A crystalline engineering thermoplastic, ACETAL is produced in Homopolymer—DELRIN® or Copolymer—CELCON®, ULTRAFORM®, etc. type. The Homopolymer is harder, tougher and mechanically stronger. Whereas the Copolymer was more resistance to higher temperatures, hot water and a broader range of pH. The recent development by E.I. Dupont of Type E Homopolymer grade has improved its resistance to hot water and alkali environments. With this advent, there is little difference in chemical resistance between Homopolymer and Copolymer. Due to lower resin cost and ease of processing; Copolymer rods, sheets, strips, tubes, etc. are less expensive. In general, ACETALS possess very good mechanical strength, dimensional stability, creep resistance, fatigue resistance and chemical resistance. One of the easiest of polymers to machine, ACETALS exhibit low moisture absorption, natural lubricity, good wear resistance (but poor grit abrasion resistance) and good electrical insulation properties. ACETALS are unusual among thermoplastics in organic solvent resistance. There are no common solvents at room temperature. Strong acids and oxidizing agents adversely affect the mechanical properties and appearance of both polymers. As with most polymers, ACETALS are degraded by exposure to UV radiation or sunlight. Pigmenting with black almost completely eliminates this problem.

With such a quality material, there are a wide variety of grades available. In addition to the standard uncompounded, un-pigmented grade there are numerous internally lubricated grades, glass filled grades and coloured materials.

ACETALS machine very well indeed, with cuttings breaking cleanly away from the tooling keeping the work piece and cutter free of turnings. Very sharp tools used at high surface speeds and slow feeds produce the best finishes and highest tolerance parts. For this reason high speed steels are recommended with all but the glass filled grades. High tool wear resulting from glass-filled grades may necessitate using carbide tooling. With their very good solvent resistance and low moisture absorption, ACETALS permit the use of a wide range of cutting fluids. Although, in many instances, a compressed gas jet directed onto the tool will provide good results in maintaining a cool cutting edge.

Copolymer CENTERING BELL used in a Soft Drink Filling Machine

With their high solvent resistance, ACETALS cannot be properly joined by cements without suitably roughening the surface. This can be accomplished using one of three methods: by sanding with 280A grit emery cloth, by a chemical etching process called SANITIZING or through a gaseous etching process. The chemical or gaseous etch produce bonds of 50% higher shear strength than the mechanical sanding method. Both contact and polar cements are useable with the polar type producing substantially higher shear strength. ACETALS can be ultrasonically and, with proper technique, hot gas welded.

Unfilled Homopolymer and Copolymer are approved by the FDA for direct food contact, at room temperature. All other grades are not approved.

BEARINGS

A convenient way of determining the suitability of ACETAL in a given bearing is to calculate the PV of the proposed bearing system. If it is lower than shown on the previous page, then the likelihood of the bearing performing well is quite high. If the PV is higher than the LIMITING PV then either the system must be changed; or materials with a higher LIMITING PV must be considered. LIMITING PV is the product of limiting bearing pressure and surface velocity; or bearing pressure and limiting velocity. The limit describes a critical, easily recognizable change that may manifest itself in one or more of the following forms: melting, cold-flow, variable friction and/or a dramatic increase in wear rate. LIMITING PV is also dependent upon ambient temperature, bearing clearances, mating material, lubrication and design of the housing. Claremont Polymer Shapes has all the necessary graphs, charts and tables available to allow us to make the necessary corrections for these factors. Contact us when you are making such calculations and we can provide the help you will need. LIMITING PV values shown on the previous page are un-lubricated, room temperature, on average hardness, 16-micron finish steel shafting using our recommended tolerances. For ACETALS, the maximum, un-lubricated surface velocity is 400 feet per minute.

ROLLERS

As an engineering material, ACETALS are quite commonly used in items like rollers. To determine the feasibility of using ACETAL, we must calculate the load capacity of the ACETAL roller. If the load capacity is greater than the actual load, then chances of a successful application are excellent. For a roller on a flat surface, the load capacity is the product of the diameter, the working width of the roller and a factor; which corrects for pressure at varying speeds. This calculation may be modified to determine the suitability of ACETAL as a roller on another roll.