DIE MAKING USED FOR PRODUCING WAX PATTERNS:
This is the most important first step in the entire investment casting process. Any error in the die is reproduced in the finished casting. Extreme care is necessary when making a die. The die should be simple in its construction. This will make it easy to use and enhance its production rates.
Certain dimensional changes are intrinsic to the process, namely:
a) Wax shrinkage.
b) Expansion of the shell during firing.
c) Cast metal shrinkage during solidification.
Allowance for each of these dimensional changes is to be incorporated in the die. Dies are usually made of aluminium.
Conventional techniques are used for making dies.
Desirable Properties for Die Materials:
a) The die material should have good thermal conductivity to enable rapid cooling of the injected wax.
b) The material should be sufficiently strong to withstand the injection and clamping pressures.
c) The material should have sufficient wear resistance to provide long tool life.
d) The material should be hard enough so that it can be polished to a high finish, and the finish has a good life.
Pattern Materials: Due to the importance of the patterns, it is very important that they have certain specific properties, such as:
a) Low ash content
b) Good fluidity at injection temperatures to produce detail.
c) Adequate mechanical strength.
d) Very low thermal contraction and expansion ranges.
e) Stability under normal working conditions.
f) Chemical compatibility with dies and shell materials.
g) Ready availability.
h) Reusability.
PRODUCTION OF WAX PATTERNS:
This is done using the Hydraulic Wax Injection Machines.
a) Wax is melted and brought to the required temperature in wax melters.<.li>
b) The molten wax is then transferred to the wax injection machines.
c) Wax is injected under pressure into the die at appropriate temperatures. In case the component is large, the dies are cooled using water jackets.
d) The wax patterns are extracted from the dies and stored in a cool place.
e) These patterns are then taken for assembly. Pattern Assembly This is the next important step. Correct design of ingates, and cluster design play a major role in the outcome of the castings. The patterns are assembled to a running and feeding system by welding with a wax-welding gun and with wax glue (stick wax).
The patterns are usually assembled in the form of a fir tree, i.e. the ingate of each pattern is fixed to the central down sprue. Assembly Procedure The assembly procedure is as follows;
1. The patterns are visually inspected before assembly any repairable damages or blemishes are patched using the ‘patch wax’.
2. Any flashes along the die parting line are carefully removed.
3. The ends of the ingates are shaped to match the shape of the sprue and welded to it.
Important notes:
1. The sprue and runners are gravity cast using recycled wax.
2. The gating and runners should be positioned to ensure easy cut – off of the finished casting.
3. The size of the cluster / tree should be properly chosen to maximise yield. The assembly should be light enough to be easily handled by the worker.
4. Enough space should be provided between the individual patterns to allow a shell of uniform thickness to form on all the surfaces of the pattern.