was 0.5 in.-0.625 in. x 0.75 in.-1 in.. Using this approach, the entire mold assembly could be made without the use of a die or wax injection press, making evaluating different pattern and sprue waxes easier. By combining a standardized test
with design of experiments testing, Pine Tree Castings was able to evalu- ate many factors and interactions with a minimal amount of tests (Figs. 1-2). The testing found three major compo- nents of the process that have a signifi- cant influence on dewax performance: mold heating rate, shell permeability and base wax formulation.
Optimizing Dewax Equipment
Molds are loaded into Pine Tree Castings’ flash fire ovens in metal baskets and pushed into the load end of the oven at regular intervals. The wax drains through the open bottom of the oven and contacts an inclined water cooler plate, reducing the wax temperature and preventing igni- tion. The oven is heated with eight burners located adjacent to the first, second and third basket. The flame from the burners directly impinges on the pour-cup end of the molds. The dewax operator must constantly monitor and adjust the loading rate of the baskets to ensure complete burnout of the wax. Pine Tree Castings had little his-
torical data on oven temperatures. The controllers were set to around 1,000F (538C), but the ovens could achieve the temperature only when the wax ignited. Most of the time, the ovens operated below their setpoint with the burners running at 100% output. To determine the effec-
tiveness of the flash fire pro- cess, the investment caster compared it to autoclave dewaxing at nearby invest- ment casting facility Uni-Cast, Londonderry, N.H. Results from the comparison tests pointed to several impor- tant points: • Pine Tree Castings’ flash fire process was a major con- tributor to mold cracking, but the defects couldn’t be eliminated simply by chang- ing to autoclave dewaxing (Figs. 3-4).
• Converting to autoclave dewaxing would require a
Fig. 3. The plotted main effects of cone cracking for the autoclave and flash fire dewaxing methods showed the severity of cracks increased with increased terephthalic acid filler level and prime slurry layers in both methods.
large capital expenditure. Pine Tree Castings would have to purchase and install two autoclaves, as well as modify or replace all of its batch ovens that did not have afterburners.
• Flash fire dewaxing had the potential to be combined with mold preheat- ing, reducing mold handling and eliminating one queue.
• The terephthalic acid filler level in the pattern wax had a large impact on crack severity.
• When the terephthalic acid filler con- tent in the stick wax was reduced, cracking did not improve, indicating the base wax also contributed to the problem. Pine Tree Castings also performed dewaxing tests in a more modern
flash fire oven at SeaCast Inc., East Greenwich, R.I. The tests showed that high temperatures were not required to successfully dewax molds. SeaCast dewaxes at 750F with no cracking.
Solution to Basket System The flash fire ovens at Pine Tree
Castings are open on the bottom to allow wax to drain from the molds, but this also allows uncontrolled outside air to enter the oven. Con- verting its basket design to ceramic trays provided the investment caster a stable platform for the molds, allowed for palletized mold storage before and after dewaxing, eliminated two mold handling steps, and provided a method for blocking airflow into the oven while allowing wax drainage (Fig. 5). A refractory floor and
wax collection chamber were added to one of the ovens. An opening was left in the floor cor- responding to the first basket position to allow for wax drainage into the collection chamber. Now, when a tray of molds is pushed into the oven, the ceramic plate covers the opening.
Temperature Sweet Spot
Fig. 4. Pine Tree Castings concluded that although the flash fire process was a major contributor to mold cracking, the defects could not be elimi- nated simply by changing to autoclave dewaxing.
MODERN CASTING / August 2010
Pine Tree Castings expected to see an im- mediate reduction in
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