The short answer
A cushion curve plots how much shock a foam passes through to the product, against how hard the product presses on that foam. The shape is a U, and both ends of it are failures.
Too little weight on the cushion is as dangerous as too much. Spread a light product over a large pad and the foam barely deflects, so it behaves like a board and transmits the impact almost intact.
Cushioning is where instinct points the wrong way. More foam and a bigger pad both feel safer, yet either can make a product more likely to break.
The curve explains why, and it is the tool engineers use to size a cushion. It is also why two shippers can use the same foam at the same thickness and get different results.
This reference covers what the curve plots, why both ends fail, how to read one into a cushion dimension, and its limits.
What the Curve Plots
The vertical axis shows transmitted shock in G, which is what the product feels on impact. The horizontal axis shows static loading, the weight the product places on each square inch of foam beneath it.
Static loading is simple arithmetic: product weight divided by the bearing area of the cushion. A 10 lb product sitting on 20 square inches of foam loads it at 0.5 psi.
Laboratories generate curves by controlled drop testing to a published method such as ASTM D1596, dropping samples repeatedly across a range of loadings and recording the peak deceleration each time.
The same foam, at one thickness and one drop height. Move along the horizontal axis by changing pad size, not by changing material.
Why Both Ends Fail
The outer box has its own ratings, and the ECT grade chart covers how those are certified. The right side of this curve is the one people expect. Load the foam too heavily and it compresses fully on impact, so the product meets the box floor through a crushed pad. The industry calls this bottoming out.
The left side catches designers, because a light product spread across a generous cushion barely compresses the foam at all. Foam absorbs energy by deflecting, so a cushion that does not deflect does not cushion, and it passes the impact straight through.
That is why adding foam can make matters worse. Doubling the pad area halves the static loading, and if the design was already near the left of the curve, the extra material moves it into the rigid zone.
The counterintuitive part in one line. A cushion only works when the product presses on it hard enough. So the cheapest safe design and the safest design usually sit in the same place, toward the right of the usable range.
Reading the Curve Backwards
Design runs in the opposite direction to the graph, because you start with the product and finish with a pad size.
- Find the fragility. The deceleration in G the product survives sets the ceiling the curve must stay under.
- Fix the drop height. Take it from the distribution environment, or from the protocol the package must pass.
- Pick the matching curve. A curve drawn at 30 inches says nothing about a 36 inch drop.
- Read the usable region. Wherever the curve sits below the fragility line, the loading performs.
- Take the highest safe loading. The right edge of the band uses the least foam.
- Convert to area. Product weight divided by that loading gives the bearing area: 10 lb at 1.0 psi needs 10 square inches.
That last step decides the cost, since a loading halfway along the region rather than at its edge can double the foam in every carton.
What the Curve Does Not Tell You
Published curves often show first-impact performance. Foam absorbs energy by deforming, and that deformation never fully recovers. So a second drop meets a cushion that has already given up part of its travel, which is why some suppliers publish separate curves for later impacts.
That gap shows up in testing, because a parcel protocol such as ISTA 3A runs a drop sequence rather than a single drop. A cushion designed from a first-impact curve can pass on paper and fail in the lab.
Vibration is a different failure entirely, because a curve describes a single shock event while transit delivers hours of low-level input. That input can fret a product against its cushion, and nothing on the graph predicts it.
Temperature and time also move the answer, since foam stiffens when cold and softens when warm. A cushion held under load for weeks takes a compression set it never recovers from.
Common Questions
Does more foam always mean more protection?
No. Extra thickness can help, though extra area lowers the static loading, and a lightly loaded cushion stops deflecting and transmits shock almost intact. More foam helps only while the loading stays inside the usable range.
What is static loading in a cushion curve?
The weight the product places on each square inch of cushion beneath it, which is product weight divided by bearing area. It is the horizontal axis, and it is what changes when you resize a pad.
Does one curve work for a different thickness?
No. A curve describes one material at one thickness for one drop height, so changing any of the three moves the characteristic, which is why suppliers publish a family of curves.
Why does the curve rise at the left?
Because foam absorbs energy by deflecting, and under a very light load it barely compresses. It then behaves like a rigid sheet, passing the impact through rather than absorbing it.
Using a Curve Well
This reference has covered what a cushion curve plots, why both ends fail, how to read one into a pad size, and the limitations of a single diagram. The design that protects the product and the one that costs least usually coincide.
Treated that way, a curve turns cushioning from guesswork into arithmetic; treated as proof that a package will survive, it carries a claim it never made.
Key takeaways
- A cushion curve plots transmitted shock in G against static loading, in a U shape.
- Loading the foam too lightly is a failure mode: undeflected foam behaves like a rigid sheet.
- Static loading is product weight divided by bearing area, so cushion dimensions are the variable.
- The highest loading that stays under the fragility line uses the least foam.
- One curve covers one material, thickness and drop height, and ignores repeated impacts and vibration.