Foamed plastics are thermoplastic or thermosetting resins with a myriad of tiny pores inside. Foaming is one of the important methods of plastic processing. The foam obtained by plastic foaming contains gas-solid two-gas and solid. The gas is present in the foam in the form of cells, and the cells are separated from the cells by a closed cell, and the connected cells are called open cells, so that there is a closed cell foam and an open cell foam. The open or closed cells of the foam structure are determined by the properties of the raw materials and their processing.
Principle of foam molding
The foaming method of plastics can be divided into two categories: physical foaming method and chemical foaming method depending on the foaming agent used. Here is a brief introduction to the blowing agent.
Foaming agent
The foaming agent can be simply and roughly classified into two types: physical foaming agent and chemical foaming agent. The requirements for physical foaming agents are: non-toxic, odorless, non-corrosive, non-combustible, good in thermal stability, no chemical reaction in the gaseous state, and the diffusion rate in the plastic melt in the gaseous state is lower than in the air. Diffusion speed. Commonly used physical foaming agents are air, nitrogen, carbon dioxide, hydrocarbons, freons, etc.; chemical foaming agents are substances that release gases such as nitrogen, carbon dioxide, etc. by heat, and the requirements for chemical blowing agents are: The gas released by decomposition should be non-toxic, non-corrosive, non-combustible, have no effect on the molding and physical and chemical properties of the product, and the speed of releasing the gas should be controlled. The foaming agent should have good dispersibility in the plastic. A wide range of inorganic foaming agents such as sodium hydrogencarbonate and ammonium carbonate, and organic foaming agents such as azoformamide and azobisisobutyronitrile are widely used.
Physical foaming
Simply put, it is the use of physical methods to foam plastic, there are generally three ways:
(1) first dissolving the inert gas in a plastic melt or paste under pressure, and then releasing the gas under reduced pressure to form pores in the plastic to foam;
(2) foaming by vaporizing a low-boiling liquid dissolved in a polymer melt to evaporate;
(3) A hollow ball is added to a plastic to form a foam to foam or the like.
The physical foaming agent used in the physical foaming method has relatively low cost, especially the low cost of carbon dioxide and nitrogen, and is flame-retardant and non-polluting, so the application value is high; and the physical foaming agent has no residue after foaming. It has little effect on the properties of foamed plastics. However, it requires a dedicated injection molding machine and auxiliary equipment, which is technically difficult.
Chemical foaming
The chemical foaming method uses a chemical method to generate a gas to foam the plastic: heating the chemical foaming agent added to the plastic to decompose and release the gas to foam; and it is also possible to utilize the chemical interaction between the plastic components. The gas released by the reaction is foamed.
The process of injection molding of a foamed plastic using a chemical foaming agent is basically the same as that of a general injection molding process. Heating, heating, mixing, plasticizing and most of the expansion of the plastic are done in an injection molding machine.
In short, no matter which kind of plastic raw material is selected, no matter which kind of foaming method is adopted, the foaming process generally has to pass through the stages of forming bubble nuclei, bubble nuclei, and foam solidification.
Structural foam molding
The structural foaming method belongs to the chemical foaming method, which is a revolution in the injection molding process technology. It retains many of the advantages of traditional injection molding processes and avoids some of the problems encountered in traditional injection molding processes, such as insufficient product strength, long production cycles, and low molding rates. In addition, the structural foaming technology can also mold large and complex products, use low-cost molds, and multi-cavity can be operated simultaneously, thereby reducing the production cost of the products. The structural foamed product is a continuous foamed material having a dense surface layer, and its unit weight strength and rigidity are 3-4 times higher than that of the same unfoamed material.
The biggest feature of the structural foaming method is that it can be injection-molded by an ordinary injection molding machine without adding equipment, but the high-pressure structural foam injection molding machine which uses the cavity expansion method to foam is more secondary than the ordinary injection molding machine. Mold holding device. In recent years, structural foam injection molding technology has been widely developed, and there are many molding methods, but it can be divided into three types: low pressure foaming method; high pressure foaming method (Note: low pressure and high pressure finger mold here) Pressure in the cavity); two-component foaming method.
Low pressure foaming
The difference between low pressure foaming and ordinary injection is that the cavity pressure of the mold is low, about 2-7Mpa, and the ordinary injection is between 30-60Mpa. Low-pressure foam injection molding generally adopts a low-injection method, that is, a certain amount (not filled with a cavity) of a plastic melt (containing a foaming agent) is injected into a cavity, and the gas decomposed by the foaming agent expands the plastic to fill the cavity. Low-pressure foam injection molding is carried out on a common injection molding machine. Generally, a chemical foaming agent is mixed with plastic and plasticized in the barrel, and a self-locking nozzle must be used. At the time of injection, since the diffusion speed of the gas is fast, the surface of the product is rough, so the injection speed of the injection molding machine is fast enough. A supercharger is generally used to increase the injection speed and the amount of injection, so that the injection action is completed in an instant.
High pressure foaming
The injection cavity pressure of the high pressure foaming method is between 7-15 MPa, and the full injection mode is adopted, that is, the injection amount is exactly equal to the volume of the mold cavity. In order to achieve foam expansion of the article, it is possible to force the cavity to be enlarged or to separate a portion of the plastic from the cavity. The cavity expansion method is generally used more often.
Compared with the ordinary injection molding machine, the injection molding machine using the enlarged cavity method adds a secondary mold clamping device, and the molten mold mixture of the plastic and the blowing agent is injected into the cavity for a period of time, and then the clamping mechanism is The movable template is moved backward by a small distance, so that the movable mold and the fixed mold of the mold are slightly separated, the cavity is enlarged, and the plastic in the cavity starts to expand and expand.
After the product is cooled, a dense skin is formed on the surface thereof, and since the foam expansion of the plastic melt is controlled by the movable template, the thickness of the dense surface layer of the product can be controlled. The movement of the movable template may be an overall movement, or may be partial movement to partially foam, thereby obtaining articles of different densities. The high-pressure foaming method has high requirements on the manufacturing precision of the mold, high mold cost, and the secondary mold clamping pressure requirement for the injection molding machine.
Two-component foaming method
Two-component foam injection molding is a special high-pressure foam injection molding method that uses a special two-component injection molding machine. The injection molding machine has two injection molding units: a core for injection molding and a surface for injection molding. In the injection molding, the skin material is first injected, and then the core material mixed with the foaming agent is injected through the same gate. Since the core material flows in a laminar flow state, this ensures that the core material is uniformly coated inside the skin layer, so that the cavity is completely filled. After the melt fills the cavity, a small amount of melt containing no blowing agent is injected to close the gate. After the product is taken out, the gate is removed to obtain a lightweight product having a non-foamed dense surface layer and a core-foamed portion. Microcellular foam molding
Microcellular foam molding is a physical foaming method. Conventional foams generally have a cell diameter greater than 50 mm and the cell density (the number of cells per unit volume) is less than 106 cells/cm3. These large-sized cells often become the source of initial cracks when stressed, reducing the mechanical properties of the material. In order to meet the industrial requirements for lowering the cost of certain plastic products without reducing their mechanical properties, injection molding is one of the main forming methods for microporous plastic products. After the plastic raw material is added into the barrel of the injection molding machine, it is plasticized under the action of the screw shearing force and the heating of the heating coil. The foaming agent is directly injected into the melting section of the injection screw and uniformly mixed with the melt, and then injected into the cavity with high pressure and high speed. Sudden depressurization in the cavity causes a large amount of supersaturated gas in the melt to be separated, foaming, expanding, forming, and shaping to form a microporous plastic product. In addition, it is also possible to form a nucleus by changing the temperature, which is easier to control than the pressure changing method, but is not applicable to a plastic whose gas solubility is not temperature sensitive.
It is more difficult to develop micro-hole plastic injection molding technology because the size of the cell is much smaller than that of conventional foam. To obtain a good micro-porous plastic product, it is necessary to ensure the supercritical fluid entering the melt of the barrel. Accurate metering requires that the plastic melt must be thoroughly mixed, homogenized, and dispersed to form a homogeneous mixture, ensuring that the nucleation point in the melt must be more than 109/cm3, and the expansion of the nucleation bubble is controlled in time. This requires very high requirements for the equipment itself and the injection molding process parameters.
The features of this technology are:
The weight of the product is reduced by about 50%;
The injection pressure is reduced by about 30~50%;
The clamping force is reduced by 20%;
The cycle time is reduced by 10~15%;
A gas input device with a metering control is used.
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