Materials & preparation
Welding rod and extrusion materials: PE, PP, PVC-U, PVDF and ECTFE
The welding rod must match the parent material, and the temperature must suit the material. Here is how you avoid faults that first manifest as a failing weld seam.
The welding rod must be the same material as the parent material
In extrusion welding, a welding rod of the same material type is fused into the parent material. PP rod in PP, PE rod in PE, PVDF rod in PVDF – and not 'something that looks like it'. A different polymer produces a weld seam that may look fine, but which does not fuse at a molecular level and will fail under load or temperature fluctuations.
Even within the same material type, differences can occur: PE for pressure pipes and PE for other applications can have different additives and melt properties. If in doubt, use a rod that is documented for the specific parent material you are welding.
Practical differences between the materials
PP is rigid and chemical resistant and is used for tanks, vessels and ducting. PE is tough and impact resistant and is used for pipes, sheeting and liners. PVDF withstands higher temperatures and aggressive media, but requires higher preheating and more precise temperature control.
The difference is particularly noticeable during preheating: PVDF requires more heat input to fuse, whereas PP and PE are more forgiving, but are on the other hand sensitive to overheating, where the material degrades and loses strength.
The precise temperatures are listed in the manufacturer's table for the specific rod and parent material. Use this – not a general rule of thumb – when the joint needs to be documented.
| Material | Density (g/cm³) | Typically used for | Preheating | Sensitivity |
|---|---|---|---|---|
| PE-HD | 0,95 | Pipes, sheeting, liners, impact-resistant components | Moderate, uniform | Moisture in parent material causes voids |
| PP | 0,91 | Tanks, vessels, ducting, chemical-resistant components | Moderate, uniform | Overheating causes degradation and odour |
| PVC-U | 1,40 | Drainage, chemical plants, ducting | Low and highly precise | Hydrochloric acid (HCl) at welding temperature – requires extraction |
| PVDF | 1,78 | Aggressive media and higher temperatures | Higher and very uniform | Requires precise temperature control |
| ECTFE | 1,68 | Aggressive media, high temperature | High and uniform | Smaller selection of welding rods and welding shoes |
| ABS | 1,05 | Vessels, ducting, components requiring machining | Low to moderate | Moisture causes voids; easily degrades at excessive temperature |
| PTFE | 2,20 | Cannot be melt-welded | Not applicable | Degrades before melting and releases toxic gases |
Densities are typical data sheet values. Temperatures and output rates must always be obtained from the manufacturer's table for welding rod and parent material.
Materials requiring special conditions – or which cannot be welded
PE-HD and PP are the two materials where extrusion welding is the standard method. The other thermoplastic types can also be welded, but each comes with prerequisites that you must clarify before starting the job.
PVC-U can be extrusion welded, but requires equipment and temperature control designed for PVC as well as effective local fume extraction, as hydrochloric acid (HCl) is released at the welding temperature. Heated tool welding of PVC-U is described in DVS 2207-12.
PVDF and ECTFE require higher melt and preheat temperatures, and the selection of welding rods and welding shoes is smaller than for PE and PP. ABS is moisture sensitive and must be welded at the lower end of the temperature range to avoid degradation.
PTFE cannot be melt-welded and therefore cannot be extrusion welded: the material degrades before melting, releasing toxic gases. If the component must withstand chemicals at high temperatures, PFA or ECTFE are the weldable alternatives – though PFA requires equipment capable of reaching high temperatures.
Standard references: heated tool welding of PE and PP is described in DVS 2207-1 and DVS 2207-11, hot gas welding in DVS 2207-3, and extrusion welding in DVS 2207-4. Welding rod consumption can be calculated using the welding rod calculator.
Welding rod diameter and welding shoe
Small extruders typically run with 3 and 4 mm rod, larger ones with 4 and 5 mm. Thicker rod yields higher output rates, but requires more heat and a shoe that can accommodate it. If you change diameter, the feed rollers and welding shoe must be changed accordingly.
The welding rod must be stored clean and dry. Dust and moisture on the rod are carried into the melt, causing voids – a frequent cause of weld seams that look fine but fail.
Typical faults and how to avoid them
Failing weld seam: incorrect material, moisture or oxide layer. Voids: moisture, excessive temperature or contamination particles. Brittle fracture: overheating or incorrect welding rod. Cold edges: preheat nozzle too narrow or feed speed too fast.
Clean the parent material, wipe it dry, use fresh and dry welding rod, and perform a test weld on a piece of the same material before starting the actual joint. See also extrusion welding in practice.