advancements in casting technology: rapid burnout investments

8/16/2019 Advancements in Casting Technology: Rapid Burnout Investments

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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


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


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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-


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


5/8


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


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.


7/8


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.


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advancements in casting technology: rapid burnout investments

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