The simpliest way I've found to cast a screw is as follows.
Step 1: Clean the screw with a wire brush or a wire wheel on a grinder.
Step 2: Soak the screw overnight in acetone to remove any remaining coating that you missed with the wire brush or wire wheel.
Step 3: Pour a layer of PR (I like silmar 41) about 1/2 inch deep into something like a pill bottle, clear tube, etc and let the resin hardened overnight.
Step 4: If the screw is a flat head screw, very neatly coat the head of the screw with thick CA and put the screw into the pill bottle HEAD first. Allow the CA Glue to dry and offgas over night.
NOTE: if the head of the screw is "rounded", take a correctly sized countersink bit and make a dimple hole in the hardened PR. Then, with a long Qtip, add enough thick CA to fill in the dimple. Then place the screw head down into the glue and let it sit overnight to cure and offgas.
Step 5: depending on the thread pitch, size, etc, it is sometimes difficult to make a cast that is bubblefree around the threads. FOR THIS REASON, I would thin the PR to a waterlike consistency. Then cast to a level above the tip of the screw.
I have cast this way, WITH and WITHOUT pressure and both seemed to work equally well.
Good Luck!