| In order to reduce vehicle weight, improve performance and increase design flexibility, automakers are increasingly favoring plastic parts. From door trims, dashboards, to engine components and hoods, plastic parts in general passenger cars account for about 10% of all parts. The key to the performance of plastic parts is to choose the right lubricant. It can reduce friction to reduce wear, noise and improve customer satisfaction. Although lubricants have been widely used on metal parts in automobiles for many years, they have not been used for plastic parts for a long time. Unlike choosing lubricants for metal parts, choosing the right lubricant for plastic parts is much more difficult. Due to friction, plastic parts degrade faster than metal parts, and choosing the wrong lubricant can have serious consequences. Types of Lubricants
The first step in choosing the right lubricant is to understand the types and uses of lubricants. Automotive lubricants can be divided into several categories, including lubricating oils, ointments, greases, dry film lubricants and solid lubricants. Lubricating oil is the most common automotive lubricant, used as engine oil, gear oil and transmission fluid. The viscosity of lubricating oil can be adjusted by adding different additives to the base oil to suit different uses. Grease is a semi-solid lubricant composed of a base oil and a thickener. Grease is easier to hold and adhere to parts than oil, so it does not require complex sealing devices. Like oil, different additives are added to grease according to different uses. Grease is widely used in various automotive parts, such as chassis, sunroof, etc. Grease is a mixture of solid lubricants and lubricating oils, which makes the use of solid lubricants more effective. Grease has relatively limited uses and is mainly used for the assembly of parts that need to bear very high loads. Dry film lubricants are paint-like lubricants made of solid lubricants dispersed in a resin binder. Dry film lubricants are used on parts where oil lubricants are difficult to adhere to or are prohibited. Solid lubricants are mainly used as additives for greases and lubricating oils to increase load capacity. Solid lubricants can also be used in metal stamping and as additives for brake pads and plastics. Physical properties of plastics
There are many types of plastics used in automotive parts, and each type of plastic has different grades and compatibility with lubricants. Compatibility is also affected by additives, fillers, and the size of the plastic molecules. For example, some grades of plastic have smaller molecules than standard grades and are less compatible with lubricants. Lubricant experts believe that the best approach is to select the right plastic based on the lubricant requirements. However, this approach is difficult to apply in practice. Since plastic raw materials are limited, it is necessary to select the right lubricant for a given type of plastic. Another important consideration is the physical properties of plastics. When choosing a lubricant suitable for plastic parts, several physical properties of plastics must be considered. Plastics have very different physical properties than metals. For convenience, functional plastics and structural plastics are selected here to explain their physical properties one by one. Functional
plastics are used in the form of lubricating component materials for automotive parts such as gears and sliders. Generally, various types of functional plastics (such as polyamide, polyacetal, etc.) have good oil resistance and good compatibility with many traditional lubricants and greases for automotive metal parts. However, since some additives in lubricants can corrode plastics, it is necessary to check the additive composition in the lubricant before use. In addition, relevant tests, such as immersion tests, are also required to verify the compatibility of the actual lubricant used with the plastic. Certain functional plastics, such as polyacetal and nylon, have self-lubricating properties, meaning they can replace metal-lubricated components without the need for lubricants. 
Figure 1 Friction of plastic parts with and without grease
Usually, the speed device will use lubricants on plastic parts. The use of lubricants can expand the upper limit of its use. Many additives in metal lubricants react with metal surfaces to form a wear-resistant lubricating film. However, since the melting point of most thermoplastics is lower than the temperature at which the additives react chemically, and the activity of the plastic surface is lower than that of metals, the performance of traditional additives cannot be fully utilized when lubricating plastics. Special care should be taken when selecting solid lubricants for plastic parts because the hardness of plastics is lower than that of metals. If an inappropriate solid lubricant is selected, the plastic will cause greater wear and damage due to friction. Suitable solid lubricants can be selected as additives to manufacture self-lubricating plastics. 
Figure 2 Friction between plastic parts and steel plate with and without dry film lubricant We should also consider the entire assembly to which the plastic part belongs. In many cases, lubrication is required not only on the plastic contact surface, but also on the plastic-to-metal or metal-to-metal part contact surfaces. In such cases, metal-to-metal and metal-to-plastic lubricity needs to be considered. In addition, the compatibility of the lubricant with the plastic material of the assembly housing must be considered, as the lubricant may come into contact with the housing during operation. Fortunately, lubricants that meet these requirements are available today. Structural Plastics
Structural plastics are used in automotive parts such as door trims, instrument panels, and device housings. Most structural plastics are polyolefins, ABS, polystyrene, and other polymers. Few applications seem to require a lot of lubrication, but proper lubrication can reduce noise, vibration, and the feeling of movement when parts slide and connect. In this case, there is no heavy motion lubrication involved. Passenger comfort is our top priority, and vibration and noise directly affect this comfort. Checking the compatibility of structural plastics with lubricants is even more important than for functional plastics. Do not choose lubricants that dissolve or corrode plastics. Also, beware of chemicals that can cause plastics to crack and break. If these factors are ignored, the appearance or function of the part will be impaired, such as brake system failure. There are several ways to test the compatibility of plastics with lubricants. All of these methods require applying the lubricant to the plastic, then bending the plastic and applying pressure. The plastic is then placed in a test fixture and left for a period of time. After several such operations, the plastic is checked for cracking.
The table above lists the compatibility between lubricant base oils and different types of plastics. Polyalphaolefin base oils are currently a popular choice as a substitute for mineral oils and are very suitable for a variety of plastics. Perfluoropolyether is a chemically inert lubricant that does not corrode plastics and can be used in many types of plastics. However, due to its high price, its use is subject to certain restrictions.
Appearance is an important consideration when selecting a lubricant. Where the lubricant is visible, a clear or translucent product is often required. A lubricant that the driver and passengers can come into contact with without getting stuck to them is often required. Dry film and semi-dry film lubricants meet this requirement. After applying the lubricating film and before curing it, compatibility needs to be checked because many dry film lubricants contain solvents that can discolor and crack plastics. Application
Noise Reduction - Using lubricants on plastic gears or sliders can reduce noise. Unlike metal parts that make noise when sliding, plastic gears make noise when they collide with each other. Lubricants such as high-viscosity grease can absorb the vibration of plastic parts due to their high viscosity, thereby reducing noise. However, automotive lubricants must work properly at low temperatures. Therefore, when considering noise reduction, its low-temperature performance should also be considered. Door panels - A variety of plastics used in door panels require lubrication, including guide rails, sliders and rollers, window lifters, door lock actuators, micromotors and exterior mirrors. Because these parts are light and small, lubricants are required to have low friction in addition to compatibility with plastics. Greases contain solid lubricants suitable for the application requirements of these parts. The window lift motor uses polyacetal gears and steel gears. Select the appropriate synthetic grease for the micromotor. The micromotor uses rubber parts to absorb vibrations. Therefore, the lubricant should be compatible with both plastic and rubber. The window regulator guide rails consist of galvanized steel guide rails and plastic sliders. High-viscosity grease is used here to meet the dual requirements of noise reduction and plastic compatibility. Door trims - Use dry film lubricants before assembling door trims to door panels because this eliminates the harsh noises that occur when the car is in motion. Dry film lubricants can also effectively reduce noise on assembly lines that use flocked plastic tape, increasing production efficiency on assembly lines. Conclusion
Some lubricant manufacturers participate in automakers' part development projects in the early stages of vehicle manufacturing. This involvement can provide automakers with different opinions and support for further testing of materials and lubrication. Because plastic manufacturers and lubricant manufacturers may have different opinions on compatibility, it is important for them to cooperate in the early stages of product development. Involving lubricant suppliers in the product development process early in the process is critical to reducing development costs and saving time.
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