supports reviewing the complete process rather than selecting a ratio alone. TotalEnergies Corbion PLA processing guidance
PCR and Recycled Materials
PCR describes the material’s origin, not a single polymer. Recycled PP, PE and PET do not share one universal screw requirement.
Before selecting a screw, identify:
- Polymer type and grade.
- Recycled content.
- Pellet, flake or regrind form.
- Bulk density and feed consistency.
- Moisture and contamination.
- Required filtration and venting.
Low bulk density can create feeding problems. Increasing compression ratio alone may not solve them. Feed preparation, drying, compaction and the feeding system may also need changes.
Recycling equipment manufacturers such as EREMA address bulk density, moisture and degassing through coordinated material preparation and extrusion systems. EREMA recycling systems
How Can You Tell Whether the Screw Is Unsuitable?
Production symptoms can help guide an investigation, but they cannot prove that compression ratio is the cause.
| Production symptom | Possible areas to investigate |
|---|---|
| High motor load | Material condition, temperature, restrictions, throughput and screw design |
| Discoloration or black specks | Degradation, contamination, deposits and residence time |
| Unmelted particles | Heating, screw speed, output and melting capability |
| Unstable output | Feeding, bulk density, wear and downstream resistance |
| Silver streaks or bubbles | Moisture, trapped air, volatiles and processing conditions |
Figure 5. Symptoms that can prompt investigation of excessive shear. They do not identify the cause on their own.
Figure 6. Poor melt quality and low output also require checks of drying, feeding, heating and equipment condition.
Record the actual melt temperature, load, output and material condition before deciding that the screw needs a different ratio.
What Can You Adjust Before Replacing the Screw?
Process changes may improve performance, but they do not change the screw’s geometric compression ratio.
Check Temperature and Screw Speed
Compare the actual melt temperature with the material supplier’s recommended range.
If shear heating is suspected, review screw speed and the barrel’s heating or cooling performance. Make controlled changes and monitor the result.
If melting is incomplete, check heater operation, temperature settings, throughput and available melting length.
There is no universal instruction to raise or lower barrel temperatures by a fixed amount.
Review Back Pressure in Injection Molding
Back pressure can affect melt consistency, mixing and recovery time.
Increasing it may help some applications, but it can also add heat or extend recovery. It does not turn a low-compression screw into a high-compression screw.
Review it within the machine and material supplier’s guidance.
Review Feedstock Preparation in Extrusion
For flakes or irregular regrind, check feed consistency and bulk density.
Drying, sorting, densifying or pelletizing may improve the feed. The appropriate approach depends on the material and production system.
A contaminated or incompatible material mixture cannot be corrected simply by choosing a different compression ratio.
Check Component Condition
Inspect the screw, barrel and relevant downstream components when process adjustments do not resolve the problem.
For injection molding, check-ring condition may also affect consistency. For extrusion, review restrictions such as filtration and die condition.
What Information Should You Give the Screw Manufacturer?
Before requesting a replacement or custom screw, prepare:
- Machine manufacturer and model.
- Injection molding or extrusion application.
- Screw diameter and available drawing.
- Material grade, fillers and recycled content.
- Feedstock form and bulk density, if relevant.
- Current output or recovery time.
- Actual processing conditions and problem symptoms.
This information helps the supplier review the complete design.
For extrusion applications, explore YINGDA’s single-screw and barrel options. For a drawing-based discussion, see custom screw design and OEM manufacturing.
The right compression ratio is the one that fits the material, machine and production process as part of a suitable screw design.