Mold opening control:
When the molten plastic is injected into the mold cavity and the cooling is completed, the mold is opened and the product is taken out. There are also three stages in the opening process. In the first stage, the mold is opened slowly to prevent the parts from tearing in the cavity.
The second stage is rapid die opening to shorten the time of die opening. In the third stage, the mould is opened slowly to reduce the impact and vibration caused by the inertia of the mould opening.
Mold closing control:
Mold closing is to close the mold with a huge mechanical thrust to resist the huge opening force caused by the high-pressure injection and filling of the mold during the injection molding process.
Close the safety door, all travel switches give signals, and the mold closing action starts immediately. First, the moving template is started at a slow speed. After a short distance, the control rod that originally pressed the slow switch is separated from the block, and the moving plate is turned to move forward quickly. When moving to the end point near the mold closing point, the pressing rod at the other end of the control rod presses the slow switch again, at this time, the movable plate rotates at a slow speed and moves forward at a low pressure.
In the process of low-pressure die closing, if there is no obstacle between the dies, it can be smoothly closed to press on the high-pressure switch. Turning high-pressure is to straighten the machine hinge and complete the die closing action. The distance is very short, generally only 0.3-1.0mm. Just turn the high-pressure knob and touch the mold closing end limit switch. At this time, the action stops and the mold closing process ends.
The mold closing structure of the injection molding machine includes full hydraulic type and mechanical connecting rod type. No matter what kind of structure, the connecting rod is fully extended to implement the closing force. The straightening process of the connecting rod is the process of opening the movable plate and the tailboard, and the process of stretching the four connecting rods.
The magnitude of the clamping force can be known from the highest value of the oil pressure gauge rising at the moment of clamping. If the clamping force is large, the highest value of the oil pressure gauge will be high, otherwise it will be low. The smaller injection molding machine is not equipped with a die closing oil pressure gauge. At this time, it is necessary to judge whether the die is really closed according to the extension of the connecting rod.
If the connecting rod of a certain injection molding machine is easily straightened when closing the mold, or "almost" fails to straighten, or one of several connecting rods is not fully straightened, the mold will expand during injection molding, and the parts will have flash or other problems.
From the nozzle of injection molding machine, the plastic melt enters the mold cavity through the main flow channel, runner and gate. The population of the cavity is called the gate. In order to prevent the solidified cold material at the end of the nozzle from entering the mold cavity, a cold well should be designed at the end of the flow channel.
runner
The runner is an important channel from the sprue to the gate, and it is also the flow channel of the molten plastic from the injection machine nozzle. The runner shall be designed for low resistance and to prevent cooling. Usually, the runner is designed to be trapezoid or circular.
For multi cavity mold, in order to get good dimensional accuracy, the design of runner is very important. The following figure is a typical runner design of multi cavity mold.
Gate
Gate system design, such as location, number, geometry and size, is very important for production efficiency and dimensional accuracy. The functions of gates are summarized as follows:
Control the volume and direction of the plastic melt flowing into the cavity
Before curing, the melt is closed in the mold cavity and the melt is prevented from flowing back to the flow channel
Generated by heat due to viscous dissipation
Easy to cut off the flow channel and simplify the post-treatment of products
Classification:
The non limiting gate is called the straight gate, as shown in the figure below, the mold design of this gate form is simple, easy to operate, easy to form and reduce shrinkage. However, the molding cycle of this kind of gate becomes longer and defects such as crack, warpage and residual stress are easy to appear.
Because of the cross-sectional area of this gate, the limiting gate is designed to cure rapidly, and the advantages of this form of gate are as follows:
Due to the reduction of residual stress and deformation around the rabbet, the crack, warpage and deformation of the product are reduced;
Because the injection pressure in the cavity is reduced, a larger projection area of the product is allowed;
Because the gate closing time is shortened, the forming period is shortened;
Because of the elimination of post-processing, the quality of products is improved.
Here are six kinds of restrictive gates
① Side gate
The thickness of the side gate is usually 30% - 40% of the wall thickness of the product. At the same time, its width is about 3 times of the product wall thickness, and the side gate can be applied to almost all plastics. The composite gate and the spoke gate are the deformation design of the side gate.
② Fan gate
The section of fan gate is wide and flat, which effectively eliminates the defects of gate, and is often used for flat products.
③ Membrane gate
The above figure is a typical membrane gate design, the gate width is the same as the product width, but the thickness is much smaller, which can effectively eliminate the residual stress and deformation of the product as the fan gate.
④ Disc gate
A thin disc gate surrounds the disc-shaped or annular products to avoid the formation of weld marks. The annular gate is the deformation of the disc gate.
⑤ Pin gate
Pin gate is usually located in the middle of the product, and it is often used in multi-point gate. Because the gate diameter is usually 0.8-1.2mm, small cross-sectional area causes high flow resistance, it is recommended to use low viscosity plastic or high injection pressure to avoid under injection.
The characteristics of pin gate are as follows:
The gate location is not strictly selected
Low residual stress around gate
It is easy to achieve gate balance for multi-mode cavity mold
For products with large projection area, multi-point pin gate can effectively eliminate warpage of products
Pin gate is easy to cut, for three plate mold, automatic gate cutting is easy to achieve, easy to achieve separation of products and gates.
