Selection of injection molding lubricants
The stable operation of injection molding machines requires high-quality lubricants, which not only reduce friction and wear on moving parts but also provide cooling, sealing, and rust prevention. The selection of injection molding lubricants requires comprehensive consideration of equipment operating conditions (such as pressure, temperature, and speed), component types (such as screws, guide rails, and hydraulic systems), and environmental factors (such as dust and humidity). Incorrect selection can lead to frequent equipment failures, shortened lifespan, and even compromised product quality. A scientific selection approach should be based on core lubricant indicators such as viscosity, extreme pressure performance, and oxidation resistance, combined with the equipment manufacturer’s recommended standards to develop a targeted lubrication plan.
Viscosity is the most fundamental performance indicator of lubricating oil, directly affecting the formation of the lubricating film and its load-bearing capacity. If the viscosity is too low (e.g., kinematic viscosity <32 cSt at 40°C), it's difficult to form an effective oil film, and moving parts are prone to dry friction, leading to increased wear. If the viscosity is too high (e.g., >150 cSt), fluidity is poor, energy consumption increases, and starting difficulties may occur in low-temperature environments (<10°C). The viscosity requirements for different components of an injection molding machine vary significantly: the hydraulic system requires a medium viscosity (kinematic viscosity 46-68 cSt at 40°C) to ensure stable operation in environments between 10°C and 60°C; guide rails and screws require a higher viscosity (100-150 cSt) to prevent oil film rupture during high-speed movement; and the gearbox is selected based on the speed: high-speed gears (>1000 r/min) use 68-100 cSt, while low-speed, heavy-duty gears use 150-220 cSt. For example, the hydraulic system of a 160-ton injection molding machine uses anti-wear hydraulic oil with a kinematic viscosity of 68 cSt at 40°C. At an oil temperature of 50°C, the viscosity is about 40 cSt, and the oil film thickness reaches 2~3μm, effectively protecting the hydraulic pump and cylinder, and controlling the wear within 0.001mm/1000 hours.
Extreme pressure and anti-wear properties are key requirements for lubricants used in heavy-loaded components, such as screws and clamping mechanisms. Additives containing active elements such as sulfur, phosphorus, and chlorine form a chemical reaction film on the metal surface, protecting against friction and wear under high loads. The clamping mechanism of an injection molding machine withstands enormous pressure (up to hundreds of tons) during mold closing. Conventional lubricants are easily squeezed out under these high pressures, necessitating the use of extreme-pressure lithium-based grease (Timken OK value ≥35 lb) or extreme-pressure gear oil (four-ball machine sintering load ≥3150 N). For example, the clamping gearbox of a large-scale injection molding machine (500 tons) reduced gear tooth wear from 0.02 mm/10,000 cycles to 0.005 mm/10,000 cycles, extending its service life by three times. For components in contact with the plastic melt, such as the screw and barrel, food-grade extreme-pressure lubricants (such as those meeting NSF H1 standards) are essential to prevent lubricant leakage and contamination of the product. This is particularly applicable to medical and food packaging applications.
Oxidation resistance and thermal stability determine the service life of lubricants. Injection molding machines operate in high ambient temperatures (hydraulic system oil temperatures can reach 60-80°C, and gearbox temperatures can reach over 90°C). Lubricants are susceptible to oxidation and deterioration at high temperatures, generating sludge and acidic substances, which can lead to component corrosion and clogging. High-quality lubricants must meet the following oxidation stability requirements: a rotating oxygen bomb life (150°C) ≥ 1000 minutes, and an acid value increase ≤ 0.5 mgKOH/g (1000-hour test). For example, in a workshop with a summer ambient temperature of 35°C, the acid value of the hydraulic oil (ordinary mineral oil) in an injection molding machine increased from 0.05 mgKOH/g to 0.8 mgKOH/g after 2000 hours of use, causing sludge to clog filters. After switching to a synthetic, oxidation-resistant hydraulic oil, the acid value only increased to 0.2 mgKOH/g after 5000 hours, eliminating the need for mid-life oil changes. For continuous production injection molding machines, it is recommended to use lubricants formulated with synthetic base oils (such as PAO and esters). Their antioxidant properties are 2 to 3 times that of mineral oils, and the oil change cycle can be extended to 8,000 to 10,000 hours.
Demulsification and rust resistance are particularly important in humid environments. Due to the presence of cooling systems in injection molding workshops, air humidity often reaches 60% to 80%. Lubricating oils are easily mixed with water (over 0.1%), causing emulsification and deterioration, loss of lubrication, and corrosion of metal parts. Lubricants with good demulsification properties (such as hydraulic oils with a water separation test of ≤30 minutes) can quickly separate water, maintaining clarity. Rust resistance is assessed through salt spray testing, where the passing standard is: no rust on steel sheets after immersion in a 5% NaCl solution for 24 hours. For example, in an injection molding workshop in a humid southern region, the use of demulsified hydraulic oil consistently kept the moisture content in the hydraulic system below 0.05%, and the rust rate on the inner walls of the cylinders decreased from 15% to 0.5%. For equipment that is not in use for an extended period, lubricants with a rust protection period of ≥6 months should be used. The oil should be circulated to all components before shutdown to form a complete oil film and prevent atmospheric corrosion.
Lubricant compatibility is a key concern when changing lubricants. Mixing different types of lubricants (such as mineral oil and synthetic oil, or lithium grease and polyurea grease) can cause chemical reactions, leading to additive inefficiency and abnormal viscosity changes. Before changing lubricants, consult the equipment manual to confirm compatibility between the new and old oils. Mineral oil is compatible with most synthetic oils, but viscosity matching is important. Mixing polyurea grease with lithium grease can cause hardening, requiring thorough cleaning of the old grease. For example, the guide rails of an injection molding machine originally used lithium grease. After switching directly to polyurea grease, the grease hardened and the rails stuck. After thorough cleaning (flushing with kerosene three times) and refilling with polyurea grease, normal operation resumed. Furthermore, the lubricant must be compatible with the equipment’s sealing materials to prevent the oil from causing seal swelling (allowable swelling rate ≤ 10%). For example, oil-resistant rubber (nitrile rubber) is compatible with mineral oil, but may swell excessively with ester synthetic oils, necessitating the use of fluoroelastomer seals.
