compression vs injection molding

Are there any major deferences between compression and injected molded midsole? Obviously the process is different, but what about quality? Consistent softness throughout? THanks for the info.

you can get more detail in injected molded EVA than CM EVA. especially on the sidewall areas that are perpendicular to the compression molding direction.

far as i know though quality should be similar. in theory an injected EVA midsole will have more consistent density throughout, but doesnt always work out that way depending on the design and injection process. i dont specifically know of any technical performance advantages of one method over the other.

i do remember some issues however with part size accuracy in injection molding though as typically the midsoles are molded some percentage smaller than final part size to allow for expansion after cooling, but would guess that the tolerance isnt enough to be concerned about in most applications.

biggest concern for choosing one or the other i’d say is volume. injection is certainly preferred for high volumes and its easier to bang out high volumes once everything is set up.

hope this helps.


perfect thanks!



1 >> COST >> compression is cheaper in lower runs (less tooling set up costs) but over a certain quantity injection becomes cheaper.

2 >> Detail >> because of the scale, some detail is lost with injection. They are injected at about 1/2 to 1/3 scale, and when the mold opens they literally expand like muffins, it is amazing. Because of that, every radius grows, making sharp form intersections difficult. Textures also grow.

I prefer compression because they are molded 1 to 1 with more perfect detail.

when injection molded eva expands, how is this controlled? is there any expansion with compression molded eva?

The expansion is calculated by the tooling engineer, it is pretty precise, but you can see examples where the compound is not just right which can lead to the part being “overblown”… or puffy. It also can react differently with thicknesses and can have a hard time with thin wall thicknesses (because of expansion)

The compression does not expand. You start with a preformed biscuit and essentially put it in a heat press.

Like most solutions, it’s at best a calculated gamble. Mike is right, rule of thumb for the injection is about 50%. The mold get tested on the sample size and tech runs, so by the time the production tooling comes around the process is honed.

And just for the record, only foams really have expansion anything close to this. Sheeted foams are milled and produced similar to wood processes so tolerances can be closer.

Waffle maker anyone?