PVC Profile Extrusion: Rigid, Flexible and Cellular PVC

1. Why PVC needs its own line

PVC is the largest volume material in profile extrusion, and it is also the least forgiving. Two properties make it a specialist material rather than just another thermoplastic.

It degrades instead of melting cleanly. Above roughly 200 °C, PVC starts eliminating hydrogen chloride. The polymer chain develops conjugated double bonds, which is why degraded PVC turns yellow, then brown, then black. The released HCl is corrosive, and once degradation starts it accelerates — the HCl catalyses further breakdown. A PVC line that stops with material in the barrel for twenty minutes can produce a barrel full of black char.

It needs a formulation, not a grade. You rarely buy "PVC" as a ready compound and run it. A PVC profile compound is resin plus stabiliser plus internal lubricant plus external lubricant plus filler plus impact modifier plus pigment, and the balance of those six decides whether the profile processes well, welds well, survives ten years of sunlight, and passes impact at −10 °C. Change one and the others usually need adjusting.

2. Three PVC families: rigid, flexible, cellular

The distinction that matters commercially is that uPVC and fPVC are the same polymer family — which is why they co-extrude together so well, and why a rigid profile with a flexible seal is the single most common co-extruded PVC product. Cellular PVC behaves differently again: the density reduction changes the cooling behaviour, the calibration requirement and the achievable surface finish.

3. Formulation: the six ingredients that decide performance

This is the part of PVC extrusion that buyers rarely see and that explains most of the price difference between two apparently identical quotes.

Loadings are indicative for a white uPVC window profile. phr = parts per hundred resin. A compound for an indoor conduit is formulated very differently from one for an exterior window profile, which is why the two are not interchangeable.

The component buyers most often get wrong is the impact modifier. A profile without adequate CPE or acrylic modifier looks fine on the line and passes a dimensional inspection. It fails when a window frame is struck at −5 °C, or when a delivery driver drops a bundle. If the part will be installed and handled outdoors in winter, the impact requirement has to be specified and tested, not assumed.

4. Lead versus calcium-zinc: the regulatory shift

The stabiliser system is the single biggest formulation decision, and it is now a compliance question rather than a technical one.

• Lead-based stabilisers — tribasic lead sulphate, lead stearate. Historically the workhorse for rigid PVC: cheap, extremely effective, forgiving of process variation. Being phased out across Europe and increasingly restricted elsewhere because of lead toxicity. In the EU, the PVC industry committed to phasing out lead stabilisers, and most European window profiles are now calcium-zinc.
• Calcium-zinc (Ca/Zn) — the lead-free alternative for most profiles. Food-contact capable, increasingly the default for window profiles, trim and any product sold into Europe. The trade-off is a narrower processing window: Ca/Zn stabilisers are less forgiving of overheating and of long residence times, so the line has to be controlled more tightly.
• Organotin — mainly for pipe and fittings where clarity and potable-water compliance matter. Effective, but restricted in some applications and under pressure in others.

If the product is exported to Europe, or touches drinking water or food, confirm the stabiliser system before the die is cut. Changing from lead to Ca/Zn is not a drop-in substitution: the lubricant package usually needs rebalancing, and the process window shifts.

5. Die and calibration — why the sleeve matters more

In PVC profile extrusion the die produces a shape that is deliberately not the final shape. The extrudate leaves the die oversized and slightly distorted, then enters a vacuum calibration sleeve where it is drawn down to final size while water cools it. Two things follow from this:

• The sleeve sets the dimension. Die wear changes the wall thickness slightly, but the calibration sleeve sets the outside profile. This is why a worn sleeve causes out-of-tolerance parts even when the die is fine, and why sleeve replacement is a routine maintenance item on high-volume PVC lines.
• The drawdown is part of the design. The die is cut larger than the finished section by a drawdown allowance — typically 8 to 20 % depending on the section — and the allowance has to be right for the compound. Change the formulation and the drawdown changes.

PVC dies also need chrome plating or hardened steel in the land area, because HCl attack and the abrasive filler both wear the flow surfaces. A die that is fine for PP will corrode on PVC.

For hollow multi-chamber profiles — window frames being the obvious case — the calibration has an extra job: it must hold the internal chambers open while the outer wall is being sized. That requires internal air pressure balanced against the vacuum, and it is the part of the tool that most often needs rework on a new profile.

6. The processing window, and what happens outside it

A typical rigid PVC profile runs with a melt temperature around 175 to 195 °C, with the die head slightly hotter than the barrel. That is a narrow band. Below it the compound does not fuse properly and the profile has poor weld strength and a dull surface. Above it, degradation begins.

The signs, in order of appearance:

1. Dull or slightly yellow surface — the first sign of thermal stress. Easy to miss under factory lighting.
2. Yellow-brown streaks, usually along a flow line or at a die land where material is hanging.
3. HCl smell — sharp and unmistakable. By the time you can smell it, degradation is established.
4. Black specks and char — advanced. The barrel needs cleaning, and the compound may need reformulating.

Residence time matters as much as temperature. A small profile running in a large barrel means the compound sits hot for longer, and the same temperature that is safe at a high output will degrade the material at a low one. This is why a supplier will sometimes decline to run a very small section on a large line.

7. Problems specific to PVC profiles

General extrusion defects are covered in our separate troubleshooting guide. These are the ones that are particular to PVC, or that behave differently in PVC.

Die build-up (plate-out)

A white or yellowish deposit accumulating on the die lip and in the calibration sleeve, which then produces drag lines and rough surface. It comes from additives migrating to the surface — usually an imbalance between internal and external lubricants, or too much filler. The fix is a formulation adjustment, not more cleaning. Cleaning alone means the problem returns in a few hours.

Poor welded-corner strength

Window profiles are welded at the corners, and the weld strength depends on how well the compound fused during extrusion. Under-fused PVC — from a melt temperature that is too low or too much external lubricant — looks acceptable but welds badly. This is why window profile specifications include a weld strength test, and why it is tested on the finished welded frame rather than on the profile.

Cold-weather impact failure

A profile that survives a hammer test in summer can shatter at 0 °C. PVC's brittle-to-ductile transition sits close to normal outdoor winter temperatures, and the impact modifier is what moves it. Specify the impact test temperature, not just the impact value.

Colour shift and chalking outdoors

White PVC exposed to UV without adequate TiO₂ and a suitable stabiliser will yellow, then chalk. The chalking is surface degradation of the polymer, and it is not reversible by cleaning. For any exterior application, the TiO₂ loading and the stabiliser system have to be specified for the exposure, not inherited from an indoor compound.

Plasticiser migration in flexible PVC

Flexible PVC loses plasticiser over time, especially at elevated temperature or in contact with other plastics, and the part hardens and shrinks. In a co-extruded seal this shows up as a lip that becomes stiff and starts to pull away. If the soft component has a long service life at temperature, specify a polymeric plasticiser rather than a monomeric one.

Vacuum marks and sink in the calibration zone

Dark or dull patches where the profile was pulled against the sleeve too hard, or where the wall was too hot to resist the vacuum. Usually a cooling or vacuum-setting problem rather than a material one — but a wall that is too thin for the section will do it regardless of settings.

8. Applications and what each one demands

9. What to specify in an RFQ

1. Cross-section drawing with wall thickness and all functional dimensions.
2. Family and formulation intent — uPVC, fPVC or cellular; and whether the stabiliser must be lead-free.
3. Colour to a reference — RAL, a physical sample, or a measured value. "White" is not a specification.
4. Service environment — indoor or outdoor, temperature range, UV exposure, contact with chemicals or drinking water.
5. Tests the part must pass — impact at a stated temperature, weld strength, Vicat softening, flame rating.
6. Length, packing and handling — straight lengths or coils, maximum bundle size, whether the profile can be nested.
7. Annual volume in metres, so tooling amortisation and line speed can be quoted realistically.

10. Frequently asked questions