LOSTWAXInvestment Casting

Wax patterns, injection parameters and tooling

Every casting begins as a wax pattern. Dimensional accuracy and surface quality are established in the die and cannot be recovered later.

Pattern waxes

Pattern waxes are engineered blends of natural and synthetic waxes, resins and, in filled waxes, inert organic fillers. The blend is chosen for low and predictable shrinkage, dimensional stability, a smooth surface, sufficient strength for handling, low ash content and clean removal during dewaxing. Filled waxes reduce contraction and sink in heavy sections; unfilled waxes are easier to reclaim and are used for runners. Wax is recycled within the foundry, typically with reclaimed material directed to the gating system and virgin or reconditioned wax to the patterns.

Injection

1 upper die half · 2 die cavity filled with wax · 3 injection channel · 4 injection nozzle · 5 locating elements · 6 parting plane

Wax can be injected as a liquid, a paste or a semi-solid. Lower injection temperatures give less shrinkage and better dimensional control but need higher pressure and good venting. The main parameters are:

  • wax temperature and die temperature;
  • injection pressure and flow rate;
  • dwell (hold) time under pressure, which compensates contraction;
  • cooling time before ejection, and release agent application.

As an indication, pattern waxes are commonly injected somewhere between about 45 and 70 °C depending on whether they are processed as paste or liquid. The parameters are established for each die and recorded so that they can be reproduced.

Tooling design

The die cavity is larger than the finished part by the combined contraction of wax, the expansion and contraction of the shell and the contraction of the alloy. This allowance differs with alloy, wall thickness and the degree to which features are restrained by the shell; it is commonly of the order of one to two and a half per cent in total and is refined after measuring first samples. For this reason dimensions with tight tolerances are best left "metal safe" in the first die so that they can be corrected.

Further design decisions are the parting plane and the position of witness lines, slides and loose pieces for undercuts, ejector positions away from functional surfaces, venting, and the location of the gate attachment on the pattern.

Cores for internal shapes

  • Soluble cores are moulded from a water-soluble wax, over-moulded with pattern wax and dissolved out, leaving internal cavities in the pattern that the shell can then fill.
  • Ceramic cores are placed in the die and remain in the shell during casting; they form passages that are too narrow or too long to be coated with slurry. They are removed chemically or mechanically after casting.
  • Assembled patterns are joined from separately injected segments when a one-piece die would be impractical.

Pattern handling and inspection

Wax continues to contract and relax after ejection. Patterns are therefore stored at controlled temperature, supported on setters where necessary, and measured only after they have stabilised. Defects such as flow lines, sink, bubbles and flash are repaired or the pattern is scrapped, since the shell reproduces every detail. See defects originating in the wax pattern.

Patterns without tooling

For prototypes and very small batches, patterns can be printed in wax-like or polymer materials or machined from wax blocks. This removes die cost and lead time but changes burnout behaviour and surface quality; see process variants.