What plastic welding actually is

Thermoplastic welding is a fusion process. Heat softens the surfaces to be joined, pressure brings the molten polymer chains into contact, and on cooling the joint becomes continuous with the parent sheet. There is no glue line, no gasket and no mechanical fastener carrying the load.

That matters most in chemical plant, where joints are an obvious weak point if they rely on a gasket or an adhesive line rather than the parent material. A correctly welded seam in polypropylene is chemically the same material as the tank wall around it, so it ages at the same rate and resists the same reagents. Our materials guide sets out how the common polymers behave in service.

Hot-gas welding with filler rod

Hot-gas welding (also called hot-air welding) uses a heated air stream to soften the joint faces and a filler rod of matching polymer, laid into a prepared vee or fillet. It is the method behind most fabricated corners, brackets, nozzles, baffles and stiffeners.

As a general guide, hot air is set somewhere in the region of 280 to 350°C depending on the polymer, with PVC run cooler than polyolefins. Speed, angle and rod pressure are controlled by hand, which makes it slower than the machine processes but far more adaptable around awkward geometry and confined access.

Extrusion welding for thick sections

Extrusion welding feeds molten polymer from a small hand extruder into a preheated joint, depositing a large weld bead in one pass. It suits thick sheet and long, straight seams where hot-gas would need multiple runs of filler rod.

Most heavy-wall tank and bund work is extrusion welded for that reason: fewer passes, a thicker weld cross-section and better consistency over distance. It is the usual choice on the seams of process tanks and vats and on structural stiffening.

Butt fusion and socket fusion for pipe

Butt fusion clamps two pipe or sheet ends against a heated plate, removes the plate and forces the melted faces together under set pressure. Done properly it produces a continuous joint in the parent material, with a characteristic bead inside and out.

Socket and electrofusion fittings do a similar job on smaller bores and where alignment is tight. Which method fits a given run depends on diameter, wall thickness, access and whether the bore must stay clear — a question worth settling early when specifying pipework and controls.

Which plastics weld, and which never weld together

Welding only works between compatible polymers. Dissimilar plastics generally cannot be fused to one another at all, however hot the air or however skilled the operator, because their melt behaviour and chemistry do not match. Where two different materials must meet, the joint has to be mechanical: flanged, bolted and gasketed.

Polypropylene (PP) — welds cleanly to PP only; the standard material for acid and alkali duty
HDPE — welds to HDPE; tough, low-friction, widely used for tanks and pipe
PVC-U — welds to PVC, and can also be solvent-cemented, which chemically softens and fuses the surfaces
PP to HDPE, or either to PVC — not weldable; use a flanged or bolted connection
PVDF and GRP — not materials we fabricate in; if your duty points that way, tell us and we will say so straight away

Why welding beats bolting, sealing or bonding

Adhesives are a poor answer on polyolefins. PP and HDPE have low surface energy, typically around 30 mN/m as a published guide figure, so most adhesives simply will not wet the surface properly and the bond peels under load or thermal cycling. Flame or plasma treatment helps, but a welded seam removes the problem rather than managing it.

Bolted and gasketed joints have their place, and are unavoidable where dissimilar materials meet or where a section must come apart for maintenance. Elsewhere they add leak paths, bolt corrosion and a gasket that needs replacing. Fusion gives a monolithic vessel instead.

Workshop fabrication welding

Most of our welding happens under our own roof in Poole, where sheet can be machined, bent and jigged accurately before a rod is struck. Working in the workshop keeps temperature, cleanliness and alignment under control, which is what a consistent weld depends on.

Tanks, ducting, sinks, hoppers and enclosures are all built this way and delivered ready for connection. See the range of fabrication work and the ducting we produce for fume extraction.

On-site welding and repair, by arrangement

Plant that cannot be moved has to be welded where it stands. Site welding is offered by arrangement: cracked tank walls, split seams, damaged bunds, altered ducting runs, new nozzles cut into existing vessels and modifications to lines that would be uneconomic to strip out.

Site conditions have to be assessed first — access, isolation, cleaning and decontamination, permits and what the surrounding process allows. That assessment is part of the free consultation. Details of repair work are set out on our plastic tank repair page.

What makes a sound weld, and how to reach us

Four things decide weld quality in every method: preparation, temperature, speed and operator skill. Surfaces must be clean, dry and freshly scraped, because oxidised or contaminated polymer will not fuse. Heat has to suit the polymer, since too little leaves a cold joint and too much degrades the material. Travel speed governs how much heat actually goes into the parent sheet, and the operator's hand controls all of it.

Beckox has fabricated in thermoplastics from Poole since 1982, designing, welding and installing in-house across Dorset, Hampshire and the wider UK. To discuss a fabrication, a modification or a repair, call 01202 736 725 or use our contact page — site consultations are free.

PAGE REVIEWED — 30 July 2026

Polypropylene Welding Services in Poole, Dorset and UK-wide