A die line is a continuous line in the machine direction that stays at the same position. It comes from something fixed at the die exit: a nick in the lip, or a particle or deposit lodged there. Die build-up is material that collects on the lip face, grows, and breaks off, leaving intermittent marks, specks or short scratches. Start by noting where the mark is and whether it is continuous, because that tells you which of the two you have.
Also described as: lines in the machine direction, streaks from the die, die drool, plate-out.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A sharp, continuous line at one fixed position. It was there from startup and is still there after the lip is cleaned. | A nick, scratch or dent in the die lip. | The lip at that position, under a light and a magnifier |
| A continuous line that appeared during the run at one position and goes away when the lip is wiped | A piece of degraded resin or a deposit lodged at the lip or in the die land. | Clean the lip at that position; look for a place upstream where resin hangs up |
| A broad, soft streak or band, not a sharp line, at fixed positions | Not the lip. Usually a weld line from a spider leg or die port, or a stream of overheated resin from inside the die. | Whether the streaks line up with the spider legs or ports; melt temperature |
| A deposit you can see growing on the lip face. Pieces break off and ride on the product as specks, lumps or short scratches. | Die build-up. | What the deposit looks like (waxy and pale, colored, or dark and hard) and how long it takes to come back after cleaning |
| Build-up is worse with a filled compound, or at a high loading of color or additive concentrate | Filler or pigment that is not fully wetted by the resin, a carrier resin that does not match, or more additive than the resin will hold. | Concentrate loading and carrier resin; a short run without the concentrate |
| Build-up is worse on humid days or with resin just brought in from the cold, sometimes with popping or steam at the die | Moisture on the pellets or in the filler. | Storage and handling; drying of filled grades and concentrates |
| Build-up rose when regrind went up or temperatures were raised | Degraded, low molecular weight material is separating from the melt at the die exit. | Regrind percentage; measured melt temperature; time the melt spends hot |
Checks in order, cheapest first
- Mark the position and note the pattern. Mark the die where the line sits. A line that never moves and never breaks is a die line. Marks that come and go, or a deposit that grows back after cleaning, are build-up.
- Clean the lip at that position. Use a brass or copper scraper or shim, never steel, which is how most nicks are made. The die is hot and the melt can spit, so wear gloves and a face shield. If the line goes and stays away, it was a lodged particle. If it never changes, inspect the lip for damage.
- Check melt and die temperatures against the supplier’s range. Measure the melt. Too hot, and resin and additives break down and come out at the lip. Too cold, and stress at the die exit rises, which also drives build-up. With PVC, overheating releases hydrogen chloride fumes, so ventilate and do not leave it standing hot.
- Check for moisture. Polyethylene and polypropylene do not absorb water, but pellets, fillers and concentrates carry surface moisture, and filled grades are moisture-sensitive. Let cold resin warm up in closed containers before opening it.
- Check concentrates and additives one at a time. Confirm the let-down ratio. Then run a short trial without the color, the filler concentrate or the additive in turn. If build-up stops, you have found the source, and it is not the base resin.
- Check regrind, then compare lots. Run a short trial on virgin resin only. If build-up arrived with a new lot, run a retained sample of the good lot on the same line and compare the COAs.
- Review purging and shutdown, then pull and clean the die. Resin left hot in the die with air present oxidizes on the metal and comes off later as lines and specks. Have nicked or worn lips repaired by a tool shop. This is the expensive check, so confirm the cheap ones first.
- Change the resin last. A die line from a nick is never the resin. Build-up sometimes is. If the die is clean, the temperatures are right, and a known good lot runs clean while the current one does not, look at the resin and its additive package.
The numbers
Two figures matter most for build-up: how hot the melt is, and whether moisture is being carried in. These are the published ranges for the resins most often extruded.
| Resin | Melt range on our processing chart | Drying, from our drying chart | Chart note |
|---|---|---|---|
| LDPE | 320–450°F / 160–232°C | Not required | Warm-air surface-moisture removal only if condensation is present. |
| HDPE | 380–525°F / 193–275°C | Not required | As above. |
| LLDPE | Not listed | Not required | As above. |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 167°F / 75°C for 2 h when required, for cosmetics only | Filled and reinforced PP grades are moisture-sensitive. Purge if idle >30 min. |
| Polypropylene copolymer | 392–500°F / 200–260°C | As for homopolymer | Filled and reinforced PP grades are moisture-sensitive. |
| Rigid PVC | 380–410°F / 193–210°C | 120°F / 49°C for 2–3 h, optional | Maximum residence 15 min. Degrades to hydrogen chloride gas. |
Typical published values from the processing chart and the drying chart. The melt ranges are written for injection molding; use them only as an outer check and set an extrusion line from the data sheet for your grade. Filled compounds and concentrates have their own drying instructions, so follow the supplier’s sheet for those.
Where die build-up comes from
- Low molecular weight material. Every resin contains some short chains and waxes, and degraded resin and repeated regrind add more. They move to the surface of the melt and deposit where it leaves the die.
- Additives. Slip, antistatic agents, stabilizers, lubricants and pigments can come out of the melt at the lip when the loading is high or the additive does not suit the resin.
- Fillers and moisture. Mineral fillers that are poorly dispersed or carry moisture collect at the exit. Steam at the die carries material onto the lip face.
- Stress at the die exit. The melt swells as it leaves the die and wipes against the lip face. Higher output and a colder melt raise the stress, and build-up usually gets worse with both.
- Heat and air finish the job. A fresh deposit is often pale and waxy. Sitting on a hot lip in air, it oxidizes, darkens and hardens until a piece breaks off.
Why a small mark at the lip becomes a line
The die lip is the last metal the melt touches. Anything that disturbs the flow there, a nick, a burr or a stuck particle, is drawn out into a line for as long as it stays. A mark made further upstream has time to heal, so it shows as a soft streak. This is why a sharp line points to the lip and a broad streak points to the inside of the die.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A polymer processing aid, in the resin or as a concentrate | It coats the die metal, lowers the stress at the exit and makes it harder for deposits to stick. | Added cost and a conditioning period. Some additives interfere with it. Many are fluoropolymer-based, and some customers and regulations now restrict fluorinated additives. Check food-contact status. |
| A different color or additive concentrate | A carrier resin close to the base resin, better dispersion or a lower loading leaves less to come out at the lip. | Color, slip, blocking and any other additive effect have to be checked and approved again. |
| A filled compound made with a surface-treated or drier filler, or a lower filler loading | Treated filler is wetted better by the resin and carries less moisture. | A higher price for the treated grade. Lower filler loading means lower stiffness and a higher resin cost per part. |
| A grade with a stronger stabilizer package | Less breakdown of the resin at processing temperature, so less degraded material at the lip. | A new grade to approve. Additive changes can affect food-contact status, sealing and printing. |
| Less regrind or recycled content | Fewer heat histories and less low molecular weight material going in. | More scrap to dispose of or sell, and a lower recycled-content figure if the customer requires one. See using regrind. |
| Prime resin in place of wide-spec or off-grade | Off-grade lots can differ in additive level and in low molecular weight content. See prime vs. off-grade. | A higher price. |
Any resin or additive change moves product properties. Surface, sealing, printing and dimensions can all move with it. Run the new material through the customer’s next step before you approve a swap.
Often confused with
- Melt fracture. A rough or matte surface over the whole width, with ridges running across the flow, is a stress problem at the die and not a deposit. See melt fracture and sharkskin.
- Scratches made after the die. A line that is not there in the melt just past the lip was made downstream, by a sizing sleeve, calibrator, roll or collapsing frame. Follow the line back until it stops.
- Gels. Gels are lumps inside the film that were in the melt before the die. Build-up marks sit on the surface and come from the lip. See gels and fisheyes in film.
- Black specks. Dark specks come from degraded resin anywhere in the extruder, and from build-up that has broken off the lip. See black specks and contamination.
Quick answers
What causes die lines in extrusion?
A die line comes from something fixed at the die exit: a nick in the lip, or a particle or deposit lodged there. It is a continuous line in the machine direction that stays at one position. If the line goes away when the lip is cleaned, it was a lodged particle. If not, inspect the lip for damage.
How do I stop die build-up?
Start with melt and die temperatures against the supplier’s range, because a melt that is too hot or too cold both drive build-up. Then check for moisture, and run short trials without the color, filler concentrate or additive in turn. With PVC, overheating releases hydrogen chloride fumes, so ventilate and do not leave it standing hot.
Why are there streaks along my extruded sheet or film?
A sharp, continuous line at one fixed position points to the die lip: a nick or a lodged particle. A broad, soft streak points to the inside of the die, usually a weld line from a spider leg or die port, or a stream of overheated resin. Intermittent marks, specks or short scratches come from build-up breaking off the lip.
Are die lines and die build-up caused by the resin?
A die line from a nick is not the resin. Build-up sometimes is. If the die is clean, temperatures are right, and a known good lot runs clean while the current one does not, look at the resin and its additive package. If build-up stops when a concentrate or additive is taken out, that is the source.
How do I clean a die lip without damaging it?
Use a brass or copper scraper or shim to clean the lip. Do not use steel, which is how most nicks are made. The die is hot and the melt can spit, so wear gloves and a face shield. Have nicked or worn lips repaired by a tool shop.
Send us a photo
A polymer chemist will look at it and reply by email. There is no charge and no obligation.