The hot pressing technology of phenolic resin has attracted much attention. Because the binder resin is in a molten state under the action of temperature, its fluidity is good, and it is easy to fill all parts of the mold cavity, so the hot pressing pressure is not high, generally 30 ~ 60 MPa. Because part of the molding pressure is consumed by the friction resistance of the mold wall and part by the water vapor volatiles overflowing from the molding material, it is difficult to ensure the appropriate pressure by the constant pressure molding method, so the grinding tool can not reach a fixed constant density. Therefore, the fixed mold forming method is often used in production, that is, the single weight of the molding material is fixed, the thickness of the abrasive tool is controlled by the size of the mold itself, and the mold is pressed in place by applying pressure. Phenolic resin is generally185 5℃; The polyimide resin is 235 5℃. When the temperature is too high and the reaction speed is too fast, it is easy to cause frustration in molding, poor bonding between matrix and binder, and sometimes even cracking of abrasive tools; The temperature is too low, the pressing time is prolonged, and the production efficiency is low.

The hardening principle is firstly phenolic resin hardening: in the first stage, thermoplastic resin reacts with urotropine to generate a bridged product containing methylene amino; In the second stage, these products continue to react with resin molecules to generate thermosetting resin with huge network structure and decompose ammonia. During the hardening process, urotropine not only reacts with thermoplastic phenolic aldehyde, but also reacts with free phenol to produce thermosetting resin. This process does not need any catalyst and can be carried out by heating to a certain temperature. Subsequently, that polyimide is harden, which is a polymerization process without a hardening agent, and the polymerization proces is also divided into two steps: the first step is to melt the polyimide prepolymer at a low temperature, and the second step is to cyclize the prepolymer into insoluble polyimide at a high temperature. There are two kinds of hardening methods for resin bond grinding, one is the primary hardening method, and the other is the secondary hardening method. The abrasive tool is heated and hardened on a hot press for 30 ~ 40 minutes to become a finished product, which is called the primary hardening method. Suitable for small, thin and irregular grinding wheels.

In order to obtain fully hardened products, some large and thick grinding wheels have been initially hardened in the hot press, but they need to be secondary hardened in the electric furnace. This method is called secondary hardening. Need to master the "maximum hardening temperature". The highest hardening temperature of phenolic resin consolidated abrasive is 180 ~ 190℃. Below 170℃, the hardening is incomplete and the chemical stability is poor. If it is higher than 200℃, the mechanical properties of the abrasive will be destroyed, and the strength, hardness and water resistance of the abrasive will be reduced. The hardening of polyimide resin bonded diamond abrasive requires the formation of cyclized polyimide chains at high temperature to increase molecular stiffness and become insoluble plastics. Therefore, its hardening temperature is high, which can reach 230℃. When the temperature is too low, the cyclization reaction is not easy to proceed and the product strength is low. The heating rate is very important, which is related to the type of adhesive, the hardening time on the hot press, the shape and particle size of abrasive. The polymerization temperature of thermoplastic phenolic resin is 100℃, while the prepolymerization temperature of polyimide is higher. The former can be heated freely before 100℃, while the latter can be heated freely before 180℃. The former reacts with curing agent after 140℃, and the latter prepolymerizes after 180℃, so the temperature rise is slow. For the long-hardened abrasives on the hot press, the volatiles have been basically discharged, which can quickly heat up; On the contrary, the secondary hardening should be heated slowly; If the abrasives are complicated at first and the granularity is fine, they should be heated slowly, otherwise they can be heated quickly.

The holding time is related to the highest hardening temperature. The higher the hardening temperature, the shorter the time and the longer the time. For phenolic resin abrasives, the holding time is only 2-3 hours at 180℃, and for polyimide resin abrasives, the holding time is generally 4-5 hours at 230℃.