The thermal conductivity of the plastic and thermal expansion

The thermal expansion of plastic related to material structure, high crystallinity, room temperature is more rigid plastic, smaller thermal expansion, thermosetting plastic, molecules of three-dimensional crosslinked structure, so the thermal expansion relatively lower thermal plastic plastic. For injection molding production of injection molding processing, parts and even flow direction and vertical direction of current flow coefficient of linear expansion also has bigger difference. Plastic material adding filler or glass fiber, can significantly reduce the thermal expansion coefficient, but in the use of fiber reinforced fiber and molecular orientation effect, can cause thermal expansion behavior of composite material is anisotropic. Asymmetric thermal expansion behavior will produce the form such as distorted or warping deformation.

the gear, bearing such moving parts, heat generated by friction and fatigue. If insufficient due to the low thermal conductivity, heat exchange, gradually accumulation of heat affect the work condition of the parts. Similar product design, therefore, should pay attention to cooling of all kinds of measures, such as wall thickness control, design for the cooling of geometry, expand the cooling area, according to a may set the metal insert, etc. Plastic materials can be added in the composition of thermal conductivity, good packing, fiber to improve and enhance performance of thermal conductivity, such as graphite, carbon fiber, metal powder and metal fiber.

in be used actually, total hope material coefficient of thermal expansion as small as possible. Thermal expansion coefficient of material, the temperature changes will affect the size of the parts under the condition of stability. Plastic material as a component when combined with other materials, the materials of the different thermal expansion coefficient of even lead to product quality accident. Of plastic parts with metal embedded parts, for example, when the temperature changes, easily because of the difference of the coefficient of thermal expansion, internal stress and stress concentration caused by plastic materials, which can lead to material craze, especially a kind of easy to crack due to stress like polycarbonate material. Design personnel in the process of product design, therefore, must take measures in advance to correction of thermal expansion mismatch problem.

with graphite filler chlorinated polyvinyl chloride (PVC), when the graphite filling amount reaches 50, 20 times thermal conductivity can be expanded. Can be used as a thermal conductive materials. Adds a special thermal conductivity of carbon fiber, PA, its thermal conductivity can be up to 10 w/( m。 K） 。 But the fiber reinforced composite material is anisotropic. One-way continuous carbon fiber reinforced composites, for instance, the thermal conductivity of the fiber direction than perpendicular to the fiber direction of more than 10 times larger thermal conductivity, and with the increase of fiber content, the difference will be bigger and bigger. In the thermal design, therefore, should be a reasonable choice component type, content and form of composite materials.

plastic material of low thermal conductivity as a benefit of plastic, widely used in many ways. Used as a cooker controller, electric tools, for example, shell, car steering wheel, etc. Closed pore structure of porous plastic has very low thermal conductivity, so commonly used as insulation. Injection molding processing materials thermal conductivity is low, and often easy to cause overheating. Some electrical appliances product in serious overload and runtime exception condition, the heat is not easy to send out in time, make the temperature rise faster, which can lead to electrical plastic mechanical strength and electrical performance degradation, serious when still can produce accident.

with similar metal materials, plastic materials also has the characteristic of thermal expansion. Characterization of parameter is the thermal expansion coefficient of thermal expansion performance. Is defined as a unit under the condition of temperature change, material on the length or volume change rate. Thermal expansion coefficient, by definition, divided into the first coefficient of linear expansion coefficient and volume expansion. Design is commonly used in linear expansion coefficient. In engineering, for those who are in a temperature change under the condition of the structure of the material, the thermal expansion coefficient is not only the use of material is an important performance, and is the key to the structural design parameters.

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