Resin Abrasive Hot Press Molding Process

The basic principle of hot pressing molding process is to press by heating. The binder is melted quickly and condensed to harden or semi-harden within the holding time. Hot pressing is a process of pressing and hardening at the same time, so it is not easy to cause delamination and foaming of the part, and it can produce abrasive tools with high hardness, tight structure and even no pores. At home and abroad, the hot pressing process attaches great importance. This method can be used to produce many special-purpose products that are difficult to produce by cold pressing.

Hot pressing molding has the following advantages compared with cooling molding:

1. It can produce fine-grained, high-hardness and high-strength abrasive tools that are difficult to produce at normal temperature (cold pressing method). For example: super-hard fine-grained threaded grinding wheels, heavy-duty billet grinding wheels, high-speed grinding discs, copper powder conductive grinding wheels, micro bearing guide wheels, etc.;

2. It can avoid the generation of waste products such as layering, foaming, and cracks during hardening; this is because under heating and pressurizing conditions, hardening or semi-hardening can overcome the expansion phenomenon caused by the volatilization and elimination of low-molecular substances during the hardening process;

3. The molding pressure is low (because the binder is in a molten state)

4. Under the same formula and process conditions, the hot pressing method can improve the strength and hardness of the abrasive tools;

5. Under the same conditions, the grinding performance of some products using the hot pressing method is better than that of the cold pressing method.

But there are also the following shortcomings:

1. Because heating and heat preservation take up time (about 3-10 times that of cold pressing method). Low production efficiency and low equipment utilization;

2. It is difficult to remove the mold and the sanitary conditions are poor;

3. The equipment is complex and inconvenient to operate (electricity, heat);

Hot presses are generally modified by installing electric heating plates and temperature control systems on ordinary presses. Most of the molds are designed using the fixed mold method, which is beneficial to heat transfer and insulation.

The hot pressing temperature depends on the properties of the resin binder and the quality requirements of the molding. For example, if thermoplastic resins such as polysulfone, nylon 1010, polyvinyl butyral, etc. are used as binders, higher hot pressing temperatures (generally 160-230°C) can be used. For example, thermosetting resins such as phenolic, epoxy, and neophenol can be used as binders. For binding agents, a lower hot pressing temperature can be used (generally 60 to 185°C).

Generally, it is selected within the temperature range where the resin can become an elastomer. The hot pressing temperature of abrasive tools for epoxy resin, phenolic resin, polysulfone, polyimide resin and other binding agents is as follows:

The final determination of the hot pressing temperature must ensure that the green body does not stick to the mold. This is related to the melting point (or softening point) of the resin powder. The hot pressing temperature must be lower than the melting point of the resin powder. Therefore, the hot pressing temperature of phenolic resin bonded flake grinding wheels and cymbal grinding wheels is usually 60 to 80°C.

The holding time is related to the specifications and thickness of the part and the polymerization speed of the resin. Parts with large specifications and thickness require time for heat transfer, and the holding time will be longer; resins containing a lot of volatiles and slow polymerization require sufficient It takes time to eliminate volatile matter and transform it into a hardened state, so a longer pressure holding time is required. This is especially true for hot pressing materials containing liquid phenolic resin. During the hot pressing process, a lot of gas is discharged and the fluidity is high. It requires multiple times before holding pressure. Deflate to eliminate moisture and other volatile matter in the reaction. It is best to pre-dry this material first, crush it and then put it into hot pressing. It can also be cold-pressed into a green body (add 2-3 mm thickness allowance), and then Dry, then put into the mold for hot pressing, the effect will be better.

Powdered resin has less volatile matter, has fast polycondensation speed at higher temperatures and is conducive to hot pressing. When the abrasive particle size is fine, the abrasive and powdery resin can be mixed in a ball mill to prepare hot pressed materials. Coarse-grained abrasives require a small amount of liquid resin or high-boiling point solvent (such as furfural, cresol) as a wetting agent to wet and mix the abrasive surface, and then add powdered resin and fillers to prepare molding materials.

Put the molding material into the mold that has been preheated to a certain temperature, quickly spread it evenly, scrape the material, put it into the upper platen and the pressure ring, and send it to the hot press machine for slight pressure. When the mold is heated to the hot pressing temperature (use a surface thermometer Measure) Apply pressure to flatten the mold, deflate it three times after 5 to 10 minutes, and maintain the pressure until the pressing process is completed.

When hot press molding of resin abrasive tools, due to the strong adhesion between the grinding wheel and the mold, it is difficult to demould. Therefore, a release agent is generally applied to the contact area between the grinding wheel and the mold to facilitate demoulding. Commonly used release agents include silicone oil, soapy water and paraffin. For some coarse-grained and high-hardness abrasive tools, if the release agent is not effective, the paper pad method can be used to solve the problem.

Since the bonding agent (resin) of the abrasive tool is in a semi-hardened or nearly hardened state after hot pressing and has a certain mechanical strength, the rapid heating and hardening method can be used to completely harden it.