PVC extrusion uses the same station sequence as any other extrusion line, and almost none of the same engineering assumptions. The difference is not the machine layout. It is what the polymer does when the line stops.
PE and PP melt, cool and remelt without much argument. PVC does not melt in the ordinary sense at all, and it will decompose if it sits at process temperature too long. Everything distinctive about a PVC line follows from that single property. What follows works through it, using the published configurations of the PVC pipe extrusion line and the PVC recycling equipment SUHUI builds.
For PE the process temperature sits comfortably below trouble. For PVC it sits next to it.
What Makes PVC Extrusion a Separate Discipline
PVC extrusion is the continuous forming of polyvinyl chloride compound into pipe, profile, conduit or sheet, and it is specified differently from polyolefin extrusion for one reason. PVC decomposes under heat rather than merely flowing, so the line has to control time at temperature as tightly as it controls temperature itself.
That phrase — time at temperature — is the whole subject. A polyolefin line cares about how hot the melt is. A PVC line cares about how hot the melt is and how long any given particle of it has been that hot, including the particles sitting in a corner of the die that are not moving at all.
Two practical consequences follow immediately. The extruder has to convey material positively so that residence time is short and predictable rather than statistical. And the flow path from feed throat to die lip has to be swept clean by the material passing through it, because anything that stagnates will eventually degrade and then leave as a defect.
The Narrow Window Between Processing and Degrading
Rigid PVC is processed in a band that sits close to where it begins to break down. SUHUI publishes the working figure for its recycling equipment as a stable 160 to 200 °C multi-zone window, and the reason the window is stated at all is that overshooting it does not just discolour the material. It starts a chemical decomposition.
The window is narrow, and time inside it is consumed as well as temperature.
When PVC decomposes it releases hydrogen chloride, which is corrosive and which accelerates further decomposition. That is why every PVC formulation carries a heat stabiliser, why SUHUI’s PVC granulating pelletizing line specifies corrosion-resistant extruder components and integrated fume extraction as standard rather than as options, and why in-line stabiliser and lubricant dosing appears in that line’s specification at all.
The stabiliser is not a permanent property of the material. It is a consumable that gets used up by heat history, which produces a rule PE and PP operators never have to think about. A PVC compound has a finite thermal budget, and every minute in a hot barrel spends some of it. Material that has already been through one line has less budget left for the next.
| PVC | HDPE and PP | |
|---|---|---|
| What heat does at the top of the window | Chemical decomposition, releasing hydrogen chloride | Reduced viscosity, then slow oxidative degradation much further up |
| Usable process band | Narrow, and shared with the degradation onset | Wide, with clear separation between melting and trouble |
| Additive dependence | Cannot be processed at all without a heat stabiliser | Processable neat; additives are for the product, not for survival |
| Effect of a stopped line | Material in the barrel keeps ageing and must be purged | Material can usually be restarted after a pause |
| Consequence of a dead spot in the flow path | Degraded material eventually breaks loose as black specks | Slow discolouration, usually cosmetic |
| Corrosion risk to screw, barrel and die | Real, and designed against with bimetallic surfaces | Negligible for clean feedstock |
Why PVC Runs on Twin Screws and Polyolefins Do Not
A single screw conveys by drag. Material moves forward because it sticks to the barrel more than to the screw, so residence time is a distribution rather than a number and some fraction of the melt lingers. For a thermally forgiving polymer that is fine. For PVC the lingering fraction is exactly the problem.
Positive displacement gives a residence time you can predict. Drag flow gives one you can only average.
A conical twin screw conveys by positive displacement. Material is pushed forward in closed chambers formed between the two intermeshing screws, so it cannot slip backwards and it cannot sit still. Residence time becomes short and repeatable, which is precisely the control PVC needs. The taper — wide at the feed end, narrow at the discharge end — also gives a large intake area for low bulk density powder and a small high-pressure section at the die.
SUHUI’s PVC pipe configurations use the SJZ conical twin screw series from SJZ55/110 up to SJZ92/188, with a parallel twin SJP130/28 at the largest sizes, covering Φ16–800 mm. The PVC profile line uses conical or parallel twin screw configurations for the same reason. Meanwhile the HDPE and PPR lines run SJ and HSJ single screws throughout. That split is not a preference. It follows the polymer.
One naming trap catches people reading a PVC quotation for the first time. In SJZ65/132 the second number is not an L/D ratio. It is the large-end diameter of the conical screws, which taper from 65 mm at the discharge end to 132 mm at the feed end. Reading it as an L/D produces an absurd number, which is the quickest way to notice the mistake. The reason PVC ends up on this architecture at all — and why a polyolefin does not — is worked through in our single screw vs twin screw extruder comparison. For PVC specifically the relevant detail is direction of rotation: counter-rotating conical screws impart less shear heat than a co-rotating pair at the same output, which matters on a polymer whose failure mode is thermal rather than mechanical. The twin screw extruder article goes into that geometry.
PVC Arrives as a Dry Blend and Not as Pellets
A polyolefin line is fed finished pellets from a resin supplier. A rigid PVC line is fed a powder recipe that the factory mixes itself, and the mixing step is part of the extrusion line rather than something that happened elsewhere. This changes both the feed hardware and where responsibility for quality sits.
The recipe is mixed on site, which puts formulation quality inside the production line.
The blend is prepared in a high-speed mixer, where impeller friction generates the heat that drives moisture off and lets the additives distribute into the resin. SUHUI’s high speed mixer is offered as a standalone hot mixer or as a hot and cold combination, the cold half existing to pull the batch temperature back down after discharge so the compound does not agglomerate before it reaches the extruder.
A typical rigid PVC recipe carries resin plus heat stabiliser, lubricant, filler, impact modifier and pigment. Each has a job that shows up in the extruder. Stabiliser buys thermal budget. Lubricant controls how the compound slips against hot metal, which is what actually sets the shear heat generated in the screws. Filler changes both cost and melt viscosity. Impact modifier changes the finished product rather than the process.
| Blend component | What it does in the barrel | What goes wrong if it is off |
|---|---|---|
| PVC resin | Provides the polymer, with the K value setting how easily it fuses | A resin change alters the whole temperature profile, not just output |
| Heat stabiliser | Absorbs the decomposition that heat starts | Yellowing, then black specks, then corrosive fume |
| External lubricant | Reduces friction between compound and hot metal surfaces | Too little causes plate-out and burn marks, too much causes slipping and surging |
| Internal lubricant | Controls shear heat generated inside the compound itself | Melt temperature climbs on its own without the heaters being touched |
| Filler | Adjusts cost, stiffness and melt viscosity | Abrasive wear on screws and dies rises with loading |
| Pigment and modifier | Set the finished appearance and impact performance | Mostly a product problem rather than a process one |
Lead-free formulation deserves a separate note because it changes the process rather than only the chemistry. Calcium-zinc stabiliser systems behave differently from the legacy alternatives, and a line has to be set up for them rather than simply switched. The SUHUI PVC profile extrusion line lists calcium-zinc compatibility as a configuration option for that reason.
Why Dead Spots in the Flow Path Are Not Cosmetic
Every extrusion die has corners where the melt moves slowly. On a polyolefin line the material sitting there slowly discolours and eventually washes out, and the operator notices nothing. On a PVC line the same material decomposes, sticks, builds up and then breaks loose as a black speck in the middle of a shift.
A corner that stagnates is a scrap generator on a delay timer.
This is why PVC tooling is designed to be streamlined and fully swept — every surface the melt touches should have material moving across it. It is also why fast changeover matters more on a PVC line than a buyer might expect. The SUHUI PVC pipe configurations use a European-style fast-locking die head, and the value of that is not only the minutes saved between sizes. It is that a die which is quick to open is a die that actually gets cleaned.
Materials of construction follow the same logic. Because decomposition products are corrosive and PVC recipes usually carry abrasive filler, the SUHUI PVC pipe line specifies bimetallic screw and barrel surfaces rather than plain nitrided steel. That combination of chemical attack and abrasion is why PVC hardware is specified differently from polyolefin hardware of the same nominal size.
Purging Before Shutdown Is Part of the Process
Stopping a PE line is uneventful. Stopping a PVC line without purging leaves compound sitting hot in the barrel and die, where it continues to age against a stabiliser budget that is no longer being replenished by fresh material. Restart then produces contaminated product until the whole flow path has cleared.
The purge displaces PVC from every surface before the heat comes off, not after.
The discipline is simple to state and easy to skip under production pressure. Displace the PVC with a purge compound or a compatible polyolefin while the line is still at temperature, run until the extrudate comes through clean, then bring the heaters down. An unplanned power failure needs the same treatment as soon as power returns rather than a straight restart.
There is a maintenance version of the same rule. Dies and screws that come out of a PVC line get cleaned while the deposit is still soft, because degraded PVC hardens onto tooling and then has to be removed mechanically, which is how die lands get damaged. None of this is exotic. It is just a category of routine that polyolefin operators moving to PVC have never had to build into a shift plan.
What the Published PVC Line Data Shows
The rules above are visible in the configuration tables rather than only in theory. Reading the SUHUI PVC pipe range end to end shows the twin screw family running the whole diameter span, and it shows something else that surprises buyers comparing quotations across polymers — the power draw per kilogram is lower than the polyolefin equivalent.
| Published configuration | Diameter range | Extruder | Output | Motor power |
|---|---|---|---|---|
| PVC63 single | Φ20–63 mm | SJZ55/110 conical twin | 180 kg/h | 22 kW |
| PVC110 single | Φ40–110 mm | SJZ65/132 conical twin | 250 kg/h | 37 kW |
| PVC250 single | Φ75–250 mm | SJZ80/156 conical twin | 400 kg/h | 55 kW |
| PVC400 | Φ160–450 mm | SJZ92/188 conical twin | 600 kg/h | 110 kW |
| PVC800 | Φ500–800 mm | SJP130/28 parallel twin | 1,100 kg/h | 160 kW |
Conical twin across the range, parallel twin at the top end.
Compare the third row against a polyolefin machine of similar output. The SJZ80/156 delivers 400 kg/h on 55 kW, where a single screw rated at the same 400 kg/h on the HDPE side draws 110 kW. Positive displacement conveying is simply more efficient than drag flow, because it is not relying on shear to do the melting.
That comparison is a useful check on quotations and a bad basis for machine selection. A lower kW figure on a PVC line does not mean the supplier has built a more efficient machine than the polyolefin quotation next to it. It means the two are different families running different polymers, and power draw is not comparable across them.
The switch to a parallel twin screw at the top of the range is worth noting too. Conical screws have a geometric ceiling on discharge pressure and torque, so the largest diameters move to constant-diameter parallel twins. That is a different machine architecture rather than a bigger conical one.
What Changes Again With Recycled PVC
Recycled PVC brings back every constraint above with less margin. Material that has already been extruded once has spent part of its stabiliser budget, so it enters the second line with less thermal reserve than virgin compound and reaches trouble sooner at the same set temperatures.
Recycled PVC arrives with part of its thermal budget already spent.
SUHUI’s PVC pelletizing line answers this with four specific measures rather than with a general claim. Multi-zone temperature control holds the 160 to 200 °C band. In-line dosing adds stabiliser and lubricant back into the melt. Corrosion-resistant components take the hydrogen chloride risk seriously. Integrated fume extraction removes what does come off. Pelletizing is air-cooled die-face hot cut to 2–4 mm, which keeps the recovered pellet dry and avoids a water stage.
Two practical points matter for anyone planning to blend recycled PVC into pipe or profile. The blend ratio has to be confirmed against the finished product standard, not chosen from a rule of thumb, because the product requirement rather than the machine sets the ceiling. And rigid and flexible PVC scrap are different feedstocks — the SUHUI line handles both without reconfiguration, but they should not be mixed into one batch and expected to behave as one compound.
For a wider view of how PVC pipe design has evolved alongside these processing limits, our guide to the PVCO pipe covers biaxial orientation, and the Vinyl Institute resource library is a reasonable starting point for formulation and regulatory background on the material itself.
Frequently Asked Questions
What is PVC extrusion?
It is the continuous forming of PVC compound into pipe, profile, conduit or sheet by pushing it through a shaped die. Unlike polyolefin extrusion it starts from a dry blend rather than pellets, runs in a narrow temperature band, and depends on heat stabilisers to survive the barrel.
What is extrusion of PVC profiles?
The same process applied to a custom cross-section rather than a round pipe. A twin screw extruder plasticises the dry blend, a profile die forms the section, and multi-section vacuum calibration plates hold the shape while it cools. Profile dies and calibration tooling are made to the customer’s drawing.
Why does PVC need a twin screw extruder?
Because a single screw conveys by drag, so some material lingers in the barrel, and PVC degrades when it is held hot. A conical twin screw pushes material forward in closed chambers, giving a short and repeatable residence time. The wide feed end also handles low bulk density powder that a single screw would bridge over.
What temperature is PVC extruded at?
SUHUI publishes a stable 160 to 200 °C multi-zone window for its PVC processing equipment. The exact profile depends on the resin K value, the lubricant package, the filler loading and the die, so it is established during commissioning rather than copied from another line.
Why does a PVC line have to be purged before shutdown?
Because compound left hot in the barrel and die keeps ageing against a stabiliser budget that is no longer being refreshed. It degrades, sticks to the flow path and comes off later as black specks. Displace it with a purge compound while the line is still at temperature, then bring the heaters down.
What plastic is best for extrusion?
There is no single answer, because the polymer is chosen by the product rather than by the process. PVC suits rigid pipe, conduit and window profile where stiffness and cost matter. HDPE and PPR suit pressure pipe where toughness and weldability matter. Each one then dictates the extruder family.
Can recycled PVC be extruded into pipe and profile?
Yes, within limits set by the finished product standard rather than by the machine. Recycled PVC has already spent part of its stabiliser budget, so it needs stabiliser dosing, tight temperature control and corrosion-resistant hardware. Rigid and flexible PVC scrap should be processed as separate feedstocks.
What a PVC Line Actually Makes
- Plastic Pipe Extrusion Line — the pipe side of the decision, with the HDPE and PPR lines alongside
- Plastic Profile Extrusion Line — where the same dry blend meets tooling cut to a customer drawing
- Plastic Extrusion Machine Range — how the twin screw and single screw families split across the whole range
- PVC Granulating Pelletizing Line — the machine behind the four measures this article’s recycling section calls for
Tem um material que precisa processar?
Envie o polímero, em que forma ele chega e a produção desejada. Retornamos com uma configuração de linha e uma faixa de orçamento realista.
