Built to Protect, Fitted to Shape
A case is two mechanisms working against each other. The shell buys time by bending two to four millimetres and springing back; the liner spends that time by holding the object inside a 20–35% compression band so it barely accelerates. Get the pairing wrong and one half does all the work until it gives out. What follows is the arithmetic that decides which half fails first: deflection budget, liner thickness, and the one to two millimetres of clearance. The figures below are typical ranges from production, not guarantees, but they are the numbers worth designing against.
01One Shell Spreads The Load, One Liner Stops The Travel
On impact the contact patch is a few square centimetres. The wall bends, and that bend carries force outward into ribs and corners. This is the shell's entire job: turn a point load into a distributed one. It is allowed to move, but only inside a narrow band. Past roughly four millimetres the polymer stops behaving elastically, and what comes back is a dent rather than a rebound.
The liner does not absorb; it restricts. Acceleration is distance divided by time, so the less an object can travel, the lower the peak it sees. Holding compression between 20 and 35% keeps the foam both loaded and recoverable. Squeeze past half its thickness and the cells collapse, after which the liner is a rigid block with no memory left to spend on the next drop.
02A Two-To-Four Millimetre Deflection Budget
Treat the wall as a loaded beam. A 3–4 mm polypropylene panel spanning 60–100 mm between ribs typically gives two to four millimetres of elastic travel before yield. More is not better. Every extra millimetre of shell movement is one the liner has to absorb, while a shell that barely flexes shortens the impact pulse until peak acceleration climbs.
There is a plain shop-floor check: measure the dent after the hit. Anything beyond 0.5 mm of permanent set means that drop crossed yield. Run the same height and orientation on all six faces, log residual deformation each time, and read the trend rather than a single number. A curve turning upward points at wall thickness or rib spacing, not at adding material.
03Compression Between 20 And 35 Percent Is Measured After Assembly
Compression ratio only exists once the case is closed. Put the object in, latch the lid, and measure how much the liner lost from its free thickness. Under 20% the object still has one to two millimetres of play, and every trip becomes a small collision that wears the cavity wall. Over 35% the cells are already flattened and repeated drops stop returning to the original dimension.
Foam density decides how usable that band is. Molded EVA in the 38–60 kg/m³ range behaves close to linearly across 20–35%. Lighter material must be squeezed a long way before it carries load, which quietly recreates the clearance you tried to remove. Die-cut walls also buckle at corners, where molded radii carry force around the bend instead. Density and cell geometry have to be chosen together, never one after the other.
04Which Side Fails First: Three Sequences That Repeat
Hard shell, soft liner: the shell hardly moves, the pulse is short, peak acceleration is high, and the liner bottoms out within a drop or two. Soft shell, hard liner: the shell gives six to eight millimetres while the liner yields barely ten percent, so corners whiten first and crack later. Both are mismatches, and both show up in the same test.
The third is quieter. When both sides are too soft, a single drop looks fine, but the liner only returns to about 90% of its free thickness each time. After a dozen handling cycles the gap has grown from one millimetre to three and the object starts sliding. Catch it by logging thickness and clearance across ten and twenty drops, then checking whether the numbers fall monotonically.
05Balance It Backwards: Liner Thickness First, Clearance Last
Start from the object. Add 15–25 mm per axis to reach the nominal cavity, then add 3–4 mm of wall and the corner radii to arrive at the envelope. Whether you sit at the top or bottom of the liner range depends on mass and on the acceleration you are willing to pass through. Heavy items trade thickness for travel; light ones do fine at 15 mm, and the saved volume is better spent on wall.
The final one to two millimetres is assembly tolerance, not design margin. It absorbs mold shrink, liner rebound and part-to-part variation. Squeeze it to half a millimetre and the line has to press the lid shut; open it to three and the preload you designed disappears. Close the case, push the object sideways: light resistance, no more than a millimetre of movement, and the set is balanced.
| Shell elastic deflection | 2–4 mm, recovers after load |
|---|---|
| Liner compression | 20–35% |
| Object clearance, per side | 1–2 mm |
| Liner thickness | 15–25 mm |
| EVA density | 38–60 kg/m³ |
| Drop verification | 1.5 m, all six faces |
| Peak impact acceleration | 40–90 G, measured range |
| Energy absorption per hit | 60–75% |
FAQ
Is one millimetre of clearance tighter and therefore better?
No. One to two millimetres covers mold shrink, liner rebound and part variation. Below 0.5 mm the lid has to be forced down and the foam stays over-compressed, losing recovery within months. Above 3 mm the object can slide, which hands the whole impact to the shell alone.
Does a thicker liner make a case safer?
Only up to about 25 mm. Beyond that the cavity shrinks, so the envelope must grow to hold the same object, adding weight and bulk. A thicker liner also has to absorb more travel at the same compression ratio and returns more slowly. For handheld gear, 15–25 mm is the practical balance.
Is a stiffer shell better in a drop?
Stiffness shortens the impact pulse, raises peak acceleration, and pushes that energy into the liner and the object. The shell needs a two-to-four millimetre elastic window, not unlimited rigidity. Stiffen the corners and hinge bosses instead, since deformation there never comes back and the lid fit goes with it.
How do you tell that one side has already failed?
Two numbers. Residual dent beyond 0.5 mm means the shell passed yield. Free liner thickness that no longer returns to 95% of original means collapsed cells. If both pass and the object is still scratched, the problem is clearance, not material.