Mirror plastic die steel is a kind of precipitated hardening aging steel that is developed to adapt to transparent plastic and rubber products with high surface quality and high corrosion. It for bainite and martensite after heat treatment of duplex mixed matrix organization, having good cold and hot machining performance, comprehensive mechanical properties and mirror performance, suitable for manufacture using temperature about 300 ℃, the hardness of 30-45 HRC, where requires a high mirror, high precision and uniform cross-section of various thermoplastic products and part of strengthening engineering plastics forming mold.
Due to the demand for some large plastic products, steel P20 is the commonly used materials for mirror plastic mold with a maximum thickness is only 400 mm. 718 steel, a steel with the addition of 1%Ni element based on P20 steel, making the maximum size from 400 mm or so increased to 1000 mm, its good structure and performance, the core surface is uniform bainite structure, hardness is fluctuating in ± 1.5HRC. For the need of super mirror plastic products of plastic mold, Martensitic plastic mold steel after tempering tempered martensite or tempered sorbsite structure is fine, uniform carbide precipitation is the production of transparent plastic products and super mirror products preferred.
The mirror processing performance of steel is the polishing performance, which not only has a high demand on the mechanical properties and physical and chemical properties of the material but also has a high demand on the polishing process. Because the shape of the plastic mold is different, the parts are in different positions in the service process, and higher requirements are put forward for the mold steel: namely, excellent surface hardness and wear resistance, excellent strength and toughness, cutting performance, polishing performance, small heat treatment deformation and good corrosion resistance, etc. Mirror processing is divided into four grades: A0 = RA0.008μm, A1 = RA0.016μm, A3 = RA0.032μm, A4 = RA0.063μm. The plastic products with low surface quality have low requirements for mold. The general requirement for the surface roughness of the injection mold is about Ra0.1~0.25 μm, and the optical mirror is Ra0.01 μm or less. For example, the roughness of the mold cavity for the manufacture of an optical disc is Ra0.01~0.02 μm, which requires the mold material to have good polishing performance.
EDM is widely used in precision cavity mold manufacturing to obtain mirror effect. The so-called “mirror EDM” generally refers to the machining surface roughness Ra<0.2 um, EDM with mirror reflection effect on the processed surface. It can replace the manual polishing process and improve the quality of parts. Not all processing types can be mirror EDM. All the discharge machining can achieve the mirror effect, and the carbon black layer produced in the discharge machining has a direct relationship, if the processing parts can easily form a uniform carbon black layer, it is easy to process the mirror effect. Its features are:
- It is easier to obtain mirrors in simple shape than in complex shape plastic EDM. The circle shape get mirror effect easiest;
- It’s easier to get a good mirror effect on the underside than on the side. The greater the depth of the cavity, the more difficult it to get the mirror effect, especially the side, the opening part and the cavity hollow part;
- The larger the processing area, the more difficult to obtain a good mirror effect;
|A||SKD61/ STAVAX (S136)/ PD555/ NAK80/ 718H||Very good|
|B||SKS3/ SKH9/ HPM38/ S55C/ H13
Not good enough
|C||SKD11/ NAK55/ HPM1/ DH2F||Bad|
|D||Bs/ Al/ Superhard alloy||Roughness|