Plastic molding equipment is developed on the basis of rubber machinery and metal die-casting machines. Since the emergence of polymer injection molding technology and simple molding equipment in the 1870s, as an industry, it did not develop rapidly until the 1930s. Plastic molding equipment gradually became commercialized, and injection molding and extrusion molding have become industrialized processing methods. Blow molding is the third largest plastic molding method after injection molding and extrusion, and it is also the fastest growing plastic molding method.
Mechanical classification
Plastic injection molding machine
Plastic injection molding is a molding method that combines injection and molding. Its equipment is called plastic injection molding machine, or injection molding machine for short. Plastic injection molding machine is the main molding equipment for making thermoplastics and thermosetting plastics into various plastic products. Ordinary plastic injection molding machine refers to the most widely used single-screw or plunger horizontal, vertical or angle single-station injection molding machine for processing thermoplastics.
Other types of injection molding machines, such as thermosetting plastics, structural foaming, multi-component, reactive, exhaust, etc., refer to some injection molding machines whose processed materials and machine structure characteristics are quite different from those of ordinary plastic injection molding machines. About 30% of the world’s plastic raw materials are used for injection molding, and injection molding machines account for about 40% of the total output of plastic machinery. They have become an important part of the plastic processing industry and the plastic machinery industry, and are one of the fastest growing, most diverse and most produced types of plastic machinery products.
Plastic Extruder
In plastic extrusion equipment, plastic extruders are usually called main machines, and the subsequent equipment plastic extrusion molding machines that match them are called auxiliary machines. After more than 100 years of development, plastic extruders have evolved from the original single screw to twin screws, multiple screws, and even screwless machines. Plastic extruders (main machines) can be matched with various plastic molding auxiliary machines such as pipes, films, holding materials, monofilaments, flat wires, strapping tapes, extruded nets, plates (sheets), profiles, granulation, cable coating, etc., to form various plastic extrusion molding production lines and produce various plastic products. Therefore, plastic extrusion molding machinery is one of the widely used machines in the plastic processing industry.
Plastic Blow Molding Machine
Blow molding consists of two basic steps: first forming the parison, then using compressed air (and stretching rods) to radially inflate (and axially stretch) the parison, making it close to (stretch) the blow mold cavity, giving the product the shape and size of the mold cavity, and cooling it.
Multilayer blow molding
There are two types of molding: co-extrusion blow molding and co-injection blow molding; stretch blow molding includes injection stretch blow molding and extrusion stretch blow molding. Stretch blow molding can be divided into one-step method and two-step method.
Presses and transfer molding machines
Compression molding and transfer molding are the main molding methods for thermosetting plastics. The press used for compression molding and the transfer molding machine used for transfer molding are the main molding equipment for various plastic products made of thermosetting plastics. The function of the press is to apply pressure to the plastic through the plastic mold. If a fixed mold is used, it also has the function of opening and closing the mold and ejecting the product. The press used for compression molding is divided into two categories: motorized presses and hydraulic presses. Since the manufacture, operation and repair of motorized presses are more complicated than hydraulic presses, and they are not suitable for compression molding of large plastic parts, they are gradually eliminated. The presses used are basically hydraulic presses. The pressure medium of hydraulic presses usually includes oil and water. The former is widely used in small and medium-sized equipment. Transfer molding equipment can use the same hydraulic press as compression molding, or a dedicated transfer molding machine. The transfer molding machine is actually a double-pressure hydraulic press with two hydraulic cylinders. According to its structure, it can be divided into two types: upper and lower double-pressure type and right-angle type.
Thermoforming Machine
There are only three methods of thermoforming plastics: female mold forming, male mold forming and counter-mold forming. Thermoforming machinery should be able to repeat the thermoforming production cycle according to a certain procedure to produce exactly the same product. There are many types of thermoforming machines, including manual, semi-automatic and automatic thermoforming machines. If the thermoformed product is large in size but small in quantity, it is appropriate to use a semi-automatic or manual thermoforming machine. On the contrary, if the product is small in size but large in quantity, it is more appropriate to use an automatic thermoforming machine.
Working principle and operation of plastic injection molding machine
The working principle of a plastic injection molding machine is similar to that of a syringe used for injections. It uses the thrust of a screw (or plunger) to inject plasticized molten (i.e., viscous) plastic into a closed mold cavity, and then solidifies and shapes the product. Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding – melting and plasticization – pressure injection – mold filling and cooling – mold opening and part removal. After the plastic part is taken out, the mold is closed again for the next cycle.
1. Locking mold: The mold plate quickly approaches the fixed mold plate (including slow-fast-slow speed), and after confirming that there is no foreign matter, the system switches to high pressure to lock the mold (maintain the pressure in the cylinder).
2. The shooting platform moves forward to the designated position (the nozzle is in close contact with the mold).
3. Plastic injection molding: The screw can be set at multiple speeds, pressures and strokes to inject the molten material at the front end of the barrel into the mold cavity.
4. Cooling and holding pressure: According to the set pressure and time period, the barrel pressure is maintained, and the mold cavity is cooled and formed.
5. Cooling and pre-plasticization: The product in the mold cavity continues to cool, and at the same time, the hydraulic motor drives the screw to rotate to push the plastic particles forward. The screw retreats under the set back pressure control. When the screw retreats to the predetermined position, the screw stops rotating, and the injection cylinder is released as set, and the expected end is reached.
6. Shooting platform retreats: After pre-plasticization, the shooting platform retreats to the designated position.
7. Mold opening: the mold plate returns to its original position (including slow-fast-slow speed).
8. Ejection: The ejector ejects the product. The basic requirements of injection molding are plasticization, injection and molding. Plasticization is the premise for achieving and ensuring the quality of molded products, and in order to meet the requirements of molding, injection must ensure sufficient pressure and speed. At the same time, due to the high injection pressure, a correspondingly high pressure is generated in the mold cavity (the average pressure in the mold cavity is generally between 20 and 45 MPa), so there must be a sufficiently large clamping force. It can be seen that the injection device and the clamping device are key components of the injection molding machine.
Molding method
Plastic molding machine processing is an engineering technology, which involves various processes of transforming plastic into plastic products. During the transformation process, one or more of the following situations often occur, such as changes in the rheology, physical and chemical properties of the polymer.
Pressing
The main equipment for compression molding is presses and molds. The most commonly used presses are self-contained hydraulic presses, with tonnage ranging from tens of tons to hundreds of tons. There are down-press presses and up-press presses. The molds used for compression molding are called compression molds, which are divided into three categories: overflow molds, semi-overflow molds, and non-overflow molds. The main advantages of compression molding are that it can mold products with larger surfaces and can be mass-produced, but its disadvantages are long production cycles and low efficiency.
Injection molding
Lamination molding is usually operated by a laminating machine, which has multiple floating hot press plates between the dynamic and fixed press plates. Commonly used reinforcing materials for lamination molding include cotton cloth, glass cloth, paper, asbestos cloth, etc., and resins include phenolic, epoxy, unsaturated polyester and some thermoplastic resins.
Extrusion
Extrusion molding is also called extrusion molding or extrusion. It is a method of forming materials by heating and pressurizing them in an extruder so that they flow continuously through a die. Extrusion is mainly used for the molding of thermoplastics and can also be used for some thermosetting plastics. The extruded products are all continuous profiles, such as tubes, rods, wire sheets, films, wire and cable coatings, etc. In addition, it can also be used for mixing, plasticizing and granulating, coloring, blending, etc. of plastics. The extrusion molding machine consists of the main parts of the extrusion device, transmission mechanism, heating and cooling system. There are two types of extruders: screw type (single screw and multi-screw) and plunger type. The extrusion process of the former is continuous, and the latter is intermittent. The basic structure of a single-screw extruder mainly includes transmission device, feeding device, barrel, screw, die head and die. The auxiliary equipment of the extruder includes three categories: material pre-treatment equipment (such as material conveying and drying), extrudate processing equipment (forming, cooling, traction, cutting or rolling) and production condition control equipment.
Pultrusion
Pultrusion molding is one of the molding methods of thermosetting fiber reinforced plastics. It is used to produce profiles with fixed cross-sectional shapes and unlimited length. The molding process is to pull out continuous fibers impregnated with resin glue through a heating mold, and then pass through a heating chamber to further solidify the resin to prepare a unidirectional high-strength continuous reinforced plastic profile.
The resins commonly used for pultrusion molding are unsaturated polyester, epoxy and silicone. Among them, unsaturated polyester resin is the most commonly used. The pultrusion molding machine usually consists of a fiber arrangement device, a resin tank, a preforming device, a die and a heating device, a traction device and a cutting device.
Molding machine maintenance
The molding machine is roughly divided into three main parts:
1. Electronic control part
Traditional injection molding machines use contact relays to switch various actions, which often fail due to loose contact screws and aging of contacts. Usually, they should be replaced with new ones after one million times of use to ensure the stability of electronic control. In particular, environmental factors such as dust adhesion and humid air will also affect the operation of the machine.
Modern injection molding machines use contactless integrated circuits, which greatly reduces the number of wire connections, significantly improves the adverse phenomena caused by wires, and improves stability.
2. Institutional part
The mechanism should be regularly maintained and lubricated to reduce the friction coefficient and wear. The nuts and locking screws on the head plate should be checked regularly to prevent the column from breaking due to uneven force.
The mold thickness adjustment mechanism should be regularly checked for deviation or looseness of the transmission shaft gear or chain, whether the pressure plate screws on the gear are loose, and whether there is enough lubricating grease.
3. Hydraulic part
In the hydraulic system, attention should be paid to the cleanliness of the hydraulic oil to maintain the quality of the hydraulic oil. Hydraulic oil with good stability and high quality should be used. In addition to regular replacement, the operating temperature should be properly controlled below 50°C to avoid affecting the stability of the hydraulic action due to deterioration.
Fault analysis and solutions
The cause of machine failure is often due to improper operation and setting, which can be easily eliminated through analysis.
Adjustment steps of clamping force and low pressure protection:
- The clamping high pressure is set to 135 bar, 10%; the low pressure speed is set to 20%, the low pressure position is set to 100 mm; the low pressure time is set to 5 seconds.
- Use mold adjustment to establish clamping force; the maximum value of the system pressure gauge when the mold is closed at high pressure shall prevail, and you can refer to the clamping force oil pressure comparison table.
- Set the low pressure position to zero after opening the mold.
- Press the mold closing button until the molds touch each other and then release it immediately. At this time, the position of the movable mold is assumed to be “X”, and the low pressure position is set to “X+1” mm.
- Then set the low-pressure protection width: the low-pressure protection width is the distance from low speed to low pressure.
- The low voltage protection time is usually set to 1.2 seconds.
