4mm and less is possible, but as you decrease the chances of chipping on either the mould or article increases dramatically. This allows for easy removal of the aprt from the mould. The deeper the mould, the more 'draw' may be required. Shallow moulds can get away with less as the article may shrink enough to allow de-moulding. The more complex the shape - the higher the initial cost for tooling, the thereafter you can exploit the advantage of easy manufacture of composite articles, providing they are easily removed from the mould.
Returns are possible by splitting the mould at the appropriate part lines to allow removal.
Each partline leaves a line on the article which can be left there or sanded and buffed out. An example of a complex mould is the one Fibreglass Developments uses to produce Dorothy the Dinosaur , which requires some 15 pieces to produce the finished article. While a complex shape can have the mould made to remove it, consider also there has to be some room to apply the laminate to the inside of the mould. This is because of low stiffness as mentioned above, and shrinkage which pulls all corners internally, regardless of whether it was internally or externally moulded. Top hats, ribs, etc may also be added as required.