Sprueing, Investing, Burn-out, Casting, Fit, Finish & Polish
Luther Ison, CDT
University of Minnesota
1 Attach sprue to wax pattern with pattern still on die. Temperature changes to pattern, once off die, distorts the pattern.(Even room temperature changes).Stay away from margins. Mark trailing edge (margins) with a dot of wax on sprue base.Sprue should point towards opposite margin.
2. Get all investing materials ready (ring liner cut and placed in ring and wet (not dripping), Beautycast investment (in cabinet under gloves in dispensing), clean, dry, investment vacumix bowl, unclogged vacumix tip,15ml distilled water (exactly).
3. Recheck margins under microscope, remove pattern from die and look for flaked margins. Attach sprue and pattern to sprue base. Sprue should be approx. 5 mm long. If using reservoir, reservoir needs to be 1 mm from pattern and 3-4 mm from sprue base.
*Your pattern is now beginning to distort so proceed as quickly as practical (i.e don’t stand around B.S.-ing).
4. Optional: Dip pattern in debubblizer on dispensing counter and gently blow off excess until dry. Wet debubblizer will cause breakdown of investment and create a rough casting. (Or do not debubblize at all).
5.Push ring on sprue base, mix investment (in special bowls provided in support labs) Hand mix quickly until all powder is saturated, Vacumix 20 seconds, exactly.
6.Pour investment into ring quickly so investment runs down inside of ring so you don’t knock waxup off sprue, until level reaches pattern, then pour slowly to insure bubbles do not attach to pattern. Fill ring completely.
Hygroscopic investing technique (Increases expansion)
7.Submerse ring with investment still liquid directly into hygrobath. Make note of which number in the hygrobath you set your ring on. Turn burn out oven on and set to 925 degrees F.
. Leave in hygrobath 30 min to 1 hour. Remove sprue base from ring, write your bench number on ring using rouge or yellow welding marker, find trailing edge dot on bottom of investment and either place ring directly into 925 F oven, or place into cold oven and bring to 925, with sprue opening down, trailing edge dot on right side, and burn out for a minimum of 1 hour.
7a. Bench set minimum of 1 hour. If over night, place ring in bag with damp paper towel. If you bench set rather than hygroscopically invested, ring must be placed in cold oven and brought up to 1000 to 1200 degrees F, with a minimum burnout of 1 hr. Some ovens will rise to temp in 30 minutes but ring must stay in oven at least an hour. As a rule, add an additional 5 to 10 minutes burnout for each ring in oven. Rings can stay in oven for several hours ( but not overnight!). Also: More water in investment = less expansion= smaller casting (tighter fit of full crown, looser fit of inlay).Higher burnout temp and less water = more expansion. (looser fit of full crowns, tighter fit of inlays and dowl and cores.
1. Light bunsen burner near casting wells.Check oven to make sure it is still at 925 degrees F (or a little more) and that it actually is hot inside!
2. Wind casting machine clockwise 2.5 to3 full winds. Pull up pin in machine to prevent unwinding. This pin can rest against counterweighted or crucible end of casting arm, whichever allows torch access to front of crucible.
3. Scrape out crucible with small knife, check for obstrucion in opening, using tongs if hot, slide crucible all the way into cradle. Slide crucible cradle away from casting ring cradle.
4. Make sure ring cradle is firmly in place in casting machine and hinge in broken arm is positioned to the right..
5. Light gas/oxygen (small tipped) torch and obtain proper blue flame (bright blue flame of approx. three fourths of an inch to one inch)
6. Preheat crucible with torch
7. Place enough gold in crucible for waxup, sprue and small button, and heat gently with bright blue flame one to two inches from metal. Try to use old button and sprue, and enough “new” metal for wax pattern. This will keep replenishing button, and insure that you will have enough new metal for all your preclinical castings.
7 a. If using old button or castings, sprinkle molten metal with small amount of casting flux to clear “skin” on surface of molten gold. If no skin appears, don’t flux.
8. When metal is approaching molten, remove YOUR ring from oven with tongs and place in casting machine with tongs straight up (this means your trailing edge is on the right in the casting machine), all the time keeping flame on metal. Gently slide crucible cradle forward into casting ring. By now the metal should be fully molten. Try not to heat metal more than 10 seconds after fully molten.
9. Firmly grasp casting arm and wind until you hear the pin drop. KEEP FLAME ON METAL!
10. Gently release casting arm and raise torch (simultaneously)allowing machine to spin. Flame must stay on metal until released.
11. When maching is done spinning, remove ring with tongs and check button. When not red, quench ring, remove investment from ring and break investment away from casting.
12. Clean remaing investment from casting with toothbrush, and/or sonicate in ultrasonic with plaster/stone remover (pink solution)and steam clean.
13. Pickle in warm acid solution for 15 minutes to remove any black oxididation. Remove with pickling tweezers and rinse with water.
Divesting, Fit, Finishing, and Polishing
1.-DO NOT PLACE CASTING ON DIE YET!!
2.- Once casting is cleaned of investment and pickled to remove oxidation, check internal surfaces with stereoscope for bubbles and defects and grind out with #2 SHP round bur.
3.-Place casting gently on die (removed from cast) and check fit. Crown should fully seat on die and margins should be fully extended to red line, as waxed. If it will not seat, check again with scope and look for areas where stone is abrading inside casting. Washable marker, or Micro-Red may be applied to inside of casting to indicate areas where casting is binding and needs adjusting.
4.- Once casting fully seats on die, remove casting from die, remove sprue as close to casting as possible with separating disk, place die on cast, and remove either the mesial or distal contact segment of cast. Try seating casting on die. Place articulating ribbon between casting and proximal contact to indicate where contact needs to be adjusted. Contact should be broad area, not pinpoint. Once one contact is adjusted, replace the opposite contact and adjust the same.
5.- Check occlusal contact and adjust with green stone as necessary until opposing fully contacts arch. Check opposite side of the arch with shim stock to make sure casting is not high.
6.-Remove casting from die and continue with polishing. If casting is rough, smooth with green stone. Refine margins with green stone, then smoothe with SHP red rubber point, then green rubber point. Finish occlusal surface by removing oxidation and refining grooves with 7801 SHP bur, green stone (if rough), SHP red Rubber Point, SHP green Rubber Point (not the intraoral brownies and greenis!!). Sharpen large Rubber Points by running them in handpiece and rubbing against a separating disk.
7.- High shine with tripoli on a SHP Robinson brush until scratch-free. Surface will appear cloudy if tripoli is present on surface of casting. Continue brushing (without applying more tripoli to brush) until cloudiness disappears and casting is shiny. Occlusal grooves are polished with shorter bristled Robinson brush (worn down) so you can get down into the grooves. Ultralustre is achieved by repeating with rouge in the same manner. Clean rouge from casting by placing in blue solution (all purpose cleaner)in ultrasonic cleaner and/or steam cleaning.
8.- Crown is now ready for patient try-in, adjusting, and cementation.