Operation sequence of continuous injection machine:
Plastic injection molding is a kind of processing method suitable for high-speed mass production of precision components. It will melt, mix, Move (3 M 's: Melt, Mix, and move) granular plastic in the barrel, and then flow, fill, and Freeze (3F' s: Flow, Form, and freeze) in the mold cavity. Its action can be divided into plastic plasticizing, filling, pressure maintaining, cooling and ejection stages.
The basic operation actions are as follows:
(1) Close the mold so that the screw starts to move forward.
(2) In the same way as the plunger ejector, the rotary screw is pushed to fill the mold cavity,.
(3) The screw continues to advance for cavity holding.
(4) When the cavity cools and the gate solidifies, the screw begins to recede and the plasticized material is ready for the next injection.
(5) Open the mold and eject the plastic part.
(6) Open and close the mold to start the next cycle.
A large amount of friction heat is produced when the plastic is extruded by the screw to form a melting state. The melt glue is accumulated in the front end of the barrel, and the temperature of the melt glue is maintained by the heater. At the beginning of the filling stage, the ejector opens the nozzle, and the screw moves forward to inject the melt glue through the nozzle into the closed mold cavity to complete the filling. When the melt enters the mold cavity, the compressed gas escapes from the ejector pin, parting line and air hole. Good filling depends on the plastic component design, gate location and good exhaust. If the fluidity of plastic is not good, or the injection pressure is insufficient, it may cause short injection; on the contrary, if the fluidity of plastic is too good, it is easy to cause burr on the parting surface of plastic parts. After the melt completely fills the cavity, continue to apply pressure to inject more melt, compensate for the plastic volume shrinkage caused by cooling, and ensure that the cavity is completely filled. At the end of filling and pressure maintaining stage, after the melt glue has completely solidified in the mold, open the mold cavity and take out the plastic part. The cooling time accounts for a very high proportion in the whole forming cycle, about 80%. The cooling time of forming products varies according to the plastic properties, shape, size, size and accuracy of forming products. When moving the formwork backward, the ejector pin will be pushed to the back plate to stop the movement, and the formed products, sprue system and waste materials will be ejected. In order to further explain the action of the ejector in the process cycle, the pressure of the hydraulic cylinder, the pressure of the die cavity, the separation distance between the male and female die and the position of the screw in different stages are drawn.
Where the number indicates: filling (injection stage)
1. Pressure maintaining and cooling, 2. Opening the mold, 3. Ejecting the plastic part, 4. Closing the lock.
The injection molding cycle time is changed according to the weight of the plastic part, the thickness of the meat, the nature of the plastic and the parameters set by the machine. Typical cycle times can range from seconds to tens of seconds. Screw operation: according to the demand, the rotary screw can set the rotation speed to plasticize the plastic particles, and press the melt glue into the mold cavity with the set screw speed, injection amount and injection pressure. The main control parameters of the injection molding of the rotary screw extruder are as follows: the back pressure is the pressure value of the plastic acting on the front end of the screw when the screw is pushed back to prepare for the next injection of plastic. When the ejector is ready to shoot out, the screw pushes the plastic at the front end into the mold cavity. After the injected plastic cools in the mold, the ejector enters the screw backward stage and starts a cycle again. In general, the ejector can adjust the maximum back pressure. When the screw moves to this preset back pressure position, the screw reverse phase will end. The preset screw stop position is set manually based on the amount of plastic required to fill the runner and cavity. Injection speed (or injection time) injection speed or screw speed refers to the forward speed of screw in injection operation. For most engineering plastics, the fastest injection speed should be set under the technical conditions of plastic part design and the economic conditions allowed by the manufacturing process. However, in the initial stage of ejection, a lower velocity should be used to avoid jet flow or turbulence. Near the completion of injection, the injection speed should also be reduced to avoid overflow of plastic parts, and it can help to form a uniform suture. The injection time is the time needed to fill the mold cavity, which is controlled by the injection speed. Although the best filling speed depends on the geometry of the plastic part, gate size and glue temperature, in most cases the glue will be injected into the mold cavity as soon as possible. Because the mold temperature is usually lower than the freezing point of the resin, too long injection time will increase the possibility of causing the plastic to set too early.
High injection rate is used for thin and thick plastic parts to prevent solidification before filling mold cavity. Sometimes, thick plastic parts or small gates will reduce the filling speed. At this time, it is necessary to keep the glue flowing through the gate continuously to prevent the gate from solidifying and then filling the mold cavity. The new research direction tries to control the injection volume, control the screw action and the closing time of the check valve, so as to achieve the purpose of controlling the component size. Screw rotation speed: screw rotation speed is the rotation speed of plasticizing screw. The faster the rotation speed is, the more intense the compression of the groove of the plastic screw will produce more shear heat. The buffer is the difference between the maximum allowable forward position of the screw and the forward position at the end. If the screw stroke is allowed to be set to the maximum value and the buffer amount is zero, the screw will advance until it touches the nozzle before stopping. Generally, the buffer is set at 3-6 mm (1 / 8-1 / 4 inch). The temperature of the melt adhesive shall be in accordance with (a) the type of resin, (b) the characteristics of the injection machine, and (c) the injection quantity. The initial set melt temperature shall refer to the resin supplier's recommended data. Generally, the melt temperature is higher than the softening temperature and lower than the melting point of the resin to avoid cracking due to overheating. The temperature of the injection area is usually higher than that of the barrel. The increased heat can reduce the injection pressure of the glue without overheating the glue. Because of the low viscosity of the melt adhesive, it is easy to fill the mold cavity without relying on the thinning caused by temperature rise. The limitation of mold temperature is to avoid the section freezing of plastic in the mold cavity and the cooling property of plastic (such as crystallization, etc.).
Therefore, the mold temperature should be a compromise between the fluidity of the melt adhesive and the mold temperature. If possible, the critical solidification position should occur at the gate. Adjusting the gate size can obtain the best fluidity at the lowest possible mold temperature. The lower die temperature can accelerate the forming cycle, so the lowest acceptable die temperature should be used as far as possible. Some injection molding needs to be cooled or condensed, some need to heat the mold to control the crystallinity and thermal stress. The mold temperature can be adjusted with coolant.
Mold temperature and coolant temperature shall be monitored. One of the purposes of using different mold temperature on the fixed side and the moving side of the mold is to control the attachment of the finished product to the mold core and facilitate ejection. Some factors that affect the temperature of the melt glue and the mold include: injection quantity - a large injection quantity requires a higher mold temperature. Ejection rate - a high ejection rate can cause a high temperature of rarefaction. Runner size - longer runners require higher temperatures. Wall thickness of plastic parts - thick parts need a long cooling time, usually using a lower mold temperature. The upper limit of the injection pressure is the capacity of the injection machine, the clamping force and the structure of the die. In general, the injection pressure and the holding pressure are set to the minimum pressure that will not cause a short injection. The injection pressure and holding pressure should be high enough to last long enough to continue filling the plastic during the shrinkage phase of the plastic part to minimize shrinkage. However, too high injection pressure can cause potential stress in the plastic part. Two stage pressurization can be used in some processes. In the first stage, high pressure is used for filling, and in the second stage, low pressure is used for pressure maintaining.
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