PVC Injection Moulding: Rigid & Flexible Grades Guide
PVC (polyvinyl chloride) is available in two fundamentally different forms for injection moulding: rigid (uPVC) and flexible (plasticised PVC). Rigid PVC has the lowest shrinkage of common moulding materials, outstanding chemical resistance, and high dimensional stability. Flexible PVC is a low-cost, chemically resistant alternative to TPE or rubber for soft-touch and sealing applications. Nordmould processes both compound families through its partner manufacturing network.
What are the mechanical and thermal properties of PVC?
PVC properties vary dramatically with plasticiser content. Rigid uPVC behaves as an amorphous, moderately stiff engineering polymer. Flexible PVC spans from tough semi-rigid to near-elastomeric depending on how much plasticiser is loaded in — typically 20–60 phr.
| Property | Rigid uPVC | Flexible PVC (medium) | Test Standard |
|---|---|---|---|
| Tensile Strength | 40–60 MPa | 10–25 MPa | ISO 527 |
| Elongation at Break | 10–50% | 150–400% | ISO 527 |
| Flexural Modulus | 2,500–4,000 MPa | 10–100 MPa | ISO 178 |
| Shore Hardness | Shore D 75–85 | Shore A 50–90 | ISO 868 |
| Heat Deflection Temp (0.45 MPa) | 65–80°C | 40–60°C | ISO 75 |
| Density | 1.35–1.45 g/cm³ | 1.20–1.35 g/cm³ | ISO 1183 |
| Mould Shrinkage | 0.1–0.5% | 1.0–3.5% | ISO 294-4 |
| Water Absorption (24 h) | 0.04–0.4% | 0.15–0.75% | ISO 62 |
| Flammability | Self-extinguishing | Self-extinguishing | UL 94 V-0 typical |
The intrinsic chlorine content of PVC makes both grades inherently flame-retardant without additives — a significant advantage in electrical and building applications.
Where is PVC injection moulding used?
PVC's combination of chemical resistance, dimensional accuracy, low flammability, and low cost drives its use across most major industries.
Plumbing and pipe fittings: Rigid uPVC is the dominant material for cold-water pipe fittings, compression joints, and valve bodies. Low shrinkage ensures reliable sealing geometries in threaded and push-fit connections.
Electrical and electronics: Cable conduits, junction boxes, plug housings, and switch gear enclosures exploit uPVC's flame retardancy and electrical insulation. Flexible PVC is standard for cable jacketing and grommets.
Medical devices: Flexible PVC is used in blood bags, tubing connectors, and IV accessories. Medical-grade compounds use phthalate-free plasticisers (DEHP alternatives) to meet EU MDR and FDA requirements.
Consumer goods: Shoe soles, grips, handles, and seals in flexible PVC; rigid outdoor furniture components, signage, and consumer electronics housings in uPVC.
Automotive: Flexible PVC for interior trim, seals, and cable harness protection; rigid PVC for fluid reservoir covers and duct components.
Construction: uPVC window profile connectors, roofline fittings, and flooring accessories are high-volume injection-moulded applications.
What are the moulding characteristics of PVC?
PVC is processed at lower temperatures than most engineering plastics, but the degradation window is narrow and the decomposition by-product — hydrogen chloride (HCl) — is corrosive to tool steel.
Melt temperature: 160–200°C for rigid uPVC; 150–180°C for flexible PVC. Degradation begins meaningfully above 200°C and accelerates above 210°C, producing HCl, discolouration, and black specks. Barrel residence time should be kept to no more than five minutes to avoid localised overheating.
Mould temperature: 20–50°C for rigid uPVC; 15–40°C for flexible PVC. Low mould temperatures reduce cycle time and minimise thermal stress, but must not cause premature solidification before the cavity fills.
Injection pressure: 80–150 MPa. Rigid PVC has relatively high viscosity; adequate injection pressure and correctly sized gate cross-sections are needed for full fill without excessive shear heating.
Drying: Rigid uPVC is hygroscopic — 2–4 hours at 60–70°C before moulding. Flexible PVC compounds absorb less moisture but should be conditioned if stored in humid conditions.
Tool steel: Hardened stainless or hard-chrome-plated tool steel is required. HCl attacks P20 and H13 directly; unprotected tooling corrodes within weeks of regular PVC moulding. Tooling steel specification is confirmed at DFM review for every PVC programme.
Venting: Generous venting (0.02–0.03 mm depth) is essential to allow HCl and volatiles to escape rather than burn trapped gas into the part surface.
Draft angles: Minimum 1° on smooth walls for rigid uPVC; flexible PVC can be stripped from shallow drafts but 1.5° or more is still recommended to reduce ejection force.
Gate design: Sub-gates and fan gates distribute shear heating. Hot-runner systems are possible but require corrosion-resistant manifolds and valve needles. Cold-runner systems are more common and easier to maintain.
Which PVC grades and variants should you consider?
| Grade / Variant | Key Characteristic | Typical Use |
|---|---|---|
| Rigid uPVC (unplasticised) | High stiffness, low shrinkage, good chemical resistance | Fittings, housings, conduit |
| Rigid uPVC + impact modifier | Improved notch toughness | Cold-environment fittings, structural parts |
| Semi-rigid PVC | Moderate flexibility, Shore D 50–70 | Tubing connectors, seals |
| Flexible PVC, standard plasticiser | Cost-effective, Shore A 60–85 | Grips, grommets, cable entries |
| Flexible PVC, phthalate-free | RoHS/medical compliant | Medical devices, food contact, children's products |
| Chlorinated PVC (CPVC) | Higher service temperature (up to 100°C) | Hot-water fittings, industrial pipe |
| Flexible PVC + flame retardant | Enhanced FR, UL 94 V-0 | Electrical cable accessories |
What are PVC's advantages and limitations?
Advantages:
- Shrinkage of 0.1–0.5% for rigid uPVC — the lowest of common injection-moulding thermoplastics, giving dimensional accuracy that other materials can only match with careful tool compensation
- Inherent flame retardancy at no additive cost — the chlorine is already in the backbone
- Outstanding resistance to aqueous acids, alkalis, and many polar solvents
- Flexible and rigid grades within the same polymer family, so the designer picks hardness at compound-selection stage without switching material families
- Competitive raw material cost, particularly for standard rigid compounds
Limitations:
- Narrow processing window: meaningful degradation begins above 200°C and becomes severe above 210°C, releasing corrosive HCl
- Requires corrosion-resistant tooling; P20 and H13 are unsuitable for sustained PVC work
- Plasticiser migration in flexible grades, particularly at elevated temperatures, alters dimensions and mechanical properties over service life
- REACH/RoHS restrictions on certain phthalate plasticisers — compound qualification is mandatory for EU and regulated markets
- Continuous service temperature of rigid uPVC tops out at roughly 60–70°C; CPVC is required for hot-water service
- Flexible grades with mixed plasticiser types are difficult to recycle at end of life
When should you choose PVC over alternatives?
PVC vs ABS: Rigid uPVC wins on flame retardancy, chemical resistance to aqueous media, and dimensional precision (0.1–0.5% shrinkage vs ABS's 0.4–0.7%). ABS is better for low-temperature impact, paint adhesion, and UV resistance without UV-grade compounds.
PVC vs PP: uPVC offers higher stiffness and lower shrinkage. PP is the better choice when living hinges, lower density, or continuous service above 70°C are required.
Flexible PVC vs TPE: Flexible PVC costs less and resists oils better than many TPE grades. TPE is the right call when overmoulding onto rigid thermoplastics, full RoHS compliance, or clean post-consumer recycling are required.
PVC vs CPVC: Use standard uPVC for cold-water and ambient-temperature applications. Upgrade to CPVC when the part sees continuous exposure above 70°C.
Recyclability and sustainability
Rigid uPVC has the most developed recycling infrastructure of any PVC grade, with established take-back schemes in the construction sector. Production waste — sprues and runners — is routinely reground and reused; rigid uPVC tolerates multiple processing cycles with minimal property change if barrel temperature is controlled. Flexible PVC recycling is more complex because plasticiser variability requires source-separated streams. In-process regrind is recovered and reprocessed where compound compatibility permits.
Frequently asked questions
What is the difference between rigid and flexible PVC in injection moulding?
Rigid PVC (uPVC) contains no or minimal plasticiser — it is stiff, dimensionally stable, and chemically resistant. Flexible PVC (plasticised PVC) contains 20–60 phr of plasticiser, giving Shore A hardness of 50–90. Both are processed by injection moulding but require different mould temperatures, pressures, and tooling steel to handle PVC's corrosive decomposition gases.
Is PVC difficult to injection mould?
PVC requires more process discipline than polyolefins. Degradation starts above roughly 200°C and releases HCl gas that corrodes tool steel. Corrosion-resistant tooling materials and strict temperature control are non-negotiable. Barrel residence time should not exceed five minutes; stagnant melt degrades faster than moving melt at the same temperature.
What wall thickness is recommended for PVC injection moulding?
Rigid PVC: 1.5–4.0 mm, with uniform walls to minimise thermal stress and degradation risk. Flexible PVC: 1.0–5.0 mm, thicker sections are acceptable as the plasticiser keeps the melt more forgiving. Avoid sharp transitions — they concentrate thermal history and increase burn risk.
Can PVC parts be used outdoors?
Standard rigid PVC has moderate UV resistance; prolonged outdoor exposure causes yellowing and surface chalking. Outdoor-grade uPVC compounds with UV stabilisers and TiO₂ pigment are standard in window and cladding applications and fully suitable for injection-moulded parts requiring outdoor durability.
Is flexible PVC food-safe?
Food-contact flexible PVC grades exist but require careful compound selection — the plasticiser type and level must comply with EU regulation (EC) No 10/2011 or equivalent. Non-food-grade flexible PVC may contain plasticisers restricted for food contact. Nordmould can advise on compound selection during the DFM review.
How does PVC compare to TPE for flexible injection-moulded parts?
Flexible PVC is generally lower in cost and offers better chemical resistance to oils than many TPE grades. TPE is preferred when RoHS compliance, full recycling compatibility, or overmoulding onto rigid thermoplastics is required. Flexible PVC service temperature is limited to roughly 60–70°C continuously.
What tolerances can rigid PVC hold in injection moulding?
Rigid PVC shrinks only 0.1–0.5%, the lowest of common thermoplastics, which allows tight dimensional tolerances — commercial tolerance typically ±0.1–0.2 mm for precision components. This makes uPVC attractive for fittings, connectors, and housings where dimensional accuracy is critical.
Can PVC be recycled after injection moulding?
PVC carries resin code 3 and is mechanically recyclable in closed industrial systems. Commingled post-consumer PVC recycling is limited by the plasticiser variety in flexible grades. Rigid uPVC recycling infrastructure is more established, particularly for construction products. Nordmould can discuss regrind allowances for production runners and sprues.
Send your PVC part design for a free DFM review — compound selection, tooling steel specification, and processing parameters are confirmed at quote stage, typically within one business day.