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Three thermal diffusion techniques to improve the life of plastic molds

Update:14-03-2020
Summary:

When we use plastic molds, the surface of the mold cavi […]

When we use plastic molds, the surface of the mold cavity often wears and corrodes. In severe cases, it will affect the appearance of the product and even cause difficulty in demolding, which greatly shortens the service life of the mold. So how to improve the service life of the mold? Surface strengthening treatment of the main forming parts of the mold can not only improve the hardness and wear resistance of the mold surface, but also improve the anti-fatigue effect of the mold, etc., thereby greatly improving the service life of the mold. Here we first introduce one of the surface strengthening technologies-thermal diffusion technology, which mainly has three treatment methods: nitriding, nitriding and nitrocarburizing.

(1) Carburizing
Different molds of different materials generally use different heat treatment methods. Plastic molds that use low-carbon steel and low-carbon alloy steel to extrude the cavity generally use the carburizing treatment method. It has fast penetration rate, deep penetration layer, The composition gradient and hardness gradient can be easily controlled and low cost. After this type of plastic mold is subjected to carburizing treatment, the carbon content in the surface (0.5 to 2.0 mm) can reach 0.8% to 1.05%. After hardening and tempering at low temperature, the hardness of this infiltrated layer is increased, and the wear resistance and fatigue resistance are also increased, thereby extending the service life of the mold. However, if the crystal grains are coarse during carburizing, the carburized layer may become brittle and the surface of the mold may peel off. At the same time, because the carburizing temperature is high and heat treatment is required after carburizing, the deformation of the mold is large. Therefore, this type of plastic mold with high precision is not suitable for carburizing. Common carburizing methods for plastic molds are solid carburizing, gas carburizing, vacuum carburizing, and ion carburizing. Because it has the advantages of high carburizing efficiency, gentle carbon concentration gradient, small deformation of the workpiece, more environmentally friendly, and the ability to handle slits and small holes, ion carburizing is currently used more in plastic molds and punches.

(2) Nitriding
Nitriding is a heat treatment method that allows nitrogen atoms to penetrate the surface of a workpiece at a specific temperature and in a specific medium. The surface hardness, abrasion resistance, fatigue strength and corrosion resistance of the mold are greatly improved after nitriding, and the mold is treated after carburizing. Nitriding is different from carburizing, it is performed at a lower temperature, the mold deformation is small, and it is suitable for molds with high accuracy requirements. The common carburizing processes of the mold are liquid nitriding, gas nitriding and ion nitriding. Compared with the traditional nitriding method, ion nitriding has fast nitriding speed, which can adjust and control the nitriding structure through different gas components, reduce the surface brittleness of the nitriding layer, the hardness distribution of the nitrided layer is smooth, and it is difficult to produce spalling and thermal fatigue. Energy saving and pollution-free. Therefore, it is suitable for almost all molds. However, ion nitriding is not effective in strengthening areas with small holes or grooves on the surface of the mold. For some plastic molds with complex shapes, it is not easy to uniformly heat, and the obtained layer is not very uniform.

(3) nitrocarburizing
Carbonitriding is a method of infiltrating carbon and nitrogen into the surface of a workpiece to form a carbonitriding layer. Using this method, the mold has small deformation, fast penetration, and low cost. The surface of a plastic mold treated by this method Extremely high hardness, high anti-adhesion, abrasion resistance, anti-seizure, and abrasion resistance. The service life of the mold can be increased by 2-5 times. However, after gas nitrocarburizing, due to the expansion of the surface compound, the mold often deforms, so it is not suitable for precision plastic molds.

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