Does your extruder occasionally spit out thick and thin areas as if it were breathing? When your extruder surges, product appearance (DIM) is affected, of course. Material waste increases, too. Dimensional instability caused by surging can ruin your entire production schedule.
Knowing what causes surging is half the battle. Once you discover the root cause, you can put measures in place to prevent it from happening again.
Here, we review the five most common reasons your extruder is surging and what you can do about it.
What Is Extruder Surging?
Periodic fluctuations in extrudate output are known as extruder surging, melt instability, or unstable output. Instead of seeing smooth extrusion, there is apparent rhythmic pulsing of material. You may experience times of over-extrusion, followed by sudden periods of under-extrusion.
Symptoms include:
- Variations in the diameter of extruded profiles or tubing
- Fluctuations in the melt pressure gauge readings
- Motor amperage or screw torque changes frequently
- Appearance defects (sharkskin, melt fracture, waviness)
- Unexpected increases in scrap rates
If you see any of these signs during extrusion, don’t panic — there is always a cause.
1. Raw Material or Moisture Content Fluctuations
Uneven raw material density, poor pellet quality, or incorrect moisture levels can cause irregular feeding, melted material accumulation, and surging.
Check:
- The consistency of incoming raw material with the supplier’s COA (certificate of analysis)
- Moisture levels of resin before processing—nylon, ABS, PC—should be dried to specification
- Pellet size and shape (are they grinding unevenly in the feed throat?) Or perhaps bridging?)
Ensuring you use quality resin from a trusted supplier and dry it as required will eliminate a large group of potential surging issues.
2. Screw and Barrel Wear
Aging screws and barrels can cause surging, especially when running abrasive fillers at high speeds. As the metal gradually wears, the clearance between flights allows the melt to flow backwards — a phenomenon known as screw slippage.
Check:
- The clearance between screw and barrel—this will indicate screw wear
- Visual deformation of the screw, corrosion pitting, or wear of mixing elements/knives
- Visible pattern of wear that could indicate barrel misalignment
While quality machines from reputable manufacturers like Deer Machine are engineered with hardened and precision-ground screws and barrels, wear eventually occurs. Implementing a screw and barrel measurement plan allows you to catch wear trends before they start impacting your process.
3. Temperature Settings
Too-hot or too-cold melt temperatures can create instability in your extrusion process. If barrel zones are too cold, unmelted “plug” material may build up and burst through at intervals. Too-hot and the resin degrades, causing low-viscosity melt that slips on the screw.
Check:
- Compare your current processing temperature to the resin supplier’s recommended temperature window
- Are your thermocouples calibrated? A bad thermocouple could be running your zone 20–30°C off without tripping any alarms
- Variances in zone temperatures can help you pinpoint bad heater bands or controller problems
- Make small incremental adjustments (5–10°C) and note the output stability after each adjustment
4. Bridging or Inconsistent Feeding
The beginning of most surging problems is often at the feedthroat. Without consistent material entering the screw, everything downstream will be affected.
Check:
- Are materials bridging? If high bulk density or oddly shaped pellets are used, the material may pack itself and create an arch above the feed throat. A feed throat vibrator or agitator can fix this
- Sticking to hopper walls while the center flows (ratholing)
- Temperature of feed throat — too high, and the pellets can soften and melt/clog before entering the screw. Keep feedthroat cooling water circulating!
- The calibration of your feeder—if using a gravimetric/volumetric feeder, ensure it is calibrated to the correct set rate and not pulsing
Our extruder designs feature optimized feed throat geometry that allows for consistent material flow into the screw, minimizing bridging without requiring user intervention.
5. Die Pressure Changes
While many causes of surging originate at the screw, issues with the die or screen pack can induce instability.
Check:
- Is the screen pack getting plugged up? A dirty screen pack causes pressure buildup and then sudden release—leading to pulsing output. Change your screen pack regularly
- Die land length/geometry—restriction from the die can exaggerate pressure pulses coming from the screw.
- Measure melt pressure before and after screen pack; increasing differential means it’s time to change
- Lint or degraded material buildup on die lips/tooling
Step by Step Process for Troubleshooting Surging
Follow this order of operations when you first experience surging:
- Confirm the basics—Is material dried properly? Is hopper full? Is screen pack changed out regularly?
- Look at your recorded data — melt pressure trend, screw RPM, motor amps can provide clues
- Divide and conquer — is the surge rhythmic or not? A regular surge indicates screw wear or temperature issue. Random surge indicates feeding
- Change one thing at a time — adjust temperature or screw speed or backpressure by one variable and note the results
- Physically inspect the extruder when possible — removing the screw can reveal many hidden problems.
Preventative Maintenance: Avoid Surging Before It Starts
Many cases of surging can be prevented by simply following a preventative maintenance schedule:
- Daily: Verify process temperatures, pressures, motor amps on startup
- Weekly: Physical hopper/feed throat/screen pack inspection
- Monthly: Thermocouple calibration and heater band verification
- Yearly: Screw and barrel pulled for measurement; review wear against OEM specs
Need help setting up a preventative maintenance plan? Deer Machine will walk you through best practices and schedules to keep your equipment running smoothly.
Final Thoughts
When has extruder surging ever occurred for no reason? Switching out the screen pack or adjusting temperatures seems like reactive maintenance; however, these are often the solution to your problem. Take a step back and examine each variable independently. More times than not, surging can be traced back to one of five causes: feeding, screw and barrel wear, temperatures, moisture in material, and die restrictions.
Don’t let sporadic surging drive you crazy. Work through this list and keep your process parameters optimized. If you’ve tried everything we have mentioned and your extruder keeps surging, it may be time to call your equipment supplier.
Deer Machine has years of extrusion experience and will work with you to troubleshoot extruder instability. Whether it’s recommending a custom screw, repairing worn components, or revising your process parameters.
FAQs
Q1: What is the most common reason an extruder starts surging after running smoothly for months?
Most likely, it’s your screw and barrel. Gradual wear and tear over time causes your flight clearance to exceed ~1% of the screw diameter, leading to melt-backflow and surging. A partially plugged screen pack is the second most common reason for sudden surging. Third on our list is thermocouple drift; if a zone is running 20–30°C off temperature, it won’t throw any alarms but will definitely cause melt inconsistency.
Q2: Can increasing or decreasing screw speed fix my surging problems? Or will it make them worse?
Either, depending on the cause. If it’s a feeding issue, reducing screw speed by 5–10% can help build consistent head pressure. If it’s your screw and barrel wearing out, changing screw speed won’t fix the problem of excess flight clearance. If it’s a temperature issue, increasing screw speed adds shear heat and could exacerbate the issue. Increasing/decreasing screw speed can help you narrow down the cause of your problem.
Q3: Moisture causes surging, How much moisture is too much moisture?
Yes! Moisture creates steam pockets in the barrel which cause surging. It takes very little moisture to start causing issues—PA 0.2%, ABS 0.1%, PC 0.02%, and PET 0.005%. Always run a dehumidifying dryer and double-check with a moisture analyzer. Dry your regrind separately from virgin material as well since it has a higher surface area and absorbs moisture more quickly.
