If sheet thickness tolerance is of importance for engineering applications, or for components to slot together, you may want to use extruded clear and not cast clear. The manufacturing process for extruded gives much tighter thickness tolerance (generally <10%), in comparison to cast acrylic manufacture. Extruded clear acrylic engraves with little contrast so is more suitable for cutting purposes only. If you require engraving also it is best to specify to use clear CAST acrylic which engraves with a white milky finish. Extruded Perspex is cheaper than cast Perspex, it thermoforms faster and glues / cements parts together easier.