The main equipment for molding production is a hydraulic press. The role of the hydraulic press machine in the pressing process is to apply pressure to the plastic through the mold, open the mold and eject the product.
Compression molding is mainly used for the molding of thermosetting plastics. For thermoplastics, due to the need to prepare the blank in advance, it needs to be heated and cooled alternately, so the production cycle is long, the production efficiency is low, and the energy consumption is large. Moreover, products with complex shapes and more precise sizes cannot be pressed. Hence the general trend towards more economical injection molding.
The compression molding machine (press for short) used for molding is a hydraulic press. Its pressing capacity is expressed in nominal tonnage, generally, there are 40t ﹑ 630t ﹑ 100t ﹑ 160t ﹑ 200t ﹑ 250t ﹑ 400t ﹑ 500t series of presses. There are more than 1,000 tons of multi-layer presses. The main contents of the press specifications include operating tonnage, ejection tonnage, platen size for fixing the die, and strokes of the operating piston and ejection piston, etc. Generally, the upper and lower templates of the press are equipped with heating and cooling devices. Small parts can use a cold press (no heating, only cooling water) for shaping and cooling. Use the heating press exclusively for thermal plasticization, which can save energy.
According to the degree of automation, presses can be divided into hand presses, semi-automatic presses, and fully automatic presses. According to the number of layers of the flat plate, it can be divided into double-layer and multi-layer presses.
A hydraulic press is a pressure machine powered by hydraulic transmission. When pressing, the plastic is first added to the open mold. Then feed the pressure oil to the working cylinder. Guided by the column, the piston and the movable beam move downward (or upward) to close the mold. Finally, the force generated by the hydraulic press is transmitted to the mold and acts on the plastic.
The plastic inside the mold melts and softens under the action of heat. The mold is filled with pressure from a hydraulic press and a chemical reaction takes place. In order to discharge the moisture and other volatiles produced during the condensation reaction of plastics and ensure the quality of the product, it is necessary to perform pressure relief and exhaust. Immediately boost and maintain. At this time, the resin in the plastic continues to undergo chemical reactions. After a certain period of time, an insoluble and infusible hard solid state is formed, and the solidification molding is completed. The mold is opened immediately, and the product is taken out of the mold. After the mold is cleaned, the next round of production can proceed.
It can be seen from the above process that temperature, pressure, and time are important conditions for compression molding. In order to improve the productivity of the machine and the safety and reliability of the operation, the operating speed of the machine is also an important factor that cannot be ignored. Therefore, the plastic hydraulic press used for pressing should be able to meet the following basic requirements:
① The pressing pressure should be sufficient and adjustable, and it is also required to reach and maintain the predetermined pressure within a certain period of time.
② The movable beam of the hydraulic press can stop and return at any point in the stroke. This is very necessary when installing molds, pre-pressing, batch charging, or failure.
③ The movable beam of the hydraulic press can control the speed and apply working pressure at any point in the stroke. To meet the requirements of molds of different heights.
The movable beam of the hydraulic press should have a faster speed in the empty stroke before the male mold touches the plastic, so as to shorten the pressing cycle, improve the productivity of the machine and avoid the reduction or hardening of the plastic flow performance. When the male mold touches the plastic, the mold closing speed should be slowed down. Otherwise, the mold or insert may be damaged or the powder may be washed out from the female mold. At the same time, slowing down the speed can also fully remove the air in the mold.
Post time: Apr-07-2023
