Low density plastics burn out well (polythene) and only require
a short addition to the burnout cycle. Other plastics require
long exposure to oxygen to reduce the ash residue, and are almost
impossible to tree cast, but can be successfully cast if all
patterns are sprued to the base, NOT on a tree. Some tips follow:
use many sprues, including one at each extremity that terminates
on the flat of the flask base, next to the gate/button/pouring
hole. This helps establish a convection pattern of air flow so no
dead air pockets exist, fresh air must get to every part of the
flask cavity, continually.
Slowly raise the oven temp to 600- 800 and hold it there for at
least 4 hours, the idea being that most of the plastic will
slowly drip out, and the convection sprues will allow air to come
in helping the viscous thick slow-moving plastic evacuate.
Position the pattern and sprues so all the plastic drips out.
Most plastics melt at about 400-450 (low densitiy polythene
melts at 350). Nylon becomes very liquid and runs out like water,
and burns out very cleanly. Polystyrene burns out almost
perfectly but is colored with colored clays (color pie) and
leaves a slight residue which doesn’t present much of a problem.
Polylstyrene is what most models (the kind you buy as kits and
glue together) are molded from and make great fun. I have enjoyed
buying small models and casting all the parts into silver (the
detail is so incredible), then soldering all the parts into the
model. The possibilities of enameling etc… casting different
metals… what fun 
The flask can be lain on its side after the plasic has dripped
out to allow better penetration of oxygen, this helps.
It might be possible to blow out the residual ash with a very
gentle air jet. High pressure, such as an air gun connected to an
air compressor set at full pressure, will disintegrate the
fragile investment. I personally haven’t had any success blowing
out residue, but know a caster who does this frequently.
Some materials may require special investments designed for
ringless casting (a dental lab procedure in which the pattern is
invested in a nylon flask, then removed and centrifugally cast).
This investment material is so hard it cannot be removed from a
metal flask, but readily breaks apart with a hammer, and is
available from any dental supplier.
To summarize; sprue in such a manner that the very viscous
plastic can easiily drip out. Some residue problems can be gotten
around by insuring that complete evacuation of the plastic is
achieved, at a temperature that doesn’t permit the plastic to
carbonize. More ash residue can be eliminated by a prolonged
burnout with the oven door slightly ajar to allow fresh oxygen to
enter.
Good luck, if I think of some more tips, I’ll post them
Jeffrey Everett
Handmade 18K, 22K, and platinum gemstone fine jewelry.
Diamond setting, rubber/metal molds, casting, lapidary
Die and mold engraving, plastic patterns for casting.
Cad jewelry design, cad/cam milling scroll filigree…
P O Box 2057 Fairfield IA 52556 515-469-6250