Introduction to Basic Manufacturing Processes and ... - always yours

236 Introduction to Basic Manufacturing Processes and Workshop Technology
(vi) Skill-ness required is less.
(vii) Moulds formed by this process can be stored until required.
(viii) Better quality of casting assured.
(ix) Mass production.
(x) It allows for greater detail and less draft.
(xi) Unskilled labor can be employed.
(xii) Future of shell molding process is very bright.
Disadvantages
The main disadvantages of shell molding are:
1. Higher pattern cost.
2. Higher resin cost.
3. Not economical for small runs.
4. Dust-extraction problem.
5. Complicated jobs and jobs of various sizes cannot be easily shell molded.
6. Specialized equipment is required.
7. Resin binder is an expensive material.
8. Limited for small size.
12.20.7 Plaster Molding
Plaster molding process is depicted through Fig. 12.15. The mould material in plaster molding
is gypsum or plaster of paris. To this plaster of paris, additives like talc, fibers, asbestos, silica
flour etc. are added in order to control the contraction characteristics of the mould as well
as the settling time. The plaster of paris is used in the form of a slurry which is made to a
consistency of 130 to 180. The consistency of the slurry is defined as the pounds of water per
100 pounds of plaster mixture. This plaster slurry is poured over a metallic pattern confined
in a flask. The pattern is usually made of brass and it is generally in the form of half portion
of job to be cast and is attached firmly on a match plate which forms the bottom of the
molding flask. Wood pattern are not used because the water in the plaster raises the grains
on them and makes them difficult to be withdrawn. Some parting or release agent is needed
for easy withdrawal of the pattern from the mold. As the flask is filled with the slurry, it is
vibrated so as to bubble out any air entrapped in the slurry and to ensure that the mould
is completely filled up. The plaster material is allowed to set. Finally when the plaster is set
properly the pattern is then withdrawn by separating the same, from the plaster by blowing
compressed air through the holes in the patterns leading to the parting surface between the
pattern and the plaster mold. The plaster mold thus produced is dried in an oven to a
temperature range between 200-700 degree centigrade and cooled in the oven itself. In the
above manner two halves of a mould are prepared and are joined together to form the proper
cavity. The necessary sprue, runner etc. are cut before joining the two parts.