Metal Finishing

Removable Partial Denture frameworks

(1) Definition of Finishing and Polishing: A logical, systematic sequence of

procedures consisting of shaping and smoothing a framework with progressively finer abrasive agents, after which a high shine material is applied to give a high luster to the framework

(2) Purpose

(a) Maintain cleanliness of appliance while in the oral cavity

(b) Maintain health and cleanliness of the oral cavity

(c) To ensure the frameworks fit the working cast and natural dentition accurately

(3) Equipment and Materials

(a) Sandblaster

– 1 Contains zircon grit

– 2 SAFETY

• a Wear safety glasses

• b Do not operate with door open

(3) Equipment and Materials

(b) High speed lathe

– 1 Used to finish chrome cobalt alloy

– 2 Automatic spindle rotates at 24,000 RPMs

– 3 SAFETY

• Wear safety glasses, mask, and ear plugs

• Pay attention to your work -

• Always hold truing stone to mounted wheels, rubber points, and stones before using them

(3) Equipment and Materials

(c) Abrasive Polishing Agents

– 1 Abrasives are substances that wear away the surfaces of softer objects.

– 2 Abrasive material can be classified according to their hardness by a scale known as Mohs scale.

(3) Equipment and Materials

(c) Abrasive Polishing Agents

– 3 Points and stones used to finish chrome cobalt metal contain carborundum.

• a Carborundum is the trade name for silicon carbide. It is composed of extremely hard blue and black crystals which closely resemble diamonds in shape.

• b On the Mohs scales diamond rates a 10 and carborundum rates a 9 ½.

(3) Equipment and Materials

(d) Electro polisher

– 1 Removes a layer of metal from the frameworks by an electrolytic deplating process

– 2 Uses an acid with electric current

(4) Procedures

(a) Recover castings from the investment

– 1 Lightly tap the outer investment away from the paint-on layer

– 2 Use the pliers to hold the sprue

– 3 Tap the pliers not the sprue

(4) Procedures

(a) Recover castings from the investment

– 3 Tap the pliers not the sprue

• a Must be careful not to hit the sprue of the framework

• b Remove only the excess investment - excessive force could break off components

• c Remaining investment will be removed during sandblasting

(4) Procedures

(b) Sandblast

– 1 Removes all investment and surface oxides from the framework and sprue

– 2 Hold the framework at least 4 inches away from the nozzle

The high pressure of air and zircon grit could warp it

(4) Procedures

(c) Important rules for use of lathe

– 1 Let the abrasive and speed of the lathe do the cutting

– 2 Avoid heavy pressure

– 3 Remove all scratches left by the previous abrasive

(4) Procedures

(d) Desprue

– 1 Use a separating disc

– 2 Be careful not to cut into bead line or clasps

(4) Procedures

(e) Rough finishing

– 1 Remove the sprue stumps

• a Use a heatless stone or 2 separating discs on a screw mandrel

• b Grind the sprue stumps down until you reach the beadline

• c Contour the stumps to the same thickness as the major connectors

• d Grind overextended stipple sheet down to the beadline

(4) Procedures

(e) Rough finishing

– 2 Remove flash and contour

• a Use mounted stones

– 1 205w - use on large areas

» a Contour and round beadline area

» b Remove excess flash and sharp areas from denture base retention areas

– 2 203w - use in areas covered by the 205w, also remove flash from clasps and around edges of lingual plating

(4) Procedures

(e) Rough finishing

– 2 Remove flash and contour

• a Use mounted stones

– 3 Go over all lingual plating to help in the rubber point stage

» a Smooth half way up the tissue side of lingual plating -Aids in seating the framework

» b Lightly dish out rests to 1mm in thickness

» c Use metal calipers to measure thickness

(4) Procedures (e) Rough finishing

– 2 Remove flash and contour

• b Use carbide burs

– 1 703 fissure bur - used to sharpen finish lines

– 2 #8 thru #2 round burs

» a Run finger across tissue side of RPD

» b Remove any bubbles with appropriate size bur

» c Remove bubbles from inside retention areas

(4) Procedures

(e) Rough finishing

– 2 Remove flash and contour

• b Use carbide burs

– 3 #1/2 rd bur - used underneath rests to remove small bubbles