Extrusion screw compression ratio describes the reduction in channel volume from the feed section to the metering section. On a conventional screw with comparable pitch and channel geometry, feed channel depth divided by metering channel depth gives a useful approximation. Choose a replacement from the complete drawing and actual feed conditions, not from a ratio alone: two screws marked 3:1 can have different channel depths and transition profiles.
By EJS Industry
- Calculate channel depth from the screw flight diameter and root diameter, not the barrel bore.
- Compare absolute depths and zone lengths before accepting two screws as equivalent.
- Describe the material entering the hopper, including regrind shape and loading.
- Investigate feeding, melting, restriction and wear before changing the ratio.
- Validate a replacement with stable production measurements, not an rpm setting alone.
What does screw compression ratio measure?
A conventional single screw has a feed section, a transition section and a metering section. The feed channel receives solid material; the transition changes channel geometry as melting develops; the metering section conveys melt toward the discharge. Compression ratio describes a geometric relationship between the beginning and end of this process. It does not measure die pressure, product density or motor efficiency.
For a simple constant-pitch screw, the workshop calculation is:
Compression ratio ≈ feed channel depth / metering channel depth
The more general definition compares channel volumes over equivalent flights. The depth shortcut needs comparable pitch and channel geometry. If those change, use the drawing to evaluate the relevant volumes rather than treating a depth ratio as an exact volumetric result. Allan Griff's explanation of screw geometry makes this distinction and warns against relying on the ratio without the actual depths.
L/D describes something different: effective working length divided by screw outside diameter. A working length of 2,700 mm and a diameter of 90 mm give 30:1. Confirm where the manufacturer starts and ends that working length. A drive extension belongs on the mechanical drawing but should not silently become extra processing length.
A specification reading “90 mm, 30:1, 3:1” therefore leaves substantial information missing. It does not identify feed depth, metering depth, transition length, pitch changes or mixing geometry. Ask for those dimensions before comparing a replacement quote with the installed screw. For the broader purchasing checklist, see the EJS extruder screw barrel guide.
How to calculate compression ratio from a screw drawing
Start with a conventional, single-start design whose feed and metering channels have comparable pitch and flight geometry. Locate the constant-depth portion of each section. Do not take the feed reading inside the taper or the metering reading from a mixer, relief or damaged edge.
For each location, calculate radial channel depth:
Channel depth = (flight outside diameter − root diameter) / 2
The division by two matters because both measurements are diameters. Using the diameter difference directly doubles the reported depth, even though the error can disappear when two equally incorrect depths are divided. The ratio might look right while the manufacturing dimensions are wrong.
A worked 90 mm example
The following dimensions are an assumed calculation example, not a recommended screw design or a production test.
| Dimension | Feed section | Metering section |
|---|---|---|
| Flight outside diameter | 90 mm | 90 mm |
| Root diameter | 72 mm | 84 mm |
| Calculated channel depth | (90 − 72) / 2 = 9 mm | (90 − 84) / 2 = 3 mm |
| Approximate compression ratio | 9 / 3 = 3:1 | Same depth-ratio calculation |
If you are inspecting a removed screw, record the axial location, measured flight diameter and root diameter together. Follow the machine's isolation and safe-removal procedure, clean the component without altering the surfaces, and use suitable calibrated instruments. Let the manufacturer's inspection procedure determine the measuring method and acceptance criteria.
Do not substitute barrel inside diameter for flight outside diameter. That calculation includes the screw-to-barrel clearance. On a worn part, do not assume the measured geometry still represents the original design: a reduced flight diameter changes the depth calculation. Obtain the original drawing and measure barrel wear separately. The EJS service-life guide explains why matching screw and barrel measurements matter.
Why two 3:1 screws can perform differently
A ratio does not identify the size of either channel. In the same assumed 90 mm example, a second screw could have a feed root of 84 mm and a metering root of 88 mm. Its depths would be 3 mm and 1 mm, so its depth ratio would also be 3:1. It would not be a dimensional copy of the 9 mm/3 mm screw.
| Comparison item | Assumed screw A | Assumed screw B | What the buyer can establish |
|---|---|---|---|
| Flight outside diameter | 90 mm | 90 mm | Same nominal screw diameter |
| Feed channel depth | 9 mm | 3 mm | Different absolute feed geometry |
| Metering channel depth | 3 mm | 1 mm | Different absolute metering geometry |
| Approximate depth ratio | 3:1 | 3:1 | Equal ratio, not equal screws |
| Transition length and pitch | Not specified | Not specified | Performance comparison remains incomplete |
This example supports a dimensional conclusion only. It does not establish a throughput difference, a melt-temperature difference or which screw should run your resin. Those questions require the remaining geometry and operating conditions.
What we check when a replacement is quoted to the same specification
In EJS replacement work, two screws carried the same diameter, L/D and 3:1 compression ratio, yet one ran clean while the other overheated a heat-sensitive compound. Their transition sections started at different positions and had different lengths. We therefore compare the zone profile, not just the numbers on the quotation.
On a replacement drawing, mark where the feed section ends and where metering begins. Then compare the depth along that interval. A ratio gives the end points; the profile shows how the channel changes between them. Neither is a substitute for the other.
This is also why “increase compression” is an incomplete design instruction. Increasing feed depth and reducing metering depth both increase the ratio, but they change different parts of the screw. Specify which dimension changes, what stays unchanged and which measured problem the change is intended to address. Otherwise, the supplier and production team may be discussing different modifications.
How resin, feed form and fillers affect the decision
A polymer name is not a complete feed specification. Record the grade, additives, particle form, moisture preparation and blend consistency. Pellets, ground edge trim and irregular flakes can carry the same polymer name while behaving differently at the feed throat. Jim Frankland's compression-ratio discussion considers feeding and melting together, including bulk density and friction rather than treating resin type as a sufficient selection rule.
The first question we ask when the feed changes
An EJS inquiry involved a 90 mm screw, 30:1 L/D and 3:1 compression ratio. The line had surging die pressure and inconsistent wall thickness, and the initial request was to copy a supposedly worn screw. We asked what entered the hopper now compared with the original production setup. The plant had started blending its own edge trim into the feed. That change belonged in the replacement specification before an unchanged drawing was accepted.
For your own line, collect representative feed samples rather than describing everything as “recycled plastic.” Measure loose bulk density using a defined container, fill method and weighing procedure. Record whether material was freely poured or compacted; do not compare two results produced by different methods. Include photographs with a scale and note the blending method.
| Feed condition | Information to collect | Design question to resolve |
|---|---|---|
| Virgin pellets | Grade, pellet shape, preparation and current stable settings | Is the established geometry still suitable for this duty? |
| Pellet/regrind blend | Blend proportion, particle-size spread, bulk-density consistency | Does the feed remain consistent throughout a run? |
| Film flake or edge trim | Shape, feeding method, bridging observations and bulk density | Can material enter and fill the feed section reliably? |
| Powder or dry blend | Formulation, flow behavior and feed-system details | Is the feeding arrangement appropriate for this material? |
| Filled compound | Filler type and proportion, preparation and wear history | What are the melting, heat and wear requirements together? |
Do not read this table as “lower bulk density always needs a higher ratio.” A geometry change cannot automatically correct bridging, segregation or an inconsistent feeder. Nor should wear resistance be selected from compression ratio. Review the EJS screw barrel materials guide separately when the formulation changes.
Too high or too low? Diagnose the process before changing the screw
Pressure surging, unmelted particles and rising melt temperature are reasons to investigate. They are not measurements of compression ratio. The useful diagnostic sequence is to establish whether the disturbance starts at feeding, melting, a downstream restriction or worn mechanical geometry.
First compare with a stable historical run using the same material and product. Keep the die, screen arrangement and measurement locations consistent. Record actual output, screw speed, melt temperature, pressure, drive load and visible defects together. Barrel temperature settings alone do not establish the melt temperature.
| Observation | Checks before a geometry change | Evidence to include in the request |
|---|---|---|
| Pressure and output fluctuate | Hopper supply, bridging, feeder consistency, blend changes and restriction | Time-aligned speed, pressure and output records |
| Unmelted particles appear | Feed rate, melting conditions, resin changes, operating speed and screw profile | Samples or photos linked to a recorded operating point |
| Melt temperature is excessive | Actual melt reading, speed, restriction, cooling and mixer condition | Measurement method, location and stable-run readings |
| Output falls at unchanged rpm | Actual feed supply, resin condition, die/screens and matching screw/barrel wear | Output per rpm plus maintenance and measurement records |
| Black specks or discoloration increase | Material history, stagnant areas, residence conditions and cleaning | Defect pattern and when it appears in the run |
Why a higher ratio is not an automatic improvement
Lin Jin and Mark A. Spalding reported a controlled LLDPE study using a 2.5-inch extruder and five compression ratios, from 2.0 to 3.6. Metering depth remained 0.125 inch while feed depth changed. At 110 to 150 rpm, the higher-ratio screws produced melt temperatures about 5 to 10°C higher than the lower-ratio designs. These are results for that equipment, resin and experiment, not a temperature prediction for an EJS replacement. See the authors' compression-ratio performance study.
A separate troubleshooting example by Mark A. Spalding describes a 6-inch, two-stage ABS extrusion screw with a first-stage ratio of 3.1. Unmelted material obstructed a spiral dam; the proposed replacement reduced that ratio to 2.5. This was a specific design response to a melting problem, not a universal ABS recommendation.
Use those examples to frame the engineering question: can the screw melt the solids it receives at the required operating point? Do not turn either example into a general resin-to-ratio chart or an assumed EJS performance result.
When a single compression-ratio number is not enough
The simple feed-depth/metering-depth comparison belongs to a defined screw geometry. Before using it, identify the screw type and feeding arrangement. A ratio reported for one design may not mean the same thing on another.
A barrier screw divides material into solid and melt channels. Its description needs the channel arrangement and barrier geometry, not just two conventional root measurements. A vented two-stage screw has more than one processing stage; ask which stage a quoted compression ratio describes. The vent region and downstream pumping section need to be evaluated as part of that complete design. Griff's screw-design discussion covers barrier channels, two-stage screws and the distinct feeding behavior of grooved barrels.
Likewise, distinguish flood feeding from controlled or starve feeding. Write the feeding mode into the specification. Otherwise a supplier may assess a full feed channel while the installed process delivers a controlled mass flow. This is a boundary condition to clarify, not a reason to apply a different universal ratio.
Twin-screw extruders also need their own geometry and process description. Do not apply the conventional single-screw depth shortcut to an intermeshing pair and assume it captures the complete compression behavior. Provide the screw arrangement, rotation mode, element or flight profile and manufacturer drawing.
Use these questions when the specification is unclear:
- Is this a conventional, barrier, two-stage or twin-screw design?
- Is the quoted ratio calculated from depths or channel volumes?
- Which feed and metering locations define the calculation?
- Does pitch or channel arrangement change between those locations?
- Does the installed feed system match the design assumption?
A specification should answer those questions without requiring the machinist to guess. If it cannot, obtain the complete profile before authorizing a copy. “3:1” is useful once its definition and application are known.
Should you copy the existing screw or redesign it?
An unchanged replacement is a reasonable starting point when the installed geometry delivered satisfactory quality and output, the duty is unchanged, and measured wear explains the deterioration. It becomes a weaker choice when the original problem existed before wear, the formulation changed or the process now runs outside its former operating range.
Separate two decisions in the quotation: restoring the original dimensions and changing the processing design. A worn screw may require replacement without requiring a different compression ratio. Conversely, a dimensionally sound screw may no longer suit the feed introduced into the plant.
Define the change before comparing proposals
Give the supplier a written problem statement: for example, pressure instability after a feed-form change, rather than “need a better screw.” Include the baseline drawing and operating records. Ask the proposed design to identify changed depths, zone lengths, pitch and any mixing features.
Keep the mechanical fit review separate. Confirm working length, drive-end connection, mounting interfaces and discharge arrangement against the installed assembly. More working length may require a machine-level change; it is not an ordinary drop-in replacement assumption. A higher L/D designation on a quote should trigger a dimension check, not an expectation of increased capacity.
Then agree on the comparison conditions and acceptance criteria. Define the material, product, operating points, quality checks and production measurements before the trial. If several design features change together, the trial evaluates the complete replacement; it cannot establish that compression ratio alone caused the result.
The EJS extruder screw barrel product page is the relevant starting point for a replacement inquiry. Supply the actual application and drawing so the discussion concerns the installed machine rather than a generic polymer range.
What to send EJS and how to validate the replacement
A useful inquiry lets the factory connect the processing requirement to a manufacturable drawing. Send the original specification even if you think it is wrong, then identify what changed. Keep revision numbers visible so that an old drawing does not silently become the production instruction.
Include the following information:
- Machine manufacturer, model, screw type and installed component references.
- Flight diameter, working length, drive-end dimensions and barrel/interface drawings.
- Feed and metering depths, zone boundaries, pitch profile and mixing features.
- Resin grade, additives, filler loading, feed form and preparation procedure.
- Feeder arrangement, normal rpm, output, melt-temperature and pressure readings.
- Defect samples, maintenance history and available matching screw/barrel measurements.
- Required product quality and the operating conditions for acceptance.
Compare stable production, not just a successful startup
After installation, follow the machine supplier's safe commissioning instructions. Confirm the approved material and setup, allow the process to stabilize, and collect readings over a defined interval. Record output by an agreed weighing method rather than estimating it from the screw speed.
For comparison, calculate specific output as measured mass output divided by rpm. Keep the output units visible, such as kg/h per rpm. Compare this value only alongside quality, actual melt temperature, pressure and drive load. A higher number does not establish an improvement if the product falls outside specification or the process becomes unstable.
| Trial record | Record consistently | Why it is useful |
|---|---|---|
| Feed | Grade, lot, blend and preparation | Identifies changes unrelated to geometry |
| Operating point | Speed, temperature setup and feed mode | Makes the comparison reproducible |
| Production | Timed mass output and acceptable product | Separates total production from useful production |
| Process condition | Actual melt temperature, pressure and drive load | Reveals trade-offs hidden by output alone |
| Quality | Agreed dimensions, appearance and defect checks | Determines whether the replacement meets the duty |
Repeat the comparison when changing operating points and keep the same measurement locations. If a problem remains, send the records back with the drawing revision and sample identifiers. Contact EJS Industry with that package rather than requesting a compression-ratio change without a defined cause.
Discuss your replacement screw
Describe the installed screw, actual feed and processing problem. The button opens an email draft to EJS; you can attach drawings and photographs before sending.
Frequently asked questions
What is extrusion screw compression ratio?
It is the ratio of feed-section channel volume to metering-section channel volume over equivalent flights. For a conventional screw with comparable pitch and channel geometry, feed depth divided by metering depth provides a useful approximation. It is a geometric ratio, not a pressure ratio or a guarantee of output.
How do I calculate compression ratio from root diameters?
Calculate each radial channel depth as flight outside diameter minus root diameter, divided by two. Then divide feed depth by metering depth. Use identified feed and metering locations, consistent units and the appropriate flight diameters. Do not use barrel bore diameter or assume a worn screw still matches its original drawing.
Is a 3:1 screw suitable for every resin?
No. The same ratio can describe different absolute channel depths and transition profiles. Suitability also depends on the specific grade, feed form, additives, feeding arrangement and operating conditions. Start with the complete geometry and a defined production requirement rather than treating a ratio as a universal resin recommendation.
Does increasing compression ratio increase output?
Not necessarily. Changing feed depth and changing metering depth affect different parts of the screw, even if both increase the ratio. Feeding, melting and downstream conditions must be considered together. Compare stable measured output with product quality, actual melt temperature, pressure and drive load before judging a replacement.
Can worn screws cause problems without changing the drawing's ratio?
Yes. The drawing describes the intended geometry, not the present condition. Measure the screw and barrel at corresponding positions and compare with the approved original dimensions. A compression-ratio label does not describe the working clearance, and copying that label does not resolve an unmeasured wear problem.
Can I use the same calculation for twin-screw or barrier designs?
Do not assume the conventional single-channel depth shortcut applies. Identify the channel arrangement, screw type and calculation locations first. Barrier channels, staged screws and intermeshing twin screws require a fuller geometry description. Ask the designer which volumes or depths define the quoted ratio before using it in a replacement specification.