Welding Options For Molded Parts

Andy Bramer
Drader Injectiweld Inc.
Edmonton, Alberta Canada

Edited by Jean M. Hoffman

It's often to a designer's advantage to make a rotationally or rotomolded part as simple as possible, then customize it with additional fittings, baffles, flanges, and various attachments using a variety of welding operations. A specialized welding system called Injectiweld also seals and permanently repairs imperfections such as blow-bys and voids.

But not all plastics are weldable, including all thermosets. Welding thermoplastics is best done with a single-resin system. In other words, welded parts should be of the same material. Welds made with different plastics may have poor strength compared to part material. But there are a number of compatible resins systems that bond. They include:
• High-density polyethylene (HDPE) to linear low-density polyethylene (LLDPE).
• Acrylonitrile butadiene styrene (ABS) to polycarbonate (PC).
• Polyvinyl chloride (PVC) to ABS. Conversely, HDPE doesn't weld to either polypropylene (PP) or ultrahigh-molecularweight PE (UHMPE). And cross-linked materials, which are in some respects similar to thermoset plastics, don't easily weld to HDPE either.

Basic welding requirements
First, the welding area must be clean. Surface oxidation and other contaminants such as grease, dust, and damaged material all must be removed. Surface preparation can be as simple as scraping a thin layer off the area to be welded.

Second, no matter what kind of welding unit is in use there are always three parameters to consider for a quality weld:

Temperature. Each thermoplastic melts within a certain temperature range to its welding point. It is important to stay between the minimum and maximum weld temperature. Drifting outside this zone will result in poor weld quality.

Pressure. Applying force lets the plastic molecules mix. The best bonding occurs when there is a physical mixing of the plastics. Too low or too high pressure will reduce weld quality.

Time. Welded plastics need a certain time to heat up and cool down. If the plastic is heated too fast, the plastic won't melt properly. And actively cooling parts with air or water can cool parts too quickly, creating large stress concentrations inside the weld bead.

Welding systems
Selecting the right welding equipment can be a challenge. Getting a good handle on the types of material that will be welded, the work area available, and the kind of assembly needed is a good first step. However, there are many things to consider so it is prudent to work closely with a welding house in order to ensure "perfect" welds.

Electric socket is mostly used to weld polyethylene pipes together. Electric power heats a copper wire inside the socket. The plastic melts and expands. The pressure of the expansion joins the pieces together.

Ultrasonic welding uses an acoustic tool to transfer vibrational energy through the plastic parts to the weld area. Friction generates heat which melts the plastic. Welding occurs as the plastic solidifies under pressure once the vibration stops.

High-frequency (HF) welding is similar to ultrasonic welding except that HF welds are continuous instead of stamped. HF welders are typically used for joining thinner materials.

Hot-plate welding joins parts with the same shape. During the process, the two joint surfaces are held against a hot plate long enough to heat and soften. After removal from the plate, the parts are joined under pressure until the weldline solidifies.

Spin welding uses frictional heat generated from rapidly spinning round parts against a stationary mate. Friction melts the surfaces as pressure is applied. Rotation stops and the parts weld together when cool.

Hot-air/hot-gas welding consists of first beveling the edges of the two parts to be joined then bringing the two edges together forming a groove into which a hot bead of thermoplastic is placed. The hot air or gas preheats the surfaces and melts the welding rod. The parts are held under pressure as the bead solidifies. However, oxidization on plastic surfaces can weaken welds. Nitrogen may replace hot air to reduce oxidation. This process is a relatively inexpensive way to join plastics, but resulting bonds may be inconsistent and relatively weak.

Extrusion welding is similar to hot-gas welding except that a hot-melt extruder applies a molten thermoplastic bead into the groove area. The extrusion welder's screw or auger mechanism moves either rods or pellets through a barrel. Inside the barrel, the plastic mixes, compresses, and melts. The auger pushes the melted plastic out of the welder creating the weld bead. Extrusion welders preheat the weld area with hot air or gas.

Injection-welding systems use a heated tip and injection pressure to form welds. The hot (interchangeable) tip melts the surface of the plastic and creates a weld zone into which molten plastic is injected. There is an actual physical mixing of the weld bead and the plastic. Injection welding is said to produce the strongest welds in this group. Because the tip cleans the plastic surface prior to welding, preparation is unnecessary unless there's heavy contamination.