PVC Injection Molding
Rigid PVC is a good moulding material with one expensive complication: it is heat sensitive, and when it overheats it releases hydrogen chloride, which corrodes mould steel. That single fact shapes the whole job — a narrow temperature window, disciplined residence time, and a tool built from corrosion-resistant steel or protected with plating. Handle it and PVC moulds into stiff, chemically resistant, self-extinguishing parts at low cost. Ignore it and the first tool is damaged before the job is finished.
Specification at a glance
- Material family
- Amorphous thermoplastic — polyvinyl chloride, rigid (uPVC) and plasticised
- Melt temperature
- Roughly 160–200 °C — low, and close to the degradation threshold at the top
- Mould temperature
- Roughly 20–50 °C
- Shrinkage
- Roughly 0.3–0.8% for rigid PVC; plasticised grades differ with the plasticiser content
- Drying
- Light — PVC is mildly hygroscopic; a short dry is normal practice
- Wall thickness
- Typically 1.5–4.0 mm
- Draft angle
- 0.5–1° typical
- Heat sensitivity
- High — degradation releases hydrogen chloride, which corrodes mould steel
- Tooling
- Corrosion-resistant steels or chrome-plated cavities are used because of the HCl risk
- Flame behaviour
- Inherently self-extinguishing, from the chlorine content
- Chemical resistance
- Good against acids, bases, salt solutions and alcohols; poor against ketones, esters and aromatic or chlorinated solvents
- Service temperature
- ~60 °C — the lowest of the common moulding plastics
- Molding route
- Through our vetted partner network, managed under one quality process; metal inserts machined on our own floor
- Mould tooling
- Quoted per drawing — mould size, shot weight, gate count and cavity steel are reviewed before the tool is cut
- MOQ
- From 50 pieces, samples from a single piece
The complication is the process, not the part
PVC’s finished properties are straightforward: stiff, cheap, chemically resistant, inherently self-extinguishing. It is what pipe fittings, conduit and window profiles are made from, and it does those jobs well.
The complication is entirely in getting there, and it comes from one property.
PVC is heat sensitive. Above its processing range — or after too long resident in a heated barrel — the polymer degrades and releases hydrogen chloride, which is corrosive to steel.
That has a direct commercial consequence: the tool has to be built for it. Corrosion-resistant mould steels, chrome-plated cavities and runners, and vents designed so that gas can escape rather than sit against the steel. A PVC tool built like an ABS tool will corrode, pit and lose its surface, and the cost of that correction is much larger than the cost of specifying the right steel at the start.
| Parameter | PVC | Compared with ABS |
|---|---|---|
| Melt temperature | ~160–200 °C | Low — ABS runs 210–270 °C |
| Degradation threshold | Close to the top of the range | Far above the processing window |
| Mould temperature | ~20–50 °C | Lower, so cycles are short |
| Shrinkage | ~0.3–0.8% (rigid) | Similar |
| Cavity steel | Corrosion-resistant or plated | Ordinary mould steel is fine |
| Service temperature | ~60 °C | ABS is 70–80 °C |
The narrow window
Because the top of the processing range is close to where degradation begins, PVC has to be run precisely rather than approximately.
Temperature is controlled, not estimated. The gap between “flows properly” and “degrades” is narrower than on any other common moulding material, so barrel temperatures are set and held rather than adjusted by feel.
Residence time is managed. Material sitting hot in the barrel degrades, so a machine that stops with PVC in it is purged rather than simply restarted. Long cycle times with a small shot weight are a particular risk, because the material in the barrel is waiting.
Purging is standard practice between materials and at shutdown, because residual PVC left in a hot barrel degrades, and degraded material contaminates the next run as well as corroding the machine.
None of this is difficult with a moulder who runs PVC regularly. It is difficult to improvise, which is why the material behaviour belongs in the review rather than being discovered on the machine.
Rigid and plasticised are two different materials
This is worth separating clearly, because the two behave differently enough that they should not share a tooling review.
Rigid PVC (uPVC) has no plasticiser. It is stiff, strong, dimensionally stable and chemically resistant, and its service temperature is around 60 °C. Pipe fittings, conduit, boxes, window profiles and drainage components are all rigid PVC.
Plasticised PVC has a plasticiser added — the amount determining how flexible the material is. That changes the hardness, the shrinkage, the service temperature and the chemical resistance. The flexible grades are what wire insulation, medical tubing and some footwear components are made from.
Two consequences:
The tooling review differs. A flexible grade shrinks differently from a rigid one, so the shrinkage allowance is not transferable.
The compliance review differs. Certain plasticisers are restricted in products that contact the body, contact food, or are handled by children, and there are separate requirements for medical devices and drinking-water contact. That is an application-and-market question rather than a materials question, and the answer depends on what the part is and where it is sold. Where a flexible PVC part falls into one of those categories, naming the application at the enquiry is what allows the grade and its documentation to be confirmed against what you have to declare.
Design rules
Draft the walls. 0.5–1° typical. PVC releases reasonably well and is not unusually sticky.
Keep the wall uniform. A thick section beside a thin one cools and shrinks differently, producing sink marks and adding to warpage. Where local thickness is needed, core it out or reduce it to roughly two-thirds of the nominal wall.
Use ribs for stiffness. Rigid PVC is stiff enough that ribs work well, and ribbing avoids the mass that would cool slowly.
Radius the corners. An internal radius of at least half the wall thickness is the standard rule.
Design for the temperature. Moulded PVC is good to about 60 °C, and that limit is reached more easily than people expect — a dark part in direct sunlight, a fitting on a warm pipe, a component near a motor or a lamp, a part bolted to something that runs warm. Where the part will run warm, the temperature belongs on the enquiry rather than being discovered in the field.
Plan the joining. PVC solvent-welds well, which is unusual among the commodity plastics and useful on assemblies. It also accepts adhesives, paints and inks directly — the polyolefins do not, so a PVC part has more finishing options than an HDPE or PP part of the same shape.
Molding route
Plastic molding runs through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Mould tooling is quoted per drawing, and on a PVC part the review includes the cavity steel and any plating, because the tooling has to be specified for the material’s chemistry rather than treated as a standard plastic tool.
Moulding holds part-scale tolerances agreed per drawing in DFM; it does not match machined metal. Where a fitting needs a precision thread or bore, the tolerance is agreed in that same review.
Where molding PVC is the right choice
- Pipe fittings, valves and drainage components at volume
- Electrical conduit, boxes and trunking — the self-extinguishing behaviour matters here
- Window and door profiles
- Water-treatment components
- Chemical plant fittings and ductwork
- Wire and cable insulation in the plasticised grades
- Medical tubing in approved grades
- Any high-volume part needing chemical resistance, stiffness and flame retardancy at modest temperature
Where molding PVC is the wrong choice
The moulder does not run PVC regularly. The material demands disciplined temperature control and a tool built for HCl. A shop treating it as a generic plastic will damage the tool and produce scrap.
The service temperature is above roughly 60 °C. The lowest limit of the common moulding plastics. Polypropylene works to about 100 °C and polycarbonate to 120 °C.
The part meets ketones, esters or chlorinated solvents. Those attack PVC — and they are the same solvents used to bond it, which is a useful way to remember where the limit is.
Flexible PVC is going into a restricted application. The plasticiser question has to be settled against the application and the market, and that is a compliance review rather than a material choice.
The volume does not justify a tool. Below the break-even point, machining is cheaper — and it avoids the HCl issue altogether, because there is no melt.
The part must be transparent or light. Rigid PVC is opaque or translucent, and it is heavy for a plastic at around 1.40–1.45 g/cm³.
What to put on the drawing
| Instead of | Write |
|---|---|
| Plastic | PVC — and say rigid or plasticised |
| PVC, flexible part | Name the plasticiser requirement if the application is restricted |
| PVC, service temperature | State it — the ceiling is only around 60 °C, and state the surface temperature, not the ambient |
| PVC, cavity steel | The tool is specified for HCl resistance — this is a tooling decision, not an option |
| PVC, meets solvents or ketones | Name the chemical — PVC is attacked by them |
| PVC, joining | State the method — PVC solvent-welds well, which most commodity plastics do not |
| PVC, needs transparency | Wrong material — acrylic or polycarbonate |
| PVC, low volume or still changing | Machine it instead — and it avoids the melt problem entirely |
The three lines that decide most PVC moldings are the service temperature, whether the grade is rigid or plasticised, and the tooling specification. The first is where PVC gives way. The second changes the material’s behaviour and its compliance position. The third is what keeps the tool alive — and it is the line most often omitted when a PVC part is quoted as if it were ABS.
Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the grade, the cavity steel, the temperature and the wall and draft where any of them need settling.
Frequently asked
- Why is PVC corrosive to injection molds?
- Because PVC is heat sensitive. Above its processing range — or after too long resident in the barrel — the polymer degrades and releases hydrogen chloride, and hydrogen chloride is corrosive to steel. In a mould, that means the cavity, the runners and the vents are exposed to an acidic gas every cycle, and a tool built from ordinary mould steel will corrode, pit and lose its surface. The standard answers are corrosion-resistant mould steels, chrome-plated cavities and runners, and disciplined processing so that degradation does not happen in the first place.
- What is the processing window for PVC?
- Narrow and low. Melt temperature runs roughly 160–200 °C, which is low for a thermoplastic — ABS runs 210–270 °C and polycarbonate 280–320 °C — and the top of the PVC range is close to where degradation begins. So the temperature has to be controlled precisely rather than approximately, residence time has to be managed so material does not sit hot in the barrel, and purging between materials and at shutdown is standard practice. A machine that stops with PVC in the barrel is purged rather than simply restarted.
- What is the difference between rigid and plasticised PVC?
- They are effectively different materials. Rigid PVC — uPVC — has no plasticiser and is stiff, strong and dimensionally stable, and it is what pipe, conduit, window profiles and electrical boxes are made from. Plasticised PVC has a plasticiser added, which makes it flexible; the amount of plasticiser determines how flexible. That changes the hardness, the shrinkage, the service temperature and the chemical resistance, so the two cannot be treated as the same material in a tooling review. Where a part is flexible, the plasticiser type and content belong on the drawing.
- Is plasticised PVC restricted in some applications?
- In some markets and applications, yes — certain plasticisers are restricted in products that contact the body, contact food, or are handled by children, and there are separate requirements for medical devices and for drinking-water contact. This is a compliance question rather than a materials question, and it depends on the application and the destination market. Where a flexible PVC part is going into any of those uses, name the application at the enquiry so the grade and its documentation can be confirmed against what you have to declare.
- How much does rigid PVC shrink?
- Roughly 0.3–0.8%, which is low — comparable to ABS and well below the polyolefins. Because PVC is amorphous, there is no crystalline phase forming and pulling the part differentially, so the shrinkage is relatively uniform and warpage from differential shrinkage is limited. That makes a first tool land close to the drawing and gives good dimensional repeatability, which is one of the reasons PVC is used for profile and fitting work where parts have to assemble.
- Does PVC need drying before molding?
- Only lightly. PVC is mildly hygroscopic, far less so than nylon or polycarbonate, so a short dry is normal practice rather than the critical, carefully controlled drying step those materials require. The moisture problem that causes hydrolysis in polycarbonate and splay in nylon is not the dominant risk on a PVC job — heat sensitivity is.
- What draft angle and wall thickness does PVC need?
- Draft of around 0.5–1° is typical for rigid PVC, and wall thickness generally sits between 1.5 and 4.0 mm. The uniformity rule applies: a thick section beside a thin one cools at a different rate, produces a sink mark and can add to warpage. Where local thickness is needed, core it out or reduce it to roughly two-thirds of the nominal wall and use ribs for stiffness. Rigid PVC is stiff enough that ribs work well.
- What is PVC injection molding used for?
- Pipe fittings, valves and drainage components; electrical conduit, boxes and trunking — where the self-extinguishing behaviour matters; window and door profiles; water-treatment components; chemical plant fittings and ductwork; wire and cable insulation in the plasticised grades; medical tubing; and shoe components. It is a high-volume, low-cost material with good chemical resistance and inherent flame retardancy.
- What is the biggest limitation of a moulded PVC part?
- Its service temperature, which is only around 60 °C — the lowest of the common moulding plastics. That limit is reached more easily than people expect: a dark part in direct sunlight, a fitting on a warm pipe, a component near a motor or a lamp, or a part bolted to something that runs warm. Where the part will run warm, name the temperature at the enquiry — it is usually the factor that decides whether PVC is the right material or whether polypropylene or another family is needed.
- When should a PVC part be moulded rather than machined?
- Moulding wins on volume and on complex geometry — a fitting with threads, a socket and a ribbed body is a moulded part, and it is also the only economical way to make one in quantity. Machining wins for prototypes, low volumes, one-offs, and large simple parts cut from sheet or rod. It is worth noting that machining side-steps the HCl problem entirely — there is no melt and no degradation — so a low-volume PVC part has an easier process route than a high-volume one. Our [machined PVC page](/materials/pvc-cnc-machining/) covers that side.
Related pages
PVC in other processes
Other Engineering Plastics materials
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