How to Qualify Thermoplastic Welds in Tower Internals

Thermoplastic tower internals are widely used in corrosive services because polypropylene, polyethylene, PVDF, and other polymers can resist chemicals that rapidly attack metals. However, the corrosion resistance of the base sheet does not guarantee the integrity of the completed distributor, support, collector, or duct.

Many failures begin at welded joints.

Thermoplastic welding is not the same as metal welding. There is no molten filler pool protected by conventional shielding gas, and the finished joint cannot be accepted by applying metal-weld criteria. Joint strength depends on polymer identity, surface preparation, heating temperature, pressure, welding speed, bead geometry, and operator technique.

A joint may look smooth and still contain cold fusion, contamination, voids, or excessive thermal degradation.

Main Thermoplastic Welding Processes

Hot-gas welding uses heated air or inert gas to soften the parent material and welding rod. It is flexible and suitable for complex fabrication, but quality depends heavily on operator control.

Extrusion welding feeds molten polymer into a prepared joint while hot gas preheats the surfaces. It can produce larger weld deposits and is commonly used for thicker sheets and structural joints.

Heated-tool butt welding joins prepared surfaces by pressing them against a heated platen and then applying controlled fusion pressure. Socket and hot-plate methods may be used for pipework and fittings.

Each process has a valid application range. A large structural seam should not automatically be accepted because the fabricator can physically deposit a bead there.

Why Thermoplastic Welds Fail

Cold welding occurs when the surfaces do not reach adequate fusion temperature or when welding speed is too high. The bead may sit on the sheet without developing molecular interdiffusion.

Overheating causes discoloration, oxidation, bubbling, polymer breakdown, and loss of mechanical properties. Some fluoropolymers require especially controlled procedures and clean conditions.

Contamination is another major risk. Dust, grease, moisture, release agents, marker ink, and mixed polymer particles can prevent fusion. Welding PP rod onto PE sheet, or mixing different grades of the same polymer without qualification, may create a weak interface.

Residual stress develops when a large weld cools unevenly or when components are forced into alignment. The completed assembly may remain dimensionally correct in the workshop but crack after thermal cycling or chemical exposure.

Typical consequences include:

Leakage through distributor seams.

Branch pipe separation.

Distortion of outlet elevations.

Cracks around nozzles and supports.

Loss of structural stiffness.

Chemical penetration into voids.

Sudden failure after months of service.

Fragments entering packing or downstream equipment.

Material Identity Comes First

Before qualifying a weld, the purchaser must confirm exactly what is being welded. “PP” is not a complete material specification. Homopolymer PP, copolymer PP, flame-retardant grades, filled materials, recycled content, and sheet from different resin batches may behave differently.

The welding rod or extrusion compound should be compatible with the parent material and traceable to an approved grade. Material certificates alone are not enough if workshop segregation is poor.

Fabricators should identify sheet, pipe, rod, and fittings throughout production. Scrap material should not be casually reused as filler.

Welding Procedure Qualification

A written thermoplastic welding procedure should define:

Polymer type and grade.

Sheet or pipe thickness range.

Joint design and preparation.

Welding process and equipment.

Hot-gas or tool temperature range.

Gas type and cleanliness.

Welding speed or extrusion rate.

Applied pressure.

Filler material.

Preheating and interpass controls.

Cleaning method.

Environmental limitations.

Cooling and restraint requirements.

Inspection and test methods.

Qualification coupons should represent the production joint. Testing a flat butt weld does not qualify a complex corner seam or pipe-to-plate connection if the heat flow and operator access are different.

Procedure variables should be recorded rather than adjusted by feel alone. Thermoplastic welding windows can be narrow, especially for thin sheets where overheating occurs quickly.

Welder Qualification

Even a sound procedure can fail when the operator cannot maintain correct angle, pressure, and speed. Welder qualification should cover the actual process and relevant joint positions.

A practical qualification can include visual examination, bend testing, tensile testing, peel testing, and sectioning. The purpose is not merely to break a coupon; it is to confirm that failure does not occur prematurely along an unfused interface.

For critical internals, production test coupons should be made using the same equipment, operator, material batch, and workshop conditions.

Inspection Methods

Visual inspection remains important. Inspectors should check:

Continuous bead shape.

Uniform bead width.

Adequate fusion at both edges.

Absence of scorching or discoloration.

No trapped contamination.

No undercut, cavities, or sharp notches.

Correct weld size.

Acceptable distortion.

Smooth transitions at starts and stops.

Visual appearance alone cannot prove fusion. Mechanical coupon tests provide stronger evidence.

Spark testing can help locate discontinuities in certain lined or fabricated components, but test voltage and applicability must be matched to material thickness and design. Excessive voltage can damage thin polymer.

Leak testing may use water, air at controlled low pressure, vacuum-box methods, or another agreed technique. The test must not impose loads that the thermoplastic assembly was not designed to withstand.

Structural Design Considerations

Polymer weld strength may be lower than parent-sheet strength, especially at elevated temperature. Long-term creep, chemical exposure, and cyclic loads must be included in the design.

A weld carrying continuous structural load should not be sized only from short-term room-temperature coupon strength. Joint geometry should reduce peel and bending stresses, which are more damaging than uniform shear.

Attachments should avoid sharp corners and abrupt stiffness changes. Large extrusion beads are not always stronger; excessive weld material can introduce heat damage and residual stress.

Procurement and Final Acceptance

The purchase specification should require procedure qualification, welder qualification, material traceability, production inspection, dimensional checks, and leak testing appropriate to service.

Before shipment, verify:

Material grade and filler compatibility.

Approved welding procedure numbers.

Operator qualification records.

Production coupon results.

Repair locations and repair procedures.

Distributor level and outlet elevations.

Joint accessibility for field inspection.

Protection from impact and UV during transport.

Absence of forced-fit assembly stress.

Final leak-test documentation.