Processes (EPS & EPP)
Design of parts and manufacturing of tools (moulds) – Once the client’s needs has been clearly identified and the feasibility study completed, a project manager will begin the product development process. This process involves several stages and includes all the necessary validation steps. Polymos engineers are also involved in the mould design stage to ensure optimal results. The moulds will then be machined and assembled in Polymos’ own workshop. Before production begins, there are one or more test steps in order to verify the conformity of the new product and validate its functionality. For more information on these initial stages of our operations, see Tooling workshop.
EPS
Pre-foaming of the material
Pre-foaming (or pre-expansion) consists in heating the EPS particles with steam. The expansion is then caused by the reaction of water vapour with the porogen (pentane). The final density, which can be as much as 50 times lower than that of the original bulk material, depends on the duration of pre-foaming and other parameters. Before being moulded, the pre-expanded beads are stored in ventilated silos for a curing period of 12–48 hours. This stage is essential to ensure the moulding process works properly. However, once pre-expanded, EPS beads become perishable and must be used within a short window of time. Care must be taken to not exceed the maximum storage time or the beads will lose their ability to fuse during moulding. The curing stage requires close management of the silos as the window of time varies depending on densities and the grade of material.
When required, colorants or antistatic additives may be added during pre-foaming in order to endow the finished product with new properties.
Moulding
The moulding process starts with the mould being completely filled with the tiny pre-expanded beads. The mould is then heated to above 110°C with high-pressure steam. This step produces a second expansion of the beads, that forces them to fuse together to take the shape of the mould. The mould is then rapidly cooled to stabilize the shape. The stabilisation time can be long; it varies as a function of the material’s density, the level of fusion desired (level of quality required), and the volume and geometric complexity of the part.
This process allows for the manufacturing of both tiny and very large parts. Furthermore, objects moulded in such a way can be simple or complex in shape, and the same part can have thin walls in some areas and thicker ones in others.
Depending on the final characteristics desired, it is possible to insert several components directly into the mould such that after moulding they form a single part with the EPS. It is also possible to co-mould another plastic part (made by thermoforming or injection) to produce a finished product with the properties of both materials. The co-moulding process is sometimes complex, and a high level of precision is needed to produce superior quality parts. Polymos is a North American leader in using these processes. See section on Molding
Cutting
To obtain parts with a more linear geometry, hot wires can be used to cut large moulded blocks of the required density to the desired dimensions. This process is often more economical for simple parts and for small-scale production since it does not require any mold. (see examples of cut parts – put with the photo: “Examples of cut parts”). See section on Wire cutting.
EPP and EPE
The transformation process for expanded polypropylene (EPP) and expanded polyethylene (EPE) is different from that for EPS.
To achieve fusion, the beads have to be compressed during injection into the mould. This process requires higher fusion temperatures and a moulding pressure up to three times higher than that required for EPS.
Several grades of these materials are available in either black or white.
It is also possible to cut blocks of EPP in the same way as for EPS.
