Fig. 5. Switching to ceramic trays to transport molds into the dewaxing oven eliminated two mold handling steps and blocked airflow into the oven while allowing wax drainage.
cracking after making modifications to its oven, but instead it produced more severe cracking. The invest- ment caster re-examined the dewax- ing process and found four factors that had the potential to affect crack- ing: temperature, burner output, burner airflow and exhaust rate. The facility performed another
trial to determine which conditions most affected cracking. Dewaxing temperature was the most important factor. Further trials were performed to determine the optimum oven setting for dewaxing with regard to cracking, mold burnout and wax recovery. Following the trials, a dewaxing sweet spot was revealed, which was found to depend on air- flow within the oven (Fig. 6). During flash fire dewaxing, the
Fig. 6. Pine Tree Castings found that oven temperature had an important impact on cone crack severity.
two components of heat transfer to the mold are convection and radia- tion. If the oven is operated at a low temperature, convection becomes the main component of heat trans- fer. If the oven is operated at high temperature, radiation becomes the main component and depends on line of sight to the burner or oven surfaces. To successfully operate at low temperatures, the airflow must be sufficient to remove the boundary layer of air surrounding the mold.
Improving Pattern Wax Pine Tree Castings uses 40% tere-
phthalic acid-filled pattern wax for all patterns. The high loading of this relatively low cost filler makes for an inexpensive pattern wax. Its high thermal conductivity allows
for fast injection cycles, produc- ing patterns with minimal sink and consistent dimensions. However, the filler was perceived to be detrimental to dewaxing. Testing revealed that the base
wax also was causing much of the cracking at Pine Tree Castings, so the investment caster looked to change the base wax before changing its terephthalic filler level. Several wax suppliers provided
the company different base waxes with 40% terephthalic acid filler. After testing, Pine Tree Castings ultimately selected the two best-performing pat- tern waxes for further trials.
Tackling Stick Wax Pine Tree Castings’ sticks and sprues are made from wax recycled from the
Pine Tree’s Steps to Better Dewaxing
• Switched from baskets to ceramic trays for mold transport into oven and added refractory floor and wax collection chamber to one of the ovens to prevent the flow of outside air into the ovens and better control temperature.
• Determined a “sweet spot” dewaxing temperature. • Switched to a modern base pattern wax.
• Incorporated a high permeability prime and backup slurry.
Fig. 7. As mold permeability increases, crack severity is reduced. 30
• Switched to a shell refractory with high thermal diffusivity.
MODERN CASTING / August 2010
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