make polyoxymethylene homopolymer, anhydrous formaldehyde must be generated. The principal method is by reaction of the aqueous formaldehyde with an alcohol to create a hemiformal, dehydration of the hemiformal/water mixture (either by extraction or vacuum distillation) and release of the formaldehyde by heating the hemiformal. The formaldehyde is then polymerized by anionic catalysis, and the resulting polymer stabilized by reaction with acetic anhydride. Due to the manufacturing process, large-diameter cross-sections may have pronounced centerline porosity. A typical example is DuPont's Delrin. Copolymer The polyoxymethylene copolymer replaces about 1–1.5% of the −CH2O− groups with −CH2CH2O−. To make polyoxymethylene copolymer, formaldehyde is generally converted to trioxane (specifically 1,3,5-trioxane, also known as trioxin). This is done by acid catalysis (either sulfuric acid or acidic ion-exchange resins) followed by purification of the trioxane by distillation and/or extraction to remove water and other active hydrogen-containing impurities. Typical copolymers are Hostaform from Celanese and Ultraform from BASF. The co-monomer is typically dioxolane, but ethylene oxide can also be used. Dioxolane is formed by reaction of ethylene glycol with aqueous formaldehyde over an acid catalyst. Other diols can also be used. Trioxane and dioxolane are polymerized using an acid catalyst, often boron trifluoride etherate, BF3OEt2. The polymerization can take place in a non-polar solvent (in which case the polymer forms as a slurry) or in neat trioxane (e.g. in an extruder). After polymerization, the acidic catalyst must be deactivated and the polymer stabilized by melt or solution hydrolysis to remove unstable end groups. Stable polymer is melt-compounded, adding thermal and oxidative stabilizers and optionally lubricants and miscellaneous fillers. Fabrication POM is supplied in a granulated form and can be formed into the desired shape by applying heat and pressure. The two most common forming methods employed are injection molding and extrusion. Rotational molding and blow molding are also possible. Typical applications for injection-molded POM include high-performance engineering components (e.g. gear wheels, ski bindings, yoyos, fasteners, lock systems). The material is widely used in the automotive and consumer electronics industry. There are special grades that offer higher mechanical toughness, stiffness or low-friction/wear properties. POM is commonly extruded as continuous lengths of round or rectangular section. These sections can be cut to length and sold as bar or sheet stock for machining. Machining When supplied as extruded bar or sheet, POM may be machined using traditional methods such as turning, milling, drilling etc. These techniques are best employed where