Casting Printed Removable Partial Denture Frames in Seven Steps
ResinVest from Whip Mix Corporation
By Craig A. Pickett, RG, CDT, TE
ResinVest phosphate investment is made specifically for burning out printed resin removable partial denture patterns. The new investment is fiber-reinforced to strengthen the resistance of the investment to cracking and finning during the burnout of the resin pattern. This high performance material offers a wide expansion range and optimal fit of the casting. It also features a rapid burnout technique in order to reduce processing time and increase productivity. Its fluid consistency promotes ease of investing, smooth casting surfaces, and easy divesting.
When printing and preparing the resin pattern, be certain that the pattern is printed correctly and that the “post-processing” procedures are closely followed for your resin. This would include complete light curing for those that are using this process. Be certain that the pattern fits the model correctly and that any defects are corrected with wax (Figure 1). This will include any areas where supports were removed and may have left a divot.
The next step is to sprue and place in the casting ring. Spruing can follow the traditional process of making removable partial dentures on a refractory cast using ResinVest as the refractory. This can also be achieved by spruing these patterns using a traditional crown-and-bridge technique using a direct or indirect method (Figure 2). The pattern may be placed horizontally or vertically if space allows, but be certain there’s a minimum of 10 mm (1 cm) of investment thickness from the edge of the sprued pattern to the side of the ring.
ResinVest is a reinforced phosphate investment that uses a colloidal silica liquid (Special Liquid Concentrate +) to begin its chemical process. Mix the investment as instructed, and pay close attention to mixing time. Ratios to determine expansion will differ depending upon environmental location and alloy used for the casting.
A pattern cleaner/bubble remover may be used but should be air dried before investing the pattern. Pour the mixed ResinVest into the assembled ring at the base of the pattern (Figure 3). From the investment in the ring, coat all intricate parts with investment using an instrument (do not use brushes, as they can leave bubbles). From there, continue to fill the ring until the pattern is covered by 10 mm (1 cm) of investment, then bench-set for the indicated amount of time.
Burnout has been problematic for many investments using printed resin patterns. Studies conducted at Whip Mix have shown that the greater the amount of resin in the pattern, the more likely the investment will fracture during burnout. This is due to the nature of the resin, not the investment. Pressures created by the burning resin cause the problem—thus the need for a reinforced investment like ResinVest (Figure 4).
Conventional gas-torch methods or induction machines work well for casting, which should be followed by standard techniques to recover the casting. You may notice that the investment is “harder” to divest than other investments and may require sandblasting. This is just the nature of reinforced phosphate. For finishing and polishing, follow your normal procedures using the alloy of your choice. Any will provide impressive results (Figure 5).
About the Author
Craig A. Pickett, RG, CDT, TE, is the Technical Education Manager for Whip Mix in Louisville, Kentucky.
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