Expandable pattern casting and plaster mold casting are two types of expendable mold casting processes. Expandable pattern casting uses an expandable polystyrene pattern that is coated, embedded in sand in a flask, and then vaporized when molten metal is poured, leaving an exact replica of the pattern. Plaster mold casting similarly uses a plaster mold instead of sand and is best for smaller, non-ferrous metal castings due to temperature limitations of plaster. Both methods can produce complex, close-tolerance castings with minimal finishing required.

1. “ExpandablePattern Casting,
&
PlasterMold Casting”

2. Expandable Pattern Casting
Also known as Expanded Polystyrene Molding or Full Mold
Process, the EPC or Lost Foam process is an economical method
for producing complex, close-tolerance castings using an
expandablepolystyrene patternand unboundedsand.
The EPC process involves attaching expandable polystyrene
patterns to an expandable polystyrene gating system and applying a
refractory coating to the entire assembly. After the coating has dried,
the foam pattern assembly is positioned on several inches of loose
dry sand in a vented flask. Additional sand is then added while the
flask is vibrated until the pattern assembly is completely embedded
insand.

3. A suitable downsprue is located above the gating system and sand is
again added until it is level to the top of the sprue. Molten metal is
poured into the sprue, vaporizing the foam polystyrene, perfectly
reproducing the pattern. Gases formed from the vaporized pattern
permeate through the coating on the pattern, the sand and finally
through theflaskvents.
In this process, a pattern refers to the expandable polystyrene or
foamed polystyrene part that is vaporized by the molten metal. A
patternisrequired foreachcasting.
Expendablemoldcastingis ageneric classificationthatincludes
sand, plastic,shell, plaster,and investment(lost-waxtechnique)
moldings.This methodofmoldcastinginvolves theuse of
temporary,non-reusablemolds.

4. Advantages
Nocores arerequired.
Reductionincapitalinvestmentandoperatingcosts.
Closer tolerancesandwallsasthinas0.120 in.
No binders or other additives are required for the sand, which is
reusable.
Flasks for containing the mold assembly are inexpensive, and
shakeout of the castings in unbounded sand is simplified and do not
require the heavy shakeout machinery required for other sand
castingmethods.
Needforskilledlaborisgreatlyreduced.
Casting cleaning is minimized since there are no parting lines or core
fins.

5. Disadvantages
The pattern coating process is time-consuming, and pattern
handlingrequires great care.
Good process control is required as a scrapped casting means
replacementnotonly of themold butthepattern as well.

6. Expandable Pattern Casting

7. Example products of Expandable Pattern
Casting

8. Plaster mold casting
Plaster mold casting is a metalworking casting process similar
to sand casting except the molding material is plaster of Paris instead
of sand. Like sand casting, plaster mold casting is an expendable mold
process; however it can only be used with non-ferrous materials. It is
used for castings as small as 30 g (1 oz) to as large as 45 kg (99 lb).
Generally, the form takes less than a week to prepare. Production
ratesof1–10 units/hrcan beachievedwithplastermolds.
Plaster mold casting is a manufacturing process having a similar
technique to sand casting. Plaster of Paris is used to form the mold for
the casting, instead of sand. In industry parts such as valves, tooling,
gears, and lock components may be manufactured by plaster mold
casting.

9. Details
The plaster is not pure plaster of Paris, but rather has additives to
improve green strength, dry strength, permeability, and castability. For
instance, talc or magnesium oxide are added to prevent cracking and
reduce setting time; lime and cement limit expansion during baking;
glass fibers increase strength; sand can be used as a filler. The ratio of
ingredientsis70–80% gypsumand20–30% additives.
Typical tolerances are 0.1 mm (0.0039 in) for the first 50 mm (2.0 in)
and 0.02 mm per additional centimeter (0.002 in per additional inch).
A draft of 0.5 to 1 degree is required. Standard surface finishes that are
attainableare1.3to4 micrometres(50–125 μin).

10. The pattern is usually made from metal, however rubber molds
may be used for complex geometry; these molds are
called Rubber plaster molds. For example, if the casting
includes reentrant angles or complex angular surfaces then the
rubber is flexible enough to be removed, unlike metal. These
molds are also inexpensive, reusable, more accurate than steel
molds, fast toproduce, and easy tochange.

11. Process
Initially plaster of Paris is mixed with water just like in the first step of the
formation of any plaster part. In the next step of the manufacture of a plaster
casting mold, the plaster of Paris and water are then mixed with various
additives such as talc and silica flour. The additives serve to control the setting
time of the plaster and improve its strength.
The plaster of Paris mixture is then poured over the casting pattern. The
slurry must sit for about 20 minutes before it sets enough to remove the
pattern. The pattern used for this type of metal casting manufacture should be
made from plastic or metal. Since it will experience prolonged exposure to
water from the plaster mix, wood casting patterns have a tendency to warp.
After striping the pattern, the mold must be baked for several hours, to
remove the moisture and become hard enough to pour the metal casting. The
two halves of the mold are then assembled for the casting process.

12. Advantages& disadvantages
Plaster mold casting is used when an excellent surface finish and good
dimensional accuracy is required. Because the plaster has a low thermal
conductivity and heat capacity the metal cools more slowly than in sand mold,
which allows the metal to fill thin cross-sections; the minimum possible cross-
section is 0.6 mm (0.024 in). This results in a near net shape casting, which can
be a cost advantage on complex parts. It also produces minimal scrap material.
The major disadvantage of the process is that it can only be used with lower
melting temperature non-ferrous materials, such as
aluminium, copper, magnesium, and zinc. The most commonly used materials
are aluminum and copper. The maximum working temperature of plaster
is 1,200 °C (2,200 °F), so higher melting temperature materials would melt the
plaster mold. Also, the sulfur in the gypsum reacts with iron, making it
unsuitable for casting ferrous materials.

13. Anotherdisadvantage is thatits long cooling timesrestrict
production volume.
Plasteris not as stableas sand, so it is dependent on several
factors, including theconsistency of the plaster composition,
pouring procedures, and curing techniques.If these factors are
not closely monitored themold can be distorted, shrink upon
drying, have a poor surface finish,or failcompletely.

14. Plaster Mold Casting

15. Example products of Plaster mold
Casting

16. Videoof ExpandablePatternCasting

18. Videoof PlasterMoldCasting