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  Documents - Welded Belts Versus Molded Belts

Often times our customers inquire as to the strength of the weld of our products.  We would like to provide some background as to why we, at Pyramid Inc., believe extruded and welded drive belts are superior to their molded counterparts. 

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 20 years, Pyramid experimented rather extensively with injection molded polyurethane drive belts and found them to lack the quality of an extruded/welded belt.  There are several important reasons for this.  First and foremost, in the molding process, molten material is introduced under pressure to a relatively cold mold.  Polyurethane in its molten state would be approximately 400° Fahrenheit, while the mold is at a relatively cold temperature in the area of 100° Fahrenheit.  As the molten material 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 the molten material travels around the mold, its leading edge is continuing to be exposed to the cold surfaces as well as picking up any contaminants that might be present in the mold.  There is also generally mold release present to facilitate the removal of the part from the mold.  The combination of this cooling effect of the leading edge of the molten material plus any debris or mold release that is present can cause a poor or nonexistent molecular bond of the materials where they meet which is called the “knit line”.  The knit line is the weak link in an injection molded drive belt.  Some molders have designed molds with multiple gates which may be somewhat beneficial but does create 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 the 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.  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 that molten state, the ends are immediately brought together to create a 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.  Therefore, we have absolutely clean molten material being brought together which allows the molecules to fully intermingle at the joint.  This welding process creates a joint that possesses superior strength when compared to the knit line of a molded belt.

In addition, we conducted side-by-side tests of both molded belts as well as extruded/welded belts in our facility, manufactured of the same resin.  The results of these tests substantiated our belief that an extruded and welded belt’s abrasion resistance far surpassed that of a molded product.  After several weeks of constant running, the extruded/welded belt showed no signs of abrasion or wear.  The injection molded product showed a noticeable amount of abrasion.

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 manufacturing these belts and believe our customers expect and deserve the very best drive belt in terms of wearability and quality.  While molded belts may be cheaper, it has been demonstrated that they do not measure up to our standards.


Susan S. Fortune

Pyramid Incorporated • 522 N 9th Ave E • PO Box 200 • Newton, IA 50208-0200