Technical Serialization - Maintenance and Maintenance of Plastic Injection Molding Machines (2)

1.4.2.3 Selection of injection pressure The injection pressure of the injection molding machine is adjusted by the pressure regulator valve. Under the condition of set pressure, the injection pressure of the high pressure and low pressure oil passages is controlled to control the injection pressure during the early and late periods.
Ordinary medium and above injection molding machines are equipped with three pressure options, namely high pressure, low pressure and low pressure after high pressure. High-pressure injection is achieved by injecting high pressure pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the mold cavity at high pressure and high speed from the very beginning. Rapid injection of plastic into the mold during high pressure injection, injection cylinder pressure gauge readings rise quickly. Low-pressure injection is achieved by injecting low-pressure pressure oil into the injection cylinder. The reading of the pressure gauge during the injection rises slowly, and the plastic enters the mold cavity at low pressure and low speed. After the high pressure, the low pressure is achieved by controlling the pressure of the pressure oil introduced into the cylinder in time according to the type of the plastic and the actual requirements of the mold.
In order to meet different plastics requirements with different injection pressures, it is also possible to use different diameter screw or plunger replacement methods. This not only satisfies the injection pressure, but also gives full play to the machine's production capacity. In large-scale injection molding machines, there are often multiple stages of injection pressure and multi-stage injection speed control functions, which can better ensure the quality and accuracy of products.
1.4.2.4 Selection of injection speed General injection molding machine control board has a fast-slow knob to meet the injection speed requirements. In the hydraulic system there is a large flow pump and a small flow pump running oil supply at the same time. When the oil circuit is connected to a large flow, the injection molding machine can realize fast opening and closing of the mold, rapid injection, etc. When the hydraulic oil circuit only provides a small flow rate, various actions of the injection molding machine are performed slowly.
1.4.2.5 Selection of ejector type There are two types of ejector ejector: mechanical ejector and hydraulic ejector, some of which are equipped with a pneumatic ejector system. The ejector ejector is provided with single and multiple ejectors. The eject action can be manual or automatic.
The eject action is initiated by a die stop limit switch. The operator can achieve this by adjusting the push-out time button on the control cabinet as required. The speed and pressure of the ejector can also be controlled by the switch on the control cabinet surface. The distance between the front and the back of the ejector movement is determined by the travel switch.
1.4.2.6 Temperature Control A thermocouple is used as a temperature measuring element, and a millivolt meter is used as a temperature control device to direct the on-off of the barrel and die heating coil current to selectively fix the temperature and mold of the barrel. temperature. Table 5 lists some plastic molding processing temperature ranges for reference.
The barrel electric heating ring is generally divided into two sections, three sections or four sections of control. The ammeter on the electrical cabinet shows the size of each segment of the electric heating coil. The reading of the ammeter is relatively fixed. If the ammeter reading is found to be low for a long time during operation, there may be a failure of the electric heating ring, or poor contact of the wire, or oxidation of the heating wire, or burning of an electric heating ring. Both will increase the resistance of the circuit in parallel and cause the current to drop.
When the ammeter has a certain reading, it is also easy to use a plastic strip to wipe the outer wall of the heating coil one by one to see if the strip is melted or not to judge whether a certain electric circle is energized or burned.
1.4.2.7 Closing Control Closing is the use of huge mechanical thrust to tighten the mold to withstand the high pressure injection and filling molds of the molten plastic in the injection molding process, so that the mold will have a huge opening force.
The safety gate is closed and each trip switch gives a signal and the closing action starts immediately. The first is the slow start of the moving plate. After a short distance, the control bar that originally pressed the slow switch is disengaged, and the movable plate moves forward quickly. When it is approaching the end of closing the mold, the lever at the other end of the control lever presses the slow switch again, and at this time, the movable board turns slowly and advances at low pressure. In the process of low-pressure mold clamping, if there is no obstacle between the molds, it can be smoothly closed to press the high-voltage switch. The high-pressure switch is to straighten the machine hinge to complete the mold clamping action. This distance is very short, generally only 0.3 ~ 1.0mm, just turn the high pressure immediately hit the mold closure limit switch, then stop the action, the mold closing process.
The mold clamping structure of the injection molding machine is fully hydraulic and mechanical linkage. Regardless of the type of structure, the clamping force is finally exerted by the complete extension of the connecting rod. The straightening process of the connecting rod is a process in which the movable plate and the tail plate are stretched, and the process in which the four rods are pulled by force.
The size of the mold clamping force can be known from the highest value of the instantaneous oil pressure table of the tight mold. If the mold clamping force is large, the maximum value of the oil pressure meter will be high, and vice versa. Smaller injection molding machines do not have mold clamping oil pressure gauges. At this time, it is necessary to judge whether the molds are actually tight according to the straightening conditions of the connecting rods. If an injection molding machine molds the connecting rod to straighten easily when it is closed, or if the “difficult” fails to straighten, or if one pair of connecting rods is not fully straightened, the expansion mold will appear during the injection molding. Flashes or other problems occur.
1.4.2.8 Open mold control When the molten plastic is injected into the mold cavity and the cooling is completed, the mold is removed and the product is removed. The mold opening process is also divided into three phases. The first stage slows the mold opening to prevent the part from tearing in the cavity. The second stage quickly opens the mold to shorten the mold opening time. The third stage slows the mold opening to reduce the impact and vibration caused by open mold inertia.
1.4.3 Control of injection molding process conditions Currently, various injection molding machine manufacturers have developed a variety of program control methods, roughly including: injection speed control, injection pressure control, injection of plastic injection into the mold cavity, screw back Pressure and rotational speed, etc., control of the plastication state. The purpose of achieving process control is to improve product quality and maximize the effectiveness of the machine.
1.4.3.1 Program Control of Injection Speed ​​The program control of the injection speed is to divide the injection stroke of the screw into 3 to 4 stages, and use the appropriate injection speeds in each stage. For example, the injection speed is slowed when the molten plastic first passes through the gate, high speed injection is used during filling, and the speed is slowed at the end of filling. With this method, flashes can be prevented, flow marks can be eliminated, and residual stress of products can be reduced.
The flow velocity is stable at low speed filling, the product size is relatively stable, the fluctuation is small, the internal stress of the product is low, and the internal and external stress of the product tends to be consistent (for example, immersing a certain polycarbonate part in carbon tetrachloride, using high-speed injection molding Parts have a tendency to crack, low-speed cracking). Under relatively slow filling conditions, the temperature difference of the material flow, especially the temperature difference between the material before and after the gate, is large, which helps to avoid shrinkage and depression. However, due to the prolonged mold filling time, it is easy for the parts to have delamination and poor weld marks, which not only affects the appearance but also greatly reduces the mechanical strength.
When high-speed injection, the flow rate is fast. When the high-speed filling is successful, the molten material fills the cavity quickly, the temperature of the material drops less, and the viscosity decreases less. The injection pressure can be lower, and it is a kind of hot material charge. Modal potential. High-speed filling can improve the glossiness and smoothness of the parts, eliminate the phenomenon of joint lines and delamination, shrink small depressions, uniform color, and ensure that large parts of the parts can be full. However, it is easy to produce products that cause blisters or blisters or yellow parts, and even burn the zoom, or cause mold release difficulties, or the phenomenon of uneven mold filling. For high-viscosity plastics, it can cause melt fractures and produce cloud spots on the surface of parts.
High-speed high-pressure injection can be considered in the following cases: (1) High plastic viscosity, fast cooling speed, and long process parts can not fully fill the corners of the cavity with low pressure and slow speed; (2) Parts with too thin wall thickness, melting material When it reaches the thin wall where it can easily condense and stay, a high-speed injection must be used to immediately enter the cavity before the melt energy is consumed in large quantities; (3) Glass-fiber-reinforced plastic, or plastic with a relatively large amount of filler material, due to poor fluidity In order to obtain smooth and uniform parts, high-speed and high-pressure injection must be used.
For high-precision products, thick-walled parts, parts with large wall thickness changes, and parts with thick flanges and ribs, multi-stage injections such as secondary, tertiary, quaternary, or even five are preferred.