In use, the hardness Plastic injection mould of economically operating molds is as low as 85 Rockwell "B" and as high as 65 Rockwell "C". At this point, the industry is consistent-completely different cavity hardness. So, if it is possible to successfully operate with a failed soft mold, why bother to harden it? In order to answer this question, heat treatment is carried out to maximize the ideal properties of the steel. The hardened surface will have a better polishing effect than the soft surface and will not be easily damaged by scratches or normal wear.
In places prone to friction and wear, such as thimble, compression cavity, slider, etc., hardened steel must be used to minimize wear or tear. In addition to increasing surface hardness, heat treatment can often improve core performance. Mild steel is easily broken or hobbed under molding pressure especially in compression molding, and heat treatment gives the mold the required strength. Obviously, the maximum hardness is not a necessary condition for a good cavity. Instead, the cavity should be heat treated to maximize toughness and sufficient surface hardness. In view of this, let us examine the various materials used to make the cavity.
The first material that may be used is mild steel. This is carburized and hardened. Although the results are satisfactory in many cases, the material is not ideal when compared to available modern tool steels. Carburizing hardens the shell, with Rockwell hardness ranging from 60 to 62, but the strength of the core is higher than before heat treatment. Alloy carburizing steel such as SAE 4615, 3120 or 3312 has usually replaced ordinary carbon steel for carburizing, except for gear hobbing steel. With proper heat treatment, the tensile strength of the core can be as high as 130,000 pounds per square inch in some of these alloy steels. As mentioned earlier, the surface hardness is usually 60 Rockwell hardness "C" or higher.
The need for greater core strength in hobbed steel has led to the use of steels such as SAE 3110 for this purpose. A steel company is developing an oil-quenched section steel that is easy to harden, but has not yet put it on the market. Oil and air hardened steel are usually used for the cavity. Depending on the heat treatment and the steel, they may range from 48 Rockwell "C" to 65. Steel that has been hardened has the greatest hardness in the quenched state. However, since the maximum hardness is not the determining factor, the steel is further heated or stretched to provide the required performance-maximum core strength. The necessary hardness reduction will depend entirely on the steel, and can be anything.