PVC Injection Molding: Properties, Process Constraints and DFM Notes
PVC is the one common resin where the material choice decides the mold steel before the part geometry matters at all. It releases hydrogen chloride at processing temperature, that gas attacks unprotected steel, and a cavity that pits within weeks produces parts no drawing will accept.
This page covers what PVC is, the two families that behave completely differently in a mold, the parts it is actually specified for, and the DFM consequences — steel, runner type, shrinkage, wear protection — that follow from the resin rather than from the design.
What PVC Is — and Why It Behaves Differently in a Mold
PVC is an amorphous commodity polymer supplied in two forms that are effectively different materials:
- Rigid PVC (uPVC) — unplasticized, high modulus, low shrinkage. This is the form used for pipe fittings, valves and electrical components.
- Flexible PVC — heavily plasticized, low modulus, very high shrinkage. The plasticizer content (often 20–50 phr) is what changes the numbers, and it is why “PVC” on a drawing is not a specification.
Two properties dominate everything else in a mold:
Thermal instability. PVC degrades at temperatures close to its own processing range, and the degradation product is hydrogen chloride. This is not a cosmetic issue — it is a corrosion issue, and it is why the steel table in the site’s material properties guide rejects P20, H13 and NAK80 for PVC and accepts only S136.
Very low rigidity-form shrinkage, very high flexible-form shrinkage. Rigid PVC shrinks 0.2–0.5%, one of the lowest values of any common resin. Flexible PVC shrinks 1.0–3.5%, among the highest. Same polymer family, an order of magnitude apart in one of the two numbers that decides cavity dimensions.
PVC Properties at a Glance
| Property | PVC (rigid) | PVC (flexible) |
|---|---|---|
| Density | 1.40 g/cm³ | 1.25–1.35 g/cm³ |
| Tensile strength | 40–50 MPa | 10–20 MPa |
| Flexural modulus | 2.5–3.5 GPa | — |
| HDT | 65–75 °C | 50–65 °C |
| Mold shrinkage | 0.2–0.5% | 1.0–3.5% |
| Relative cost | $ | $ |
Note on HDT. These values are quoted from the resin reference data without a load specification. HDT is load-dependent — a rigid PVC figure at 0.45 MPa and one at 1.8 MPa describe different service conditions. If heat deflection matters for your part, confirm the test load before it goes on a drawing.
Density deserves its own line item. At 1.40 g/cm³ rigid PVC is roughly 55% denser than PP (0.90) and 35% denser than ABS (1.04). The same part volume costs more in material than the per-kilogram price suggests, and it weighs more on the robot and in the pack-out.
Where PVC Is Specified
Most PVC volume worldwide goes to extrusion and calendering, not injection molding. Injection molding is the smaller share, and it concentrates where a discrete, dimensionally defined part is needed:
Rigid PVC
- Pipe fittings and valve bodies — the largest injection-molded PVC application. Solvent-weld sockets demand tight dimensional control, which is where the 0.2–0.5% shrinkage works in the molder’s favour.
- Electrical junction boxes, conduit fittings and cable trunking components — PVC’s flame behaviour and electrical properties are the reason it is specified here at all.
- Window and door hardware components — profile end caps, drainage components, gasket carriers.
Flexible PVC
- Cable strain reliefs and grommets — the plasticized grades exist for flexibility and grip.
- Seals, boots and bellows — where a soft part must flex repeatedly.
- Tool grips and handles — overmolded or insert-molded onto a rigid core.
- Medical tubing connectors — where the tubing itself is PVC and the connector must match it.
The common thread: PVC is chosen for chemical resistance, flame behaviour, electrical insulation or cost — not for mechanical strength. At 40–50 MPa tensile (rigid) it is a commodity, not an engineering resin.
PVC Programs at JBRplas — What We Can and Cannot Publish
JBRplas runs PVC injection molding programs. Every one of them is covered by a customer non-disclosure agreement, so this page contains no PVC part photographs, no client names and no program-level data.
That is a deliberate limit, not an oversight. The same discipline that keeps a competitor from reading a customer’s part geometry is the discipline that will apply to your drawings — NDA signed before file review is standard practice here, and it applies to every material, not only PVC.
What we can publish is the process: which steel the resin forces, why the runner type is not a choice, what the two shrinkage ranges do to cavity sizing, and where the wear surfaces are. That is the content of this page, and it is the part of a PVC program that a buyer can actually evaluate before awarding a tool.
If you need to see evidence of PVC capability beyond this page, ask for it directly in an RFQ — reference material and process references can usually be discussed under NDA even when parts cannot be shown.
DFM Notes Specific to PVC
The Steel Is Decided by the Resin
This is the first decision on a PVC tool and it has no alternatives. Per the site’s steel compatibility matrix, PVC accepts S136 (420SS) and rejects everything else:
| Mold steel | Suitable for PVC | Reason |
|---|---|---|
| P20 (718H) | No | Unprotected steel is attacked by HCl off-gas |
| H13 | No | Same corrosion mechanism — hardness does not help |
| S136 (420SS) | Yes | Stainless, 48–52 HRC — the only acceptable choice |
| NAK80 | No | Not specified for corrosive resins |
The mechanism is documented in the mold steel selection guide: PVC releases hydrochloric acid during processing, which pits unprotected steel within weeks, and a pitted cavity produces a textured part where the drawing calls for a gloss surface. The failure is not gradual and not cosmetic — it is a scrapped tool.
No Hot Runner — It Is Not a Preference
PVC is rated unacceptable for hot runner in the site’s hot runner vs cold runner comparison: HCl off-gassing at melt temperature destroys hot runner components. A hot runner also holds a residence volume of PVC at temperature for the whole shift, which is exactly the condition that generates more HCl.
The consequence for the part: cold runner, and a runner layout designed to be short. A cold runner in PVC means sprue and runner weight is real material cost at 1.40 g/cm³, so runner balance and sprue length are cost decisions, not just flow decisions.
Residence Time Is a Process Variable, Not a Detail
Since HCl generation is a function of temperature and time, the process window for PVC is narrower than for a stable resin: more purge, less dwell, and no running at the edge of the temperature range to chase a cosmetic defect. A PVC tool that needs to be pushed hot to fill is a tool with a runner problem, not a temperature problem.
Two Shrinkages, Two Different Tools
Rigid PVC at 0.2–0.5% and flexible PVC at 1.0–3.5% cannot share a cavity-sizing assumption. On flexible grades the plasticizer level is the dominant variable, so the shrink figure must come from the actual compound and its actual plasticizer content — not from a generic “PVC” value.
Density Changes the Economics
At 1.40 g/cm³, a rigid PVC part is heavier than the same part in ABS or PP by a wide margin. That affects three things: material cost per part, required shot capacity, and handling/packing. It also means a PVC program quoted on a per-kilogram basis can look competitive while costing more per part than an ABS alternative.
Chrome Plating and Vent Protection
The site’s mold maintenance guidance classifies PVC as high wear aggressiveness with chemical corrosion as the primary mechanism, and lists the mitigation as S136 or stainless steel with chrome plating mandatory. Vents are where the off-gas leaves the cavity, so vent surfaces carry the same exposure as the cavity and need the same protection — a vent that corrodes changes the fill pattern silently.
Design Around the Flame and Electrical Requirement
Buyers rarely specify PVC for its mechanicals. It gets specified for flame behaviour, electrical insulation or chemical resistance — so the drawing should state which of those the part actually has to meet, and to what standard. That determines whether a standard rigid grade is sufficient or a formulated grade is required, which in turn can shift shrinkage.
Related Process Pages
- Injection molding services — machine range and process capability
- Mold manufacturing — how the tool is built, and where steel selection is locked in
- Mold design — runner, gate and cooling layout
- Mold steel selection guide — the S136 rule and the corrosion mechanism in full
- Hot runner vs cold runner — why PVC sits on the cold-runner side
- Material properties guide — the full resin and steel compatibility tables
Getting PVC Right on Your Part
The three decisions that decide a PVC program — steel, runner type, and which shrinkage the cavity is cut to — are all made before the tool is cut, and two of them are forced by the resin rather than chosen by the designer.
Send the drawing and the resin grade, ideally with the flame or electrical standard the part must meet. We will confirm the steel and runner configuration, tell you which shrinkage we would cut to, and flag the DFM issues that PVC specifically creates on your geometry.
NDA is signed before file review as standard. Request a quote →