Surging is extruder output that rises and falls in a cycle. It shows as thickness or dimensions that pulse along the length of the product, with head pressure and motor load moving in step. Time the cycle before you change anything, because the length of the cycle points to the cause. Most surging starts in the feed section of the screw. It is usually not the resin.
Also described as: output pulsing, thickness variation, gauge bands, unstable pressure.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Head pressure and thickness pulse quickly, in step with each turn of the screw | The screw flight passing the feed opening, or a worn or damaged screw. It shows most when head pressure is low. | Count the pulses against screw speed |
| An irregular swing over several seconds to a minute or so | Unsteady feeding or melting: pellets bridging at the feed throat, or the solid bed breaking up in the screw. | Feed throat cooling; hopper flow; rear barrel zone temperatures |
| A slow, regular swing over several minutes | Barrel or die heaters cycling on and off, or the hopper loader dumping cold resin in batches. | Actual zone temperatures and loader timing against the pressure trace |
| Output falls steadily over hours while pressure ahead of the screens climbs | The screen pack is loading up. | Pressure before and after the screen pack |
| Thickness varies but head pressure and motor load are steady | Not the extruder. Haul-off speed, a slipping puller or nip, winder tension or cooling. | Haul-off speed reading; belts and nip rolls for slip |
| Surging started when regrind or fluff was added, or the ratio changed | Bulk density of the feed is changing. The feed section delivers by volume, so lighter feed means less output. | Regrind ratio, particle size and how well it is blended |
| Output per screw turn has dropped over months, and surging gets worse at high head pressure | Worn screw flights and barrel. Melt leaks back over the flights. | Output at a set screw speed against old run records; flight clearance |
Checks in order, cheapest first
- Time the cycle. Watch head pressure and motor load and time the swing. Compare it with screw speed, with heater on and off cycles and with the hopper loader. A match tells you where to look.
- Check that the haul-off is steady. If head pressure and motor load are steady and the gauge still moves, the cause is downstream. Check puller or nip speed, belt and roll slip and winder tension before you touch the extruder.
- Check the feed throat and hopper. The throat cooling water must be on and flowing. A warm throat softens pellets so they stick and bridge. Keep the hopper level steady and the magnet and grate clear. Stop the screw and lock it out before you clear a bridge, and never put a hand or a tool into the feed throat of a running machine.
- Check barrel temperatures, zone by zone. Compare actual with set point. Look for a failed heater, a loose thermocouple or a zone that overshoots and cycles. Then change the rear zone temperature in small steps and watch the pressure trace. Feeding depends on the pellets gripping the barrel, and that changes with barrel temperature.
- Measure the melt temperature. Compare it with the supplier’s range for the grade. A melt that swings in temperature swings in viscosity, and output follows.
- Change the screen pack. A loaded pack raises back pressure and cuts output slowly. A torn pack lets pressure fall suddenly. Note the pressure with a clean pack so you know when the next one is due.
- Take regrind out, then compare lots. Run a short trial on virgin pellets only. If the surge stops, work on regrind particle size and a steady ratio. If the problem arrived with a new lot, compare melt index on the COAs and run a retained sample of the good lot.
- Measure the screw and barrel, then consider the resin. Wear is the expensive check, so confirm the cheap ones first. If the machine is in tolerance, settings are right and a known good lot runs steady while the current one does not, the resin is the next place to look.
The numbers
Output at a given screw speed depends on melt viscosity, so a shift in melt index between lots or blend components shows up as a shift in head pressure and gauge. Bulk density of the feed matters as much, and it is not on a COA.
| Resin | Melt index (g/10 min) | Density (g/cm³) | Melt range on our processing chart |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | Not listed |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | 320–450°F / 160–232°C (LDPE) |
| HMW-HDPE film, pipe and sheet grades | Very low, often <0.1 at 190°C/2.16 kg. High-load melt index ~5–15 at 190°C/21.6 kg for film grades | ~0.945–0.960 | 380–525°F / 193–275°C (HDPE) |
| Polypropylene homopolymer | See the data sheet | See the data sheet | 374–525°F / 190–274°C |
| Polypropylene copolymer | See the data sheet | See the data sheet | 392–500°F / 200–260°C |
| Rigid PVC | Not specified by melt index | See the data sheet | 380–410°F / 193–210°C |
Typical published ranges from our LLDPE, LDPE film and HMW-HDPE pages. Melt index is measured at 190°C/2.16 kg unless stated. The melt ranges come from the processing chart, which is written for injection molding; use them only as an outer check and set an extrusion line from the data sheet for your grade. The density in the table is the density of the solid resin. Bulk density, the weight of loose pellets or regrind that fills a given volume, is much lower and is what the feed section responds to. Measure it yourself by weighing a container of known volume filled with the feed.
Why output swings
- The feed section meters by volume. A single-screw extruder moves solid pellets forward by friction. The pellets have to grip the barrel and slide on the screw. Anything that changes that friction or the weight of feed in each flight changes output: pellet shape, regrind, surface additives, barrel temperature, a warm feed throat.
- Melting can be unsteady. The packed bed of solids in the screw can break up before it has melted. Pieces of solid then move forward in bursts, and pressure and melt temperature swing with them.
- Wear lets melt leak backward. As the gap between flight and barrel opens up, more melt flows back over the flights. The leak grows with head pressure, so a worn screw loses output and becomes more sensitive to every other disturbance.
- Viscosity follows temperature. A zone that cycles a few degrees changes melt viscosity, and output and die pressure move with it.
- Thickness is output divided by line speed. A steady extruder with an unsteady haul-off makes the same defect as a surging extruder with a steady haul-off. Head pressure tells the two apart.
Material and equipment changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| Feed in a more uniform form: pellets in place of fluff, or regrind cut to a size close to the pellets | Steadier bulk density, so each screw flight carries the same weight. | Repelletizing adds a heat history and a cost. Finer screens on the granulator slow it down and make more dust. |
| A gravimetric blender or weight-based control of the extruder | It holds the blend ratio and can adjust screw or line speed to keep the weight per length constant. | Equipment cost and setup. It corrects slow drift, not a fast surge. |
| A melt pump between the extruder and the die | It meters the melt at a fixed volume, so the die sees steady flow even when the extruder does not. | Capital cost, more line length, more time for the melt at temperature, and another item to maintain. Heat-sensitive resins such as PVC need care. |
| Rebuild or replace a worn screw and barrel | Restores output per turn and the stability the line had when new. | Downtime and capital cost. Settings have to be found again afterward. |
| A melt index range on the purchase specification, and one lot per run where possible | Lot-to-lot viscosity shifts are a common cause of gauge that moves at a lot change. | Nothing in the product. It takes an incoming check, and it may narrow your choice of sources. |
| A grade with a different slip or lubricant level | Additives on the pellet surface change how the pellets grip the barrel. On some machines a high-slip grade feeds less steadily. | Slip, blocking and friction of the finished film or part change, and the customer has to approve the grade. |
Any resin change moves the process. A different melt index or density changes head pressure, output per screw turn, draw-down and finished dimensions. Set the line again and check the product before you approve a swap.
Often confused with
- Bubble instability. In blown film, a bubble that breathes, sways or spirals changes gauge with a steady extruder. Check head pressure: if it is steady, look at the air ring, the cooling and the melt strength. See bubble instability.
- Gauge bands across the web. A thick or thin lane that stays at one position across the width is a die gap, die temperature or cooling problem. Surging varies along the length.
- Stick-slip melt fracture. Alternating glossy and rough bands with a swinging head pressure start at a set output rate and come from the die, not the feed. See melt fracture and sharkskin.
- Draw resonance. In cast film and extrusion coating at high draw-down, thickness can pulse regularly while the extruder is perfectly steady. It depends on draw ratio and cooling, and it goes away when line speed is reduced.
Quick answers
Why is my extruder surging?
Most surging starts in the feed section of the screw, and it is usually not the resin. Time the cycle first, since its length points to the cause. A quick pulse in step with the screw points to the flight or a worn screw. An irregular swing points to feeding. A slow, regular swing points to heaters or the loader.
What causes gauge variation in extrusion?
Thickness is output divided by line speed, so gauge varies when either one is unsteady. If head pressure and motor load pulse with the gauge, the extruder is surging. If they are steady and the gauge still moves, the cause is downstream: haul-off speed, a slipping puller or nip, winder tension or cooling.
Why does my film thickness keep changing?
Film thickness that pulses along the length usually follows extruder output, with head pressure and motor load moving in step. Check head pressure first. If it is steady, look at the haul-off, or in blown film at the air ring and cooling. A thick or thin lane at one position across the width is a die or cooling problem.
Can regrind cause extruder surging?
Regrind can cause surging because it changes the bulk density of the feed. The feed section delivers by volume, so lighter feed means less output. Run a short trial on virgin pellets only. If the surge stops, work on regrind particle size and a steady ratio.
How do I clear a bridge at the feed throat safely?
Stop the screw and lock it out before clearing a bridge, and do not put a hand or a tool into the feed throat of a running machine. Bridging usually comes from a warm throat, which softens pellets so they stick. The throat cooling water must be on and flowing, and the hopper level kept steady.
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