Mohawk College - Introduction to Metal Casting
...
MATL MTB70 Module 3 - Course Notes...
Mold Types &
Methods
This section will
largely be a text based listing of General Mold types and the various methods
associated with each, as well as the advantages and disadvantages found
with them.
The Text is a fair
reference, but the material in the video clips is the best so far in the
course... Very much the sort of thing a Metal-Casting Specialty channel would
carry. I've opted to not make illustrations for this Module as it would be a few
days of work, and likely wouldn't come close to the videos, lecture note images
or the text...
Conventional
Molding - Green Sand
This
is the traditional Cope & Drag Flask method, where a re-usable pattern is
placed in a flask, sand is rammed up, the flask is rolled, the other flask half
is applied, rammed with sand, split and the pattern extracted to leave a cavity
into which the molten metal is poured.
The
Ramming process can be termed either as a Jolt or Squeeze method.
Advantages
- a wide range of metals can be cast with this method, the cost of materials and
patterns is comparatively lower than other methods/types of mold making, the
process is scalable to higher production run requirements and the molding
material (bonded Sand) is easily recycled.
Disadvantages
- The complexity of the casting is limited by the requirement of draft, parting
lines and the ability to position cores, Part tolerances and dimensional
accuracy may be lower than other types/methods requiring additional labour costs
for finishing/machining of castings,
Conventional
Molding - High-Density
Molding
This
method employs mechanical ramming of bonded sands with either hydraulic or air
pressure into the flask over the pattern. By using sand with a lower moisture
content higher densities and mold hardness are achieved.
Advantages
- Again a diversity of Metal types can be cast, but with closer dimensional
accuracy while improving surface finish over the previous method. The other benefit
is higher production rates realized by the mechanization of the ramming process.
Disadvantages
- Higher capital cost of equipment, higher labour cost associated with
maintenance and training of staff and tighter quality control.
Alternative
Sands
This section describes the family of No-Bake or Quick-Set
binders. The binder or Bonding material is a Polymer or Silica Gel that
replaces the Clay & water combination found in the Green Sand family. The
objective of these synthetic binders is to achieve greater binding strength.
Binder types - Each has
unique properties relating to pH, appropriate selection for alloy type, strength
and shake-out to be discussed later
-
Alkyd Urethane
-
Furan
-
Sodium Silicate
-
Acid Cured Phenolic
-
Phenolic Urethane
-
Phenolic Ester
Skin-Dried & Dry Sand Molding
The primary purpose of Skin Dried molds is to reduce moisture
and gas forming materials at the surface of the mold where the metal sand
junction occurs. The sand typically has special bonding agents applied prior to
drying such as GM Bond (presumably a proprietary agent developed by General
Motors???)
The Mold face exhibits superior mechanical strength, while
retaining comparible "Shake-out" characteristics to Green Sand
molds. This method is employed for medium to Heavy and Heavy castings.
The Surface can be dried with either Hot Air, Gas or Oil
Flame and the mold face can be washed with as refractory coating (so that's a
refractory coating on a refractory type material???)
Advantages - Reduced Gas and Moisture , ie porosity/pinhole
gas defects, and a stronger mold.
Disadvantages - are Labour and material costs compared to
Green Sand molding, reduced production rates due to drying process of molds.
Dry Sand Molding
This is the Green Sand process, but with the addition of heat
to bake the mold at 400 to 600F, the baking process allows for greater
mechanical strength, and higher casting complexity. This process is employed for
medium to heavy castings such as gears, housings etc...
Advantages - The mechanical strength of the mold, tighter dimensional
accuracy, and better surface finishes due to the reduction mold moisture and/or
the application surface coats.
Disadvantages - are the capital cost of Bake Ovens, cost of
operation, and reduced production relative to Green Sand.
Shell Molding or Croning Process
This process uses a heat activated Resin/Catalyst combination
with silica sand, The pre-coated sand is applied to a heated pattern plate where
the resin hardens to form a shell that is approx 3/4" to 1" in
thickness. The 2 halves of the mold are glued together with an appropriate
adhesive.
Advantages - are Superior dimensional accuracy and surface
finish.
Disadvantages - Capital and labour costs, as well as reduced
productivity due to the process being relatively slow...
this is the end of the "Conventional
Molding types
Precision
Molding and Casting
Permanent Mold - Gravity feed
This class of mold is typically made of tool steel. cast iron,
graphite, Copper or aluminum (mold material selection is based on the metal type
to be cast) for repeated usage. The metals to be cast generally are restricted
to lower Melting Temp alloys due to the issues inherent in the handling and
mechanization of high temp alloy handling. This process is well suited to the
Aluminum, Zinc and Magnesium family of alloys.
This type of process is further divided into 2 (two) variants
Static and Tilt Pouring. This process also exhibits superior dimensional
accuracy and surface finish over the Shell process just discussed.
Static Pour - The mold is stationary along a fixed plane while
the molten metal is poured into a sprue.
Tilt Pour - The mold is placed into a machine that can tilt
back once the receiving cups/reservoirs are filled with molten metal, this semi-rotational
action allows for more complex mold creation then possible with a static mold.
Low Pressure Mold
This method employs a feed mechanism that exerts a 5 to 15 PSI
head on the molten metal forcing it into the mold. It should be noted that an
Aluminum casting with a wall thickness of 5/32" or greater is achievable
with this method.
Advantages - Good dimensional accuracy and casting complexity
compared to previous methods discussed and better economy of scale on a per
casting basis (beyond the initial tooling cost recovery)
Disadvantages - are typically the capital cost of
equipment and Mold production, as well as the restriction of alloys that can be
cast.
High Pressure Molding
This is a High Volume casting process for low-temp alloys that
offers superior dimensional accuracy and complexity of castings. The Metal is
injected into the mold at pressures up to 5000PSI (5,000PSI that's high!!!) with
such pressures the susceptibility of porosity defects is increased.
This class also can be divided into 2 (two) classes Cold
Chamber & Hot Chamber processes.
Advantages - Low cost of castings when extended across very
large production runs with part complexity, surface finish and dimensional
accuracy at a high level.
Disadvantages - Castings are limited to typically 75lbs and
require additional QC procedures to avoid Porosity defects, stringent Mold
Design requirements and the Very High cost of tooling and equipment that must be
extended across high volume production runs to recover the initial costs.
this concludes the section on Permanent
Molds
Investment Casting
This class of molding describes the process of
"Investing" a pattern in a suitable molding material. The Pattern is
typically burned out of the mold leaving a cavity that molten metal can then be
poured into.
Lost Wax Process
A wax duplicate of the desired casting is created to be
invested into a "Ceramic Slurry". the slurry covered investment can be
dipped into alternating coatings of sand & slurry until a suitable thickness
of shell is achieved that can hold the molten metal after the investment is
burnt out.
The "Burn-Out" process requires that the investment
and coating are inverted in an oven that is fired to 1800F so that the
investment can flow out and be recovered. The refractory coating is also cured
in this procedure.
Once the investment is lost, and the Refractory is cured the
mold is removed and poured immediately while it is still hot.
Advantages - High level of accuracy and flexibility of design
due to no draft, parting lines and the ability to mold under-cuts previously
impossible with other techniques. The process is scalable from small to high
production requirements and offers reduced finishing costs with exception surface
quality.
Disadvantages - include a physical limitation on the size of
the casting as determined by the strength of the cured ceramic slurry (even when
backed by sand) and a higher material and labour cost (which can be offset by
savings in finishing and machining).
Evaporative Pattern Casting
This is it, LFMC (Lost Foam Metal Casting) where it all
started for me, and is a great process that has served me well.
Patterns are produced in EPS (Encapsulated Polystyrene), the
pattern receives a sprue or feeder system (also of EPS) and can be either placed
directly into loose dry sand, or invested into a ceramic slurry. The slurry is
air dried or in a low temp oven (but the foam pattern is retained).
In either case the pattern is surrounded by loose dry sand
that is being constantly agitated by a vibratory mechanism as the sand is added
to the container that holds the pattern. The vibratory motion creates a degree
of fluid movement within the surface of the sand that is able to completely fill
all crevices, under-cuts, openings and orifices thus removing the requirement of
otherwise complex cores.
Once the pattern (and refractory coating) is completely held
in a container of sand, the metal is poured into the sprue. The heat of the
molten metal evaporates the foam in it's path and accurately fills in behind the
foam as it advances down through the pattern. The vapourized EPS is vented into
the loose sand through the refractory coating (if used).
Of note is that the Lecture notes, text and videos all specify
the use of a refractory coating, but I know for a fact that it can be omitted
with a minor degradation in surface finish and complexity of casting. Castings
with wall thickness of 120 thou" that's 5 thou less than an 1/8" are
possible... just fantastic...
Advantages - are (but not limited to...) coreless molding,
very high casting complexity, comparatively low capital and operating
investment, simplified shakeout, ease of sand recovery and as mentioned
0.120" wall thicknesses. The Video and notes reflect this as a relatively
environmentally benign process... (I have concerns to the contrary)
Disadvantages - Pattern coating requires additional labour and
material costs, patterns require care as they can be fragile due to
construction, Strict safety procedures to be followed when handling loose sand
post pouring. The last item applies to all loose sand handling with regard to
the hazards of Silicosis, but especially when handling sand that has burnt EPS
covering the grains. The strong chemical stench of the sand screams of future
respiratory ailments if caution is not exercised (personal observation &
belief)