CASTING WITH EPOXIES
Epoxy
compounds are used to cast various shapes and sizes of components. Although
epoxies are relatively simple to use, there are some basic steps that must be
followed to establish a trouble free casting operation.
Epoxy
casting compounds must be carefully selected to suit the application at hand.
The following are the basic considerations for selecting an appropriate epoxy
compound:
-
Processing limitations
-
Tool (mould) design
-
Viscosity
-
Reactivity
-
Exotherm
-
Vapour
pressure
-
Shrinkage
-
Expansion characteristics
-
Thermal
shock capabilities
-
Thermal
stability characteristics
Processing limitations
Consideration must be given to the availability of appropriate processing
equipment. Epoxy systems are available to suit hand casting, automated casting,
heat curing and higher temperature (oven) curing. Epoxy compounds may contain
fillers or may be unfilled liquids.
Tool (mould) design
The
following are the basic considerations for tool design in casting with epoxy
compounds:
-
The pour hole should
be located for the shortest path into the cavity being filled.
-
Provide vent openings
to allow air to escape as the epoxy fills the cavity. Air vents should be
provided for all areas where air might be trapped such as flat surfaces and
around intricate inserts.
-
A reservoir of
material should be available to draw from as the epoxy shrinks during the
gellation process. Most often the reservoir will be located
over the pour hole which is usually the largest opening into the mould.
-
Avoid sharp corners
and undercuts wherever possible as these are areas conducive to air
entrapment.
-
Provide uniform wall
thickness (mass) throughout the tool to allow for uniform heating and to
prevent large temperature variations due to different rates of cooling.
Viscosity
The lower
the mixed viscosity the easier it is to process the epoxy. The mixed viscosity
of two component epoxy compounds can be reduced significantly by separately
heating the resin and hardener prior to mixing together or warming the mixture
itself.
Storing
materials containing fillers under heat will require constant agitation to
prevent filler settling and the possibility of an off ratio mix. Heating will
normally result in shorter pot life.
Reactivity
"For
every 10oC
rise in temperature the reaction rate doubles"
This means that for every 10oC rise
in temperature, the pot life (the time during which the mixture remains
pourable) is reduced by (half) 50%. For example; if an epoxy system is
formulated to gel (become firm) in 30 minutes at 25oC
(77oF), then if warmed to 35oC
(95oF) it will gel in approximately
15 minutes.
Exotherm
Exotherm is the heat generated by the compound,
above the cure temperature, during the reaction. The amount of heat (exotherm)
generated depends on the epoxy formulation and the (mass) amount of product
being reacted at one time. The exotherm generated by resins and/or hardeners
heated to reduce the viscosity will be greater than without the application of
heat. As a general rule, fast reacting epoxy systems generate higher exotherm
during the reaction.
Vapour pressure
If a curing epoxy system becomes too hot, it will
generate gas bubbles which, if the bubbles form just before gellation
(hardening), can become trapped in the structure. Vapour pressure is also a key
consideration for the use of vacuum to remove air from the epoxy mixture.
Certain key ingredients such as accelerators or portions of the curing agent
itself can be stripped out of the mixture under vacuum.
Shrinkage
Shrinkage is the reduction in volume during cure.
Excessive shrinkage will cause internal stresses and serious degradation in the
performance of the solidified epoxy. Epoxy systems containing fillers shrink
less than unfilled epoxies. As a rule, the higher the filler content the lower
the shrinkage. Providing a reservoir of product to replenish the shrinking epoxy
improves the end result.
Thermal expansion characteristics
This must be considered especially if the casting
contains embedded component such as inserts within the cured epoxy. Large
differences between the expansion characteristics of the epoxy and the embedded
inserts can be the cause of cracking and reduced thermal cycling abilities. In
general, the more flexible the cured epoxy the more it will expand and the
higher the filler content the lower the rate of expansion.
Thermal shock capabilities
The higher the elongation capabilities of the cured
epoxy the better the thermal shock capabilities. Unfortunately, highly
flexibilized systems exhibit rather poor thermal stability. The epoxy system
must be formulated to achieve a suitable compromise between cured hardness,
tensile strength and elongation to achieve the desired characteristics for the
application.
Thermal stability
The
thermal stability of a cured epoxy system is its ability to operate at elevated
temperature while maintaining a specified set of minimum properties. A common
test method to determine thermal stability is % weight loss in a period of time
at a given temperature.
Since
every manufacturing process is somewhat unique and the parts manufactured must
meet different performance requirements, it is not possible to provide hard and
fast rules that could apply to all processes and materials. The following is a
general set of trade-offs caused by specific changes in the handling of epoxy
compounds:
|
Action |
Result |
|
Mixed Viscosity |
Pot Life |
Gel Time |
Exotherm |
Shrinkage |
De-mould Time |
Prob. of Cracking |
|
|
Heating the mix |
Lower |
Shorter |
Shorter |
Higher |
More |
Shorter |
Higher |
|
|
Cooling the mix |
Higher |
Longer |
Longer |
Lower |
Less |
Longer |
Less |
|
|
Heating the mould |
Lower |
NE |
Shorter |
Higher |
More |
Sorter |
Higher |
|
|
Cooling the mould |
Higher |
NE |
Longer |
Lower |
Less |
Longer |
Lower |
|
Recommended actions based on symptoms
experienced
|
Symptom |
Recommended corrective actions |
| Excessive shrinkage during gellation |
Reduce mould temperature, reduce
mix temperature, increase reservoir size. |
| Bubbles in casting. |
Mix the resin and hardener together more
gently avoiding turbulence, de-air the mix before use, pour slowly into one
area, make sure inserts are dry and free of any volatile substances, avoid
high humidity. |
| Skinned over cavities in the casting. |
Reduce mould temperature, reduce insert
temperature, increase reservoir size. |
| Surface blemishes. |
Clean tool surfaces, reduce the amount of
mould release applied to the tool, slightly reduce the mould temperature. |
| Part sticking in tool. |
Clean tool surfaces from baked on deposits,
apply a thin even layer of mould release. |
| Cracking |
Reduce mould temperature, Reduce the
difference between the insert temperature and the mould temperature. |
| Soft spots in part |
Probably an improper or off ratio mix. Mix
resin and hardener more thoroughly scraping the sides of the mixing
container. |
| Excessive yellowing of part after post cure. |
Some products will experience surface
oxidization in post cure. For best results cure immersed in an inert oil. |
The above
represent a brief summary of possible corrective measures for problems
encountered in casting with epoxy products. The list is not exhaustive and
please contact us for specific solutions.
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