What separates one foamed plastic from another is not only the material but the forming route. Extrusion produces continuous sheet and roll, injection moulding produces one finished part at a time, and bead moulding makes small beads first and fuses them into a block. Whether you are buying “a 0.5 mm sheet” or “a packaging box with a latch” is answered at the route, before any material is discussed.
Three routes
The route decides which shapes can be made, and in turn which parameters can be negotiated.
| Route | How it forms | Typical output | Parameters you can negotiate |
|---|---|---|---|
| Extrusion | The melt expands as pressure drops at the die and is drawn off continuously | Roll, sheet, board | Thickness, width, density, length |
| Injection moulding | The melt fills a closed cavity and expands and sets inside it | Moulded parts, housings, structured pads | Part dimensions, wall thickness, tooling and batch size |
| Bead moulding | Expandable beads are pre-expanded, then steam-fused in a mould cavity | Blocks, moulded packaging, board | Block size, density, moulding tool |
Choose the route by the output shape first, then discuss the material. The same base travelling through extrusion and through injection moulding can produce very different thicknesses and structures.
Extrusion: continuous sheet and roll
The extruder melts the base, mixes it, releases the pressure at the die so the gas expands the melt into cells, and then draws, cools and winds the result. The output is continuous: length is set by the winder, thickness and width by the die.
The strength of this route is thinness. Our XPE can be made in single layers of 3–15 mm, IXPE at 0.1–15 mm and IXPP at 1–6 mm; anything thicker is multi-layer heat lamination, to 200 mm for XPE and IXPE and 100 mm for IXPP. Width is 500–2000 mm, and 500–1500 mm for IXPP. Cutting it into sheets, winding it into rolls or die-cutting it into parts are all operations after this stage.
The cost is that the shape can only be a surface. Extrusion cannot produce a three-dimensional structure with bosses, latches or internal ribs; to get those you either die-cut and laminate them up, or change to injection moulding.
Injection moulding: one finished part at a time
The melt is injected into a closed cavity, foams and fills it, and sets; opening the tool releases a finished part. The route can produce wall-thickness changes, reinforcing ribs and latches with no secondary operation.
The cost is in tooling and batch size. One tool corresponds to one part, and changing the shape means changing the tool, so injection moulding suits parts with high volume and a stable shape; at sample stage it does not pay. Foaming inside a cavity is also affected by wall thickness — thick walls are harder to keep uniform — so where an equal-wall design is possible, use one.
Bead moulding: make beads, then fuse them into a block
The resin is first made into expandable beads, which are loaded into a mould cavity and steamed so that they expand and fuse together; cooling gives one integrated foam block. The strength of the route is low-density blocks and moulded packaging.
Precisely because the beads have fusion interfaces between them, the cell structure differs from the extrusion route, and the distribution of mechanical properties differs with it. For moulded packaging where cushioning matters, the route is common.
What is left to decide after the route
Shape comes from the route; the remaining parameters come from the material.
- Density and expansion ratio. Our XPE runs from 3× (300±40 kg/m³) to 40× (26.8±3 kg/m³).
- Cross-linked or not. This decides whether it can be made thin and whether it can be remelted — see PE foam and the cross-linked grades.
- Closed cell or open cell. This decides insulation and vapour control versus sound absorption and breathability — see closed-cell vs open-cell foam.
- Converting. Foil facing, adhesive, winding or die-cutting; see laminated foam.
What the process fixes
Five things follow from the route and cannot be negotiated later.
Density. The expansion ratio sets it directly: an extruded sheet at 3× sits near 300 kg/m³, and the same formulation at 40× near 26.8 kg/m³.
Cell type. Extruded thermoplastic foam comes out closed-cell because the die and the cooling rate decide how the walls set. Open-cell structure comes from formulations that let the walls rupture as the gas escapes.
Shape. Extrusion gives continuous sheet and profile, moulding gives a finished part, bead moulding gives a block. The process, not the polymer, decides which of those a buyer can order.
Skin. A moulded part forms an integral skin where the melt met the tool, so the surface is denser than the core. A cut sheet exposes open cells on the cut faces, which is why a cut edge takes adhesive differently from a skin — a distinction that matters when an acceptance clause has to name which face is tested.
Thickness ceiling. A single extruded layer has a practical limit; beyond it, layers are heat-bonded. That is how XPE and IXPE reach 200 mm, and the bond layer becomes part of the specification.
Common questions
Extrusion or injection moulding — which is cheaper?
The question has to be split. Extrusion is cheaper for small batches and samples because no tool is needed: a prototype is a few metres of sheet. Injection moulding is cheaper at volume with a complex shape, because forming removes the secondary operations. Where the crossover falls depends on the part shape and on how far the tooling cost is spread, not on the material.
Why is thin foamed plastic hard to make?
Below a certain point the cell walls are drawn so thin during expansion that they cannot hold, and cells merge or rupture. Cross-linked grades tie the chains first and can therefore go thinner — IXPE reaches 0.1 mm in a single layer, where uncross-linked PE foam cannot approach that scale. It is a structural limit, not a question of how good the extrusion line is.
Can extruded sheet be processed further?
Yes. The usual operations are foil facing or adhesive coating into a laminate, die-cutting into gaskets or pads, and multi-layer heat bonding into a thicker board. Cross-linked grades can be heat bonded; uncross-linked material can also be remelted, which is why it is the easier one to recycle.
Roll or sheet?
Choose by the next operation. Continuous die-cutting and automated placement want roll; laying a whole floor or taking one piece at a time wants sheet. It is only the difference between winding and cutting, and the parameters do not change — thickness, width and density come off the same table.
Which route Bofeng Foam uses
Bofeng Foam is a manufacturer of cross-linked polyolefin foam, with a 10,000 m² plant, five production lines and 5,000 tonnes of annual capacity in Zhuhai, Guangdong. We use the extrusion route: roll and sheet in XPE, IXPE and IXPP, expansion ratio 3× to 40×, IXPE single layer down to 0.1 mm, width 500–2000 mm, multi-layer lamination to 200 mm for XPE and IXPE and 100 mm for IXPP; then die-cut, laminated or wound to order.
Injection moulding and bead moulding are not on our lines. When a project needs them we say so rather than substituting a different process.
See the cross-linked foam range · Ask about a specific grade
See also what foamed material is, the types of foam material, what foam is made of, PE foam, laminated foam and PP foam.