The injection molding process has good adaptability to the processing of various silica gels, has high production capacity, and is easy to realize automation. Today, with the rapid development of the silicone industry, injection molding machines play an important role in terms of quantity and variety, and thus become one of the fastest growing and most produced models of silicone machinery.
Silicone injection molding technology was developed from the end of the 19th century to the beginning of the 20th century based on the principle of die casting, and it is one of the most commonly used methods in silicone processing. This method is applicable to all thermoplastic silica gel and some thermosetting silica gel (accounting for about 1/3 of the total silica gel).
Classification of injection molding machines
- Injection molding machines are divided into plunger injection molding machines and screw injection molding machines according to the plasticization method.
- According to the transmission mode of the machine, it can be divided into full hydraulic machine, full electric motor, and electro-hydraulic compound type.
- According to the number of cylinders, it is divided into single-color machine, double-color machine and multi-color machine. A single-color machine refers to an injection molding machine with one barrel, a two-color machine refers to an injection molding machine with two barrels, and a multi-color machine refers to an injection molding machine with more than two barrels.
- Injection molding machine can be divided into vertical type, horizontal type and vertical-horizontal compound type, angle type injection molding machine, multi-mode rotary table type injection molding machine according to the arrangement of injection device and mold clamping device
Working principle of liquid silicone injection molding machine
LIMS (Liquid Injection Molding System) liquid silicone injection molding system is a new processing system formed by combining liquid silicone rubber with excellent performance and equipment capable of precise and stable injection molding. Just put A/B two kinds of liquid materials into the equipment, and C is the liquid injection molding color paste, which is fully automatic from mixing to molding. While pursuing simplified process and shortened processing time, it is very convenient to realize the production of high-quality products. processing. Not only that, by utilizing the various properties of liquid silicone rubber, it can be widely used in various industries represented by electrical, electronic, automobile, and food. Focusing on the improvement of production capacity and the reduction of labor costs, LIMS perfectly reflects the excellent economic efficiency. The working principle of the 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 silicone in a molten state (ie, viscous fluid state) into the closed mold cavity. The process of obtaining products after curing and setting.Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding - melt plasticization - pressure injection - mold filling and cooling - mold opening and picking. After taking out the plastic parts, the mold is closed again, and the next cycle is carried out.
LIMS molding conditions
- Curing temperature: Under normal circumstances, curing can be achieved at 90°C to 210°C. Injection pressure: 40~120Kg/CM2.
- Curing time: at 150°C, the curing time for a 1mm test piece is within 10s.
- Linear shrinkage: 100℃～150℃, the linear shrinkage is 2%～3%. Due to the low viscosity and high fluidity of LSR, in order to avoid glue overflow, there are high requirements for the mold sealing position and parting surface.
In order to reduce air bubbles, it is best to install a vacuum structure in the mold to ensure that the air in the mold cavity is discharged in time.
The structure of liquid silicone injection molding machine
Injection molding machines are divided into plunger type injection molding machines and screw type injection molding machines according to the plasticizing method, and can be divided into hydraulic type, mechanical type and hydraulic-mechanical (connecting rod) type according to the transmission mode of the machine, and automatic type according to the operation mode. , Semi-automatic and manual injection molding machines.
(1) Horizontal injection molding machine: This is the most common type. The mold clamping part and the injection part are on the same horizontal center line, and the mold is opened along the horizontal direction. Its characteristics are: short fuselage, easy to operate and maintain. The center of gravity of the machine is low and the installation is relatively stable. After the product is ejected, it can automatically fall by gravity, which is easy to realize fully automatic operation. At present, most injection molding machines on the market use this type. Even the mainframe has no height limit for the plant due to its low body. In the case where the product can be automatically dropped, automatic forming can be realized without using a robot. Due to the low body, convenient material supply and easy maintenance. The mold needs to be installed by a crane. With multiple units arranged side by side, the molded products are easily collected and packaged by the conveyor belt.
(2) Vertical injection molding machine: the clamping part and the injection part are on the same vertical center line, and the mold is opened in the vertical direction. Therefore, it occupies a small area, it is easy to place inserts, it is more convenient to load and unload molds, and the materials falling from the hopper can be more evenly plasticized. However, after the product is ejected, it is not easy to automatically fall, and it must be taken off by hand, which is difficult to realize automatic operation. Vertical injection molding machines are suitable for small injection molding machines. Generally, injection molding machines below 60 grams are used more, and large and medium-sized machines are not suitable for use. Main features of vertical injection molding machine:
- The injection device and the clamping device are on the same vertical center line, and the mold is opened and closed along the up and down direction. Its floor area is only about half of that of the horizontal machine, so the productivity is about twice that of the floor area.
- Easy to implement insert molding. Because the mold surface is facing upwards, insert positioning is easy. Using a model with a fixed lower platen and a movable upper platen, combined with a belt conveyor and a robot arm, fully automatic insert molding can be easily realized.
- The weight of the mold is supported by the horizontal formwork for up and down opening and closing actions, and there will be no phenomenon that the formwork cannot be opened or closed due to the forward fall caused by the gravity of the mold similar to the horizontal machine. Conducive to durability to maintain the precision of machinery and molds.
- The cavities of each plastic part can be taken out by a simple manipulator, which is conducive to precision molding.
- Generally, the mold clamping device is open around, and it is easy to configure various automatic devices, which is suitable for automatic molding of complex and delicate products.
- The pull belt conveying device is easy to be installed in the middle of the mold, and it is convenient to realize the automatic production of molding.
- It is easy to ensure the consistency of the fluidity of the resin in the mold and the temperature distribution of the mold.
- Equipped with rotating table, moving table and inclined table, etc., it is easy to realize insert molding and in-mold combination molding.
- During trial production in small batches, the mold structure is simple and cost-effective, and it is easy to disassemble.
- Withstood the test of many earthquakes. Due to the low center of gravity, the vertical machine has better shock resistance than the horizontal machine.
(3) Angle injection molding machine: The injection direction and the interface of the mold are on the same surface. It is especially suitable for flat products that do not allow traces of the gate in the machining center. It occupies a smaller area than a horizontal injection molding machine, but inserts placed in the mold are prone to tilting and falling. This type of injection molding machine is suitable for small machines. The axis of the injection screw of the angle type injection molding machine and the movement axis of the mold clamping mechanism template are arranged perpendicular to each other, and its advantages and disadvantages are between the vertical type and the horizontal type. Because the injection direction and the parting surface of the mold are on the same plane, the angle injection molding machine is suitable for asymmetrical geometric molds with side gates or products that do not allow gate traces in the molding center.
(4) Multi-mode rotary disc injection molding machine: It is a special injection molding machine with multi-station operation. It is characterized in that the mold clamping device adopts a rotary disc structure, and the mold rotates around the shaft. This type of injection molding machine gives full play to the plasticizing ability of the injection device, which can shorten the production cycle and increase the production capacity of the machine, so it is especially suitable for large-volume products that have a long cooling and setting time or require more auxiliary time due to the placement of inserts. production. However, due to the large and complex mold clamping system, the mold clamping force of the mold clamping device is often small, so this kind of injection molding machine is widely used in the production of plastic shoe soles and other products.
(5) Two-color injection molding machine: an injection molding machine that can inject two colors at one time, which can meet consumers' requirements for appearance and make users feel more comfortable using the product. 5. Multi-station molding machine The injection device and mold clamping device have two or more working positions, and the injection device and mold clamping device can be arranged in various ways.
(6) Electric injection molding machine: The all-electric injection molding machine can not only meet the needs of special purposes, but also has more advantages than ordinary injection molding machines. Another advantage of the all-electric injection molding machine is that it can reduce noise, which not only benefits the workers, but also reduces investment and construction costs in the soundproof production workshop.
The action procedure of the liquid silicone injection molding machine: mold closing→premolding→retraction→nozzle forward→injection→holding pressure→nozzle back→cooling→mold opening→ejection→opening the door→taking the workpiece→closing the door→closing the mold.
Liquid silicone injection molding machine operation items: Injection molding machine operation items include three aspects: control keyboard operation, electrical control cabinet operation and hydraulic system operation. Carry out injection process action, feeding action, injection pressure, injection speed, selection of ejection form, monitoring of temperature, current and voltage of each section of barrel, adjustment of injection pressure and back pressure, etc.
Liquid silicone injection molding machine injection process action selection
Generally, injection molding machines can be operated manually, semi-automatically and fully automatically.
Manual operation is in a production cycle, each action is realized by the operator toggling the operation switch. Generally, it is only used when testing the machine and adjusting the mold.
During semi-automatic operation, the machine can automatically complete a working cycle, but after each production cycle, the operator must open the safety door, remove the workpiece, and then close the safety door, so that the machine can continue the production of the next cycle.
In fully automatic operation, the injection molding machine can automatically enter the next working cycle after completing one working cycle. In the normal continuous working process, there is no need to stop the machine for control and adjustment. However, it must be noted that if full-automatic work is required, then (1) Do not open the safety door halfway, otherwise the full-automatic operation will be interrupted. (2) Add materials in time. (3) If the electric eye sensor is selected, care should be taken not to cover the electric eye. In fact, it is usually necessary to temporarily stop halfway in the fully automatic operation, such as spraying release agent to the machine mold.
During normal production, semi-automatic or fully automatic operation is generally used. At the beginning of the operation, the operation mode (manual, semi-automatic or fully automatic) should be selected according to the production needs, and the manual, semi-automatic or fully automatic switch should be toggled accordingly.
The semi-automatic and fully automatic working procedures have been determined by the circuit itself. The operator only needs to change the speed and pressure, the length of time, the number of thimbles, etc. Make work procedures confusing.
Before each action in a cycle is adjusted properly, manual operation should be selected first, and semi-automatic or fully automatic operation should be selected after confirming that each action is normal.
Liquid silicone injection molding machine pre-molding action selection
According to whether the injection seat backs up before and after pre-plastic feeding, that is, whether the nozzle leaves the mold, the injection molding machine generally has three options.
(1) Fixed feeding: the nozzle is always attached to the mold before and after pre-molding, and the injection seat does not move.
(2) Pre-feeding: the nozzle presses against the mold for pre-plastic feeding. After the pre-molding is completed, the injection seat moves back and the nozzle leaves the mold. The purpose of choosing this method is to use the injection hole of the mold to hold the nozzle during pre-molding, so as to prevent the molten material from flowing out of the nozzle when the back pressure is high. The relative stability of their respective temperatures.
(3) Post-feeding: After the injection is completed, the injection seat retreats, the nozzle leaves the mold and then pre-molding, and the injection seat moves forward after pre-molding. This action is suitable for processing silica gel with a particularly narrow molding temperature. Due to the short contact time between the nozzle and the mold, heat loss and solidification of the molten material in the nozzle hole are avoided.
After the injection of the liquid silicone injection molding machine is finished and the cooling timer is finished, the pre-molding action starts. The screw rotates to extrude the silicone melt to the front of the screw head. Due to the function of the one-way valve at the front end of the screw, the molten silica gel accumulates at the front end of the barrel, forcing the screw back.
When the screw retracts to a predetermined position (this position is determined by the travel switch, the distance of the screw to be retracted is controlled to realize quantitative feeding), the pre-molding stops, and the screw stops rotating. Followed by the retraction action, the retraction means that the screw makes a slight axial retreat. This action can relieve the pressure of the molten material gathered at the nozzle and overcome the "salivation" phenomenon caused by the imbalance of pressure inside and outside the barrel. .
If the retraction is not required, the retraction should be stopped, the switch should be adjusted to the appropriate position, and the pre-molding should be stopped. At the same moment when the switch is pressed, the retract stop switch is also pressed. When the screw moves backward to press the stop switch, the retraction stops. Then the injection seat began to retreat. When the injection seat retreats until the stop switch is pressed, the injection seat stops retreating. If the fixed feeding method is adopted, attention should be paid to adjusting the position of the travel switch.
LIMS Common Problems and Countermeasures
1. Bubbles or swelling
Q: Insufficient vulcanization
A: Extend the vulcanization time and increase the temperature
Q: Insufficient molding pressure
A: Increase the holding pressure
Q: Generate bubbles
A: Evacuate the air and adjust the injection speed
Q: Uneven mold heating
A: Adjust the heating device to make the heating even.
2. Lack of material, surface spots, uneven color mixing
Q: Insufficient vulcanization
A: Extend the vulcanization time and increase the temperature
Q: The air is not exhausted
A: Take out the air fully
Q: Gas cannot be excluded
A: Check the equipment to seal the muzzle
Q: The mold temperature is too high
A: Reduce temperature, average temperature distribution
Q: mixed spots
A: Adjust the injection speed and check the feeder
3. Clamping line problem
Q: Improper mixing ratio
A: Check the feeding system
Q: uneven mixing
A: Adjust the injection speed
Q: Poor molding pressure
A: Increase the pressure and decrease the temperature
Q: The injection time is too long
A: shorten the injection time
Q: Poor exhaust at the welding part
A: Using a vacuum machine
Q: Unbalanced runner
A: Balanced runner
4. Poor gloss, dull color
Q; Insufficient vulcanization
A: Extend the vulcanization time and increase the temperature
Q: The surface of the mold is rough
A: Mold polishing or release agent is too strong
5. Poor release performance
Q: Insufficient vulcanization
A: Extend the vulcanization time and increase the temperature
Q: The surface of the mold is not good
A: Mold surface treatment
Q: Surface temperature distribution is uneven
A: Check the mold heating method
6. Burr overflow glue
Q: Too much glue injection
A: Reduce the amount of glue injected
Q: parting surface problem
A: Check the parting surface
Q: Clamping force is too low
A: Increase the clamping force
Q: Mold temperature is too low
A: Increase mold temperature
Q: The injection pressure is too high
A: Reduce the injection pressure
Q Holding time is too long or too high
A: Reduce the holding time
7. Color
Q: Spots appear
A: Add color: 1. Control and stabilize color paste 2. Increase the amount of color paste 3. Stirring time is prolonged 4. Increase the screw speed 5. Static mixer 6. Cleaning 7. Check the color port (too big/blocked)
Q: There are flow marks
A: 1. Color paste dispersion 2. Change the batch number of color paste 3. Change the shape of the injection port (membrane needle-shaped film injection port)
Q: Matte surface
A: Accumulation of volatile matter, regular cleaning
