Dental Lab steps from mould preparation to finish/ dental education in india

GOOD MORNING
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

LAB STEPS-MOULD
PREPARATION TO
FINISH

CONTENTS
 INTRODUCTION
 SPRUING
a) DEFINITIONS
b) BASIC REQUIREMENTS
c) PRINCIPLES OF SPRUING
 INVESTING
 WAX BURN OUT
 CASTING
 TRIMMING AND FINISHING
 CONCLUSION
 REFERENCES

Structural faults in dental castings
can result from any combination of
problems related to spruing, investing,
wax elimination, alloy melting, casting
& solidification of the casting.
The ability to produce smooth and
well fitting castings require strict
adherence to certain fundamentals.....

SPRUING
“Spruing is an art which is not well
understood…..”
Off all the procedures involving
casting work, the spruing technique
has a paramount importance in
producing a complete & dense casting.

DEFINITIONS
Sprue : the channel or hole through
which plastic or metal is poured or
cast into a gate or reservoir and then
into a mould
Sprue button : the material remaining in
the reservoir of the mold after a
dental casting
Sprue former : a wax, plastic, or a metal
pattern used to form the channel or
channels allowing molten metal to
flow into a mold to make a
casting……. (GPT-8)

BASIC REQUIREMENTS
To form a mount for the wax pattern
To create a channel for elimination of wax
during burn out
To form a channel for entry of molten alloy
during casting
Provides a reservoir of molten metal which
compensates for alloy shrinkage during
solidification

Taggart (1907), introduced the lost wax technique
Asgar & Peyton (1959), stated that flaring should
occur at the sprue/wax pattern junction
Strickland et al (1959), stated the importance of the
type, shape location & direction other than the
size of the sprue
Bruce (1964), tested the different methods of
spruing patterns
Rousseau (1964), described a casting method with a
constricted sprue

MATERIALS
 Wax
 Plastic/Resin
 Metal sprues

FACTORS TO BE CONSIDERED
 diameter
 Shape
 Number
 Attachment
 Location
 Angulation

DIAMETER
 Usually large-diameter sprue is
recommended
 The diameter of sprue should be equal to
the thickest portion the wax pattern.
 There are various gauges of sprue former
 gauge 6, 8, 10, 12, 14, 16, 18 (0.4cm -
0.1cm)
 Usually  for molar and metal ceramic
restoration  10-gauge (2.5mm)
  Premolars and partial coverage
restoration  12-gauge (2.0mm)

SHAPE
 The sprue former should be straight to reduce
chances of creating turbulence in molten metal
entering the mold.
 High turbulence of alloy cause porosity.

NUMBER
 Usually a single sprue is used for small castings.
 When two thick sections of a pattern are
connected by thin part of wax, 2 separate sprues
should be attached to each thick portion

LOCATION OF ATTACHMENT
 Attachment of sprue former and wax pattern: -

ANGLE OF ATTACHMENT
 The sprue should be attached to
pattern such that it makes 45 degrees
to the walls of mold, which decreases
the turbulence of molten alloy.
 If the sprue is placed perpendicular to
the mold wall, it induces high
turbulence in molten alloy, leading to
creation of a hot spot on mold wall.
This results in localized shrinkage
porosity.
 It should be placed away from thin /
delicate parts of pattern to prevent
fracture of investment.

ATTACHMENT MORPHOLOGY
 The transition should be smooth
 Usually it is flared for high density gold alloys but
restricted for low density alloys. Flaring acts as
reservoir and facilitates the entry of molted alloy
into the mold area.

SPRUING TECHNIQUE
 Once spruing is done
the wax pattern and
sprue former assembly
is carefully removed
from the die.
 During removal of
pattern no pressure
should be applied to
prevent its distortion.

 Positioning the pattern
on the crucible former
 Application of
surfactant
 Ring liner increases the
setting expansion
 Pattern must be
positioned sufficiently
away from the end of
the ring

INVESTING

GPT TERMINOLOGY…..
 Investing : The process of covering, enveloping,
wholly (or) in part an object such as denture tooth,
wax form, crown, etc with a suitable material
before processing, soldering, casting.
 Dental casting Investment: Material consisting
principally of an allotrope of silica and a bonding
agent. The bonding substance may be gypsum (for
use in lower casting temperature) (or) phosphates
and silica (for use in higher casting temperatures).
 Refractory : (adj) - Difficult to fuse/corrode,
capable of enduring high temperatures
 Refractory investment : An investment that can
withstand high temperature using a
soldering/casting.

ESSENTIALS…..
 It should produce mould expansion to
compensate for casting shrinkage.
 Should have sufficient strength at
room & high temperature.
 Inner surface of the mold should not
break at a high temperature.
 Should exhibit sufficient strength,
to withstand the force of molten
alloy entering the mold.
 Inner surface of mold should be
smooth.

BASIC COMPOSITION…..
Refractory Materials :
The most commonly used refractory material is silicon
dioxide in form quartz, crystoballite.
Binder : commonly used binders are
 - calcium sulfate hemihydrate
Others are – Sodium silicate, ethyl silicate, ammonium
sulfate, sodium phosphate.
Other chemical modifiers
Reducing agents – Ex : Charcoal
Sodium chloride, boric acid – to  shrinkage, during
dehydration of calcium sulfate dihydrate.
Potassium sulfate (accelerator)
Copper powder(reducing agent)/magnesium oxide

 When classifying investments by binder, three
groups are recognized: gypsum-bonded, phosphate-
bonded, and silica-bonded investments.
 Each has specific applications.
 The gypsum-bonded investments are used for
castings made from ADA Type II, Type III, and Type
IV gold alloys.
 The phosphate-bonded materials are recommended
for metal-ceramic frameworks.
 The silica-bonded investments are for high-melting
base metal alloys used in removable partial denture
castings.

EXPANSION…..
This property of investment is
needed for compensation of
casting shrinkage of alloy.
The expansion occurs because
of:-
… Normal setting expansion
… Hygroscopic setting expansion
… Thermal expansion.

ARMAMENTARIUM
 Vacuum mixer and bowl
 Vibrator
 Investment powder
 Water or colloidal silica
 Spatula
 brush

CRUCIBLE FORMER
 Helps to hold & place the sprue in
the center of the casting ring
 Helps to contour the investment
surface
 They may preformed or contoured
with wax

CRUCIBLE
 Carbon crucibles
 Clay crucibles
 Quartz /zirconia - alumina

Clay crucibles are used for
crown and bridge alloys such as
high noble and noble alloys
.carbon crucibles are used for
high noble crown and bridge
alloys and for high fusing gold
based metal-ceramic alloys
Quartz and Zirconia-alumina
crucibles are used for high
fusing alloys or alloys that are
sensitive to carbon
contamination www.indiandentalacademy.com

CASTING RING LINER
 A ring liner is placed on the inner
surface of the casting ring
 Should be placed a few mm short of
the ring
 It can be either
asbestos liners (used earlier)
non-asbestos liners (cellulose,
ceramic-cellulose,
alumino silicate)

CASTING RING
Choice of the ring:
Rubber ring
Metal ring
The diameter and the length of the ring should be
selected rightly.

RINGLESS CASTING SYSTEM
Versatile system for accurate & simple
spruing, investing & casting with
plastic or metal rings
Produces consistent, accurate &
predictable castings
Designed to allow unrestricted expansion

ASSEMBLING THE RING
This step should be coordinated with
spruing & choosing the size of the
ring
Wetting agent is applied ( to reduce air
bubbles) on the wax pattern

PREPARATION OF THE INVESTMENT
MIX
Mixing of the investment is done by:
Hand mixing (brush technique)
Vacuum mixing

BRUSH TECHNIQUE
Add investment powder to the liquid in the
mixing bowl and quickly incorporate with
the hand.
Attach the vacuum hose to the bowl,
evacuate the bowl and mechanically
spatulate
Coat the wax pattern with the investment
Carefully coat the internal surface & the
margin of the pattern
Fill the ring slowly, starting from the
bottom

VACUUM TECHNIQUE
First hand spatulate the mix
With the crucible former and pattern in
place attach the ring to mixing bowl
Attach the vacuum hose & mix accordingly
to the manufacturers recommendations
Invert the bowl & fill the ring under
vibration
Remove the vacuum hose before shutting of
the mixer
Immediately clean the bowl & mixing blade
under running water

SETTING OF INVESTMENT
After the ring is filled to the rim
allow the investment to set.
If hygroscopic technique is used, the
ring is placed in a 37 c water bath
for 1 hr.

TIME OF PLACEMENT OF
INVESTMENT FOR BURN OUT
after 20 mins
after 2 hrs
After 1 hr
Castings were undersized
Castings were satisfactory
Were found to be the best

WAX BURN OUT
 For Gypsum investment - 6500 –
Slowly heated for 60 min &
held 15 – 30 min at upper
temperature

FOR PHOSPHATE INVESTMENTS
 2% or more expansion is required for Metal –
Ceramic Prosthesis
 Burn Out temperature – 7500 – 10300C
 Slow – till 3150 C & quite rapid thereafter

BURN OUT FURNACE

REMOVAL OF MOLD FROM FURNACE

CASTING
 Casting machines requires a heat source to melt
the alloy and a casting force
 Heat source can be either the reducing flame of a
torch or electricity
 Present day casting machines still use either air
pressure or centrifugal force to fill the mold

TORCH
MELTING/CENTRIFUGAL
CASTING MACHINE

 Conventional alloys can
be melted with gas air
torch
 Metal ceramic alloys in
a higher melting range
need a gas oxygen torch
 Base metal alloys need
a multiple orifice gas
oxygen torch

TORCH MELTING/CENTRIFUGAL
CASTING MACHINE
 Most common
 it rapidly spins the mold,crucible and molten alloy in a
circle.
 Casting machine spring is first wound from 2-4 turns
 the rotating arm is composed of two parts connected by a
pivot point and thus is called a broken arm casting
machine.
 The alloy is melted by a torch flame in ceramic crucible
attached to the broken arm of the casting machine.

Once the metal has reached the
casting temperature ,the machine
is released and the spring triggers
the rotational motion.
The broken arm accelerates the
initial rotational speed of the
crucible and the casting ring, thus
increasing the linear speed of the
liquid casting alloy as it moves into
the mold.

QUENCHING & CLEANING THE
CASTING
Casting appears dark
with oxides
The process to remove
this oxides is called as
pickling
50% HCl is used to
remove the oxides

RECOVERY OF THE CASTING
 Trimming is done from
the button end of the
ring
 Investment is being
pushed out of the
casting ring
 The mold is broken
open
 Investment is removed
from the casting

RAPID CASTING

COMMON CAUSES OF CASTING
FAILURE
problem Possible causes
1.Rough casting
2.Large nodule
Improper
finishing of wax
pattern
Excess surfactant
Improper w/p
ratio
Excessive burn
out temperature
Air trapped
during
investment
procedure

problem possible causes
Multiple nodules
Nodules on occlusal surface
Marginal discrepancy
Suck back porosity
Inclusion porosity
Inadequate vacuum during
investing
Improper brush technique
Lack of surfactant
Excessive vibration
Wax pattern distortion
uneven expansion
Improper pattern position
narrow long sprue
Particle of investment
dislodged during casting

problem Possible
causes
fins Increased
w/p ratio
Pattern too
near edge of
the
investmment
Premature
heating
Too rapid
heating

BACK PRESSURE POROSITY
 entrapped air porosity
seen on the inner
surface of the casting
produce large concave
depressions
 Inability of the air in
the mold to escape
through the pores in
the investment
 The entrapment is
found on the cavity
surface of the crown

INCOMPLETE CASTING
 Molten metal is prevented in completely filling the mold
 Insufficient venting
 High viscosity of fused met
 Mold is filled and alloy is solidified in 1 sec yet it is quite soft
during early stages.

 Incomplete elimination of the wax residue from
the mold
 Pores in the investment gets filled and air cannot
be vented completely.
 Margins are rounded and quite shiny rather than
dull ,this is due to the strong reducing
atmosphere created by carbon monoxide left by
the residual wax.

TRIMMING AND
FINISHING OF
METAL AND METAL
CERAMIC
RESTORATIONS

TRIMMING
PROCESS OF REMOVING EXCESS
MATERIAL BY CUTTING OR GRINDING A
MATERIAL WITH ROTARY INSTRUMENTS TO
PROVIDE A DESIRED ANATOMIC FORM
Finishing
Process of removing surface defects or
scratches created during the contouring
process, through the use of cutting or
grinding instruments or both

 The technique of finishing and polishing castings is a
systematic reduction of surface irregularities.
 The finished casting must maintain the fundamental
requirements of fit ,form, function,and marginal integrity

 Ease in finishing a casting depends directly on the quality of
wax pattern,the investing technique,and the casting
technique
 The smoother the wax pattern ,the smoother the casting;the
denser the casting,the easier the finishing

DIVESTED CASTING

Divested casting is sand blasted with 90-120µm alumina

Castings after sand blasting

Equipment

ARMAMENTARIUM
Initial gross trimming instruments

Stones for smoothening the rough marks left by trimmers

Wire brushes, rubbers and bristle brushes for finishing

RECOMMENDED SEQUENCE FOR FINISHING A
CAST RESTORATION

RECOMMENDED SEQUENCE FOR
FINISHING OF A CAST RESTORATION
 Zone 1. Internal margin
 Zone 2. Internal surface(INTAGLIO)
 Zone 3. The sprue
 Zone 4. Proximal Contacts
 Zone 5. Occlusal Surface
 Zone 6. Axial Walls
 Zone 7. External Margins

INTERNAL SURFACE
(INTAGLIO) INSPECTION OF
CASTING
The best time to check for nodules is immediately after
removing the investment from the casting and before
attempting any trimming.

NEVER FORCE
THE CASTING
ONTO THE DIE,
USE GREAT
CAUTION WHEN
FITTING THE
CASTING
When removing a
nodule, remove slightly
more than the defect to
ensure complete seating
of the restoration

THE SPRUE
 To reestablish proper coronal morphology and
function,the sprue must be sectioned,and the
casting must be recontoured in the area of its
attachment

Castings being seperated by gentle twisting

Thick carborandum disk followed by TC trimmers

PROXIMAL CONTACTS
 Special care is needed to prevent the finishing
procedures from producing an overreduced and
consequently inadequate proximal contact.

Articulating paper

Cross section showing
properly finished
connectors
Twine impregnated with
polishing compound is
an efficient way to polish
this hard to reach area.

OCCLUSAL SURFACE
 Occlusal contacts are reestablished in static and
dynamic relationships to the opposing arch.
 Occlusal morphology must ensure positional
stability and satisfy all funtional requirements

Metal caliper is used to check for even thickness of coping

Check for contacts

AXIAL WALLS
 The axial walls should be smoothly contoured
and highly polished,enabling the patient to carry
out optimum plaque control

Stones Rubber wheel Rubber point

EXTERNAL MARGINS
 Margin finishing is critical to a restoration’s
longevity and therefore deserves special attention.
 The objective of all cast restoration finishing is a
highly polished metal surface without ledges or
steps as transition is made from restoration to
unprepared tooth.

SUMMARY AND CONCLUSION
The accuracy of the fit of a cast restoration is essential for
its longevity
This allows for less plaque accumulation at the marginal
area
It also provides better mechanical properties like stability
and resistance, less microleakage and good esthetics
 Correct handling of the wax pattern with proper investing
and casting technique should be used to get a accurate
casting.

REFERENCES
 Phillips’science of dental materials – Kenneth
J.Anusavice,11th Edition.
 Removable partial prosthodontics-
McCracken,9th Edition.
 Dental laboratory procedures – Fixed partial
dentures -Robert M. Murrow, Kenneth D.
Rudd, John E. Rhoads.
 Notes on Dental materials – E.C.Combe,6th
Edition.
 Contemporary Fixed Prosthodontics –
Stephen F. Rosenstiel, Martin F. Land,
Junhei Fujimoto,3rd Edition.
 Restorative dental materials – Robert
G.Craig, John M. Powers.

Thank you
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Dental Lab steps from mould preparation to finish/ dental education in india

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