Putting the Metal into RPDS

Question 1

What type of technique is used for model casting?

Answer 1

lost wax technique

Question 2

What is the difference between casting for rpds and casting for crowns, bridges or implant bars?

Answer 2

RPDs involve a refractory model whereas other castings are waxed on a definitive cast and cast indirectly from the cast itself

Question 3

What is the refractory cast?

Answer 3

It is the second cast made for the metal framework and is based on the study cast

Question 4

What is the refractory cast made in?

Answer 4

phosphate bonded investment material

Question 5

What are phosphate bonded investments stronger than?

Answer 5

gypsum bonded investments

Question 6

Why must refractory casts be treated by hardening the surface of the model?

Answer 6

ON HEATING THEY ARE SUBJECT TO SURFACE WEAR

Question 7

What are the methods the refractory model can be hardened?

Answer 7

• BEESWAX – THE MODEL IS HEATED AND DIPPED INTO HOT WAX
• RESIN – THE MODEL IS HEATED AND DIPPED INTO COLD RESIN
• AEROSOL – THE COLD MODEL IS SPRAYED

Question 8

What is the most common hardener method used?

Answer 8

resin

Question 9

How is refractory cast hardened?

Answer 9

• THE MODEL IS HEATED TO 200°C FOR 45 MINUTES AND THEN DIPPED FOR 10 SECONDS IN THE MODEL HARDENER
• THE HARDENER IS ABSORBED INTO THE MODEL–NO DIMENSIONAL CHANGE TAKES PLACE
• THE MODEL IS ALLOWED TO COOL (15-20 MINUTES)
• THE WAX PATTERN CAN BE PLACED DIRECTLY ONTO THE COLD MODEL SURFACE

Question 10

What are wax patterns used for?

Answer 10

The pattern for the metallic bases and used to establish a minimum thickness in certain base areas, such as the lingual and palatal bars, and to achieve the desired contour on the lingual bar.

Question 11

When adapting the palate what must be considered?

Answer 11

if the vault of the platal is deep the wax sheet will stretch and thin out too much therefore two sheets should be placed overlapping and the junction should be removed by burnishing

Question 12

What does a sprue do?

Answer 12

CONDUCTS THE MOLTEN METAL INTO THE MOULD

Question 13

Where is the spure attached?

Answer 13

To the thickest areas of the wax and ingress of metal

Question 14

Why should the metal cool from the exterior to the centre?

Answer 14

preventing cooling shrinkage which will cause voids in the casting

Question 15

What is the sprue attached to?

Answer 15

a cone which both the sprue and cone are higher than any part of the wax pattern

Question 16

After the sprue attachment what is prepared?

Answer 16

A MOULD IS PREPARED AROUND THE REFRACTORY CAST USING THE SAME INVESTMENT AS THE REFRACTORY.

Question 17

What is the temperatue of the furnace increased from?

Answer 17

FROM 0C TO AROUND 1100C IN APPROXIMATELY 3-3.5 HOURS

Question 18

What temperature does CoCr alloy melt?

Answer 18

1400 c

Question 19

Why must the mould be hot too?

Answer 19

the mould must be hot to recieve the molten metal or the mould will expand and crack or explode

Question 20

How is the cooled mould removed from the metal?

Answer 20

tapping the residue of the casting button

Question 21

How is the casting cleaned?

Answer 21

using a sandblasting cabinet with aluminim oxide

Question 22

What can air bubbles on the surface of the castt be caused by?

Answer 22

possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast

Question 23

Why should abrasions of the cast be attended to by the technician?

Answer 23

gypsum plaster is softer than real enamel therefore the real enamel will be damaged and cause more abrasion to the metal

Question 24

What should be done before fitting to study casts?

Answer 24

finishing
trimming

Question 25

After fitting the casting what happens?

Answer 25

THE CASTING IS ELCTRO- POLISHED (ANODE-REDUCTION) BY PLACING THE CASTING INTO A POLISHING UNIT.

Question 26

What does the anode do in polishing?

Answer 26

THE ANODE REDUCES ITS SIZE WHICH PRODUCES A POLISHED SURFACE ON AREAS ON THE IMPRESSION SURFACE THAT CANNOT BE HAND POLISHED ACCURATELY.

Question 27

What is the non-impression surface polished using?

Answer 27

rubber cynlinders and wheels