Plastic journeyman

Thermoplastic can be welded because the material softens when heated and melts together when two surfaces are brought into contact under pressure and then cooled. This distinguishes thermoplastic from thermoset which cannot be welded. Common materials are PE (polyethylene), PP (polypropylene), and PVC used for pipes, sheets, containers, and membranes.

You can only weld plastic types that fit together — PE is welded to PE, PP to PP. If the materials are confused, the joint becomes weak or without bonding. Therefore, the material is identified before work, and the welding parameters are adjusted to the specific plastic, because melting temperature and behavior vary from type to type.

The plastics manufacturer SIMONA puts it precisely: Only the same type of plastic can be welded together, e.g. PP with PP. The materials must be in the same or an adjacent melt index group and have the same density. The melt index tells you how easily the plastic flows when it is melted. The colour does not matter.

With hot air welding heated air is blown at the seam and a filler wire of the same material is pressed into the softened seam. The method is widely used for sealing membranes foil joints and minor sheet work and requires a stable interaction between temperature air velocity feed rate and pressure.

With mirror welding the pipe ends are heated against a hot mirror (a flat heating plate) after which the mirror is removed and the ends are pressed together and held under pressure while the joint cools. The method is used for PE and PP pipes and gives a strong homogeneous joint when the ends are clean flat and aligned correctly.

Extrusion welding adds molten filler material from an extruder and is used for heavier sheet joints and tanks. Electrofusion uses fittings with built-in resistance wires: when power is applied, the wires heat the plastic so that the fitting and the pipe fuse together. The method is widespread for PE pipes, but it is very sensitive to dirt and poor preparation. Incorrect preparation of the pipe is the most common reason an electrofusion weld fails.

Whatever the method, clean, dry and correctly prepared surfaces are crucial. Before electrofusion, the PE pipe is scraped with a rotary scraper: at least 0.2 mm, and at least 0.3 mm on pipes above DN225. A hand scraper must not be used for scraping. After welding, the joint is inspected visually, and depending on the task a leak test may be carried out. Welding parameters and the process are often documented, especially on pipelines.

The table shows SIMONA's recommended temperatures. For butt fusion welding and socket welding it is the temperature of the heating element. For hot air it is the air temperature, measured 5 mm inside the nozzle.

• 01Butt fusion welding: Plane the pipe ends. The shaving must be no more than 0.2 mm thick.
• 02Never touch the planed surfaces with your hands.
• 03Electrofusion welding: Scrape the pipe with a rotary scraper. A hand scraper may only be used to deburr the edge.

A joint must be inspected and documented. And anyone who welds plastic pipes must have a valid welding certificate with the right endorsement.

Ground-source heat collectors are PE pipes in the ground. The chart shows where in the country there are most buildings with ground-source heating — and therefore where there are PE pipes that must be joined correctly.