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Spruing ,

Investing and

casting

DR. MAHMOUD N. ALMUGHANY

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

The process consists of surrounding the

wax pattern with a mold made of heat-

resistant investment material. eliminating

the wax by heating and then introducing

molten metal into the mold through a

channel called the sprue.

Wax Pattern Construction

Spruing

Sprue

Definition:

Its a channel through which molten alloy can

reach the mold in an

invested ring after the wax

has been eliminated .

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

Requirement of sprue :

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)

Diameter

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

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 .

Location

Should be attached at angle to allow the incoming

molten metal to flow freely to all portions of the

mold, (About 45 degree ).

Attachment

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

Length

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

Venting

usually made of rubber, which serves as a

base for the casting ring during investing

and the sprue is attached to it .

Crucible Former

The pattern is painted with surface tension reducer

and then carefully coated with vacuum-mixed

investment

Spruing technique

Direct In Direct

The flow of molten metal is

straight ( direct from casting

crucible to the pattern.

Using aconnector or (runner

bar) which the wax pattern

sprue is attached .

Number of Sprues

Single Double Multiple

Casting

Ring and

Liner

Casting Ring

The casting ring serves as a

container for the investment

while it sets and restricts the

setting expansion of the mold.

Types of ring according to

shape Complete

ring

Split ring

Round

Oval

Rigid : Metal , plastic

Flexible: Rubber

Metal

plastic

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.

Material used as liner :

Asbestos liner

Cellulose liner ( paper )

Ceramic liner ( aluminum silicate )

Combination of ceramic and cellulose liner.

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.

Hygroscopic Expansion

Max expansion: immerse investment-filled ring in

water bath at 38C.

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 :

Investment

material

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.

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

Types:

Gypsum bonded

Phosphate bonded

Silica bonded

Gypsum Bonded Investments

use with alloys that melt below 1,000C.

Used with Type I, II, III gold alloys

Composition:

Phosphate Bonded Investments

Much stronger and can withstand much higher burnout temps

(800C +)

For alloys with casting temp > 1150C

Silicate Bonded Investments

base metal alloys such as chrom-cobalt and steel

A silica-bonded investment material can be heated up to

1200C

The reason is that these alloys melt at temperatures ranging from 1250 - 1400

Investing

Hand mixing Vacuum mixing

Technique

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 .

Investment material

pouring

1- casting ring is filled up to its rim .

2- allow investment to set for about 1 hour .

3- crucible former is carefully removed .

Burn out

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 wax-

elimination procedure, heating must be

continued, and casting must be completed.

Bring the furnace to 200C (400F), and hold this

temperature for 30 minutes. Most of the wax is by then eliminated.

Its 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 .

Burnout ovens

manual semiautomatic fully programmable

controls

Casting

Melting and casting

technique

Requires :

heat source : to melt the alloy

Casting force : to drive the alloy into the mould

Molten the alloy

Using Torch Using electric furnace

Types of torch flame :

Gas air torch

Gas oxygen torch

Oxy- acetylene torch

Hydrogen oxygen generator

Types of torch tips :

Multi orifice

Single orifice

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)

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 .

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

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 dont boil it .

Type of acids :

Hydrochloric acid

Sulfuric acid

Ultrasonic devices

Divesting and Finishing

Polishing

Thank you