In addition, keeping the part at a reasonable temperature during curing is critical. Putting a part into a cold environment while it is curing can stop the cure, and even post curing after the fact will not push the cure to a high level. I have seen companies take a demolded part and move it into areas that were 30 to 50 F so they have room in the shop to make more parts. Unfortunately, the resin just would not finish curing, and even post curing the parts did not make an acceptably cured part.
Post curing parts shortly after making them is another source of energy. Consider it an insurance policy to overcome some of those variations in temperature or other factors that drive the cure. Normally, the sooner you can post cure a part, the easier it is to achieve a high degree of cure.
Select Peroxides Carefully
The selection of the peroxide and the level used is another energy source. There is a large range of peroxides that can be used in room temperature processes, including methyl ethyl ketone peroxides (MEKPs), cumyl hydroperoxide (CHP) and acetyl acetone peroxide (AAP), as well as blends of these. It’s important to note that these are not pure chemicals like some of the peroxides used in elevated temperature cure, such as tertiary butyl peroxybenzoate (TBPB). Room temperature peroxides can have different carriers and other active ingredients that can change how they disperse in the resin and how the resin cures.
MEKP peroxides are not all the same and won’t perform the same on your production line. Some of the performance differences include changes in gel time, hardness development, exotherms, demold time, trim time, sensitivity to temperature, sensitivity to changes in thickness and/or combinations of these parameters. It’s been my experience as a resin manufacturer that even though people often claim that MEKP peroxides are equal, they are not. The differences may be minor and not detected on your production line, but some of them can wreak havoc.
Achieving a good cure requires a suitable amount of peroxide to initiate the reaction and push it to a higher degree of cure. Using too low a level of peroxide may give a good gel time, but it may not push the cure far enough, unless there is additional energy added to the system to compensate. This energy can come from the exotherm of the resin or outside sources of energy. For example, when a part is thick – like a marble matrix more than one inch thick – blanketing the part with a thermal media to hold in the exotherm, heating the mold or putting the parts in under heaters can produce energy.