Short answer: use linear vibration welding when the part is medium to large, the joint lies in one plane (or close to it), and you need a sealed weld along a long flange or across internal walls. It is usually faster than hot plate welding on the same part and covers a larger weld area than ultrasonic. It is a poor fit for round parts that can simply be spun, for thin flexible walls that move with the tool instead of rubbing, and for joints that wrap around a strongly 3D surface.
The rest of this note covers what decides whether a vibration weld is good: the flange, the material pair, the settings and the usual ways it goes wrong.
What actually happens at the joint
One half sits in a fixed lower fixture. The upper half is clamped in a fixture on the vibrating head and moved back and forth in a straight line while the two are pressed together. At first it is dry friction. As the surface heats, a thin melt film forms and the process becomes viscous shearing of that film. Molten material is pushed out to the sides as flash and the parts move closer together. That movement is the weld depth, or meltdown. When the target depth is reached the vibration stops, the head brings the parts back into alignment, and the machine holds pressure while the melt solidifies.
The steady phase is the one to watch. Once melt is being squeezed out at the same rate it is created, extra time only adds flash and depth. So controlling the weld by depth rather than by time gives far more repeatable parts. On SHNEHP machines welding depth is controlled in closed loop with a BALLUFF or GEFRAN sensor, and the worktable position is tracked by a depth sensor.
Joint design comes first
On a new project, most vibration welding problems are decided at the mould design stage, not at the machine.
- The joint has to lie in the plane of vibration. A small out-of-plane slope can work, but walls that run parallel to the motion get little relative movement and weld poorly.
- Give the flange enough width to carry the clamping load and to stay stiff. A narrow lip on a thin wall flexes with the tool and never builds heat at the interface.
- Plan where the flash goes. Flash traps or a tongue-and-groove style joint hide the squeeze-out, which matters on visible parts and on parts where loose particles are not allowed downstream.
- Leave clearance in the direction of motion for the amplitude you plan to run. Bosses, ribs and clips near the joint must not collide with the other half while it moves.
- Internal walls can be welded in the same stroke as the outer flange, which is one of the main reasons the process is used for multi-chamber parts. Keep their weld surfaces at the same height as the perimeter so they all start rubbing together.
- Design the part so the fixture can grip it close to the joint. Support far away from the weld line lets the wall bend instead of rub.
Materials: what welds well and what needs care
Vibration welding works with most thermoplastics, both semi-crystalline grades like PP, PE and PA and amorphous grades like ABS and PC. SHNEHP's product page lists PP, TPE and glass-filled materials as typical, and these are where the process is most often chosen over the alternatives.
A few practical points:
- PP and PE are forgiving. They melt readily and the main risk is too much flash rather than a weak weld.
- PA, filled or not, absorbs moisture. Wet parts can foam and bubble in the weld zone, so moulded parts should be welded dry or within a controlled time after moulding.
- Glass fibre does not cross the weld line in a meaningful way, so a glass-filled joint will be weaker than the base material. Design the flange width with that in mind instead of expecting parent-material strength.
- Both halves should be the same polymer, ideally the same grade. Dissimilar pairs only work if the two materials are chemically compatible and melt in a similar range.
- Fillers, pigments and mould release on the joint surface all change friction. If a new resin lot behaves differently, check the surface before changing settings.
How the settings trade off
There is no single correct recipe. These are the levers and what each one costs you.
| Setting | Raise it and you get | The cost |
|---|---|---|
| Amplitude | Faster heating, shorter cycle | More flash and particles, more stress on bosses and clips near the joint |
| Weld pressure | Faster meltdown | A thinner melt film and, if too high, weaker welds and more squeeze-out |
| Weld depth | More of the surface welded, better seal on uneven parts | More flash, longer cycle, dimension change |
| Hold time and hold pressure | A properly set joint before release | Longer cycle |
Frequency is mostly set by the machine design and the tooling mass. On SHNEHP's XN VW series the theoretical frequency is approx. 210 to 270 Hz on VW15 to VW100 and approx. 70 to 120 Hz on the large VW200, with amplitude adjustable from 0 to 1.8 mm on all models. The usual approach on a new part is to fix the amplitude near the middle of what the part tolerates, then tune pressure and weld depth, and only then shorten the cycle.
Typical parts
The process earns its place on parts with long sealed joints and internal walls. Examples from SHNEHP's application areas include air ducts and other flow channel parts, water tanks for appliances and automotive reservoirs, multi-port manifolds and distribution housings, and interior trim parts such as covers, brackets and decorative pieces.
If the part is round and both halves can rotate relative to each other, spin welding is normally cheaper. If the joint follows a deep 3D contour, hot plate welding is usually the better choice even though the cycle is longer. Small parts with short joints are often faster on an ultrasonic machine.
Common failure modes and where to look
| What you see | Likely cause | Where to look first |
|---|---|---|
| Leaks at corners or at the ends of a straight run | Flange flexing or uneven clamping at the corners | Fixture support close to the joint, flange stiffness |
| Weak weld with little flash | Not enough meltdown, or the wall moves with the tool | Weld depth target, part gripping in the upper fixture |
| Heavy flash and loose particles | Amplitude or depth too high for the joint | Reduce amplitude first, then depth, add flash traps in the next mould revision |
| Bubbles or porous weld on PA | Moisture in the parts | Drying and time between moulding and welding |
| Halves offset after welding | Parts released before the melt set, or poor realignment | Hold time, fixture locating features |
| Cracked clips or bosses near the weld | Collision or vibration damage | Clearance in the direction of motion, amplitude |
| Good welds on trial, scatter in production | Welding on time instead of depth, resin or moisture variation | Switch to depth control, check incoming material |
A sealed part should not leave the cell on visual inspection alone. SHNEHP builds welding and testing fixtures, including sealing test fixtures that close off the air ports and apply pressure to check weld integrity.
How SHNEHP sizes a machine
The VW range runs from the XN VW15, rated for a welding area of 120 cm2, to the XN VW200 at 650 cm2, with mould weight, table opening and clamping force growing across the range. Each machine and fixture is configured around the plastic material, part geometry, weld area, inspection requirements and target cycle time. A typical project goes from technical review to sample welding and process validation, then machine, electrical and fixture design, in-house testing, delivery, installation and operator training.
SHNEHP provides global service, with service partners in the USA, Brazil, India, Thailand, Egypt and Qatar. We respond remotely within 24 hours.
To check whether your part suits vibration welding, send the 3D drawing, resin grade and target cycle time through the inquiry form. If you have a few moulded parts, send them over and we will weld samples free of charge before you commit to a machine.
FAQ
Can vibration welding make an airtight joint?
Yes. A continuous flange welded to a controlled depth gives a hermetic seal, which is why the process is used on tanks, reservoirs and ducts. The seal still has to be confirmed by a leak test on each part or on a sampling plan.
Vibration welding or hot plate welding?
Choose vibration welding for flat or near-flat joints when cycle time matters. Choose hot plate welding when the joint follows a deep 3D contour or when the part cannot take vibration.
Is a glass-filled nylon vibration weld as strong as the part?
No. Fibres do not bridge the weld line, so expect a lower strength than the base material and size the flange to suit.
Does SHNEHP weld sample parts?
Yes, free of charge, using your own parts. Sample welding and process validation are part of SHNEHP's normal project workflow.
Request a Technical Consultation
Send your part drawing, plastic material and target cycle time. Our engineers will check whether the part suits vibration welding and recommend a machine and fixture configuration.
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