Disc mold vacuum brazing

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Discs used in DVD players are thermoformed using molds. In order to ensure the quality of the disc, high technical requirements are placed on the mold. The surface hardness of the mold after quenching is greater than HRC58, and the long-term stability of the mold size must also be maintained. During the thermoplastic forming process, the mold needs to dissipate heat, so a water cooling chassis is installed under the forming panel of the mold. According to the connection structure between the panel and the chassis, it can be divided into two forms: 1. Mechanical fixing, rubber ring sealing; 2. Brazing connection sealing. The density of brazing connection is stable, so it is commonly used by optical disc manufacturers.

The mold is brazed by vacuum brazing, and brazing and quenching can be performed. However, after the mold is brazed, the quenching is rapidly cooled at high temperature, and a large internal stress will be generated inside the mold, which is prone to cracking of the brazing seam. Therefore, the brazing and quenching of the optical disk mold has become the key process of the optical disk mold manufacturing.

Process flow:
The outer diameter of the disc mold is 150mm, the thickness of the panel is 4mm, and the thickness of the cooling chassis is 28mm. The ratio of the unequal thickness of the panel to the chassis reaches 1:7, and the welded joint is in the form of a flat butt welded joint, which will increase the tendency of the brazing joint to crack. The mold material is martensitic stainless steel with high carbon content, such as 440. From the heat treatment and quenching process curve of 440 martensitic stainless steel, it can be concluded that the heating temperature of martensitic stainless steel cannot exceed 1100 °C.

To avoid grain growth. The quenching temperature is set at 1050℃, and the hardness of about 60HRC can be obtained by gas quenching. Martensitic stainless steel contains Cr, Ni, Mn and other elements. In order to remove the oxide film, the vacuum degree of vacuum brazing should not be lower than 2×10-2Pa.

The parameters of vacuum brazing include vacuum degree, heating rate, stabilization rate, holding time, cooling method, and furnace temperature. The process parameters of the disc mold are formulated according to the heat treatment specification of martensitic stainless steel and the structural characteristics of the mold, so that the brazed joint has sufficient penetration rate and reasonable alloying reaction.

The vacuum brazing process of the mold is carried out in two parts: the rise and the cooling of the furnace temperature. From normal temperature to 1050 ℃, set up 6 heating curves to complete the brazing process. Fill with argon gas at 1050 °C, start the vacuum furnace fan to cool to 100 °C, and complete the mold gas quenching process.

The brazing filler metal for vacuum brazing is selected from nickel-based brazing filler metal H82NiCrMnSi. The nickel-based brazing filler metal has good technological performance and reliable joint performance. Moreover, the brazing temperature is consistent with the quenching temperature, and the brazing temperature is selected at 1050 ℃, so that the fluidity of the brazing material is in the best state.

Brazing with nickel-based brazing filler metal requires strict requirements on the gap of the brazed joint, and the increase of the brazing gap will increase the brittle compound. The increase of brittle compounds can significantly reduce the tensile strength of brazed joints. In order to reduce the generation of intermetallic compounds, the clearance of the flat butt joint of the mold is required to be no more than 0.1mm.

Vacuum brazing process:
Cleaning: Degrease the surface of the parts and clean with gasoline.
Filling: brazing filler metal – Hni82CrMnSi powder, 140 mesh ~ 200 mesh. Mix the adhesive and powder solder into a paste, and use a syringe to fill the solder.
Drying: electric heating box 150℃x30min, or room temperature 2h.
Assembly: Remove excess solder with a blade, then proceed to mold assembly.
Furnace loading: The mold is placed on the rack of the vacuum furnace.
The vacuum brazing heating curve is carried out according to the parameters of the process card.
Cooling method: Air-cooled at 1050°C with argon to 100°C.

Cold treatment and tempering process:
After the mold is quenched, there is still retained austenite in the material. During the long-term natural aging process, the retained austenite slowly decomposes, and when it is transformed into martensite, it expands, and the martensite precipitation carbide shrinks. Without treatment, the mold will slowly undergo phase transformation and dimensional changes during use. Using cold treatment and tempering process, the retained austenite can be transformed into martensite to achieve dimensional stability of the mold

Main Specifications:
Operating temperature range: normal temperature~-180℃
Temperature control accuracy: ±5℃
Loading capacity: 120kg
Refrigerant: Liquid nitrogen gasification
Generally, cold treatment can be carried out: -78℃x2h
The tempering temperature of 580℃x2h needs to be carried out twice, which can achieve the stability of the mold size.
Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.