Welded
Belts vs. Molded Belts
Often
times our customers inquire as to the strength of our welded
products in comparison to that of a molded belt. The following
remarks explain and substantiate why we feel the PYRATHANE Power
Transmission Stretch Belt is superior to a molded product as
well as provide some background as to how we have arrived at
these conclusions.
Pyramid
Inc. has a great deal of experience with injection molding even
though we exclusively use the extrude/weld method to manufacture
our PYRATHANE Power Transmission Stretch Belts. When Pyramid
was founded in 1967, we provided both molded and extruded products
to customers. We continued to manufacture products using both
methods until selling our molding equipment in early 1987. During
those twenty years, Pyramid experimented rather extensively
with injection molding polyurethane belts. Believe me, if we
thought a high quality molded belt was possible, we would have
been producing, and would still be producing, that belt. In
our opinion, however, a high quality, durable, molded polyurethane
drive belt is not a possibility.
I
believe there are several important reasons why we, as well
as others, have not been successful at producing a high quality
drive belt using the injection molding process. First and foremost,
one must understand the molding process itself. In the molding
process, molten plastic is introduced under pressure to a relatively
cold mold. Polyurethane in its molten state would be approximately
400.... F., while the mold is at a relatively cold temperature
in the area of 100.... F. As the molten plastic is forced into
the mold, the material travels around the O-ring configuration
in the shape of a “C”, traveling in both directions from the
gate, or entrance into the O-ring. As this molten material travels
around the mold, its leading edge continues to be exposed to
cold surfaces, as well as picking up any debris or contamination
that might be present in the mold. Polyurethane also tends to
stick quite tenaciously to the clean steel mold surface. Because
of this, mold release is commonly used to facilitate the removal
of the part from the mold. Mold release, however, is very detrimental
to the bonding process required at the point where the material
meets, which is called the “knit line.” The cooling effect on
the leading edge of the molten material, plus any contamination
or mold release that is present, can cause a poor or nonexistent
molecular bond of the material. The knit line is the weak link
in an injection molded drive belt and the area of greatest concern.
Some molders have designed molds with multiple gates, which
may be somewhat beneficial but also creates multiple knit lines.
Another
point of concern is the area of the belt at the gate. As the
molten material is forced into the mold through a relatively
small port or gate, there is turbulence created at that gate,
which can also create weak links in a drive belt. Because of
the great amount of flexural stresses present in drive belt
applications, these weak links are often failure points.
Pyramid
manufactures drive belts through the extrusion/welding method
by first producing long lengths of extruded cord stock. We extrude
all of our own cord stock within our facility. This cord stock
is then cut to the desired length just prior to welding and
handled so as to keep the freshly exposed cut ends clean. These
lengths are then loaded into a welding device that exposes the
freshly cut, clean ends to a heat source that returns the material
to its molten state. Once that material has reached the molten
state, the ends are immediately brought together to create a
molecular bond, or weld, and then allowed to solidify in that
condition. Using this method, the cut ends of the cord are not
cooled by touching any foreign object or contaminated by mold
releases or debris prior to or during the welding process. We,
therefore, have absolutely clean, molten material being brought
together, which allows the molecules to fully intermingle at
the joint, creating a molecular bond. This welding process creates
a joint that possesses at least 80% of the strength of the base
cord.
Another
important aspect of Pyramid’s manufacturing process which contributes
to its quality is that the Pyrathane drive belt is an extruded
product and this process aligns the molecular structure of the
polyurethane as it passes through filtration-type strainers
producing a belt of much greater durability. Belts that are
injection molded have more of a molecular jumble within the
part as the material is forced into the mold in a rather random
pattern. Scientific substantiation may be hard to find, but
it is the only conclusion I can reach after conducting side
by side tests of belts that were both molded as well as belts
that were extruded and welded in our facility of the same resin.
These tests proved beyond any doubt that an extruded and welded
belt’s abrasion resistance far surpassed that of a molded product.
After only a few weeks of testing, the injection molded product
showed a noticeable amount of abrasion, while the extruded/welded
belt showed no signs of abrasion or wear.
It
should be noted that Pyramid’s entire business is, and has been,
based on Drive Belts. We utilize manufacturing methods that
have historically proven to be the best at building Drive Belts,
not O-ring seals, etc., but Drive Belts. We believe our customers
expect and deserve the very best drive belt in terms of wearability
and quality, and while molded belts may be cheaper, it has been
demonstrated that they do not measure up to our standards. We
firmly believe the PYRATHANE Power Transmission Stretch Belt,
in the right application, is the finest stretch belt available
to industry today and we pledge our ongoing efforts to make
that true tomorrow as well.
Thomas
M. Tripp
President
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