The main equipment for molding production is a hydraulic machine. The role of the hydraulic machine in the pressing process is to apply pressure to the plastic through the mold, open the mold and eject the product.
Molding is mainly used for the molding of thermosetting plastics. For thermoplastics, since the blanks need to be prepared in advance, they need to be heated alternately and then cooled, so the production cycle is long, the production efficiency is low, the energy consumption is large, and the products with complicated shapes and relatively accurate dimensions cannot be pressed, so that it is generally more economical to adopt. Injection molding.
Pressing presses for molding (referred to as presses), hydraulic presses, the pressing capacity is expressed in nominal tons, generally 40t, 63t, 100t, 160t, 200t, 250t, 400t, 500t and other series of presses . Multi-layer laminators have more than a thousand tons. The main contents of the press specifications include the operating tonnage, the tonnage, the die size for the fixed die, the stroke of the operating piston, and the ejection of the piston. The upper and lower stencils of the general press are equipped with heating and cooling devices. Small parts can be used for cooling by cold press (no heating, only cooling water), and the heating press is specially used for thermoplasticization, which can save energy.
According to the degree of automation, the press can be divided into a hand press, a semi-automatic press, and a fully automatic press; according to the number of layers of the flat plate, it can be divided into a double layer and a multi-layer laminating machine.
(1) Working principle of hydraulic machine Hydraulic machine is a pressure machine powered by hydraulic transmission. When pressing, the plastic is first added to the open mold, and then the pressure oil is introduced into the working cylinder. The piston and the movable beam are guided by the column, and move downward (or upward) to close the mold, and finally the force generated by the hydraulic machine is transmitted to the piston. The mold acts on the plastic. The plastic in the mold is melted and softened under the action of heat, and the mold is filled with a pressure applied by a hydraulic machine to perform a chemical reaction. In order to discharge the moisture and other volatiles generated by the plastic during the condensation reaction, the quality of the product is ensured, and pressure relief and exhaust are required. Then, the pressure is raised and maintained. At this time, the resin in the plastic continues to undergo a chemical reaction. After a certain period of time, a hard solid state which is insoluble and infusible is formed, and the solidification molding is completed, and then the mold is opened, and the product is taken out from the mold. After cleaning the mold, you can proceed to the next round of production.
It can be seen from the above process that temperature, pressure and time are important conditions for press molding. In order to improve the productivity of the machine and the safety and reliability of the operation, the running speed of the machine is also an important factor that cannot be ignored. Therefore, the plastic hydraulic press for pressing should meet the following basic requirements.
1 The pressing pressure should be sufficient and adjustable, and it is 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 necessary when installing the mold, preloading, splitting, or failure.
3 The movable beam of the hydraulic press can be used for speed control and working pressure at any point in the stroke. To meet the requirements of different height molds.
The movable beam of the hydraulic machine should have a faster speed in the empty stroke before the male mold is in contact with the plastic, so as to shorten the pressing cycle, increase the productivity of the machine and avoid the plastic flow performance degradation or hardening. When the male mold contacts the plastic, the mold closing speed should be slowed down. Otherwise, the mold or the insert may be damaged or the powder may be rushed out of the female mold. At the same time, the slowing down speed may further eliminate the air in the mold.
(2) Structure and classification are the structure of a typical down-pressure hydraulic machine. It consists of the fuselage (including the upper beam, the lower beam, the column, etc.), the working cylinder, the movable beam, the ejection mechanism, the hydraulic transmission and the electrical control system. The working cylinder is mounted on the upper beam. The movable beam is integrally connected with the piston of the working cylinder, and is guided up and down by the column (the frame hydraulic machine is guided by the guide rail), and transmits the force generated in the working cylinder to apply pressure to the plastic. In view of the convenience of loading and handling, the hydraulic machine generally adopts the form of a vertical opening and closing mold. Hydraulic presses used in plastic molding can be classified in several ways.
1 According to the action mode, it is divided into upper pressure hydraulic machine, lower pressure hydraulic machine and special hydraulic machine.
2 According to the fuselage structure, it is divided into column hydraulic machine and frame hydraulic machine.
3 According to the operation mode, it is divided into manual hydraulic machine, semi-automatic hydraulic machine and automatic hydraulic machine.
4 According to the transmission form, it is divided into pump direct transmission hydraulic machine and pump accumulator transmission hydraulic machine. From the current use and development of plastic hydraulic presses, the most commonly used is the semi-automatic column or frame hydraulic press that directly drives the upper pressure pump.
The upper pressure type working cylinder is mounted on the top of the machine, and the lower cross member is fixed, which is convenient to use under the lower pressing type of the working cylinder. Domestic plastic hydraulic presses are basically of this type, but for laminated plastics, there are generally more presses. For forming large and medium-sized thermosetting plastics, hydraulic pumps driven by pump accumulators are often used, that is, centralized supply of high-pressure oil, so that each press is equipped with a separate hydraulic system. This saves money and increases the utilization of hydraulic equipment, but requires a high pressure accumulator and a central pressure supply system to balance the need for high pressure liquids at low loads and peak loads.
(3) Although there are many performance parameters used to characterize the hydraulic machine, many of them are similar to the parameters of the mold clamping part of the injection molding machine. Therefore, several important parameters of the hydraulic machine are discussed here.
1 Maximum total pressure (nominal pressure). The maximum total pressure of the hydraulic press or the tonnage of the hydraulic press is the main parameter indicating the pressability of the machine. It is generally used to indicate the machine specifications. For example, the domestic SY-250 hydraulic press represents a plastic (S) hydraulic press (Y) with a maximum total pressure of 250t.
2 The pressure of the working fluid. Working fluid pressure is an important parameter in the design of hydraulic presses. The pressure of the working fluid affects the size of the working cylinder, the structural size of the hydraulic machine, the design of the hydraulic transmission system, and the maintenance of the machine. Therefore, when determining the working pressure, we must consider all aspects of the factors, according to the pressure standard series to determine. At present, the working pressures used by domestic plastic hydraulic presses are 16.7 MPa, 29.4 MPa, 31.4 MPa, 49 MPa, etc., and most of the working pressure is about 29.4 MPa. The working pressure of the hydraulic machine should not be too low. Otherwise, in order to meet the needs of the maximum total pressure of the hydraulic machine, a large working cylinder is required, so that the hydraulic machine has a large structure and an increased quality.
3 Maximum return force. That is, the force required by the hydraulic movable beam on the return stroke. The return force can be selected according to actual experience. The maximum return force of the hydraulic machine is generally 20%~50% of the hydraulic machine tonnage.
Boost time. Although the boost time is not listed in the performance parameters of some hydraulic machines, it is a relatively important technical indicator for hydraulic presses. Because in the plastic pressing process, not only the hydraulic machine needs to generate sufficient force, but also when the pressed plastic is in the lowest viscosity and the best fluidity, the pressure of the hydraulic machine can be quickly reached to the maximum value, so as to ensure that the plastic is filled with the mold and the satisfactory product is obtained. . Therefore, the working pressure of the hydraulic machine must be raised to the required value within a certain period of time. According to experience, the current plastic press below 500t requires a boost time of less than 10s.
For other parameters such as the maximum distance between the movable beam and the workbench (lower beam), the piston stroke (ie the movable beam stroke), the low pressure line speed (ie fast), the high pressure line speed (ie low speed), the table size, the column or Items such as the distance between the guide rails are parameters indicating the performance of the machine. Their meaning can be found in the relevant chapters of the injection molding machine.
In order to meet the needs of pressing different plastics, the high-pressure speed of hydraulic presses is often adjustable, and the adjustment range is between 0~6mm/s.
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