Introduction
PP welding fuses two polypropylene surfaces into a single homogeneous piece — the weld bead and the parent material are chemically identical, so the joint has exactly the same corrosion resistance and mechanical properties as the unwelded sheet. This is the fundamental advantage of thermoplastic welding over every other joining method: there is no adhesive to degrade in acid, no gasket to compress and leak, and no mechanical fastener to corrode. Hot gas welding using a hand-held welding gun at 230–260°C with matching-grade PP welding rod is the standard method for fabricating PP duct, scrubber shells, tanks, and fittings from sheet. Extrusion welding uses a motorized extruder to feed a continuous bead of molten PP at higher deposition rates for long straight seams and thick sections. This guide covers both methods — temperature parameters, rod selection, joint preparation, step-by-step technique, and common defect identification — so that every weld in a PP system matches the 15+ year design life of the parent material. For the PP sheet grades that the welding rod must match, see our PP plastic sheet properties guide.
Key Takeaways
– Hot gas welding temperature is 230–260°C at the nozzle — below 220°C, the rod does not fuse with the parent sheet. Above 280°C, the PP thermally degrades and the weld is brittle.
– The welding rod must match the parent sheet grade — homopolymer rod for homopolymer sheet, FR-grade rod for FR-grade sheet. Mismatched grades produce welds that are weaker than either material.
– Joint preparation is half the weld quality — surfaces must be clean, dry, and beveled at 30° to create a 60° V-groove. Contaminated or un-beveled surfaces produce weak, porous welds regardless of technique.
– A good weld is glossy and uniform; a bad weld is matte and rough — insufficient temperature is the most common cause of weld failure, and the visual difference between a strong weld and a weak one is obvious once you know what to look for.
Hot Gas Welding — The Standard Method
Hot gas PP welding uses a hand-held welding gun that heats compressed air or nitrogen to 230–260°C and directs it through a nozzle onto the joint area. A PP welding rod is fed into the preheated V-groove, where both the rod surface and the parent sheet surface melt and fuse into a single molten pool. As the gun moves along the joint, the pool cools and solidifies into a homogeneous weld bead.
Equipment
- Welding gun — with adjustable temperature control (200–300°C range, ±10°C stability). A gun that drifts more than ±20°C during use produces inconsistent welds. Use a 5–8mm round nozzle for sheet welding; a speed-welding nozzle (with integrated rod guide) for long straight seams.
- Compressed air or nitrogen — clean, dry, oil-free. Air compressor output must be filtered to remove oil aerosols that contaminate the weld. Nitrogen produces marginally better weld quality (no oxidation) but is rarely justified except for the highest-purity chemical service.
- PP welding rod — 3mm diameter for root passes, 4–5mm for cover passes. Triangular rod has a larger cross-section for faster fill; round rod feeds more smoothly for detail work. Store in sealed packaging; rod exposed to humidity for more than 24 hours must be dried at 60°C for 2 hours before use.
Joint Preparation
Both surfaces must be clean, dry, and free of oil, grease, dust, or moisture. Wipe with isopropyl alcohol. Do not use acetone, MEK, or solvents that soften PP. Bevel each edge at 30° to create a 60° V-groove. Use a hand scraper or rotary deburring tool — not sandpaper, which embeds abrasive particles in the surface.
Welding Technique
- Set temperature to 230–260°C per DVS 2207-1 hot gas welding standards. Test on a scrap piece: the rod should melt and flow into the groove with a glossy surface. Adjust up if the rod sits on top without melting; adjust down if the PP discolors (turns brown) or smokes.
- Tack-weld at 3–4 points to hold alignment.
- Root pass — 3mm rod, fed at 45° angle to the surface. Move the gun in a slight weaving motion to heat both sides of the groove evenly. The rod tip should form a small molten bead that flows into the groove. Forward speed: approximately 100–150 mm/minute.
- Cool to ambient (5–10 minutes) between passes. Welding over a hot root pass overheats the parent material and produces a weak, oxidized cover pass.
- Cover pass(es) — 4–5mm rod, same technique. Build a slight crown 1–2mm above the surface. Do not grind flat.
- Visual inspection — glossy, uniform bead, no porosity, no cracks, smooth transition at edges.
Extrusion Welding — High-Speed Long Seams
Extrusion PP welding uses a motorized hand-held extruder that feeds PP welding rod through a heated barrel, melting and extruding a continuous bead of molten PP at rates of 2–4 kg/hour — approximately 5–10× faster than hot gas welding. Extrusion welding is used for long straight seams on large-diameter duct, tank walls, and thick-section scrubber shells where the deposition volume makes hot gas welding impractical. For the installation procedures where welding is the primary joining method, see our PP duct installation guide.
The extruder preheats the joint area with a hot gas nozzle, then deposits the molten PP bead into the V-groove through a PTFE shoe that shapes the bead profile. The operator controls the extrusion rate, shoe pressure, and travel speed to produce a uniform bead.
Advantages: 5–10× faster deposition, consistent bead quality (machine-controlled temperature and feed rate), better for thick sections (>10mm) where multiple hot gas passes would be required.
Disadvantages: higher equipment cost ($2,000–5,000 for a professional extruder vs $200–500 for a hot gas gun), less maneuverable for tight corners and complex geometries, requires 3-phase power for larger units.
When to use extrusion vs hot gas: extrusion for straight seams longer than 2 meters on material 10mm or thicker. Hot gas for all joints shorter than 2 meters, all curved or corner joints, and all material below 10mm thickness. Most PP fabrication shops use both methods — hot gas for detail work, extrusion for production seams.
Common Weld Defects — Identification and Cause
| Defect | Appearance | Cause | Fix |
|---|---|---|---|
| Porosity | Small bubbles in weld bead | Moisture in rod or on surface | Dry rod, clean and dry surfaces |
| Cold joint | Matte, rough surface; rod sits on top | Temperature too low (<220°C) | Increase gun temp to 230–260°C |
| Burned weld | Brown discoloration, smoke smell | Temperature too high (>280°C) | Reduce temp, increase travel speed |
| Centerline crack | Crack along weld center | Cooling too fast (draft, cold air) | Shield weld from drafts during cooling |
| Incomplete fusion | Rod separates from parent at edges | Insufficient preheating of groove sides | Weave gun more to heat both sides |
| Undercut | Groove at weld edge into parent material | Excessive temperature or dwell time | Reduce temp, increase travel speed |
Any weld showing porosity, cracks, or incomplete fusion must be ground out completely and re-welded. A porous weld that passes visual inspection will fail in service when the stored chemical penetrates the porosity and attacks the weld from the inside.
Frequently Asked Questions
What temperature should PP welding be done at?
230–260°C at the welding gun nozzle. Below 220°C, the PP rod does not fully melt and the weld is a cold joint — the rod sits on top of the groove rather than fusing into it. Above 280°C, PP thermally degrades (oxidation, chain scission) and turns brown, producing a brittle weld with significantly reduced strength.
Can I weld PP with a soldering iron or heat gun?
No. A soldering iron does not reach the controlled, sustained temperature required for PP welding and will produce cold, weak joints. A general-purpose heat gun does not provide the focused, temperature-controlled airstream that a purpose-built PP welding gun delivers. Use a proper PP welding gun with adjustable temperature control.
Does PP welding rod need to match the sheet color?
Color is not a reliable indicator of material grade. White, beige, and grey PP sheets may all be homopolymer — or the grey may be copolymer. Always verify the sheet grade (homopolymer, copolymer, FR-grade) from the material specification, and use matching-grade welding rod regardless of color. Grey rod on beige sheet produces a visible grey weld bead but full structural strength if both are homopolymer.
How strong is a PP weld compared to the parent material?
A correctly executed PP weld achieves 90–95% of the parent material tensile strength. The weld is not the weak point — the heat-affected zone adjacent to the weld (where the parent material was heated but not melted) is slightly weaker due to polymer chain relaxation. This is accounted for in the design safety factor and does not affect the 15+ year service life.
How should PP welding rod be stored?
In sealed, moisture-proof packaging in a dry indoor area. PP welding rod exposed to ambient humidity for more than 24 hours absorbs sufficient moisture to cause porosity in the weld bead. Rod that has been exposed can be dried at 60°C for 2 hours and used immediately. Do not store rod in direct sunlight — UV degrades the rod surface and contaminates the weld.
Conclusion
PP welding — whether by hot gas gun at 230–260°C for detail work or by extrusion welder at 2–4 kg/hour for production seams — is the joining method that makes PP the superior material for corrosive chemical equipment. The weld shares the chemistry of the parent sheet, so there are no weak interfaces for acid to attack, no gaskets to degrade, and no fasteners to corrode. Correct technique — clean dry surfaces, 30° bevel for a 60° V-groove, root pass followed by cover passes after cooling, and visual inspection for gloss and uniformity — produces a joint that will outlast the equipment it serves. For additional welding method details including rod material specifications and storage requirements, see our PP welding rod and equipment guide. For PP welding rod, welding guns, and technical support, contact Xicheng EP at factory-direct pricing. Send us your fabrication drawings, and we will supply the PP sheet cut to size with matching welding rod — or fabricate the complete equipment in our factory with every weld inspected and guaranteed.
Get PP Welding Rod & Equipment →
Written by Corbin, a senior process engineer whose career has spanned over a decade supervising PP welding and fabrication for industrial scrubbers, ductwork, tanks, and fittings across three continents. Every welding parameter, defect identification criterion, and material compatibility rule in this article is drawn from documented outcomes of our 500+ completed installations.
