DOC KB-FOM-003REV 2026-09-21CLASS: PUBLIC
Foam Inserts for Protective Cases: EPE vs EVA vs PU vs CNC-Cut
Direct answer
EPE is the low-cost general cushioning foam; EVA is firmer and holds precision-cut cavities, making it the standard for layered CNC inserts; open-cell PU is the material of "pluck foam" trays that users tear to shape; anti-static (conductive-dissipative) grades of EPE/EVA/PU protect electronics. For OEM electronics, specify a CNC-cut layered EVA insert with anti-static grade.
§01The four foam families
| Foam | Feel / density | Best for | Watch out for |
|---|---|---|---|
| EPE | soft, ≈20–35 kg/m³ | general cushioning, void fill, low cost | poor edge definition; ages faster under compression |
| EVA | firm, ≈30–80 kg/m³ | precision CNC cavities, presentation cases | cost; heavier than EPE |
| PU (open-cell) | very soft | pluck-foam trays, lens/equipment wraps | absorbs water if the case floods; crumbles over years |
| Anti-static grades | same as parent foam | PCBs, instruments, any ESD-sensitive load | must be specified at order time — often pink or black conductive |
§02Layered insert anatomy
A typical OEM insert stacks three or more layers: a thin top "cap" layer for looks, a middle layer with CNC-cut cavities matching the product silhouette, and a dense base layer under the load. Recessed finger notches near each cavity let users remove items one-handed. Edge-glued layers should use contact adhesive compatible with the foam, or heat-lamination for EVA.
§03How much cavity clearance?
- Allow 2–5 mm of clearance around the item silhouette for insertion and extraction.
- Compress foam by roughly 20–30 % of its thickness at rest to hold the load — more than that fatigues the foam, less lets the item migrate.
- Heavy items need the cavity in the lower half of the stack so shock loads transfer into the base layer and shell, not into a thin foam bridge.
Spec tip: ask your supplier for the foam's compression-set value and thickness tolerance. Two "EVA 40 kg/m³" inserts from different factories can behave very differently after 500 open/close cycles.