The joint decides most of it. A round joint where one half can rotate points to spin welding. A small part with a short joint line points to ultrasonic. A long flange that lies more or less in one plane points to vibration welding. A large part with a joint that climbs over a 3D contour, or a material that does not like friction, points to hot plate welding. Bosses that have to hold a metal bracket or a circuit board point to hot staking. Induction welding is the specialist option for hollow, thin-walled or fragile parts where you can put a conductive implant in the joint.
Material and cycle time come second. They usually decide between two candidates rather than pick the process from scratch.
Quick comparison
| Process | Joint shape it suits | Typical part size | Where it struggles |
|---|---|---|---|
| Hot plate | Any shape the platen can follow, including 3D contours | Medium to large | Longest cycle of the group, heated tooling to maintain |
| Vibration friction | Flat or near-flat flanges, internal walls in the same plane | Medium to large | Round parts, very thin flexible walls, deep 3D joints |
| Ultrasonic | Short joints designed with an energy director or shear joint | Small to medium | Large weld areas, long joints, very soft materials |
| Spin | Circular, axis-symmetric joints | Small to medium round parts | Anything that is not round |
| Hot staking | Plastic posts formed into rivet heads | Covers, panels, parts with many bosses | It joins by mechanical lock, it does not seal |
| Induction | Joints with a conductive implant | Hollow, thin-walled, complex parts | Needs the implant and a custom coil for each part |
Hot plate welding
A heated platen melts both mating surfaces, retracts, and the two halves are pressed together until the melt cools. Because the platen can be machined to follow the joint, it handles large parts and joint lines that move in three dimensions, and it can give a hermetic seal when the joint is designed for it. Water tanks, lamp housings and large containers are typical work. SHNEHP also uses large-format hot plate welding for HDPE and PP pallets, where the platen melts the internal ribs and the perimeter of the top and bottom deck at the same time.
Choose something else when cycle time is the main pressure and the joint is flat. Vibration welding will usually be faster on the same part.
Vibration friction welding
One half moves back and forth in a straight line under pressure. Friction melts the interface, then the motion stops and the parts are held until they set. It welds a large area in one stroke, internal walls included, which is why it is common on ducts, manifolds and tanks with several chambers. SHNEHP's vibration page names PP, TPE and glass-filled materials as typical, and its XN VW range covers welding areas from 120 cm2 to 650 cm2.
The joint has to allow relative motion. Walls running parallel to the vibration get little rubbing, and a thin wall that bends with the tool never heats up. Our note on vibration welding joint design goes through flange design and defects in detail.
Ultrasonic welding
The horn passes high-frequency vibration into the part and the energy concentrates at the joint, which melts in a fraction of the time the other processes need. There is no glue, no screws and no drying time. It suits small and medium parts such as housings, covers and filter parts, and SHNEHP offers 15 kHz, 20 kHz and 35 kHz machines plus a servo model.
It needs joint design up front. Without an energy director or a proper shear joint the energy spreads into the part and welds come out weak or marked. It is also limited by horn size, so long joints or large weld areas usually move to vibration or hot plate.
Spin welding
One half is held still, the other rotates under axial load, and the friction melts the circular interface. Rotation stops, pressure holds, and you get a sealed round joint, often faster than hot plate welding for round assemblies. SHNEHP's spin page lists PP and PE circular components as a good fit. When the two halves have to end up in a set angular position, choose a servo head that can stop at a set angle.
If the joint is not round, spin welding is simply not an option.
Hot staking
Molded plastic posts pass through a metal bracket, a circuit board or another plastic part, and heated tooling forms them into rivet heads. There is no high-frequency vibration, so it is safe for electronics, lenses, displays and high-gloss surfaces, and there is no ultrasonic dust. Multi-point heads form many rivets in one cycle. SHNEHP builds heads that form 36 points at once and multi-angle heads for contoured parts.
It holds parts together mechanically. If the assembly needs to be airtight, staking alone will not do it.
Ultra high frequency induction welding
A conductive implant sits in the joint. A copper coil shaped to the part sets up an electromagnetic field that heats the implant while the surrounding plastic stays largely unaffected, and the plastic melts from the inside out under clamping pressure. Because there is no vibration and no external hot tool, it is gentle on fragile, hollow and thin-walled parts and leaves little external flash.
The trade-off is part-specific tooling: every new part needs its own implant and coil design.
Material checks before you decide
Most thermoplastics can be welded by more than one of these processes. SHNEHP's hot plate parameter table, for example, lists ABS, PP, ASA, PC, PC/ABS, PEI, PES, Acrylic, PMMA, PPO, PS, PSU, rigid PVC, SAN, LCP, POM, Nylon, PA, PBT, PET, PEEK, PE and PPS. A few rules of thumb narrow the choice:
- Both halves should be the same polymer. Dissimilar pairs only weld well when the materials are compatible and melt in a similar range.
- Semi-crystalline materials such as PP, PE and PA melt over a narrow range. Ultrasonic can weld them, but it needs more energy and a good shear joint. Vibration, hot plate and spin welding are usually easier.
- Amorphous materials such as ABS and PC are the easiest to weld with ultrasonic.
- PA absorbs moisture. Weld it dry or soon after molding with any process.
- Glass fibre does not bridge the weld line, so the joint will be weaker than the base material.
- Soft or very flexible grades damp vibration. Hot plate or induction is often the safer choice for them.
When the answer is still not clear
Two processes often look possible on paper. A water tank with a flat flange could be hot plate or vibration welded. A small round cap could be spun or ultrasonically welded. In those cases the decision comes from sample welding: weld real parts both ways, leak test them, check the cycle time and the flash, then choose.
That is how SHNEHP works on new projects. Each machine and fixture is configured around the plastic material, part geometry, weld area, inspection requirements and target cycle time, and a project starts with a technical review followed by sample welding and process validation. Send your drawing, resin grade and target cycle time through the inquiry form. If you can send a few molded parts, we will weld samples free of charge before you choose a process.
SHNEHP provides global service, with service partners in the USA, Brazil, India, Thailand, Egypt and Qatar. We respond remotely within 24 hours.
FAQ
Which plastic welding process is fastest?
Ultrasonic welding usually has the shortest weld time, but only on small to medium parts. On larger parts vibration welding is normally faster than hot plate welding.
Which process gives an airtight joint?
Hot plate, vibration, spin, ultrasonic and induction welding can all give a hermetic seal if the joint is designed for it. Hot staking does not seal.
Can one machine do several processes?
Usually each process has its own machine type. Several processes can be combined in one automated line, for example welding followed by leak testing, and SHNEHP builds such custom machines when a part needs it.
How do I know which process my part needs?
Start from the joint shape and part size in the table above, then confirm with sample welds on your own parts.
Request a Technical Consultation
Send your part drawing, resin grade and target cycle time. Our engineers will help you choose a welding process and recommend a machine and fixture configuration. If you can send a few molded parts, we will weld samples free of charge.
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