Acetal Copolymer – Compression vs. Extruded

Acetal copolymer users have decided that the characteristics of copolymer offer the best choice for their machining needs. They see advantages in the fact that acetal copolymer offers no centerline porosity and better dimensional stability because of lower crystallinity. Acetal copolymer also provides improved hot water and chemical resistance and is available in medical grade formulations. With the decision made to use acetal copolymer, the question becomes whether to choose extruded or compression molded sheet.

Because of the improvements made in the extrusion process, limitations that were once associated with acetal copolymer extruded sheet are no longer valid. For example, a few years ago common sizes ranged from 48” to 96” maximum. Today extruded sheet is available in sizes up to 48” x 120”. In addition, a look at the mechanical properties of both products and a comparison of stress levels indicates that extruded acetal copolymer sheet may be the better choice for a wide range of applications.

To make the comparison between compression molded and extruded, two sizes of compression molded and extruded acetal slab stock were analyzed (0.5” thick and 1.5” thick) for basic mechanical properties as well as dimensional stability and dimensional consistency. For the 0.5” plate stock there were minimal differences in tensile strength and tensile elongation. Tensile strengths were approximately 8800 – 8900 psi. Elongations at break were approximately 20 – 24%. Tensile modulus however was significantly higher for the extruded product than with compression molded acetal (394 kpsi vs 300 kpsi).

Acetal copolymer plates with thickness of 1.5” were also analyzed in a similar fashion. The extruded product had a tensile strength of 9800 psi vs 8600 psi for compression molded plate. Tensile elongation was almost twice as large for the extruded product than for compression molded plate (20 vs 12.9%). Tensile modulus was much closer for extruded and compression molded product (370 kpsi vs 357 kpsi) Similarly flexural strength and modulus were comparable in both extruded and compression molded product, approximately 420 kpsi.

The overall internal stress was analyzed for the extruded plates. Samples 2” square were taken from five areas across the width of the plate to determine the variation of stress in the part. Measurements were taken along the extrusion direction, perpendicular to extrusion and through the thickness. The compression molded product measurements were taken in a similar fashion. The parts were heated to 300ºF. After six hours, the specimens were cooled slowly to room temperature. The measurements were repeated and the percent dimensional change was calculated.

For the 0.5” compression molded plate the dimensional change was approximately 1% for the in-plane dimensions and 4.5% for the thickness. The high stress in the thickness would be expected since that would be the direction of compression in the manufacturing process. The 0.5” thick extruded plate exhibited significantly smaller dimensional changes. The extruded plate showed a 0.05% dimensional change in the extrusion direction, a 0.09% change perpendicular to extrusion and 0% change in the thickness. This indicates that the extruded plate had a very low stress level.

The 1.5” thick plates showed a similar result. The 1.5” compression molded plate had dimensional changes of 1%, 0.7% and 2% in the in-plane dimensions and thickness directions respectively. The extruded slab again displayed much lower stress levels. The extrusion and perpendicular dimensions only changed approximately 0.1%. The thickness dimension changed 0.07%. Per ASTM D 6100 the allowable dimensional change is 0.4%.

The testing data indicates that the extruded acetal copolymer products tested exhibited lower internal stresses than the compression molded acetal copolymer stock shapes tested. Lower internal stresses translate into flatter material after machining and minimal dimensional change. Therefore parts can be machined to higher tolerances and minimal rework is required, thus reducing machining costs. Extruded acetal copolymer stock shapes have excellent machinability. Standard high speed tooling can be used for milling, drilling, turning and cutting. The material has good chip forming capabilities when machined and the low internal stress allows machining to very high tolerances. Extruded acetal copolymer shapes have excellent wear characteristics. They are ideal for bearing and bushing applications. These extruded products have an excellent combination of mechanical and thermal properties which make them an excellent choice for many applications in general industrial, food contact and non-implant medical applications. The extruded acetal copolymers that were tested meet or exceed the requirements of ASTM D 6100 S-POM 0211LP and they comply with the appropriate FDA regulations for direct food.

Written by Kenneth S. Grier, Manager, Technical Resources, Ensinger-Hyde, a leading manufacturer of plastic sheet and cast nylon shapes. Manufacturing capabilities include extrusion, injection molding, compression molding and cast nylon.
He can be reached at 856-227- 0500 ext. 1111, Fax: 856-232-1754, E-mail: kgrier@ alhyde.com.