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