Delrin® Homopolymer
Delrin® homopolymer also known as POM is a crystalline plastic that offers an excellent balance of properties between metals and plastics. Different chemical companies with slightly different formulas produce this material using names such as Delrin® and Celcon®.
Characteristics of Delrin® are its high tensile strength, hardness, and rigidity to −40 °C. It exhibits high fatigue resistance and creep resistance as well as high toughness and stiffness. Delrin® homopolymer has excellent dimensional stability and low friction. Because of the high crystalline composition of POM, it is intrinsically white in color and is opaque. It also comes in black.
Development
Hermann Staudinger, a German chemist discovered Delrin®. He was awarded the Nobel Prize in Chemistry in 1953. During the 1920s, he studied the polymerization and structure of POM. POM was not commercialized at that time because there were problems with the thermostability of the material.
Dupont
Research chemists at DuPont synthesized a version of POM in the early 1950s. Called Delrin®, it is a homopolymer. DuPont gives credit to R. N. MacDonald as the inventor of high-molecular-weight Delrin®. Patents by MacDonald and coworkers describe the preparation of the material, but these also lacked sufficient thermal stability to be commercially viable. Stephan Dal Noagre finally developed a heat-stable version of the POM homopolymer. By 1960, DuPont had built a plant in Parkersburg, West Virginia to produce its version of acetal resin.
Celanese
Also by 1960, Celanese had completed its own research into acetal resins. A plant to produce Celcon was built in Kelsterbach, Hessen by 1962 with Hostaform offered a year later. Both materials remain in production under the auspices of Celanese.
Fabrication
Delrin® homopolymer resin can be made into the shapes required by injection molding or extrusion into a sheet or rod. It is also possible to make shapes by rotational molding and blow molding. POM sheets can be cut cleanly and accurately using an infrared laser, such as a CO2 laser cutter. POM is commonly extruded as continuous lengths of round or rectangular sections. These sections can be cut to length and sold as a bar or sheet stock for machining.
Machining
Delrin® homopolymer lacks the rigidity of most metals so care should be taken to use light clamping forces and sufficient support for the workpiece. When supplied as extruded bar or sheet, POM may be machined by turning, milling, drilling, etc. While soluble cutting lubricant is not required, it is recommended.
Pom Burns
POM has a UL94 HB. It can have undesirable characteristics when burned. The flame is not self-extinguishing, shows little to no smoke, and the blue flame can be almost invisible. Burning also releases formaldehyde gas, which irritates the nose, throat, and eye tissues.
Center-line Porosity
One negative of Derlin® is center-line porosity. There are two types of acetal. One is a homopolymer and the other a copolymer. Delrin ® is a homopolymer. Homopolymers are plastics with molecular chains containing identical DNA repeating sequences. Homopolymers tend to develop centerline porosity.
Centerline porosity occurs when a homopolymer begins to cool during the manufacturing process as the exterior portions solidify first. This creates a loss of volume in the interior. In sheets and rods, this center line will appear as a white stripe or band in the center or middle of the shape. Centerline porosity will appear as a small circle in the middle of a part.
What Causes Center-line Porosity
Gasses in the material try to escape and form voids and pockets. This causes center-line porosity. Centerline porosity can cause a weakening of the part. It can also provide a route for liquids and gasses to leak. Additionally, it can permit bacterial growth in food processing and/or medical applications.
Copolymers are not as susceptible to developing center-line porosity. However, it is important to keep it in mind as it does happen. Then you may want to consider a different material if you have the option.
Applications
Typical applications for injection-molded POM include high-performance engineering components e.g. gear wheels, as well as ski bindings, fasteners, lock systems, and even zippers and yoyos. Delrin® homopolymer is made into springs, screws, and nuts as well as parts for televisions and telephones. In medicine, it is used to make insulin pens and metered dosage inhalers. Delrin® is used in the music industry to make picks, Irish flutes, bagpipes, and practice chanters. It is widely used in the automotive and consumer electronics industries. And there are hundreds of other applications.
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