So I am creating a relief in a model that should become a Strong & Flexible Plastic in Inventor. My goal is to put another Strong & Flexible Plastic inside this relief, that has another color. At first I thought about inverting the sketch but then I realized that it could not fit. Is material shrinkage sufficient for it to fit in or should I create an offset? If an offset is necessary, which amount should I have?
Yes, an offset is necessary. How much will depend on your specific geometry, but I have found 0.3mm to be a good starting point.
You will definitely want to make offset. 0.2-0.3mm is good start. However, the problem would is not material shrinkage, but small random surface imperfections.
Depending what size the print is, I have found that larger prints have about a 0.4% shrinkage to the model as the inner and outer diameters had different results from the designed sizes. I have had inner diameters shrink by 0.03" on some big prints.