The weld that joins two PP sheets into a scrubber shell is the difference between a vessel that contains acid gas at 80°C for 15 years and one that leaks at the seam within months of commissioning. PP extrusion welding — when executed correctly — fuses the parent material into a single homogeneous piece with the same chemical resistance as the original sheet. The weld does not weaken the vessel; it strengthens it at the joint. But when executed poorly — insufficient penetration, surface contamination, incorrect temperature — the weld becomes the failure initiation point for the entire system.
This guide covers PP welding methods for industrial fabrication: extrusion welding (the standard for scrubber vessels and tanks), hot gas welding, butt fusion, and the quality inspection criteria that distinguish a 15-year weld from one that will separate under the combined load of acid exposure, thermal cycling, and hydraulic pressure.
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Key Takeaways
- PP extrusion welding is the standard method for scrubber vessel and tank fabrication. A heated stream of molten PP — the same material as the parent sheet — is deposited into the weld joint, fusing both surfaces into a single homogeneous piece. The resulting weld has the same chemical resistance as the parent material because it is the parent material.
- Weld quality determines vessel service life. A correctly executed PP extrusion weld lasts 15–20 years in acid gas service with no degradation. A poorly executed weld — insufficient penetration, oxidation from excessive temperature, surface contamination — fails within months. The weld is the limiting component in every PP fabrication.
- Five inspection criteria identify weld defects before they become leaks. Surface appearance (smooth, uniform bead with consistent color), penetration depth (minimum 80% of sheet thickness), porosity (no visible air bubbles), oxidation (no brown discoloration), and dimensional accuracy (no undercut or overlap at the weld toe).
- Welding temperature for PP homopolymer is 200–230°C at the nozzle. Below 200°C, the melt does not penetrate the parent sheet surface — producing a cold joint with zero structural strength. Above 230°C, the polymer oxidizes (visible as brown discoloration), reducing the weld’s chemical resistance. The welding gun must maintain temperature within ±10°C of the setpoint.
- Hot gas welding and butt fusion serve specific roles where extrusion welding is impractical. Hot gas welding is used for thin sheets (2–6 mm) and detailed geometries where the extrusion gun is too large. Butt fusion joins pipe and duct sections end-to-end. Extrusion welding remains the standard for structural seams in vessels, tanks, and duct headers.
Why PP Welding Quality Determines Scrubber Service Life
A PP scrubber vessel operates under continuous chemical and mechanical stress. The scrubbing liquid — a chloride-rich brine at pH 7–12 and 50–65°C — is in constant contact with the vessel interior. The packing weight and liquid holdup apply sustained vertical load to the vessel floor. Thermal cycling between operating and shutdown temperatures expands and contracts every welded joint. Each of these stresses concentrates at the welds — the interfaces where two sheets of PP were joined into a continuous vessel.
A correctly executed extrusion weld distributes these stresses across a homogeneous material section. The weld bead and the parent sheet are chemically identical — polypropylene with the same molecular weight distribution, the same crystallinity, and the same chemical resistance. The weld interface, when proper penetration is achieved, is not an interface at all — the polymer chains from both sheets have interdiffused across the joint during the molten phase, creating a continuous molecular network upon cooling. This is the fundamental difference between PP welding and mechanical fastening or adhesive bonding: a correctly welded joint has no dissimilar material, no stress concentration at a fastener hole, and no adhesive interface that the scrubbing chemistry can attack. For the full material properties that make PP the standard for acid-gas service, see our scrubber material selection guide. For PP ductwork fabrication specifications, see our PP duct system design guide.
Extrusion Welding: The Standard Method for PP Vessel Fabrication
Equipment and Setup
PP extrusion welding uses a handheld or machine-guided welding gun that feeds PP welding rod — typically 3–4 mm diameter round rod — through a heated barrel. The rod melts inside the barrel at 200–230°C and is extruded through a shaped nozzle (round, triangular, or flat) into the weld joint. Simultaneously, a stream of hot air from the gun preheats the parent sheet surfaces to near their melting point. The molten extrudate fuses with the softened parent surfaces, and the assembly cools into a single piece.
The critical parameters are temperature, speed, and preparation. Temperature at the nozzle must be 200–230°C for PP homopolymer — verified with a contact thermometer, not the gun’s dial setting. Travel speed must be 0.1–0.3 m/min for a 4 mm weld bead — faster speeds produce insufficient penetration; slower speeds overheat the parent material. Sheet edges must be clean, dry, and free of oxidation. PP sheet stored outdoors without UV protection develops a chalky surface layer that must be removed by scraping or sanding before welding, or the oxidized material contaminates the weld.
Joint Preparation
V-groove preparation is standard for structural seams. The two sheets are butted together, and a V-shaped groove — typically 60–90° included angle, 2–3 mm root face — is machined or scraped along the joint. The groove provides a clean, fresh PP surface for the weld filler and creates a geometry that guides the extrudate into full contact with both parent surfaces. For sheets thicker than 10 mm, multiple weld passes are required — each pass filling approximately one-third of the groove depth — with cooling between passes to prevent excessive heat buildup that softens the surrounding material.
Other PP Welding Methods
Hot Gas Welding
Hot gas welding uses a heated air or nitrogen stream (200–250°C) to melt both the parent sheet surface and a separate PP welding rod that is hand-fed into the joint. The method is used for thin sheets (2–6 mm), detailed geometries where the extrusion gun is too large, and repair welding in confined spaces. Hot gas welding produces lower weld strength than extrusion welding — approximately 70–85% of parent material strength versus 90–95% for properly executed extrusion welds — because the heat input is lower and the intermixing of molten polymer from the rod and sheet is less complete. For structural seams in scrubber vessels and tanks, hot gas welding is used only for tack welding or preliminary assembly prior to extrusion welding.
Butt Fusion Welding
Butt fusion joins PP pipe and duct sections end-to-end. The pipe ends are faced (machined flat and parallel), heated against a temperature-controlled plate (200–220°C) until a melt bead forms on both surfaces, and then pressed together under controlled pressure (0.15–0.25 MPa) and held until cooled. The resulting joint is homogeneous — the two pipe ends have fused into a single continuous section. Butt fusion is the standard method for PP ductwork installation because it produces joints with zero leakage and the same chemical resistance as the pipe. AWS G1.10M provides the standard evaluation criteria for thermoplastic welds including butt fusion joints. ISO 10121-2:2013 provides standardized test methodology for gas-phase air cleaning media — relevant when specifying PP packing that must maintain surface area after welding-induced thermal exposure.
Weld Quality Inspection: 5 Defects to Check Before the Vessel Enters Service
1. Surface appearance. A high-quality PP extrusion weld has a smooth, uniform bead with consistent color — the natural off-white of PP homopolymer. Brown discoloration indicates oxidation from excessive temperature. Gray or dull finish indicates surface contamination. A rough, irregular bead profile indicates inconsistent travel speed or feed rate.
2. Penetration depth. The weld must penetrate a minimum of 80% of the parent sheet thickness into the root of the V-groove. Incomplete penetration creates a stress concentration at the unwelded root that will initiate cracking under thermal cycling. Penetration is verified by sectioning a test weld — destructive testing of a sample fabricated from the same PP sheet batch and the same welding parameters as the production vessel.
3. Porosity. Visible air bubbles in the weld bead indicate moisture contamination in the welding rod, insufficient preheat of the parent material, or excessive travel speed. Each bubble is a stress concentration point. More than 3 pores per 100 mm of weld bead is cause for rejection.
4. Oxidation. Brown or yellow discoloration anywhere in the weld bead or heat-affected zone indicates that the PP has been thermally degraded — the polymer backbone has been broken by excessive heat. Oxidized PP has reduced chemical resistance and mechanical strength. An oxidized weld must be ground out and re-welded.
5. Dimensional accuracy. The weld bead must not undercut the parent sheet (a groove at the weld toe that reduces the effective sheet thickness) or overlap the parent sheet without fusing (a cold lap — the extrudate sits on top of the sheet without penetrating the surface). Both defects are detected by visual inspection with a bright light angled across the weld surface.
Frequently Asked Questions
What is the best welding method for PP scrubber vessels?
Extrusion welding is the standard for structural seams in PP scrubber vessels and tanks. It produces a homogeneous weld with 90–95% of parent material strength and identical chemical resistance. Hot gas welding is used for thin sheets and detailed geometries where the extrusion gun cannot access. Butt fusion is used exclusively for joining PP pipe and duct sections end-to-end.
What temperature should PP be welded at?
PP homopolymer extrusion welding requires 200–230°C at the nozzle, verified with a contact thermometer. The parent sheet surfaces must be preheated to 150–180°C immediately before the extrudate is deposited. Welding below 200°C produces a cold joint with minimal structural strength. Welding above 230°C causes polymer oxidation visible as brown discoloration.
How do I check the quality of a PP weld?
Five-point visual inspection: surface appearance (smooth, uniform bead with consistent color), porosity (fewer than 3 pores per 100 mm), oxidation (no brown or yellow discoloration), dimensional accuracy (no undercut at weld toe, no cold lap), and penetration depth (minimum 80% of sheet thickness, verified by destructive testing of a sample weld). For critical applications, a dye penetrant test or vacuum box test confirms leak-tightness.
Can PP be welded to other plastics?
PP can only be reliably welded to PP. Welding PP to HDPE, PVC, or PVDF produces a joint with negligible structural strength because the polymer chains of dissimilar materials do not interdiffuse during the molten phase. For applications requiring material transitions — such as a PP scrubber connecting to PVC ductwork — use a flanged mechanical connection with a chemically compatible gasket, not a welded joint.
How long does a PP weld last in acid gas service?
A correctly executed PP extrusion weld lasts 15–20 years in HCl, H₂SO₄, and NaOH service at temperatures up to 80°C — the same service life as the PP parent sheet. The weld does not degrade faster than the sheet because it is chemically identical to the sheet. Welds that fail prematurely in acid service typically fail because of fabrication defects — insufficient penetration, oxidation, or contamination — not because of chemical attack on properly welded PP.
Conclusion
PP welding is a fabrication process with engineering consequences that last 15–20 years. The weld that joins two PP sheets into a scrubber shell will be in continuous contact with hydrochloric acid, caustic soda, and dissolved chloride salts at 50–65°C for the entire service life of the system. The quality of that weld — the penetration depth, the absence of oxidation, the uniformity of the bead — determines whether the vessel remains gas-tight or develops a leak path at the seam. There is no repair procedure that restores a failed weld to original strength without removing the damaged section and re-welding.
Three practices separate professional PP fabrication from failures waiting to happen. First, verify welding temperature at the nozzle with a contact thermometer — the gun’s dial setting is not a measurement. Second, section a test weld from the same PP batch and the same welding parameters as the production vessel — destructive testing of a sample is the only way to confirm penetration depth. Third, inspect every linear meter of weld bead against the five criteria: surface appearance, porosity, oxidation, dimensional accuracy, and penetration. A weld that passes all five will survive the chemical and mechanical environment of an acid gas scrubber for the full 15–20 year design life. A weld that fails any one of them should be ground out and re-welded before the vessel enters service — because the cost of re-welding in the fabrication shop is measured in hours; the cost of repairing a leaking weld in an operating scrubber is measured in days of downtime.
For a technical consultation on PP welding procedures, material selection, or fabrication quality standards — Request Your Welding Consultation →
Next read: For the material selection logic that determines whether PP, FRP, or SS304 is the right material for your acid gas service, see our scrubber material selection guide.
