It is always a good idea to lubricate plastic components whenever possible because it reduces friction and wear, lowers power consumption, and increases part life. For example, studies have shown that lubricated plastic sliding bearings last up to five times longer than non-lubricated ones. When choosing a lubricant for plastic parts, the most important factor is whether the lubricant is compatible with the plastic material.
To verify compatibility of lubricants with plastics, manufacturers monitor changes in physical properties of the plastic material under conditions of speed, load, and environment. These properties include weight, volume, hardness, strength, and elongation. Each manufacturer determines the limit on allowable changes, but typically the range is 7% to 10%. When evaluating these tests, be sure they reflect your worst possible conditions because both lubricants and plastic materials are more prone to change in high temperatures and adverse environments, particularly with high dynamic loads. Choosing an incompatible lubricant for plastic parts can cause the plastic to develop stress cracking, become discolored, or lose dimensional stability or structural integrity.
There are three main criteria for choosing a lubricant for plastic parts:
A lubricant’s compatibility with plastic is largely determined by its chemical structure. Lubricants based on silicone, perfluorinated PFAE, mineral oils, and synthetic hydrocarbons (SHC or PAO), typically work well with plastics. Esters and polyglycols are generally not compatible with plastic, although there are exceptions based on the type of plastic material. Sometimes additives can cause an undesirable or unintended reaction between the lubricant and the plastic. Solid additives like graphite and molybdenum disulfide can penetrate and weaken a plastic part. Conversely, PTFE solid additives can be beneficial in certain cases, such as providing dry lubrication or reducing startup friction.
High viscosity oils – those with ISO VG of 100 or more – are less likely to penetrate, crack, or otherwise adversely affect plastic materials. Lighter loads require oils with lower viscosity to prevent viscous drag, while higher loads require higher viscosity oils to maintain a lubricant film from start to stop. For greases, a consistency of NLGI 1 or 0 helps reduce friction and grease-induced noise (grease slap).
Lubricants are more likely to attack plastic parts as they age. That is why synthetic lubricants, which have a high aging resistance, are the best choice for long-term applications. Outgassing byproducts of plastic, particularly formaldehyde and styrene, accelerate the lubricant aging process and should be kept to a minimum if possible.
Mineral-oil-based lubricants don’t attack most plastics and offer excellent performance for an economical cost. However, if your application requires high operating speeds, high temperatures, or long operation, a better choice would be synthetic lubricants such as hydrocarbon (PAO) types. PAOs are compatible with most plastics, have high aging resistance, and offer long-term lubrication within a temperature range of -60º to 320ºF.
Silicone-based lubricants also show excellent compatibility and are a great choice for low load applications and a wide temperature range (typically -90º to 425ºF). PFAE lubricants are extremely compatible, even with hard-to-match plastics, and can be used in extreme temperature applications, up to 500ºF. However, because of their high cost, PFAE lubricants should only be used where necessary.
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