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1. |
Did you know that exotherm
(the heat generated during reaction) is proportional to the gel time and
the mass of the casting? |
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The faster the gel time the
higher the exotherm. |
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Thick sections have higher exotherm. |
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The higher the mass, the higher the exotherm. |
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2. |
Did you know that it is the responsibility of the user
to confirm that all the products they purchase is on the TSCA Inventory
(USA) or on the DSL (Canada)? |
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Assuming that the supplier has the products listed is not
an adequate defense with the EPA. |
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3. |
Did you know that Epoxies and Polyurethanes are mix
ratio sensitive? |
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Unlike Polyester materials, adding more or less hardener
will not speed up or slow the reaction but will cause degradation
in the cured properties. |
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4. |
Did you know that not all epoxies are hard after cure? |
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Epoxies can be formulated to yield a
variety of hardness, down to the Shore A range with elongation up to 100%.
However, if a product turns out to be softer than indicated on the
technical data sheet, there is a good chance that it was not mixed
in accordance with the stated mix ratio. |
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5. |
Did you know that HDT (heat deflection temperature)
and Operating Temperature are not the same thing? |
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The HDT is the temperature where the material begins to
lose its mechanical strength however, this is only significant in
applications where mechanical strength is required at that temperature. In
other cases, such as potting, where mechanical strength may not be
necessary at elevated temperatures a material with a lower HDT may be
sufficient. |
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6. |
Did you know that only 50-60% of the epoxy groups have
reacted at the point of gellation? |
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At this point the linear molecules have become large
enough to become solid at room temperature, but the product does not have
significant mechanical properties. Depending on the product, the gelled
material may have developed enough properties to be handled in production
but not enough for its intended end use. In addition, a material in its
gelled state is vulnerable to stresses caused by thermal cycling.
Inadvertent thermal cycling can occur during processing if the part is
allowed to cool prior to post cure. |
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7. |
Did you know that embedded sharp corners are one of
the major causes of cracking? |
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Embedded sharp corners represent areas of concentrated
stress. Small cracks are initiated during gellation as the material
shrinks. Subsequently, these micro cracks will propagate during thermal
cycling. |
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8. |
Did you know that temperature has a major impact on
processing? |
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The viscosity of epoxy resins decrease rapidly with
temperature. Lower viscosity will aid in de-airing products while
resulting in shorter pot life and faster gellation. As a rule the curing
reaction will double (the pot life is halved) for every 100C
rise in temperature. |
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9. |
Did you know that soft spots in a cured product are
most often the result of improper mixing? |
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Thorough and complete mixing of two component materials
is extremely important to avoid either resin rich or hardener rich areas
in the mix. Under catalyzed or over catalyzed areas contain excess,
un-reacted material resulting in soft spots. If the components are mixed
by hand or drill it is important that the material from the sides and
bottom of the container is properly mixed in. If the materials are mixed
with automated equipment, it is important to have the correct mix head
(the correct number of elements) and the mix ratio is checked frequently. |
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10. |
Did you know that shrinkage occurs in two ways? |
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The first shrinkage that occurs is during cross-linking as
the molecules attach to each other and take up less space. More
significantly, the greater shrinkage occurs due to thermal contraction
while cooling down from the
exothermic reaction. As a rule, slower reacting or products containing large amounts
of fillers shrink less. |
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11. |
Did you know that epoxies and polyurethanes are
brittle immediately after gellation? |
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Almost all epoxy and polyurethane compounds pass through
a stage, just before the molecules completely align themselves, during
which the product is extremely brittle. Depending on the type of material,
this stage is passed either with time or the application of additional
heat such as post cure. A component or module will be prone to cracking if
subjected to undue stress, thermal cycling or thermal shock immediately
after the gellation phase. |
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12. |
Did you know that, in order to compare the dielectric
strength of two different materials, you must also consider the thickness
of the test specimens used to determine the published values? |
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It often happens, during material selection, that a
product is ruled out because the published dielectric strength is lower.
In fact, the published figure is just about meaningless unless the
thickness of the test specimen is also stated. A thin test specimen will
always yield higher figures because there are less impurities
contained within the structure. In order to compare "apples to apples" the
two specimens have to be tested in the same thicknesses under the same
conditions. |
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13. |
Did you know that some materials are more suited to
automated dispense equipment than others? |
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Without question, unfilled product are most suited to
automated equipment. However in most cases they are not suitable
for the part under consideration due to their inherently higher shrinkage
and lower thermal conductivity. Materials containing fillers can be
formulated for reduced wear on automated equipment by employing
"non-abrasive" fillers. Crystalline fillers will cause the most severe
wear in dispense cylinders while "Amorphous" fillers are the least
damaging. It is important for the supplier to know how the material will
be dispensed in order to provide the most appropriate material for the
process at hand. |
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