The white deposit that rebuilds on your die within hours of every clean-off is not made of the compound running through the machine.
That finding, from a Loughborough University analysis of eight deposits taken off commercial rigid-PVC lines, upends the usual troubleshooting sequence. The lubricant is the vehicle, not the load, which is why cutting calcium stearate sometimes clears the die and sometimes changes nothing.
Reading the deposit before touching the recipe narrows the search to one variable family. Where the deposit sits and what it is made of tell you which family to open first.
What Plate-Out Deposits Contain
Plate-out deposits from PVC extrusion lines consist of pigment (titanium dioxide), filler (calcium carbonate) and metal-soap stabilizer residue, transported to the tooling by a molten wax and stearate carrier phase. PVC itself is not generally detectable. Outside PVC, the same die build-up is called die drool or die bleed.
A white film on the finished part is surface blooming, not plate-out. Plate-out sits on the tooling and rebuilds within hours of cleaning. Degraded polymer carries PVC and discolors; plate-out contains neither.
A deposit on the die and a deposit in the calibrator are two different failures. Die plate-out is inorganic-led: pigment, filler and stabilizer residue carried out of the melt by a molten wax and stearate phase. Calibrator plate-out is mainly organic, produced by volatilization and condensation of lighter fractions onto the cooler tool surface.
DSC separated the organic fraction by melting point: fatty-acid mixture at 40-55 °C, hydrocarbon wax at 80-85 °C, zinc stearate at 125 °C, calcium stearate at 140 °C. If the deposit is at the calibrator, check the volatile end of the recipe first. If it sits on the die, the inorganic load is the starting point.
Which PVC Additives Produce the Deposit
Above 175 °C, molten hydrocarbon dissolves stabilizer and calcium stearate into a complex that coats polar inorganic particles. In decompression zones, that conglomerate precipitates as temperature falls back below 175 °C, deposits on metal surfaces, loses its hydrocarbon, and becomes the site for the next layer.
| Cause family | Deposit signature | First check |
|---|---|---|
| External lubricant (Ca stearate, PE wax) | Carrier phase, small by mass | Internal/external ratio; wax type |
| Metal-soap stabilizer (Ca-Zn residue) | Strong Ca and Ti, Zn only as a trace | Solid vs liquid stabilizer system |
| Filler and pigment | CaCO3 and TiO2 in every sample | TiO2 loading (increases deposit); CaCO3 reduces it |
| Resin grade | Same package, different result | Porosity and K-value |
In a Ca/Zn die deposit from the same study, SEM-EDX showed strong calcium and titanium with zinc only as a trace. The stabilizer’s metal contributes to the deposit, but at far lower concentration than the filler and pigment around it.
Increasing calcium carbonate loading was observed to reduce plate-out through abrasive scouring of the extruder walls. Increasing titanium dioxide increased it. Fine-particle silica works as an anti-plate-out additive on the same abrasion logic.
PE waxes and metal stearates form alloys in the melt. Swapping wax type at the same phr is a real formulation change, not a like-for-like substitution.
How much lubricant reaches the metal depends partly on the resin. A K-66 pipe grade has about 0.25 cc/g porosity; a K-70 soft-compound grade about 0.41 cc/g.
The lower-porosity particle absorbs less lubricant internally, so the same package can run clean on one suspension PVC grade and plate out on another. Before cutting lubricant, check the deposit against your resin grade’s porosity spec.
Process Conditions That Trigger Plate-Out
Melt temperature is the process lever with the clearest direction: plate-out increases as temperature rises, driven by the viscosity drop that lets additives migrate. The processing window is narrowest between 175 and 195 °C, where primary particles are still present in the melt and the compound has not finished fusing.
Higher extrusion torque increases plate-out. Torque is the shear-history proxy on a running line, and it responds to screw speed, compound viscosity and barrel temperature together.
Residual moisture is a first-order trigger. A 1983 patent trial on unplasticized PVC held the additive package constant across compositions and still saw different plate-out ratings depending on which PVC grade was used. Adding 44.5 ml of water during a six-minute run pushed the best-performing composition from a rating of 3 to 5.
Die geometry concentrates the deposit independently of the recipe. A longer die land increases exit stress, and the exit surface runs cooler than the die body, so a surface thermocouple there gives a more honest reading than a standard die thermocouple upstream.
Metal traces from the extruder barrel appeared in only two of eight industrial deposits examined. Check the recipe and the thermal profile before ordering a barrel inspection.
What to Check Before Cutting Lubricant
The usual reflex is cutting calcium stearate. That throttles the carrier, and the deposit may slow, but the inorganic load stays in the melt.
If you see this defect, start from where the deposit sits and what it looks like. A die deposit points at the additive balance and the thermal profile; a calibrator deposit points at what is volatile in the recipe.
Match the deposit to one of the four cause families, then check the process window around it. Formulation balance, not one ingredient’s phr, decides whether the run stays clean or the deposit rebuilds every shift.