Process description

The plaster mold casting process is a split-pattern technique that is similar to the common sand-casting process. The primary differences between the two processes are that plaster mold patterns are accurately machined from metallic or acryllic materials as opposed to being fabricated from wood, and also use a plaster slurry as mold material instead of sand. As a result, the dimensional accuracies and surface finishes that are typically obtained are significantly superior to those realized through sand casting.

Tooling consists of a permanent pattern machined into two parts. The casting shape is divided along a parting line that is established to provide the best tolerance advantage with no undercuts in either half. If undercut portions are needed, cores are produced separately in precision-built core boxes and placed in the mold halves to form any undercuts.

Plaster molds are made by mixing a metalcasting plaster, other refractory materials, and water to form a slurry. The slurry is then poured over the patterns and sets firmly within five to fifteen minutes.

When the plaster mold halves have been drawn from the patterns, they are placed in drying ovens, where both free and chemically-bonded water molecules are driven off. Molds may be dried in batch-type ovens, or in conveyorized ovens that have been programmed to remove water at a precise rate to assure complete drying without thermal shock. Cores are dried in a similar manner, usually at the same time as their parent molds.

After cooling, molds are carefully broken away from the castings, and gates and risers are removed.

Finishing of the castings is accomplished by removing the parting lines, and either abrasive-blasting or acid-etching the parts prior to shipment.

Aluminium base alloys

ALLOY

TEMPER

TENSILE

(p.s.i)

YIELD

(p.s.i)

%ELONG

REMARKS

360

N/A

46 000

24 000

3.5

Very good casting charateristics

380

N/A

49 000

27 500

3.8

Good corrosion resistance and very good retention of as-cast or heat-treated mechanical properties at elevated temperatures up to 300 F.

713

T5

32 000

22 000

3.0

Good corrosion resistance, excellent anodizing properties. This alloy is self-aging.

A356

T51
T6
T71

25 000
30 000
30 000

20 000
20 000
25 000

1.5
2.0
1.0

Good corrosion resistance, good pressure tightness, good weldability.

A357

T6

38 000

28 000

5.0

Good pressure tightness, good corrosion resistance.

 

 

 

 

 

 

T51 -Simulates die casting applications
T6 - Normal engineering
T71 -Exceptional dimensional stability after machining

 

Different molding methods

REQUIREMENT

GREEN SAND

PERMANENT MOLD

DIE CASTING

SAND-SHELL

CERAMIC MOLD

INVESTMENT CASTING

PLASTER MOLD

Cost, large quantities

Low

Low

Lowest

Medium high

Highest

Medium

Medium

Cost, small quantities

Lowest

High

Highest

Medium high

High

High

Medium

Pattern cost

Lowest

High

Highest

Medium high

Medium

High

Medium low

Prototype adaptability

Lowest cost

High cost

Highest cost

Medium cost

Low cost

High cost

Low cost

Lead Time

Short

Long

Longest

Medium

Short

Long

Short

Weight maximum

Unlimited

100 lb

30 lb

250 lb

Over 100 lb

30 lb

250 lb

Tolerances, in.

0.012

0.03

0.01

0.01

0.003

0.003

0.004

Thinnest section, in.

0.150

0.125

0.03

0.1

0.05

0.05

0.04

Surface finish

Fair

Good

Best

Good

Good

Very Good

Very good to best

Complex shapes

Fair

Fair

Good

Good

Very Good

Best

Very good to best

Ease of design change

Best

Poor

Poorest

Fair

Fair

Fair

Very good

Alloys

Unlimited

Copper-base and lower melting alloys preferred

Zinc, aluminum and lower melting alloys preferred

Unlimited

Unlimited

Unlimited

Aluminum, magnesium, zinc, Duralcan

Précicast Ltée
102, rue Joseph-Gagné Nord, Ville de La Baie (Québec), G7B 4T1
Téléphone: (418) 544-8448
Télécopieur: (418) 544-9391
Courrier électronique : precicast@precicast.ca
Personne contact: Marc Tremblay