Main parts of injection molding machine
There are many types of plastic injection devices for injection molding machines, but plasticizing with screws is common to them. Different types of foam injection molding machines have different functions and structures of screws. Some screws have only plasticizing functions, and their operation can be continuous or intermittent; some screws have plasticizing and pressing. Double function, screw operation can only be intermittent; the use of physical foaming agent and the use of chemical foaming agent on the screw structure requirements are also different. Chemical foaming is generally done by first mixing the chemical foaming agent with other raw material components before adding it to the plasticizing cylinder for plasticization. Physical foaming is often used to directly inject the foaming agent from the front end of the barrel. In the plastic melt. For this purpose, the screw should have a special structure not only in the injection section, but also a mixing element to sufficiently mix the blowing agent with the plastic melt.
The nozzle of the injection foam molding machine should meet the following special requirements. The nozzle controls the opening and closing and minimizes flow resistance. Generally, the closed nozzles are controlled separately. In order to reduce the accumulation of materials, it is more suitable to use needle valves to lock the nozzles. Some nozzles also have a mixing function. In general, the nozzles have various types of structures, and the quality of the foamed products is obvious. influences.
When the foam is injected, the delayed opening of the nozzle has a significant effect on the quality performance of the product. The so-called delayed opening nozzle means that the injection nozzle is delayed when the injection pressure is started to be applied to the plastic melt. The effect of the delayed amount on the foamed product is shown in the figure. The effect of the time delay on the tensile strength of the foamed article is shown in (1) of the figure. In the figure, the horizontal coordinate is the time when the plastic melt is compressed before injection, that is, the time when the nozzle is delayed. It can be seen from the figure that the product with long delay has stable performance, the quality is relatively uniform, and the low limit strength is improved. Figure (2) shows the effect of the delayed opening nozzle on the uniformity of the skin thickness, and the horizontal coordinate is the dimension of the product in the flow direction. It is obvious that the delay can improve the uniformity of the thickness of the skin layer. The figure (3) shows the effect of the delayed opening nozzle on the smoothness of the cortical surface. It is obviously delayed by about 0.5s, the improvement effect is obvious, and the effect is further increased.
There are many types of nozzles. The figure on the right shows the most general controllable lock type nozzle. The core frame in the nozzle is provided with a core rod that can move axially, that is, a thimble or a needle valve stem. The nozzle hole is closed, the movement of the core rod is controlled by a joystick, and the joystick is driven by a hydraulic system or a linkage mechanism. When the injection is not performed, the core rod is moved forward, and the nozzle outlet, that is, the nozzle hole, is blocked, so that the plastic melt in the cylinder cannot flow out even under a high pressure. When injecting, the mandrel is pushed back by the lever and the nozzle outlet is opened to allow the plastic melt to flow out. The action of the joystick can be linked with the action of the injection plunger, so that the control is relatively reliable.
The core rod is also controlled by a spring, which is called a spring-type locking nozzle. It is less used because of the large resistance and difficult to control when the material flows.
Multi-cavity or multi-gate molds often use a multi-nozzle mechanism. In order to prevent the foam accumulated in the nozzle from foaming, a needle valve should be provided for each nozzle to control the opening and closing of the nozzle opening, and the overall transmission can be adopted for the sake of simplicity. The hot runners can also be used by moving the respective core rods together and opening and closing the nozzle openings.
The above picture shows an injection nozzle with a mixing function. The main channel is divided into two sections. Cone, the small end diameter is about 6mm, the big end diameter is about 9mm. When the melt flows from the small flow passage to the large flow passage, the cross-sectional area increases sharply, and the melt produces a strong expansion process. Here, the foaming agent is injected through some radial gaps. It is intensively mixed with the expanded melt to form a plurality of vesicles; the slanting rods in the nozzle control the inlet and the inlet so that they open and close simultaneously.
The mold is a molding and shaping equipment for the production of plastic products. The mold used in the injection foam molding machine is similar to the mold used in the ordinary non-injection foam molding machine. It is also composed of two parts, a movable mold and a fixed mold, including a concave mold. The design principles of common molds such as punch, guide, demoulding, runner, exhaust and heating and cooling are also applicable to foaming molds, but the latter mold must also meet the special requirements of foam molding process. . For example, in order to increase the expansion ratio of the foamed product, the uniformity of the density distribution, the surface quality of the product, etc., it is necessary to increase the filling speed of the melt and shorten the flow. For this reason, the pouring system and the row of the injection foam molding die Gas systems, heating and cooling systems, and the shape and size of the cavities all place more demands.
When selecting a mold material for injection foam molding, it is necessary to select a suitable material according to the working conditions and requirements of the mold. For example, for the mold for low-pressure injection molding, since the cavity pressure is relatively low during molding and shaping, the plastic melt does not need to be filled with the cavity, so the injection pressure is not directly transmitted to the mold wall, and the mold wall only bears the plastic melt. The expansion force of the bubble is much lower than that of the cavity when the ordinary injection is not foamed. The cavity pressure is 30-60 MPa for ordinary injection molding, and the cavity pressure is only 2 for low-pressure injection molding. ~7MPa. Therefore, the mold for low-pressure injection molding can use materials with relatively low strength and hardness, which reduces the cost. When high pressure injection molding is used, the plastic melt must be filled into the cavity. The injection pressure will be transmitted directly to the cavity wall. However, it is not exactly the same as ordinary non-foaming injection molding. Its melt does not need to be compacted, and the pressure in the cavity is the highest in the compaction stage. Therefore, the mold for foam molding is injected by high pressure method. Although the cavity pressure is higher than the low pressure method, it is lower than the ordinary non-foaming injection molding, generally taking 7-15 MPa, and some may take higher, mainly depending on the filling process, so the material of the high pressure method injection molding mold Materials with certain strength and hardness requirements should be used. In addition, the number of batches of foamed products, the corrosiveness of the product materials, the complexity of the shape of the products, the accuracy of the dimensions, and the surface finish are all the basis for selecting materials. Since the thermal conductivity of the foam is low and the wall thickness of the product is relatively large, the cooling condition must be intensified in order to shorten the cooling time. For this reason, a material having a large thermal conductivity and a small specific heat capacity should be selected as the mold material, and the ordinary injection can be selected when selecting the material. The principle of selection of molds.