Investing and CastingFixed Prosthodontics

Done by: Lina Nashawati

Introduction

Lost wax-technique: is a method of using a wax pattern to define a space within a stonelike material, which eliminates that pattern and then casts the space into metal.

The resulting casting must be a highly accurate reproduction of the wax pattern in both surface details and overall dimension.

Spruing

Investing

Burnout

Casting

Finishing &

Polishing

sprue is a channel through which molten metal is fed to the mold cavity

Wax pattern elimination so that a mold is created into which a molten alloy can be placed

a process by which a wax pattern of a shape is converted into metal.

the process of covering or enveloping wholly or in part an object such as denture, tooth, wax form, crown, etc. with a suitable investment material

before processing, soldering, or casting.

Spruing

Spruing Technique Armamentarium

A. SprueB. Sticky WaxC. Rubber Crucible

FormerD. Casting RingE. Ring LinerF. Bunsen BurnerG. Pattern CleanerH. Scalpel BladeI. Forceps

1- Sprue

Three basic requirements: 1. The sprue must allow the molten wax to escape from the mold.2. The sprue must enable the molten metal to flow into the mold with as little

turbulence as possible.3. The metal within it must remain molten slightly longer than the alloy that

has filled the mold. This provides a reservoir to compensate for the shrinkage that occurs during solidification of the casting alloy.

The sprue can be made from wax, plastic, or metal. Wax sprues are preferred for most castings because they melt at the same rate as the pattern and thus allow easy escape rate as the molten wax.

Diameter:A relatively large-diameter is recommended because this

improves the flow of molten metal into the mold and ensures a reservoir during solidification.

Molars and metal-ceramic patterns = 2.5 mm sprue Premolar and partial-coverage restorations = 2.0 mm sprue

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.The point of attachment should allow a stream of metal to be directed to all parts of the mold without having to flow in an opposite direction to the casting force.

2- Crucible Former

The sprue is attached to a crucible former (AKA: sprue former) usually made of rubber which serves as a base for the casting ring during investing.

Remove the pattern form from the die using extreme caution not

to distort it.

Insert it into the hole in the crucible former. Lute it into place with wax. Use of a surfactant enhances wetting of the

pattern during investing.

3- Casting Ring and Liner

The casting ring serves as a container for the investment while it sets and restricts the setting expansion of the mold

Therefore, a liner is placed inside the ring to allow for more expansion of the investment material during setting and burnout since the liner is somewhat compressible.

In case of wetting the liner, the absorbed water help in hygroscopic expansion. The amount of expansion depends on the number of the liners used. Types of Liners:

1. Asbestos liners have carcinogenic potential (used earlier)2. Non-asbestos:

Cellulose paper liner Ceramic ring liner (alumino-silicate fibrous material) Combination of cellulose and ceramic ring liner

Note: no liner expansion is toward the center of the mold distorted cast

Line the casting ring, keeping it flush with the

open end, and moisten the liner.

Place the ring over the pattern to ensure that it’s long enough to cover the pattern with about 6

mm of investment.

Positioning the pattern closer to the end of the ring increases the risk of the casting alloy breaking

through the end of the investment.

Material Science: Several investment materials are available for fabricating a dental

casting mold. Typically consist of a refractory material (usually silica) and a binder

material provides strength. Refractory: A material that withstand high temperature and resist

corrosion. Any addition material that a manufacture adds, it’s only to improve

handling characteristics. When we want to classify the investment material by it’s binder, we

have three groups:-Gypsum-bonded: for casting made from ADA type II, type III, & type IV gold alloys-Phosphate-bonded: recommended for metal-ceramic restorations-Silica-bonded: for high-melting base metal alloys used in casting partial removable dental prostheses

Investing

Investing: Vacuum mixing of investment materials is recommended to obtain

results in casting with minimal surface defects.

Armamentarium:A. Vacuum mixer and a bowlB. VibratorC. Investment powder(gypsum or phosphate bonded)D. Water or colloidal silicaE. SpatulaF. BrushG. SurfactantH. Crucible formerI. Casting ring and liner

Procedure: Brush technique:• Pattern is first painted with a surface tension reducer, the surface

must be wet completely.

Add investment powder to the liquid in the mixing bowl, and quickly

incorporate it by hand

1. Attach the vacuum hose to the bowl

2. Evacuate the bowl and mechanically spatulated.

Coat the entire pattern with investment and gently vibrate

through the application

Place the lined casting ring over the pattern and pour the investment in

the ring.

Allow the investment to set after the ring has been filled to the rim.

1. Remove the filled ring and crucible former from the bowl.

2. Clean the bowl and mixing blade immediately under running water

Burnout

Wax elimination (burnout):

1. Bring the furnace to 200 ◦C2. Hold the temperature for 30

minutes Until most of the wax is eliminated.

3. Increase the heat to final burnout temperature 650 ◦C

4. Hold the temperature for 45 minutes because the heating rate affects expansion

5. The mold id now ready for casting

Casting

Casting: A casting machine requires a heat source to melt the alloy and a

casting force. For a complete casting: The casting force must be high enough to overcome the high surface

tension of the molten alloy, as well as the resistance of the gas with in the mold.

Armentarium:• Centrifugal casting

machine• Crucible• Goggles• Tongs• Casting alloy• flux

Steps:

1-The crucible is placed in the casting machine and

preheated

Enough balls of metals must be used in casting to fill the mold, sprue, and part of the crucible former to ensure

sharp and complete details in the casting.

The alloy is added and melted until it is ready to cast.

1.The casting ring is removed from the furnace

• The mold is placed in the cradle of the casting machine

• The casting machine arm is released to make the casting ring.

• The casting machine is spun until it stops.

1.Ring is removed with casting tong

After the red glow has disappeared….

The casting ring is washed under cold running water

A knife is used to trim the investment at the button end of the ring .The investment is then pushed out of the ring.

Final traces of oxide layer can be removed ultrasonically.

FINISHING & POLISHING

Finishing and polishing

A cut off disc is used to remove the sprue.

The internal surface of the investment is examined to check if there are any nodules or irregularities left in there and are removed.

1. Make sure that the margins of the investment fits perfectly on the die

2. Do some polishing with polishing burs to ensure that there are no irregularities on the surface of the investment.

FINAL RESULT

THE END

Bibliography: Contemporary Fixed Prosthodontics