Plastic injection molding machine

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Issue Time: Apr 21,2021

Summary

Plastic injection molding is a molding method of 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 that makes thermoplastic and thermosetting plastics into various plastic products. Ordinary plastic injection molding machine refers to the most widely used horizontal,

Other types of injection molding machines, such as thermosetting plastics, structural foaming, multi-component, reactive, venting, etc. injection molding machines, refer to the materials and machine structural features that are quite different from ordinary plastic injection molding machines. Injection molding machine. About 30% of the plastic raw materials in the world are used for injection molding, and injection molding machines account for about 40% of the total output of plastic machinery, and have become an important part of the plastic processing industry and plastic machinery industry, and it is a growth in plastic machinery products. One of the fastest models with the most varieties and specifications and the most production quantities[1].

Plastic extruder

In the plastic extrusion molding equipment, the plastic extruder is usually called the main machine, and the subsequent equipment that is matched with the plastic extrusion molding machine is called the auxiliary machine. After more than 100 years of development, the plastic extruder has been derived from the original single-screw, multi-screw, and even non-screw and other models. The plastic extruder (host) can be used with various plastic molding auxiliary machines such as pipe, film, holding material, monofilament, flat wire, strapping, extruding net, board (sheet) material, profiled material, pelletizing, cable coating, etc. Matching to form various plastic extrusion molding production lines to produce various plastic products. Therefore, plastic extrusion molding machinery is one of the widely used models in the plastic processing industry.

Plastic blow molding machine

Blow molding consists of two basic steps: firstly, the parison is formed, and then compressed air (and stretch rod) is used to radially inflate (and axially stretch) the parison to make it close to (stretch) blow molding The mold cavity gives the shape and size of the mold cavity to the product and cools it.

Multi-layer 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.

Press and transfer molding machine

Compression molding and transfer molding are the main molding methods for thermoset plastics. The press used for compression molding and the transfer molding machine for transfer molding are the main molding equipment for making various plastic products from thermoset 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 presses used for compression molding are 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 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. The transfer molding equipment can be the same hydraulic press as the compression molding, or a dedicated transfer molding machine can be used. The transfer molding machine is actually a double-pressure hydraulic press with two hydraulic cylinders. According to its structure, it can be divided into up and down double pressure type and right angle type.

Thermoforming machine

There are only three types of plastic thermoforming methods for thermoforming machines, namely, female mold forming, male mold forming and pair-molding. The thermoforming machine should be able to repeat the thermoforming production cycle according to a certain procedure to produce exactly the same products. There are many types of thermoforming machines, including manual, semi-automatic and automated thermoforming machines. The volume of thermoformed products is large and the quantity is small, so semi-automatic or manual thermoforming machines are suitable. On the contrary, if the volume of the product is small and the quantity is large, it is more appropriate to choose an automated thermoforming machine

Working principle and operation of plastic injection molding machine

The working principle of the plastic injection molding machine is similar to that of the syringe used for injection. It uses the thrust of the screw (or plunger) to inject the plasticized plastic in the molten state (that is, the viscous fluid state) into the closed mold cavity. The process of obtaining products after curing and shaping. Injection molding is a cyclic process. Each cycle mainly includes: quantitative feeding-melting and plasticizing-pressure injection-mold filling and cooling-mold opening and part removal. After taking out the plastic part, the mold is closed again for the next cycle.

1) Locking the mold: the mold board quickly approaches the fixed mold board (including slow-fast-slow speed), and after confirming that there is no foreign matter, the system is turned to high pressure, and the template is locked (maintaining the pressure in the cylinder).

2) The shooting table moves forward to the position: the shooting table advances to the specified position (the nozzle is close to the mold).

3) Plastic injection: The screw can be set to inject the molten material at the front end of the barrel into the mold cavity at multiple speeds, pressures and strokes.

4) Cooling and maintaining pressure: According to the setting of various pressures and time periods, the pressure of the barrel is maintained, and the mold cavity is cooled and formed.

5) Cooling and pre-molding: The products in the mold cavity continue to cool, while the hydraulic motor drives the screw to rotate to push the plastic particles forward. The screw retreats under the control of the set back pressure. When the screw retreats to the predetermined position, the screw stops rotating and injects The oil cylinder is released according to the setting, and the expectation is over.

6) The shooting table retreats: After the pre-plasticizing, the shooting table retreats to the designated position.

7) Mold opening: the mold board moves back to the original position (including slow-fast-slow speed).

8) Ejection: The thimble ejects the product. The basic requirements of injection molding are plasticization, injection and molding. Plasticization is a prerequisite for realizing 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 cavity (the average pressure in the cavity is generally between 20 and 45 MPa), so there must be a large enough clamping force. It can be seen that the injection device and the clamping device are the key components of the injection molding machine.

Molding method

The processing and use of plastic molding machines is an engineering technology that involves various processes that transform plastics into plastic products. During the transformation process, one or more of the following situations often occur, such as the rheology of the polymer and changes in physical and chemical properties.

Pressing

The main equipment used for compression molding is a press and a mold. The most frequently used presses are self-contained hydraulic presses with tonnage ranging from tens of tons to hundreds of tons. There are down-pressing presses and up-pressing presses. The molds used for compression molding are called compression molds, which are divided into three categories; overflow molds, semi-flush molds, and non-overflow molds. The main advantage of compression molding is that it can mold larger flat products and can be mass-produced. Its disadvantages are long production cycle and low efficiency.

Injection molding

Laminate molding is usually operated by a laminator, which is equipped with a multi-layer floating hot plate between the dynamic pressure plate and the fixed pressure plate. The commonly used reinforcement materials for lamination are cotton cloth, glass cloth, paper, asbestos cloth, etc. The resins include phenolic, epoxy, unsaturated polyester and some thermoplastic resins. Injection blow molding, extrusion blow molding, stretch blown film coating, hot melt deposition, fluidized spraying, flame spraying, electrostatic spraying, plasma spraying.

Extrusion molding

Extrusion molding is also called extrusion molding or extrusion molding. It is a method in which materials are continuously passed through a die in a flowing state by heating and pressing in an extruder. The extrusion method 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, granulating, coloring, blending, etc. of plastics. The extrusion molding machine is composed of extrusion device, transmission mechanism, heating and cooling system and other main parts. 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 single screw extruder mainly includes transmission device, feeding device, barrel, screw, head and die. The auxiliary equipment of the extruder includes three types of material pre-processing equipment (such as material conveying and drying), extrudate processing equipment (formation, cooling, traction, cutting or roll) and production condition control equipment.

Pultrusion

Pultrusion is one of the molding methods of thermosetting fiber reinforced plastics. It is used to produce profiles with a fixed section shape and unlimited length. The molding process is to pull out the continuous fiber impregnated with resin glue through a heating die, and then further solidify the resin through a heating chamber to prepare a unidirectional high-strength continuous reinforced plastic profile.

There are three types of resins commonly used for pultrusion: unsaturated polyester, epoxy and silicone. Among them, unsaturated polyester resins are most used. The pultrusion machine usually consists of a fiber arrangement device, a resin tank, a pre-forming device, a die and a heating device, a traction device, and a cutting device [2].

Molding machine maintenance

1. The molding machine is roughly divided into three main parts:

1) Electric control part:

◎Traditional injection machines use contact relays to switch various actions. They often fail due to loose contact screws and aging contacts. Usually, they should be replaced after one million times of use to ensure the stability of the electronic control. In particular, environmental factors such as dust adhesion and humid air will also affect the machine's operation.

◎Modern injection machines use contactless integrated circuits, the connection of wires is greatly reduced, the bad phenomena caused by the wires are obviously improved, and the stability is improved.

Controller

2) Institutional part:

◎Parts of the mechanism should be regularly maintained and lubricated to reduce the friction coefficient and reduce wear. The nuts and locking screws on the head plate should be checked regularly to prevent the elder pillar from breaking due to uneven force.

◎The mold thickness adjustment mechanism should regularly check whether the large gear or chain of the drive shaft is offset or slack. Whether the screw of the pressure plate on the gear is loose, whether the lubricating grease is enough, etc.

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. The stable and high-quality hydraulic oil should be used. In addition to regular replacement, its working temperature should be properly controlled below 50C to avoid deterioration. And affect the stability of hydraulic action.

When the injection molding machine is in operation, the controller will sound an alarm when there is any abnormality in the system, and a line of warning messages will appear at the bottom of the camp screen, as shown below

Failure problem analysis and solution editing

The cause of machine failure is often caused by improper operation and setting, and easy troubleshooting can be done through analysis.

Adjustment steps of clamping force and low pressure protection:

1. The mold clamping high pressure is set to 135bar, 10%; the low pressure speed is set to 20%, the low pressure position is set to 100mm; the low pressure time is set to 5 seconds.

2. Use the advance and retreat of the mold adjustment to establish the clamping force; the maximum value of the pressure gauge rising when the system pressure gauge is closing the mold is subject to the maximum value, and you can refer to the clamping force hydraulic pressure comparison table.

3. After opening the mold, set the low pressure position to zero.

4. Press the closing mold until the mold is in contact and release immediately. At this time, the movable mold position is assumed to be "X", and the low pressure position is set to "X+1" mm.

5. Then set the low-voltage protection width: the low-voltage protection width is the distance from low speed to low pressure.

6. Set the low-voltage protection time is usually 1.2 seconds