Ultrasonic welding vs adhesive
Welding compared with a glue layer.
Ultrasonic welding vs adhesiveProcess guide
Solvent Bonding
Look at an IV set: tubing pushed into molded connectors, with no clips and no visible glue. Most of those joints are solvent bonded, and when they are done right the tube and the fitting become one piece of plastic.
How it works
Dissolve, press, and let the solvent leave
The joint has no glue layer. The two plastics themselves grow together.
A solvent is applied to one or both surfaces. It swells and softens the top layer of the plastic, freeing the polymer chains to move. When the parts are pressed together, chains from each side mix across the joint. As the solvent evaporates or soaks away, the plastic hardens again with the joint line gone.
The process only works on plastics a solvent can dissolve, which mostly means amorphous resins: PVC, ABS, acrylic, polycarbonate, polystyrene, and PETG. Polypropylene, polyethylene, and acetal resist almost every solvent, so they need welding or mechanical joints instead.
Application
How the solvent gets to the joint
The method is chosen by joint shape and how much solvent the part can tolerate.
| Method | Used for | Risk |
|---|---|---|
| Dip or swab | Tube ends and connector bores | Excess solvent pooling inside the bore |
| Needle or metered dispense | Controlled beads on housings and fittings | Needs automation to stay repeatable |
| Capillary wicking | Close fitting flat parts; solvent runs into the gap on its own | Only works when the gap is tight and even |
| Solvent cement | Solvent thickened with dissolved resin, for gaps and pipe fittings | Slower to cure; a visible bead if overapplied |
The main risk
Stress cracking: the failure that shows up later
A solvent that dissolves a plastic will also attack any area already under stress. Molded in stress plus solvent is how bonded parts crack days after assembly.
- Tight interference fits add hoop stress right where solvent sits
- Parts molded fast and cold carry more internal stress into bonding
- Solvent vapor trapped in closed parts keeps attacking long after the joint is made
- Polycarbonate is especially sensitive; annealing before bonding is a common fix
Joint design
Molded features that make bonding reliable
Most of these are decided when the part is designed, long before the solvent bottle is opened.
A slip fit or light press fit at the joint, not a heavy interference
A lead in chamfer so the tube or mating part does not scrape solvent away
A vent or gap so vapor can escape from closed joints
A stop feature that sets how far parts go together
Clean, release agent free surfaces at the joint
Keep reading
Related guides
Other joining methods, and resins that bond well with solvent.
Laser welding plastics
Another joint with nothing added.
Laser welding plasticsAnnealing molded parts
Relieving stress before bonding.
Annealing molded partsPolycarbonate
Tough, clear, and stress sensitive.
PolycarbonateAcrylic (PMMA)
Bonds cleanly for clear assemblies.
Acrylic (PMMA)Medical device housings
Enclosures for handheld and bedside devices.
Medical device housingsProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about solvent bonding
Is solvent bonding the same as gluing?
No. Glue leaves a layer of a different material between the parts. Solvent bonding leaves nothing behind once the solvent has gone; the joint is the base plastic.
Can two different plastics be solvent bonded?
Sometimes, if one solvent dissolves both. Common pairs include PVC tubing into PC or ABS connectors. Dissimilar pairs need testing for strength and cracking.
How long before a bonded joint can take load?
It handles light handling soon after assembly, but full strength comes only when the solvent has fully left the joint. Thick joints and closed parts take the longest.
Why is it so common in medical devices?
Tubing sets and fluid paths need leak free joints with no added material that could react with drugs or fluids. Solvent bonded PVC and PC joints are a long standing answer.
Next step
Send both parts and the joint
Upload CAD for the parts that meet and note the resin. The review looks at ultrasonic welding and assembly after molding as the joining route. NetProto manufacturing is ISO 9001 certified.