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With China Plastic Machinery
With China Plastic Machinery
After the blow molding machine sprays out the liquid plastic, it uses the wind force blown out by the machine to blow the plastic body into a mold cavity of a specific shape, thereby making a product. The blow molding machine is composed of a plastic processing machine, including a screw extruder, a film forming machine, an air ring blowing cooling machine and a traction machine, and finally the product is wound into a roll by a winder.
Blow molding (blow moulding) is also called hollow blow molding, a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is placed in a split mold while hot (or heated to a softened state). After the mold is closed, compressed air is immediately introduced into the parison to inflate the plastic parison and cling to the inner wall of the mold. After cooling and demolding, various hollow products are obtained. The manufacturing process of blown film is very similar to that of hollow product blow molding in principle, but it does not use molds. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion. During World War II, the blow molding process began to be used to produce low-density polyethylene bottles. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology has been widely used. The volume of hollow containers can reach thousands of liters, and some production has been computer-controlled. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and the resulting hollow containers are widely used as industrial packaging containers.
According to the method of making the parison, blow molding can be divided into extrusion blow molding and injection blow molding. The newly developed ones are multi-layer blow molding and stretch blow molding.
The energy saving of the blow molding machine can be divided into two parts: one is the power part and the other is the heating part.
Energy saving of the power part: Most of them use inverters. The energy saving method is to save the residual energy consumption of the motor. For example, the actual power of the motor is 50Hz, but you actually only need 30Hz in production to produce enough, and the excess energy consumption is wasted. The inverter changes the power output of the motor to achieve energy saving.
Energy saving in heating part: Energy saving in heating part mostly adopts electromagnetic heater, and the energy saving rate is about 30%-70% of the old resistance coil.
1. Compared with resistance heating, electromagnetic heater has an extra insulation layer, and the utilization rate of heat energy is increased.
2. Compared with resistance heating, electromagnetic heater directly acts on the material pipe for heating, reducing the heat transfer heat energy loss.
3. Compared with resistance heating, the heating speed of electromagnetic heater is more than one-fourth faster, reducing the heating time.
4. Compared with resistance heating, the heating speed of electromagnetic heater is fast, the production efficiency is improved, and the motor is in a saturated state, which reduces the power loss caused by high power and low demand.
The above four points are why Feiru electromagnetic heater can save up to 30%-70% of energy on blow molding machine.
Blow molding machine can be divided into three categories: extrusion blow molding machine, injection blow molding machine and special structure blow molding machine. Stretch blow molding machine can belong to each of the above categories. Extrusion blow molding machine is a combination of extruder, blow molding machine and clamping mechanism, which consists of extruder and parison die head, inflation device, clamping mechanism, parison thickness control system and transmission mechanism. The parison die head is one of the important components that determine the quality of blow molded products, usually with side feed die head and central feed die head. Storage cylinder parison die head is often used for blow molding of large products. The minimum volume of the storage cylinder is 1kg and the maximum can reach 240kg. The parison thickness control device is used to control the wall thickness of the parison, with a maximum of 128 control points, generally 20 to 30 points. Extrusion blow molding machine can produce hollow products with a volume range of 2.5ml to 104l.
Injection blow molding machine is a combination of injection molding machine and blow molding mechanism, including plasticizing mechanism, hydraulic system, control electronics and other mechanical components. Common types include three-station injection blow molding machine and four-station injection blow molding machine. The three-station machine has three stations: preform, blow and demoulding, and each station is 120бу apart. The four-station machine has one more preforming station, and each station is 90бу apart. In addition, there are double-station injection blow molding machines with stations 180бу apart. The plastic containers produced by injection blow molding machines are accurate in size and do not require secondary processing, but the mold cost is relatively high.
Special structure blow molding machines are blow molding machines that use sheets, molten materials and cold billets as billets to blow hollow bodies with special shapes and uses. Due to the different shapes and requirements of the products produced, the structures of blow molding machines are also different.
1. The screw center shaft and cylinder are made of 38CrMoAlA chromium, molybdenum, and aluminum alloys treated with nitrogen, with the advantages of high thickness, corrosion resistance and wear resistance.
2. The die head is chrome-plated, and the screw mandrel structure makes the discharge more even and smooth, and better completes the blown film. The complex structure of the film blowing machine makes the output gas more uniform. The lifting unit adopts a square frame platform structure, and the height of the lifting frame can be automatically adjusted according to different technical requirements.
3. The unloading equipment adopts peeling rotation equipment and center rotation equipment, and uses a torque motor to adjust the film smoothness for easy operation.
In the production process of blown film, the uniformity of film thickness is a very critical indicator. The longitudinal thickness uniformity can be controlled by the stability of extrusion and traction speed, while the transverse thickness uniformity of the film generally depends on the precision manufacturing of the die head and changes with the changes in production process parameters. In order to improve the transverse thickness uniformity of the film, an automatic transverse thickness control system must be introduced. Common control methods include automatic die head (thermal expansion screw control) and automatic air ring. Here we mainly introduce the principle and application of automatic air ring.
The automatic air ring structure adopts a double air outlet method, in which the air volume of the lower air outlet is kept constant, and the upper air outlet is divided into several air ducts on the circumference. Each air duct is composed of a wind chamber, a valve, a motor, etc. The motor drives the valve to adjust the opening of the air duct and control the air volume of each air duct.
During the control process, the thickness probe detects the film thickness signal and transmits it to the computer. The computer compares the thickness signal with the current set average thickness, calculates according to the thickness deviation and the curve change trend, and controls the motor to drive the valve to move. When the film is thin, the motor moves forward and the air outlet is closed; on the contrary, the motor moves in the opposite direction and the air outlet is increased. By changing the air volume of each point on the circumference of the air ring and adjusting the cooling speed of each point, the lateral thickness deviation of the film is controlled within the target range.
The automatic air ring is an online real-time control system. The controlled objects of the system are several motors distributed on the air ring. The cooling airflow sent by the fan is distributed to each air duct after being constant-pressured by the wind ring air chamber. The motor drives the valve to open and close to adjust the size of the air outlet and air volume, change the cooling effect of the film blank at the die head discharge, and thus control the film thickness. From the control process, there is no clear relationship between the film thickness change and the motor control quantity. The thickness change of different thickness films and valves at different positions and the control quantity are nonlinear and irregular. Each adjustment of a valve has a great impact on adjacent points, and the adjustment has a lag, which makes different moments interrelated. For such a highly nonlinear, strongly coupled, time-varying and control uncertain system, its accurate mathematical model is almost impossible to establish. Even if a mathematical model can be established, it is very complicated and difficult to solve, so it has no practical value. Traditional control has a better control effect on a relatively certain control model, but it has a poor or even powerless control effect on highly nonlinear, uncertain, and complex feedback information. In view of this, we chose the fuzzy control algorithm. At the same time, the fuzzy quantization factor is changed to better adapt to the changes in system parameters.
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