Most casting resins can be filled with various types of filler. Fillers are used for wall thicknesses from about 25 mm in order to achieve the following:
lessen the amount of shrinkage,
reduce the weight of the cast piece,
change the working attributes of the material (putties),
influence the final attributes or the appearance of the material,
reduce the reaction temperatures and thereby the working life when casting large items.
The mixing ratio between the already mixed casting resin mixture and the filler depends on the type of filler being used and the attributes you want to achieve. A PUR quick casting resin which has a mixing ratio of 1:1 (e.g. G26) lends itself to the following mix preparation: 1 kg resin to 1 kg hardener with 2 to 3 kg filler. When adding filler, one half of the dry filler should be stirred into the resin and the other into the hardener. In the case of polyurethane casting materials, you should begin with the component which does not contain isocyanate so that the water-sensitive isocyanate component can remain closed for as long as possible.