PS and HIPS: Where the Window Sits on a Low-Cost Resin

PS and HIPS are common in packaging, household goods and appliance internals. They are cheap and easy to mould, and among the easier resins to weld ultrasonically, but a low-cost resin brings its own traps.

PS: a wide window, but brittle

General-purpose PS is amorphous with a broad softening range, so it fuses across a wide span of settings. The difficulty is that it is brittle and notch sensitive.

Sharp corners on the joint are the first killer. The root of an energy director and the corners of a shear joint, left unradiused, leave a ring of stress concentration; transport vibration or a drop cracks there first.

Do not be generous with interference. Take shear joint interference at the low end; PS will not carry large side-wall deflection.

Pad the fixture contacts. Hard locating points on PS press in white marks, and stress whitening is microscopic cracking — the start of a crack.

HIPS: tougher, but hungrier for amplitude

HIPS adds a rubber phase to PS for impact resistance, and that brings two changes: toughness rises, and so does internal damping.

Higher damping means the wave attenuates faster. For the same joint, HIPS needs somewhat more amplitude than PS, and far-field welding is harder.

The other change is at the surface. The rubber phase leaves HIPS softer than PS, so horn impressions show more. On cosmetic parts, put the horn contact on a non-visible face — a drawing-stage decision that beats adjusting parameters afterwards.

Two shared traps

Regrind ratio. Parts in these resins commonly contain regrind, often at a ratio that is not fixed. Lower molecular weight means lower melt viscosity, so the same energy produces more flash and a thinner joint. Fix the ratio and write it into the work standard rather than running 10% one day and 30% the next.

Stress cracking. Both resins are sensitive to solvents. Residual stress from welding, plus cleaners, label adhesive or oil downstream, produces cracks days later. The test is simple: remove the suspected chemical contact and see whether the cracking stops. If it does, work both routes — lower weld force and longer hold to reduce stress, and change the incompatible chemical.

Compatibility

PS welds to HIPS. PS is partially compatible with SAN and ABS. Neither joins to any semi-crystalline resin, so where a design pairs them, use staking or mechanical fastening.

The compatibility matrix is in the weldability handbook on the downloads page. Force control and approach speed settings are on the ultrasonic welding machine pages.

Send the grade, regrind ratio and drawings to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.