dnetal materials lecture

1. Spruing ,
Investing and
casting
DR. MAHMOUD N. ALMUGHANY

2. Curriculum :

Wax pattern construction .

Sprue ( Def, Role, Requirements and technique )

Casting ring and liner ( types, mold expansion )

Investment material (requirement, composition and
types )

Investing ( mixing and pouring )

Burn out

Casting ( Requirement, alloy melting, casting )

Quenching

Pickling

Divesting and finishing

Polishing

3. 
The process consists of surrounding the
wax pattern with a mold made of heatresistant investment material. eliminating
the wax by heating and then introducing
molten metal into the mold through a
channel called the sprue.

5. Wax Pattern Construction

6. Spruing

7. Sprue
 Definition:
It’s a channel through
which molten alloy can
reach the mold in an
invested ring after the wax
has been eliminated .

8. Role of a sprue :

Holds the wax pattern to avoid its distortion.
Creates a channel to allow the molten wax to
escape from the mold.

Enables the molten alloy to flow into the mold
which was previously occupied by the wax
pattern.

Sprue can made from : Metal , plastic and wax

9. Requirement of sprue :
Diameter
In general, a relatively large diameter sprue is
recommended because this improves the flow of
molten metal into the mold.
Molar around 2.5 mm ( 10 gauge), premolar 2 mm (12
gauge)

10. 
Reservoir :
Is a small amount of
additional wax
added to sprue
former 1 mm below
wax pattern .
Function :
to compensate for the
shrinkage occurs
during solidification
of the casting.
Reservoir

11. Location
The sprue should be attached to the
bulkiest noncritical part of the
pattern. away from margins and
occlusal contacts .
Normally, the largest nonfunctional
cusp is used.
Fuctional cusps :
Buccal cusps of the mandibular posteroir
teeth & Lingual cusps of the Maxillary
posterior teeth .

12. Attachment
Should be attached at angle to allow the incoming
molten metal to flow freely to all portions of the
mold, (About 45 degree ).

13. Length
The length of sprue former should keeps the wax
pattern 6 mm from the casting ring length to
Provides adequate bulk of investment to withstand
force.
Allows gases to escape from end of mold.
The pattern should be placed as possible to the
center of ring

14. Venting
Small auxiliary sprues or vents have
been recommended to improve
casting of thin patterns and may
helps in :
gases escape during casting.
compensate for the shrinkage during
solidification

15. Crucible Former
usually made of rubber, which serves as a
base for the casting ring during investing
and the sprue is attached to it .
The pattern is painted with surface tension reducer
and then carefully coated with vacuum-mixed
investment

16. Spruing technique
Direct
The flow of molten metal is
straight ( direct from casting
crucible to the pattern.
In Direct
Using aconnector or (runner
bar) which the wax pattern
sprue is attached .

17. Number of Sprues
Single
Double
Multiple

18. Casting
Ring and
Liner

19. Casting Ring
 The
casting ring serves as a
container for the investment
while it sets and restricts the
setting expansion of the mold.

20. Types of ring according to
shape
• Round
• Oval
Complete
ring
Split ring
•Rigid : Metal , plastic
•Metal
•Flexible: Rubber
•plastic

21. Ring Liner :

Creates a space to allow for investment
expansion

Allow setting of investment material under water,
to give hygroscopic expansion

Regulate heat transfer through the investment
material.

Facilitate removal of investment from the casting
ring after casting.

Thickness of liner should be around 1 mm not less.

One or two layers.

3.0-mm short of both ends of ring will allow
supporting contact of investment with ring after
liner has burned out.

22. 
Material used as liner :

Asbestos liner

Cellulose liner ( paper )

Ceramic liner ( aluminum silicate )

Combination of ceramic and cellulose liner.

23. 
Four mechanisms to produce mold expansion:
Setting Expansion of Investment
Hygroscopic Expansion
Wax Pattern Expansion
Thermal Expansion

Setting Expansion
Occurs as a result of normal gypsum crystal growth in
air.
About 0.4% but partly restricted by metal investment
ring.

24. 
Hygroscopic Expansion
Max expansion: immerse investment-filled ring in
water bath at 38°C.
Water in bath replaces water used by hydration
process space between growing crystals is
maintained crystals grow longer  outward
expansion of mold.
About 1.2 to 2.2% max with expandable ring.

Wax Pattern Expansion
While investment is still fluid, expansion occurs when
wax is warmed above Temp at which it was
formed.

Thermal Expansion
when investment is heated in burnout oven.
Two purposes :

25. Investment
material

26. Requirements:

Precise reproduction of wax pattern.

Sufficient strength to withstand burnout
and casting.

Sufficiently porous to allow escape of
gases.

Expand enough to compensate
solidification shrinkage of alloy.

27. Composition of Investment

Refractory material or filler
Material that resist high temperature
Expand upon heating
Its problem have no cohesion to maintain the
shape of investment when dried

Binder
Mixed with filler to give some strength

Chemical modifier

28. Types:
Gypsum bonded
Phosphate bonded
Silica bonded

29. Gypsum Bonded Investments
use with alloys that melt below 1,000°C.

Used with Type I, II, III gold alloys

Composition:
Phosphate Bonded Investments
Much stronger and can withstand much higher burnout temps
(800°C +)
For alloys with casting temp > 1150°C
Silicate Bonded Investments
base metal alloys such as chrom-cobalt and steel
The reason is that these alloys melt at temperatures ranging
from 1250 - 1400
A silica-bonded investment material can be heated up to
1200C

30. Investing

31. Technique
Hand mixing
Vacuum mixing

32. Vacuum mixing

Advantages :
Reduce the amount of porosity in the investment .
Texture of the surface casting is smoother with better
detail reproduction .
Compressive strength of the investment is increased .

35. Investment material
pouring
1- casting ring is filled up to it’s rim .
2- allow investment to set for about 1 hour .
3- crucible former is carefully removed .

36. Burn out

37. Wax Elimination

Wax elimination or burnout consists of heating the
investment in a thermostatically controlled
furnace until all traces of the wax are vaporized.

Once the investment is heated during the waxelimination procedure, heating must be
continued, and casting must be completed.
Bring the furnace to 200°C (400°F), and hold this
temperature for 30 minutes. Most of the wax is by
then eliminated.

38. 
It’s advisable to begin the burn out procedure
while the mould is still wet because water
trapped in the pores of the investment reduces
the absorption of wax .

39. Burnout ovens
manual
semiautomatic
fully programmable
controls

40. Casting

41. Melting and casting
technique



Requires :
heat source : to melt the alloy
Casting force : to drive the alloy into the mould

42. Molten the alloy
Using Torch
Using electric furnace

43. Types of torch flame :

Gas air torch

Gas oxygen torch

Oxy- acetylene torch

Hydrogen oxygen generator
Types of torch tips :

Multi orifice

Single orifice

45. Flame zones :
(OXY- ACETYLENE flame)
1- Neutral Flame (Acetylene oxygen in equal
proportions)
2. Oxidizing Flame (Excess of oxygen)
3. Reducing Flame (Excess of acetylene)

48. 
Air pressure casting machine :
Alloy is melted in situ in crucible hollow of the ring ,
followed by applied air pressure on the melt

Centrifugal casting machine:
Alloy is melted in crucible , then forced in the mold
by centrifugal force .

51. Quenching

After the red glow has disappeared from
the button, the casting ring is plunged
under running cold water into a large
rubber mixing bowl

52. Pickling :
Surface of the casting appears dark with oxides and
tarnish. This surface film removed by these process
( pickling ) .
By place the casting in a dish and pour acid over it .
Heat the acid and don’t boil it .

Type of acids :
Hydrochloric acid
Sulfuric acid
Ultrasonic devices

53. Divesting and Finishing

54. Polishing

55. Thank you