Injection molded EVA foam products are made by a pellet to product process, injecting a melted pellet into a closed mold foaming the formula into a multi-dimensional design. Compression molded EVA foam ( Compression molding EVA foam) can’t come close to creating the same complex designs. However, when injection molding produces the same, or similar, product to compression molding here’s how they differ.
When it comes to a finished look, the overall appearance of a Compression molded EVA (vacuform/thermoform) foam part cannot compare with that of an injection molded foam part. The increased density, and perfect “mold” finish of an injection mold, produces a part with far superior aesthetics. Because the foam used for compression molded parts is outsourced, the quality of the foam is also likely to be variable, affecting the final aesthetics of the part. On the other hand, because the injection molded foam is manufactured in-house with the foam and part made simultaneously, the quality is always consistent.
Specific variables that affect quality in compression parts, but not injection parts, are:
- inconsistent cell count within the foam, variable pigmentation and density in the raw materials causing color variation
- inconsistent part to part processing time, temperature and degree of compression causing variation in part definition.
The ability to produce a consistent dimensional part is based on how well the molding process can be controlled. The pellet to product process means that the manufacturing of the foam, and final part, are one and the same. Most processing conditions can be controlled and duplicated, resulting in minimal part to part variation. Compression molding, however, starts with a bun or sheet of foam that is heated and molded into a different shape, subject to far more variable processing conditions. In addition, the need for final trimming of the compression molded part can also be a major contributor to a low quality image.
If the final product has varying height profiles, a compression molded part must start with a sheet (bun) of foam that is the height of the tallest dimension of the part. If the foam chosen for the compression molding process does not come in a thick enough bun, laminating sheets together is required to form thicker buns. This can result in that some parts have a lamination line which can delaminate with time. Injection molded foam parts, however, can be made to any height, not limited by the process, without need for lamination.
Post Mold Stability:
Another area where the two processes differ significantly is what happens to the part after it is processed. Injection molded parts are extremely soft and malleable when they finish their molding cycle. During the post molding phase, a cooling fixture may be required to help the part maintain its shape. Unless the part is subjected to external forces that causes it to change shape, the final cooled part will be exactly what was expect. On the other hand, compression molded foam parts can suffer post molding shrinkage and warp, even if cooling fixtures are utilized.
Cost and Value:
Compression molded EVA foam parts will almost always cost less than injection molded foam parts. Not only is the tooling and manufacturing equipment less expensive but the parts as well. Because compression molding uses bun stock, low density (inexpensive) foams are often chosen. This results in lower quality products that are great for a give-away or other temporary product. Injection molded foam products, while more expensive, are attractive, with long lasting value.
Both EVA compression molded foam and EVA injection molded foam have their place in the world of manufacturing. Choose the foam process that is best for your product.