advancements in casting technology: rapid burnout investments
TRANSCRIPT
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
1/8
NEW TECHNOLOGY
Advancements in Casting Technology:
Rapid Burnout Investments
Nannette Boyd, CDT
rian
J. Knopf,
S
Chem. MBA*
b u r n o u t i n ve s tm e n ts h a ve
evolved
in
response
to the
labora-
tory s nee d
to
reduce processing
and
The new
g e n e r a t i o n
of
can
produce superb
in
minimal time. However,
all
phos-
are
sensitive
to the
invest-
ng technique. Rapid burn out investments
re typically even mo re s ensitive
to
tech-
due to the
more reactive nature
of the
A knowledge of phosphate
and the
rapid burnout
the
dental
s success wh en us ing
a
r ap id
a
on the
chemistry
and
composit ion
and
outl ines
the
15 in-
Mix
Corp , Lou isville,
KY .
Technical Représentative, Whip Mix Corporation, Louisville,
Kentucky.
of Lab
Sen/ices, Wh ip
Mix
Corporation,
Louisville, Kentucky,
nt reque sts: Ms Nannette Boyd, W hip Mix C orporation,
• Phosphate nvestment
Most laboratories in the United States use
phosphate investment
for all
a l loys—pre-
cious
and
nonprecious.
The
main advantage
of phosphate over gypsum investment
is
that
it
can
withstand
the
much higher heating
re-
quired for non-gold and low-gold alloys as
well
as
ceramics. Expansion
can
also
be ad-
justed with
the
colloidal silica liquid.
As a re-
sult, phosphate investments
are
compatible
with many different alloys
and
ceramics.
The phosphate investment binder chem-
istry
is
fairly complex
and
results
in a
very
energetic reaction. Am mo nium phosphate
and magnesium oxide react
in the
presence
of water
to
form
an
ammonium magnesium
phosphate co mp ound . This is an aggressive
reaction that generates
a
great deal
of
heat-
Some investments will even produce steam
during
the set.
W hen heated above 850°C
during burnout,
the end
product
is
primarily
magnesium pyrophosphate (based on x-ray
diffraction rate), which
is a
strong crystalline
complex.
The
basic chemistry
is
as follows :
NH.HjPO,
MgO
5Hfi
-
NH.MgPO,
•
ÓHp
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
2/8
I Boyd/Knopf
A typical phosphate investment system
consists of three key co mp onen ts:
1 .
Binder The binder system consists pri-
mar i ly o f ammonium phosphate and
magnesium ox ide . As s ta ted before ,
this is a very energetic reaction and the
kinetics of this reaction are crucial to
casting quality. Other chemicals are typ-
ically added to control the reaction at
key stages.
2.
Refractory The refractory system is pri-
marily quartz and another allotrope of
silica called cristobalite. Other refractory
oxides may be used to some extent.
When developing the refractory system,
the consistency or feel of the mix, ther-
mal expansion of the investment, and
densi ty or porosi ty requirements are
taken into consideration.
3. Liquid. The liquid contains colloidal sil-
ica, which contributes to the setting and
thermal expansion. Unlike a gypsum in-
vestment, the casting size can be con-
trolled by the use of the proper concen-
tration of liquid.
The binder system is the most important
component in a phosphate investment be-
cause it can have the greatest effect on the
overall investment performance. It is even
more important in the rapid burnout invest-
ments. The binder chemistry can affect all of
the main properties that are considered im-
portant for an investment:
• Setting time
• Setting expansion
• Strength
• Thermal expansion
• Pattern expansion
• Cracking and spalling
• Casting size
Though the refractory system is important,
it tends to be more stable and less sensitive
to external factors. The refractory system can
affect:
• Fluidity or feel of the mix ie, wh ether it
feels coarse or s m ooth, thick or thin)
• Densi ty or porosi ty, which wi l l im
cracking and spalling
• Casting size
Many technicians think that therma l o
ting expansion will dictate the casting
This is not so. After analyzing hundre
data points, the authors have found a st
cal ly poor correlation between therm
setting expansion and casting size. The
son is that there is a third component—
tern expansion—that must be consid
Pattern expansion is the expansion tha
curs as the wax pattern is heated durin
initial stage of the binder reaction befor
investment has fully set. If the invest
sets before sufficient heating, the castin
have a tight fit. If the pattern heats too
before the set, the casting will be loose
may have a rough surface.
Without knowledge of the pattern ex
sion which we cann ot measure), there
eno ugh inform ation to pre dict casting
without making castings. An understan
of the factors that affect the binder rea
should help the technician overcome
lems with casting fit and finish.
Factors ffecting Binder Reaction
Before the mold fully sets, phosphate in
ments are sens itive to changes such
addition or removal of energy from th
vestment, or activities that affect the c
growth. As a result, phosphate investm
are sensitive to the preparation techn
Variations in mixing tim e o r mixing spe
the temperature of the equipment, can
or subtract energy, which can slow or a
erate the reaction. To demonstrate this
t ing expansion gauges, made f rom
metal and p last ic, were eq uip pe d w
transducer and a therm oco uple l inked
computer to monitor the setting expa
and temperature profi le. It was found
the metal sett ing-expansion gauge,
absorbs heat better than plastic, rem
energy from the investment and slowe
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
3/8
RAPID BURNOUT INVESTMENTS
g 1 T ime f rom invest ing to cast ing w i th rap id com-
ed to s tandard investme nt .
ced the maximum tem perature o f t h e re-
This exp er imen t emphasizes the
metal ring than in a plastic or paper ring.
Phosphate investments are also sensitive
de much bette r security.
Many liquids also are not freeze stable and
y freeze durin g sh ipp ing . Even freeze-
To achieve op t im um perfo rm an ce, one
turer s instructions. If the desired per for-
t ech-
e resp ons ib le ; mo st l i ke ly the system
Per iod ica l ly eva luat ing the mix ing
s the authors have fou nd th at a worn bow l
Most laboratories in the US have switched
from the ring technique to the ringless
tech-
nique for phosphate investment casting. The
investment slurry is poured into a plastic or
paper liner that is removed before burnout.
The ringless technique is considered to be
cleaner to work with and easier to divest.
However, investments designed for the ring
technique may not work in the ringless tech-
nique. Investments may be more prone to
cracking without the extra support from the
metal ring.
• Rapid Burnout Investment
The most significant advance in crown and
bridge investments has been the develop-
ment of the rapid burnout technique. With
this technique, the bench-set time is short-
ened and the mold is placed directly into a
heated furnace. From a manufacturer s per-
spective, these conditions present a special
chal lenge. The short bench-set t ime and
rapid heat ing put extreme stress on the
mold. As a result, the binder chemistry and
refractory system density become even more
important. Dental laboratories are increas-
ingly demanding this type of investment. The
reason is simple: time = money.
Figure
shows the typical difference in time
between the rapid burnout and standard cast-
ing techniques. The bench-set time is reduced
from about 45 to 15 minutes for the fastest in-
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
4/8
3oyd/Knopf
Fig 2 FastFire 15 investme
system.
Fig 3 Sprues are attached
patterns using pliable sticky
Fig 4 Patterns mounted in
crucible former base are spr
with Whip Mix SmoQthex wa
pattern cleaner.
Fig 5 Ring is placed on for
base.
Table 1 Sugge sted Liquid Conc entrat ions
for
Mixing*
Alloy
Base
(Ni-Cr, Ni-Cr-Be)
Noble
(Ag-Pd,
Au40 )
Liquid
concentration
90
75
60
lómLyóOg
14 mL liquid
2 mL water
12 mL liquid
4 m L water
10 mL liquid
6 mL water
Liquid powder ratio
24 mL 90 g
21 mL liquid
3 mL water
18 mL liquid
6 m L water
14 mL liquid
10 mL water
27mL 100g
23 mL liquid
4 m L water
20 mL liquid
7 m L water
16 mL liquid
11
m L
water
•Recommended conce ntrations are approxima te an d can
be
adiusted
to
optimize fit. To increase
ex
pansion, use more liquid and less water. To decrease exp ansion, use less liqu id an d m ore water. Al-
ways maintain total liquid/water volume.
the mold is placed in an oven at room temper-
ature and slowly heated to maximum temper-
ature,
is
el iminated.
The
mold
is
placed
di-
rectly into an oven at maximum temperature
and allowed to soak for approximately 30 min-
utes. The rapid burnout technique reduces this
stage of the casting operation from about 2
hours to 40 minutes. The time from the start of
investing to the completion of casting is less
than 1 hour. This results in more casting per
day and less processing time. It is also of value
• Rapid Bu rnout Investment Techn
The fol lowing technique outl ines
th
burnout process using FastFire 15 in
ment. FastFire 15 is a fine-grain phos
investment that requires only a 15-m
bench set before placing molds into
oven
for
rapid burnout.
It may be
used
either
the
ringless
or
metal ring techn
Op t i m u m fit is achieved for all crown
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
5/8
RAPID BURNOUT INVESTMENTS
Fig 6 Powd er ¡s inc orp ora ted
into l iqu id by hand spatu lat ion.
Fig 7 Investmen t is mi xed under
vacuum at low speed.
Fig 8 Casting ring Is f i l l ed usin g
low vibra tion.
Fig 9 Wax pat tern s are cov ere d
with no less than 0.25inch (7 mm]
of investment.
Preparation
• Attach sprue to the wax pattern using pli-
able sticky wax (Fig 3).
• Mount patterns on crucible form er base.
• Spray with Whip Mix Smoothex wax pat-
tern cleaner. Gently blow excess cleaner
from pattern (Fig 4).
Place ring on to fo rm er base (Fig 5).
Fill casting ring using low vibration, cover-
ing wax patterns with no less than 0.25
inch (7 mm) of investment (Figs 8 and 9).
Bench Set
• Bench set 15 minutes, then remove m old
from ring and base (Fig 10).
ixing
shown in Table 1, Distilled water is recom-
mended for dilution.
Rinse the bo wl w ith water and shake ou t
excess.
corporate powder by hand spatulation 10
to 15 seconds (Fig ó).
Mechanically mix un de r va cuu m at low
speed (350 to 450 rpm) from 60 to 90 sec-
Burnout
• Tr im glaze off top of mold and r inse
under tap water (Fig 11). Note Molds al-
lowed to set more than 24 hours should
be re-wet by soaking in water for 3 to 5
minutes.
• Place m old in preh eate d oven at recom -
m en ded tem pera ture (maximum 900°C/
1650T), For higher temperatures, place
m old in oven a t 900°C (1650°F), the n h eat
to fin al tem peratu re at 14 to 20°C {25 to
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
6/8
Boyd/Knopf
Fig 10 Mold is removed from the ring and ba
after bench set.
Fig 11 Glaze is trimmed off top of mold then
under tap water.
Fig 12 Mold is placed in preheated oven at
maximum temperature of 900°C ¡1Ó50T).
Fig 13 Casting is done according to alloy
manufacturer s instructions.
Fig 14 Investment is broken from metal
framework/castings.
Figs 15a and 15b Metal is prepared for porce
application with separating disk and noncontam
nating aluminum oxide abrasives.
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
7/8
RAPID BURNOUT INVESTMENTS
Heat soak at final tempe rature for 30 min-
utes; add 10 m i n u t e s per a d d i t i o n a l
mold.
to alloy manufacturer's in-
structions (Fig
13).
e vesting
Break investment from metal framework/
castings (Fig
14).
Clean castings according to alloy manufac-
turer's suggestions (aluminum oxide
50 to
60 ^im or ultrasonic solution}.
• Using a separating disk and noncontami-
nating aluminum oxide abrasives, prepare
metal
for
porcelain application (Figs
15a
and 15b).
• Place co m ple ted cas tings on master cast
for porcelain application (Fig 16).
Table 2 Physical Properties of FastFire 15
Investment*
Liquid/powder ratio
Initial setting time
Setting expansion
Thermal expansion
Compressive strength, wet
(45 mm)
•Measured at 100 liquid concentration.
• Conclusion
27mLyiOOg
7-9 min
1.0
1.0
500 psi (3.4 MPa) .
Rapid burnout investments can provide ex-
cellent castings
in
minimal time.
The
physical
properties
of the
FastFire
15
investment
sys-
te m
are
shown
in
Table
2. A
knowledge
of
phosphate investment chemistry
and the in-
vesting technique will enable the technician
to ensure better control over
the
casting
process and take full advantage of the bene-
fits from the new generation of rapid burnout
investments.
-
8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments
8/8