Failing the Drop Test: Impact Is Not Static Loading

Handheld products, appliances and packaging all face drop testing. The familiar situation is a large margin on the pull test and a crack along the joint after a one-metre drop.

The strain rate effect

Plastics are viscoelastic, and loading speed changes how they behave. Pulled slowly, the chains have time to rearrange and the material yields and draws; under impact the strain rate is orders of magnitude higher, the chains cannot move in time, and the material behaves brittly and simply breaks.

So a joint with generous ductility in a static test may have almost no plastic capacity under impact. Extrapolating impact performance from static data starts in the wrong direction.

Temperature compounds it: materials are more brittle when cold, which is why many drop specifications require testing at low temperature — it is the most severe combination.

Notch sensitivity is amplified

Under impact, sensitivity to notches rises markedly. Three notches at the joint cost the most here.

A sharp joint root. Statically it may reduce strength slightly; under impact it is the fracture origin.

The flash edge. Likewise, a geometric notch acts as a crack at high strain rate.

The end of an unfused region. Local lack of fusion may average out in a pull test; under impact it is the crack source.

The remedies are still radii, flash traps and uniform fusion, but impact-loaded parts demand far more of all three than static ones.

The energy has to go somewhere

A drop is an energy event: the kinetic energy has to be absorbed by something. Design has two routes.

Let structure absorb it. Design energy-absorbing features away from the joint — ribs, crush features, soft feet — so the impact energy is spent before it reaches the weld. This is the most effective route.

Do not let the joint be the only load path. If every impact load must pass through one weld, that weld eventually becomes the failure point. Snap fits or screws as parallel paths improve drop performance substantially.

Material trade-offs

Impact-resistant materials such as ABS, PC and toughened grades are usually straightforward to weld, but grades with a high content of impact modifier can weld weaker than the standard grade of the same base, because the modifier does not take part in fusion at the interface.

Glass-filled grades depend on direction: good along the fibres, poor across them, and fibres never cross the weld interface in the first place. Drop failures on filled parts frequently occur at the joint.

These trade-offs belong in material selection alongside the structure, not in a revision after the drop test fails.

How to validate

Set the drop test from the real service conditions: height, floor material, orientation for corner, face and edge drops, temperature and number of drops. Edge and corner drops are usually more severe than face drops, because the energy concentrates into a small area.

Section failed parts and read the fracture: brittle fracture is flat with radiating marks and the origin is usually clearly identifiable. Finding the origin tells you whether to change the radius, the flash or the fusion.

The line still relies on consistency: monitor post-weld height and energy on every part and remove under-fused ones before they become drop test or field failures.

Joint design and notch control are in the handbooks on the downloads page. Energy and depth modes and per-part monitoring are on the ultrasonic welding machine pages.

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