Delrin® homopolymer is used to make screws, hex nuts, threaded rod, set screws, as well as many other fasteners and custom parts. Delrin® is a crystalline plastic that offers an excellent balance of properties between metals and plastics. It is also known as polyoxymethylene or POM. It is produced by different chemical companies with slightly different formulas and sold by a variety of 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.
Hermann Staudinger, a German chemist discovered Delrin® . The Nobel Prize in Chemistry was awarded to him in 1953. During the 1920s, he had studied the polymerization and structure of POM. POM was not commercialized at that time because there were problems with the thermostability of the material.
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 product its version of acetal resin.
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.
Delrin® homopolymer resin can be made into the shapes required by injection molding or extrusion into 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 bar or sheet stock for machining.
Delrin® homopolymer lacks the rigidity of most metals so care should be taken to use light clamping forces and sufficient support for the work piece. 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 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 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 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 midle 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. Center line 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.
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® homoplymer 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.