Plate-Out in PVC Extrusion: Causes, Die Build-Up and the Plasticizer's Role

Plate-out in PVC extrusion is the name processors give the material that leaves a compound and settles on metal: die lips, calibrators, cooling rolls, screw flights, screen packs. Die build-up is the same event seen at the die. The line runs clean for a few hours, then a ring appears at the lip, the surface picks up die lines or matte streaks, and someone reaches for a brass scraper.

Short answer: the deposit is almost always a mixture, and the plasticizer is one candidate among several. Identify what the deposit is made of before changing the formula. Then check six inputs in a fixed order: resin fusion, filler, stabilizer and lubricant, pigment, plasticizer, and the die and process. Time-to-failure, not opinion, decides whether a change worked.

This article is written from the additive side. Shandong Changxing supplies DOTP (CAS 6422-86-2) to cable compound, film and sheet, profile and synthetic leather plants. The process points below are general extrusion practice; the plasticizer rows are ours, and we say plainly where a plasticizer change cannot fix plate-out.

What plate-out and die build-up are

Plate-out is a deposit on equipment. Die build-up is that deposit at the die. In polyolefin extrusion the same behaviour is often called die drool; in PVC the older plant word is plate-out, and it covers every metal surface the melt touches.

Where it lands depends on the product. A film and sheet line usually shows it at the die lip and on the first cooling roll. A cable line finds it at the die and inside the calibrator. Profiles pick it up at the die and the sizing sleeve. Calendered synthetic leather can carry it on the rolls, where it shows as haze or a repeating mark.

The symptoms on the product are always the same: die lines, matte patches, loss of gloss, a rough hand, weld-line marks that were not there before, or a sudden change in dimensional pull. None of these tells you the cause. The deposit itself does.

How to read a plate-out deposit before you change the formula

The deposit narrows the list of possible inputs, so read it before you touch the formula. Scrape a sample with a clean brass tool onto aluminium foil, note the position, the running hours and the die temperature at the time, then look at it.

Appearance narrows the field of plate-out sources; it does not close it. FTIR identifies the organic part of the deposit. EDS or XRF gives the element pattern: calcium points to filler or calcium stearate, titanium to pigment, and zinc to a Ca/Zn stabilizer. TGA tells you how much of the sample is volatile. A plant that sends one deposit sample for this analysis usually saves more time than it spends.

One caution on sampling. A scraper blade is also a source of metal. Use brass or a plastic tool on the die lip, and never scrape a chrome-plated surface with steel.

Six inputs that feed plate-out

Most plate-out investigations end with two contributors, not one. Work through the list in this order and you start to see how they interact.

The table is not a ranking of plate-out risk. In a plant that has just changed filler supplier, filler is the first suspect. In a plant that has run the same formula for a year and changed nothing, the die temperature or a worn screw is more likely than the recipe.

Why the deposit lands on the die

Molten PVC is viscoelastic, and it flows through the die in layers. The layer against the wall moves slowly; the core moves fast. Low-surface-energy species such as lubricants, stearates and plasticizer drift toward the slow layer under shear. That migration is normal. Plate-out begins when the wall can no longer hold them in solution.

Two conditions push the wall past that point. The first is temperature: die lips and adapters often run cooler than the melt, and solubility falls as the metal cools. The second is concentration: an over-loaded or under-fused compound carries more free plasticizer than the resin can keep. Volatiles condense on the same cool metal, and degradation products react with metal soaps to form chlorides that stick and darken.

This is why plate-out takes hours to show. The first layers are thin enough that they do not disturb the flow. Once the deposit reaches a critical thickness, it alters the flow boundary, and the defect appears all at once. It is also why cleaning alone brings temporary relief: the surface is clean, but the conditions that caused the deposit are unchanged.

A seven-step field check order for plate-out

The order below moves from cheap evidence to expensive changes, one variable at a time, and it is written for a line that is running. The first three steps need only a scraper, a sample bag and a contact probe.

1. Define the failure in numbers

Write down the defect, its position, the running hours before it appears, and whether it clears after purging. "Plate-out" is a category; "a matte band at the die lip after four hours, cleared by purging, returns at the same position" is a case you can test.

2. Take the deposit sample

Sample the die lip and, separately, the calibrator or first roll. If the two deposits differ, you are probably looking at two mechanisms, not one.

3. Verify the temperature profile

Measure the die lip, adapter and melt with a contact probe. Controller readings and real metal temperature are not always the same, especially on an older die with a failing cartridge. A die running below the rest of the line is a classic build-up site.

4. Check residence time and output

Running well below rated output lengthens residence time; running above it raises shear. Both change how much low-molecular material reaches the wall, and so how fast plate-out builds. Compare the current rate with the rate at the last clean run.

5. Check incoming lots against the COA

Filler coating, stabilizer lot, pigment lot and plasticizer lot. A supplier change that looks minor on paper can move the deposit. Keep the certificates next to the trial record.

6. Bisect the formula, one variable at a time

On a lab compounder or a small line, reduce or remove one component per run and stop when the deposit disappears. Start with external lubricant, then filler coating, then pigment, then plasticizer lot. Changing two variables at once gives you a result you cannot explain.

7. Clean, re-baseline, and record time-to-failure

Clean the die properly, photograph the lip, and run the baseline again. Record the hours to the same defect. That number is the only fair comparison for the next change.

What the plasticizer does, and what it does not

Plasticizer is a low-molecular, low-surface-energy component. It can migrate to the wall like any other plate-out source, and the risk rises when the compound cannot hold it. Three things decide whether your plasticizer is part of the problem.

Solvating power and fusion

DOTP is a slightly less solvating plasticizer than DOP for PVC. Most compounds need a few phr more, or a little more heat and work, to reach the same hardness. Run a DOTP compound on the old DOP temperature profile and the result can be under-fused: plasticizer sits free in the melt instead of being absorbed, and free plasticizer is exactly what migrates. The compounding side is covered in the wire and cable application guide and the PVC film guide.

Purity rows that matter here

Three delivery rows are worth reading when plate-out is the complaint.

• Acid value at ≤0.035 mg KOH/g limits acidic species that can attack the stabilizer system and the metal.
• Moisture at ≤0.1% limits flashing at the die.
• Ester content at ≥98% limits the residual alcohol and monoester that volatilise and condense on cool metal.

Our DOTP is specified on all three, and the full row set sits in the technical properties guide.

Loading and absorption

More plasticizer is not more safety margin. Past the point the compound can hold at that temperature, the excess behaves like a free liquid. Set loading from the hardness and cold-flex target, then confirm that the dry blend absorbs it before the compound reaches the die.

The limits matter as much. A new plasticizer will not fix a die that runs cold, a filler carrying excess stearate, or an over-lubricated formula. If the deposit is white and chalky, swapping the lot changes nothing. Change the plasticizer only when the plate-out evidence points there.

Choosing a DOTP grade when appearance matters

When plate-out puts the plasticizer under review, grade selection comes up at the same time. Colour ceiling is the row this decision turns on: 150 APHA, 230 APHA or 450 APHA.

Grade is a colour-ceiling decision, not a plate-out rating. A lower colour ceiling does not make the plasticizer less likely to migrate. A dark compound on qualified grade and a pale one on superior grade can plate out the same way if the die is cold or the formula is over-lubricated.

Superior

150 APHA ceiling

First-grade

230 APHA ceiling

Qualified

450 APHA ceiling

How to run a trial that proves the change

A plate-out trial only answers the question if everything except the variable stays the same. Use the same resin lot, filler lot, stabilizer package and die. Change one thing: the plasticizer lot, the loading, or the lubricant level.

Run until the same defect appears, or until the baseline time-to-failure has passed with no deposit. Photograph the die lip at fixed intervals under the same light. Weigh nothing unless you have a repeatable method; a visual rating and an hours count are more useful than a number nobody trusts.

Repeat the run. A single result can come from a die that was cleaned differently or a room that was warmer. The second run is what tells you whether the first one meant anything.

Frequently asked questions