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High-Shock Electronics Cushioning: Why Shock and Vibration Need Different Foams
Defense electronics face two mechanical threats engineers often lump together: steady vibration, which an isolation mount tunes out, and shock, a single fast transient that surviving is about absorbing energy within the cushion's crush, not tuning a frequency. A foam optimized for one is frequently wrong for the other. This article explains what a shock event does to a cushion, why stiff foams transmit and soft foams bottom out, the difference between slow and fast rebound, and how to leave room to crush.
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Shock and vibration are different problems, and they want different foams
Defense electronics face two mechanical threats that engineers often lump together, to the detriment of the design. Vibration is a steady, repeating input that an isolation mount handles by tuning its natural frequency below the disturbance. Shock is a single, fast transient that delivers a burst of energy in a few milliseconds, and surviving it is about absorbing that energy within the cushion's available crush, not about tuning a frequency.
A foam optimized for one is frequently wrong for the other, so the first step in cushioning electronics is deciding which threat dominates.
What a shock event actually does to a cushion
A shock pulse tries to decelerate a mass very quickly, and rapid deceleration means high force. A cushion's entire job is to stretch that deceleration over more time and more distance, trading a brief, violent spike for a longer, gentler push that the hardware can survive. The cushion does this by crushing: it gives way under the pulse, absorbs the energy as it deflects, and lowers the peak force the protected part ever feels.
How well it works depends on whether the foam has enough crush, at the right resistance, to absorb the whole event before it runs out of travel.
Why a stiff foam transmits and a too-soft foam bottoms out
There is a window. Too stiff a foam barely deflects under the pulse, so it passes the spike almost undiminished into the hardware, the cushioning equivalent of mounting the part on a brick. Too soft a foam crushes completely, runs out of travel, and then the part slams into whatever is behind it; the foam has bottomed out and the last of the energy arrives as a second, hard impact.
The right grade sits between: firm enough to resist progressively, soft enough to use its full thickness, so the energy is spent within the crush and the peak force stays low. Finding that window is the heart of shock-cushion design.
Slow rebound versus fast rebound
Slow-rebound grades for transient events
Slow-rebound microcellular urethane, such as the ShockSeal 4790-79 grade in the PORON family, absorbs a transient and recovers slowly. Because it does not spring back immediately, it does not store and return the shock energy as a rebound, which is exactly what you want for an infrequent, high-energy event: absorb it, dissipate it, recover for next time.
Fast-rebound grades for repeated cushioning
Where the input is repetitive rather than a single transient, a faster-rebounding grade of PORON urethane or a resilient silicone foam recovers quickly between cycles and keeps cushioning. Closed-cell vinyl nitrile adds a sealing function where the cushion also keeps out moisture. The rebound behavior, not just the firmness, is part of the selection.
Static loading sets up the dynamic response
A cushion does not start its shock job from a relaxed state; it starts from wherever the static weight of the hardware has already compressed it. If the part is heavy and the foam is thin, the static load may already use most of the crush, leaving little travel for the shock and inviting a bottom-out. This is why the supported mass and the installed deflection matter as much as the grade: the foam has to carry the static load on the firm part of its curve while keeping enough crush in reserve for the transient.
Specify enough thickness that the static load leaves room to work.
Reading a Cushion That Is Not Protecting
| Symptom | Likely cause | Direction |
|---|---|---|
| Hardware feels the full spike | Foam too stiff; barely deflects | Softer grade or more thickness to use the crush |
| Second hard impact after the pulse | Foam bottomed out; ran out of travel | Firmer grade or more thickness; reduce static deflection |
| Energy returned as rebound | Fast-rebound grade on a one-shot event | Slow-rebound grade to dissipate, not return |
| Works at first, degrades over time | Compression set eroding the crush | Low-compression-set grade; verify installed deflection |
| Low peak force, full recovery | Grade and thickness matched to the event | Working as intended; hold the tuning over life |
Directional guidance only; the actual response depends on the mass, the pulse, the crush space, and the geometry of your hardware. Confirm with your own shock analysis.
Designing the cushion: leave room to crush
The design rule follows from the mechanism: give the cushion enough thickness that it can use its full crush to absorb the event, choose a firmness that resists progressively without bottoming out under the worst-case pulse, and match the rebound behavior to whether the input is a single transient or a repeated one. Account for the static weight first so the dynamic crush is what is left after the hardware settles in. The material supplies the energy-absorbing crush; the analysis of mass, pulse, and travel is what sizes it.
How H-O Products helps
H-O Products die-cuts, waterjet-cuts, and laminates cushioning materials to your drawing in Winsted, Connecticut; we do not manufacture the foams in-house. We cut PORON, ShockSeal, silicone-foam, and vinyl-nitrile cushions to thickness and footprint, build up multi-layer cushions, back them with adhesive, and kit them by hardware location, all under our ISO 9001:2015 certified quality management system with lot-coded traceability and the maker's data on the order.
Frequently asked questions
Why is shock cushioning different from vibration isolation?
Vibration is a steady, repeating input handled by tuning a mount's natural frequency below the disturbance. Shock is a single fast transient, and surviving it is about absorbing the burst of energy within the cushion's available crush, lowering the peak deceleration force. The two jobs can want different grades, firmness, and rebound behavior, so it is worth deciding which threat dominates before choosing a foam.
Why does a stiff foam make shock worse?
Because it barely deflects under the pulse, so it passes the spike almost undiminished into the hardware, like mounting the part on a brick. A cushion protects by crushing, spreading the deceleration over time and distance; a foam too stiff to crush cannot do that. The fix is usually a softer grade or more thickness so the foam can use its travel to absorb the event.
PORON ShockSeal versus standard PORON: when do I specify each?
Slow-rebound ShockSeal absorbs a transient and recovers slowly, so it dissipates rather than returns the energy, which suits infrequent, high-energy events. A faster-rebounding standard grade recovers quickly between cycles, better for repeated cushioning. Match the rebound behavior to whether the input is a one-shot transient or a repeating one; both are converted to your drawing the same way.
What happens if the cushion bottoms out?
It runs out of travel mid-event, and the protected part then slams into whatever is behind it, so the last of the shock energy arrives as a second, hard impact. Bottoming out usually means the foam was too soft or too thin, or the static weight of the hardware already used most of the crush. More thickness, a firmer grade, or less installed deflection keeps travel in reserve.
Does H-O sketch or analyze the shock environment for me?
No. H-O supplies converted cushioning materials to your drawing and the maker's force-deflection data; we do not perform shock analysis, qualification testing, or weapons-hardware design. Send the supported mass, the pulse your hardware must survive, and the crush space available, and we will help point at a grade and thickness and convert the cushion accurately.
Have a shock cushion reviewed
Send the supported mass, the crush space available, and whether the input is a single transient or repeated, and we will help point at a grade and thickness and quote converted cushions to your drawing.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
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Related reading
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D3574 (flexible cellular urethane, force-deflection and compression set); ASTM D1056 (flexible cellular rubber); ASTM D2240 (durometer).
- MIL-STD-810 (environmental and shock test methods). Cited by designation for orientation; H-O performs no qualification testing and designs no weapons hardware.
- Manufacturer technical data sheets: PORON microcellular urethane and ShockSeal, silicone foam, vinyl nitrile. Energy-absorption behavior is standard cushioning theory, summarized qualitatively.
This article summarizes material-selection principles for missile, UAV, and weapons-system hardware using the families and method designations cited on the parent application page. H-O Products supplies converted materials only; we do not design, integrate, or test weapons systems, and nothing here describes weapon performance. Comparisons are qualitative; values are per each grade's current technical data sheet, and no test results are quoted.
MIL and FAR designations are named for orientation only; H-O makes no qualification, compliance, or airworthiness determinations and holds no claim to program-specific certifications. All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a converter, not a raw-material producer.