Investment casting wax materials are blend of numerous complex compounds. Each compound has been included to influence the final properties of the wax in some way. A few points that affect the quality of a casting wax and hence pattern production are listed.
1. Contraction and cavitation: Stable results on contraction and cavitations of a casting wax are extremely important to the foundry. We have already discussed how structure and composition affects contraction. This highlights the importance of both the wax manufacturer and foundry’s quality control tests.
2. Congealing point or melting point: Congealing point and melting point are temperatures at the beginning and end of the semi-liquid state respectively. They have a major influence on the injection temperature & pressure settings of the injection machine.
3. Ash content: Most foundries would be aware of the importance of using and maintaining wax with low ash content and of the detrimental effect of ash. The limit generally recommended is 0.05% maximum.
4. Hardness and elasticity: Casting wax must have sufficient hardness and elasticity to help reduce the possibility of rejects due to breakages, bending or other undesirable phenomena during the subsequent processing of the wax pattern.
5. Viscosity: The viscosity of a casting wax compound is critical to successful pattern production. Where large fine sections need to be produced then often a low viscosity wax is required to enable the wax to penetrate into the finest spaces in the die. For heavier sections a less fluid wax may be preferred. Viscosity is generally directly related to injection temperature.
6. Good surface finish: A good surface finish is an important property for successful pattern production. In general, unfilled wax has a glossy surface, emulsified wax has more surface smoothness, whereas filled wax has a slightly rough surface. Surfaces that could prove detrimental, are the ‘soft easily damaged’ surface or the ‘pitted’ surface usually associated with coarse particle sized filler being used.
7. Setting rate: On one extreme, some production parts require a very fast set and release from the die, whereas on the other extreme a slower setting wax is an advantage.
8. Oxidation stability: Oxidation or breakdown of certain compounds in wax due to the action of heat or simply ageing will markedly change the overall properties and the wax may become unsuitable for use. It is necessary for the manufacturer to use antioxidant materials where this could occur and foundries must be aware of this.
9. Reclaimibility: The Reclaimibility of wax are important economic and ecological issues. While stating it is possible to reclaim and reconstitute all three categories of wax, strict quality control over the process is recommended. The above points considered should cover the majority of properties of an investment casting wax and how these can affect quality of wax and wax pattern production.
WAX PATTERN PRODUCTION AND THE MONITORING OF FAULTS
If problems with wax pattern production are being encountered, it is very important to consider with the wax supplier a number of fault guidelines. The most common faults encountered during wax injection are:
1. Flow lines: are usually associated with; a) Cold die b) Cold wax c) Incorrect injection pressure d) Injecting a thick section through a thin section
2. Trapped air: is usually associated with; a) Wax too hot – causing turbulence during injection b) Flow rate too high – the wax flowing into the die faster than the air escaping through the joints, thus becoming trapped. c) Air entrapped in the wax in the machine, causing air bubbles to be injected with wax. d) Air trapped in the patching wax when filling in slots in ceramic cores.
3. Lubricant marks: can be associated with over – lubrication of the die, allowing wax to push lubricant into the folds or creases, giving the appearance of flow lines.
4. Chill breakthrough: is usually associated with; a) Chill too large b) Distorted chill c) Chill too small (floating to one side) d) Pips missing from the chill e) Sinks on the chill in pip location area f) Chill movement due to force of wax, especially if located near the sprue.
5. Incomplete coverage of chill: is associated with; a) Too much lubricant on chill b) Trapped air around the chill (injection rate too fast) c) Insufficient injection pressure
6. Orange peel effect is associated with; a) Die too cold b) Wax too cold c) Insufficient injection pressure
7. Misrun is usually associated with; a) Cold wax b) Cold die c) Injection rate too low d) Wax flow restriction in the die, predominately with thin wall sections
8. Cavitation is usually associated with; a) Die temperature too high b) Wax temperature too high c) Insufficient injection pressure d) Sprue too small e) Sprue in wrong position a. Chill left out of die b. Chill required c. Injecting a thick section through a thin section. This long list only highlights the many variables that exist during wax injection technology and the object is to illustrate how important it is for the foundry to check each area thoroughly.