How spin welding works

One part spins. The other part stays still. Friction at the circular joint turns into heat. The heat melts a thin layer of polymer at the joint face. The spin stops. Pressure rises. The melt cools into a solid bond.

The joint itself does the work. The parts do not need to be round. Only the joint interface needs to be circular. A square housing can still spin weld if its joint ring is round. That detail opens the method to housings, caps, and containers with flat sides.

A tongue-and-groove guide lines the parts up before the spin starts. It also gives an even bearing surface while the joint melts. A burr trap sits beside the joint. It catches the small bead of melt that squeezes out. The flash stays hidden inside the trap instead of showing on the outside face.

Most spin-welded parts start in the injection molding process with a molded joint ring. The ring must be true and clean. A warped ring melts thin on one side and thick on the other. The joint then leaks at the thin spot.

The weld melts more material than most other joining methods. It moves up to about 3x more material at the joint. That extra melt fills small gaps and seals the joint. It is one reason spin welds suit hermetic seals and pressure-tight joints.

Spin-weld joint designs that hold

Four joint shapes cover most spin-weld work. Shear joints melt along a vertical wall. Tongue-and-groove joints guide the parts and trap the melt. Scarf joints spread the melt over a sloped face. Butt joints sit flat face to face. They are rare in spin welding because the melt has nowhere to go.

A spin welding joint design guide from Emerson shows each shape in section. Read it before you draw the joint. A shear joint suits tall walls. A tongue-and-groove joint suits parts that must line up fast.

Size the weld area to match the wall. The weld area should equal or exceed a typical wall section. A weld thinner than the wall breaks first. A weld far thicker than the wall wastes melt and cycle time.

Which plastics spin weld well

Spin welding works on almost any thermoplastic, filled or unfilled. It works on amorphous resins and on semi-crystalline resins. ABS and PC weld good to excellent. Both melt clean and flow at the joint. Glass-filled grades weld well too, though the fiber wears the joint face faster.

Delrin acetal and UHMWPE are difficult. Both resist melting at the joint and slip instead of gripping. Hard parts need more torque to hold the spin. PC and acetal sit in that hard group. If the driver cannot hold the torque, the part slips and the joint never heats.

Part size sets the other limit. Spin welding joins parts from 0.5 to 18 inches in diameter. Small parts need a light touch and a short spin. Large parts need a strong driver and a steady fixture. Check the diameter before you promise a spin weld on a drawing.

Do not mix a spin weld up with a mold defect. A spin weld is a deliberate joint. It is not a weld-line defect. Weld lines form where two melt fronts meet in the mold. A spin weld forms after molding, on purpose, at a joint you drew.

How solvent bonding works

Solvent bonding softens a small area on each joining surface. The solvent lets the polymer chains on both faces mix. The solvent leaves. The chains lock together. The joint is the same polymer on both sides, so it can look near-invisible on clear acrylic.

It works on acrylic and PC. It also works on PVC, PETG, PS, and ABS. It does not work on polyolefins. PE and PP resist solvents. PTFE resists solvents too. No solvent softens those resins enough to bond. A buyer who asks for solvent bonding on a PP part will get a joint that falls apart in the hand.

Solvent choice limits the resin list. For two different plastics, the same solvent must work on both. That rule kills most dissimilar pairs. A thickened solvent cement can take 24 to 48 hours for full cure. Plan the cure time into the schedule, not after it.

PC needs one extra step. Let vapor disperse after bonding. Trapped solvent gas can attack PC and craze the surface. Leave the part open to air. Do not pack it tight while vapor is still leaving.

Solvent bonding suits low-volume runs and rapid prototypes where a weld fixture is not yet built. The parts need no spin fixture and no glue gun. They need a brush, a solvent, and a jig that holds them still while the joint sets.

How adhesive bonding works

Adhesive bonding adds a glue layer between the parts. One-part glues cure on their own. Two-part glues cure when the parts mix. The glue sticks to each mating surface and cures into a solid layer. The joint strength comes from the glue and from how well it grips each surface.

Adhesive bonding is more flexible for dissimilar materials. It joins plastic to metal and glass. It also joins plastic to fabric. Suppliers list glues by resin family. Match the glue to the resin on the drawing, not to the glue on the shelf.

Surface energy decides whether the glue wets the surface. PC and ABS sit near 42 dyne per cm. PP and PE sit near 30. That gap matters. Low-energy plastics need flame or plasma treatment before the glue will grip. Skip the treatment, and the glue peels off in a sheet.

UV-cured adhesives cure in seconds. They work best with transparent parts. The light must reach the bond. Shadowed joints are the main limit. If the joint sits behind an opaque wall, UV cannot cure it. Use a two-part glue there instead.

A plastic adhesive guide from Permabond lists glues by resin family. Weigh cure time, cost, and method needs before you pick a glue. A fast cure helps a line move. A slow cure lets you reposition the part. The right pick depends on the line, not on the glue catalog.

Solvent vs adhesive: which one fits your part

Both methods bond without heat or spin. Both are permanent. The split comes from the resin pair, the look, and the cure. Solvent bonding needs a matching solvent. Adhesive bonding needs a matching glue and a clean surface.

Pick solvent bonding when the joint must disappear on a clear part. Pick adhesive bonding when the parts are different materials. When neither fits, spin welding may still work if the joint is round.

Choosing between spin welding solvent and adhesive bonding

Start with the joint shape. A round joint points to spin welding. A flat face with a matching solvent points to solvent bonding. A mixed-material joint points to adhesive bonding. The shape answers the question before the resin does. That is the full choice in spin welding solvent and adhesive bonding.

Then check the resin. ABS and PC spin weld well. Acrylic bonds clean with solvent. PP and PE need adhesive bonding with surface treatment. Delrin and UHMWPE are difficult to spin weld. Check the glue supplier before you promise a bond on either resin.

Then check the run. Spin welding suits repeat runs with a fixture. Solvent bonding suits small runs and clear parts. Adhesive bonding suits mixed materials and parts that cannot spin. We ask for a drop test and a pressure test before we sign off a joint.

Send an injection moulded product sample with the joint ring molded in. A sample tells the supplier more than a drawing note. It shows the wall, the ring, and the resin in one piece.