Thermoplastic resin powders of cellulose acetate esters in which essentially 80% or more of the particles pass through a 100 mesh screen (U.S. Sieve Series) and essentially 50% pass through a 200 mesh screen (U.S. Sieve Series) are desirable and useful as a filler material for investment casting waxes. The cellulosic thermoplastics resin powders are environmentally safe when used as filler material for investment casting wax compositions since the powders produce essentially no carcinogenic materials on combustion. The powders are as effective as acid fillers and set relatively fast thus improving cycle time in the die but do not produce an adverse effect of shell cracking nor react with the slurry.

Thermoplastic moulding materials consisting of (1) 70 to 99.9% by weight of a cellulose ester or cellulose mixed ester (2) 0.1 to 30% by weight of a rubber polymer built up in the form of blocks from recurring structural units (I) with molecular weights of 10,000 to 1,000,000 wherein A denotes bifunctional polysiloxane segements of the formula in which R.sup.1 and R.sup.2 =aryl (in particular phenyl) or alkyl (in particular methyl) and x is a number such that the molecular weight is .gtoreq.500, and B denotes bifunctional aliphatic polyester and/or aliphatic polyester segments with molecular weights of .gtoreq.800, and the glass transition temperatures of the polymeric blocks A and B are less than 20.degree. C., which contains additionally (3) 0 to 30% by weight based on the mixture of (1) and (2) of a plasticizer.

Edge-supported and fully supported membranes having substantially uniform thickness, and substantially parallel surfaces, and having the capacity to transmit at least about 90% of incident light with very little diffraction, dispersion or absorption of incident light are made by forming a polymer/solvent mixture, spin-coating a film of the polymer from the mixture onto a rotatable surface, which may have a release agent thereon, and, where a release agent is present, joining a frame or other support to the membrane and removing the membrane with its attached support from the surface. Apparatus for making such membranes includes a dispenser for dispensing a polymer/solvent mixture onto a rotatable support; a device for spinning the rotatable support at gradually increasing speeds from a first to a second speed; and a device for maintaining the speed of rotation of the support at the second speed until the membrane forms.