Delrin, Hostaform, and Nylon 6 - An introduction to Plastic Gears
Delrin, Hostaform, and Nylon 6 - What’s the difference between Accu’s Plastic Gears?
Accu’s standard range of gears comprises products manufactured from a selection of reliable, high-quality engineering plastics, each with unique qualities which benefit particular applications.
In comparison to gears manufactured from metal alternatives such as mild steel and stainless steel, plastics are lightweight and generally, though not always, more chemically resistant - this means that in certain applications, gears are manufactured from engineering thermoplastics such as Delrin and Hostaform acetals, and more generalised plastics such as Nylon 6. A summary of these three widely-used plastics can be found below:
Delrin 500 is an acetal homopolymer, meaning that it consists of single repeating unit of monomers within the polymer chain.
Increased Strength and Stiffness
As a homopolymer, Delrin 500 has a composition of repeated, uniform units of monomers, which increases crystallinity and increases the tensile strength and rigidity of the polymer. By contrast, copolymers such as Hostaform C have reduced crystallinity, which reduces tensile strength as a trade-off for other advantageous qualities.
Short-Term Heat Resistance
Susceptible to Centerline Porosity
Delrin 500 has a higher melting point and a greater resistance to short-term temperature spikes when compared to Hostaform C, but is weakened more easily by regular, long-term exposure to high temperatures.
When formed, Delrin 500 cools and forms more gradually than Hostaform C. As materials generally cool from the outside in, this creates stresses within a gear as it is stretched from the outside, creating microscopic (or potentially larger) voids within the cooling plastic at the core of the gear. This effect, known as centerline porosity, can weaken a gear, and is more common in larger gears, which take longer to cool.
Delrin 500 is an acetal, which means it cannot be easily combined with many of the common flame-proof additives such as chlorine and bromine due to its violent reaction to these elements. For this reason, acetals have notoriously poor flame resistance.
Hostaform C is an acetal copolymer. A copolymer is made up of two or more repeating units of monomers within a polymer chain - a copolymer essentially functions similarly to a metal alloy, in that the addition of ingredients imbue the material with specific advantageous qualities.
Better Chemical Resistance
Hostaform C is more chemically resistant than Delrin 500. One major test for identifying homopolymers and copolymers is to expose the material in question to a heated bath containing a strong base such as triethylamine (TEA) - an environment where homopolymers dissolve rapidly, whereas copolymers do not.
This increased resistance to chemicals is particularly useful in strong acid and alkali environments, which could damage a homopolymer alternative such as Delrin 500. Chlorinated water is a common environment in which Delrin 500 would be damaged, and Hostaform C would not. For these reasons, Hostaform C is more suitable for use in food and medical industries.
Less Susceptible to Centerline Porosity
Centerline porosity is caused by a gear being stretched as it cools, creating miniscule pockets of air within the forming plastic. Hostaform C cools more rapidly than Delrin 500, which limits the window of time in which centerline porosity can occur within a cooling gear. For this reason, Hostaform C is often preferable for larger plastic gears, where centreline porosity is of greater risk.
Long-Term Heat Resistance
Hostaform C has a lower melting point than Delrin 500, but is more thermally stable under prolonged exposure to heat. Because of this, Hostaform C has greater long-term resistance to temperature when compared to Delrin 500.
Like Delrin 500, Hostaform cannot be made flame-resistant due to its violent reaction to chlorine, bromine, and other common flame-proofing additives. For this reason, acetals are susceptible to flammability.
Nylon 6 is a polyamide resin, meaning that it is built up of repeating units linked by amide bonds.
Nylon 6 is slightly porous, and has a tendency to absorb moisture from the air in humid environments. This means that Nylon 6 is not recommended for applications in regular contact with water.
As a side-effect of water absorption, the qualities of Nylon 6 are modified, for better or worse, dependent on humidity. In dry conditions, Nylon 6 has higher tensile strength and lower impact strength; in humid conditions Nylon 6 has lower tensile strength, and higher impact strength. This characteristic provides greater versatility to the material, and can be either advantageous or disadvantageous, dependent on application.
Nylon has a UL94 flammability rating of V-2, which means that it does not burn continuously, and is less flammable than acetals.
Delrin 500 is an acetal homopolymer suitable for applications requiring gears of increased tensile strength and rigidity in small sizes. Delrin 500 spur gears are used in applications requiring a lightweight, high-strength engineering plastic.
Hostaform C is an acetal copolymer commonly used in harsh chemical environments. Hostaform C can be manufactured in larger sizes than Delrin 500. Hostaform C bevel gears are commonly used in strong acid and alkali environments.
Nylon 6 is is a strong, lightweight, and cost-effective material, which has reduced tensile strength but increased shock absorbance in humid environments. Nylon is a versatile gear material which can be applied in a range of additional applications, including Nylon screws.