The basic principle of hot pressing is to press the part using heat. 

This causes the binder to melt rapidly, then condense and harden or semi-harden during the dwell time. 

Hot pressing simultaneously compresses and hardens the part, minimizing delamination and foaming. 

This allows for the production of molds with high hardness, a dense structure, and even zero pores. 

Hot pressing is highly valued both domestically and internationally, as it can be used to produce many specialized products difficult to produce using cold pressing.

Hot pressing offers the following advantages over cold pressing:

1. It can produce molds with fine grain, high hardness, and high strength, which are difficult to produce using cold pressing at room temperature. 

Examples include: super-hard, fine-grained thread grinding wheels, heavy-duty steel blank grinding wheels, 

high-speed grinding discs, copper powder conductive grinding wheels, and miniature bearing guide wheels.

2. It can avoid delamination, blistering, and cracking during hardening. 

This is because, under heating and pressurizing conditions, 

hardening or semi-hardening overcomes the expansion caused by the volatilization and elimination of low-molecular substances during the hardening process.

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

4. Under the same formulation and process conditions, hot pressing can improve the strength and hardness of the abrasive tool.

5. For certain products, hot pressing can achieve better grinding performance than cold pressing under the same conditions.

However, there are also the following disadvantages:

1. Heating and holding time are required (approximately 3-10 times that of cold pressing). Low production efficiency and equipment utilization;

2. Difficulty removing molds and poor sanitation;

3. Complex equipment and inconvenient operation (electrical and thermal);

Hot presses are generally modified conventional presses by installing electric heating plates and temperature control systems. 

The molds are often designed with a fixed mold method to facilitate heat transfer and insulation.

The hot pressing temperature depends on the properties of the resin binder and the quality requirements for the molding. 

For thermoplastic binders such as polysulfone, nylon 1010, and polyvinyl butyral, higher hot pressing temperatures (generally 160-230°C) can be used. 

For thermosetting binders such as phenolic, epoxy, and neophenol, lower hot pressing temperatures (generally 60-185°C) can be used.

The hot pressing temperature must be determined to ensure that the blank 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 below the melting point of the resin powder. 

Therefore, the hot pressing temperature for thin-sheet and cymbal grinding wheels with phenolic resin bonds is typically 60-80°C.

The holding time depends on the part's specifications, thickness, and the resin's polymerization rate. Larger and thicker parts require time to transfer heat, 

so the holding time is longer. Resins with high volatile content and slow polymerization rates require ample time to expel these volatiles and transform into a hardened state, 

thus requiring a longer holding time. This is particularly true for hot-pressed materials containing liquid phenolic resin. 

During hot pressing, they emit a large amount of gas and exhibit high fluidity, requiring multiple venting cycles before holding to remove volatiles such as water. 

It is best to pre-dry and crush such materials before hot pressing. 

Alternatively, cold pressing can be performed to form a green body (with a 2-3 mm allowance), followed by drying and placement into a mold for hot pressing, 

for better results.

Powdered resins have fewer volatiles and rapidly condense at higher temperatures, making them more suitable for hot pressing. 

When the abrasive particle size is fine, the abrasive and powdered resin can be mixed in a ball mill to prepare the hot pressing material. 

For coarse-grain abrasives, a small amount of liquid resin or a high-boiling-point solvent (such as furfural or cresol) is used as a wetting agent to wet and mix the abrasive surface. 

Powdered resin and fillers are then added to prepare the molding compound.

The molding compound is placed into a preheated mold, quickly spread evenly, scraped, and placed on the upper platen and pressure ring. 

The mold is then placed in the hot press and gently pressed. 

Once the mold reaches the hot-pressing temperature (measured using a surface thermometer), pressure is applied to flatten the mold. 

After 5-10 minutes, the air is released three times, and the pressure is maintained until the pressing process is complete.

During hot-pressing of resin molds, the strong adhesion between the grinding wheel and the mold makes demolding difficult. 

Therefore, a release agent is generally applied to the contact area between the grinding wheel and the mold to facilitate demolding. 

Common release agents include silicone oil, soapy water, and paraffin wax. 

For some coarse-grained, high-hardness abrasives, if the release agent is ineffective, a paper backing can be used.

Since the bonding agent (resin) of the mold is in a semi-hardened or nearly hardened state after hot pressing and has a certain mechanical strength, 

it can be fully hardened by the method of rapid temperature rise hardening.