Foam plastic is a thermoplastic or thermosetting resin as the matrix, which has a myriad of tiny pores inside the plastic. Foaming is one of the important methods of plastic processing, plastic foam obtained foam containing gas-solid two - gas and solid. Gas In the form of foam cells present in the foam, the cells and the cells are called closed cells isolated from each other, known as open pores, so there are closed-cell foam and open-cell foam. Open foam structure Or closed cell is determined by the raw material properties and its processing technology.
The technology of plastic foaming has a long history. The earliest foam bakelite in the early 1920s was made from a method similar to the production of foamed rubber; rigid polyurethane foam and polystyrene foam appeared in the 1930s; polyethylene, polyvinyl chloride in the 1940s. Epoxy resin, phenolic foam; in the 50's there were expandable polystyrene foams and flexible polyurethane foams.
Now, virtually all plastics, including both thermoplastic and thermosetting, can be foamed into foams. Industrial preparation methods include: extrusion foaming, injection foaming, molded foaming, calendering, powder foaming and Spray foaming and so on. Among them, injection foaming is one of the most important molding methods. Here focus on injection molding foaming.
Foaming principle
The foaming method of plastics can be divided into physical foaming method and chemical foaming method according to the different foaming agents used. First, the foaming agent is briefly introduced here.
Foaming agent
The foaming agent can be roughly divided into physical foaming agents and chemical foaming agents. The requirements for physical foaming agents are: non-toxic, odorless, non-corrosive, non-flammable, heat stable, and not under gaseous state. Chemical reactions take place. The rate of diffusion in the plastic melt in the gaseous state is lower than the diffusion rate in the air. Commonly used physical blowing agents are air, nitrogen, carbon dioxide, hydrocarbons, freon, etc.;
A chemical foaming agent is a substance that releases gases such as nitrogen, carbon dioxide and the like from heat energy. The requirements for a chemical foaming agent are: The gas released from its decomposition should be non-toxic, non-corrosive, and non-flammable. Forming and physical, chemical properties have no effect, the speed of release of gas should be able to control, foaming agent should have good dispersion in plastics. The widely used inorganic foaming agents such as sodium bicarbonate and ammonium carbonate, organic foaming Agents such as azoformamide and azobisisobutyronitrile.
Physical foaming
In simple terms, it is to use physical methods to foam plastics. There are generally three methods:
(1) The inert gas is first dissolved in a plastic melt or a paste under pressure, and then the gas is released after decompression, thereby forming pores in the plastic and foaming;
(2) Foaming by vaporizing the low-boiling liquid dissolved in the polymer melt to vaporize;
(3) adding hollow spheres in plastic to form a foam and foaming.
The physical foaming agent used in the physical foaming method has a relatively low cost, and in particular, the cost of carbon dioxide and nitrogen is low, and it can be flame-retardant and non-polluting. Therefore, the application value is high; and the physical foaming agent has no residue after foaming. The impact on the performance of foam plastics is not significant. However, it requires a dedicated injection molding machine and auxiliary equipment. The technical difficulty is very high.
Chemical foaming
Chemical foaming is the use of chemical methods to generate gas to foam plastics: The chemical foaming agent added to the plastic is heated to decompose and release the gas to foam; Alternatively, chemical interactions between the plastic components can also be used. The reaction releases gas and foams.
The process of foaming plastics injection using chemical foaming agents is basically the same as the general injection molding process. The heating of plastics, heating, mixing, plasticizing and most of the foaming expansion are done in injection molding machines.
In short, regardless of which plastic raw material is selected, no matter which foaming method is adopted, the foaming process generally involves the formation of a bubble nucleus, expansion of bubble nucleus, solidification of the foam body, and the like.
Structural foam molding
Structural foaming is a chemical foaming process. It is a revolution in 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 Product strength is not enough, production cycle is too long, molding rate is low.
In addition, the use of structural foaming technology can also be used to mold large and complex products. Using low-cost molds, multi-cavity cavities can be operated at the same time, thereby reducing product manufacturing costs. Structured foam products are a conjoined foam material with a dense surface layer. Its unit weight strength and stiffness are 3 to 4 times higher than that of unfoamed materials of the same kind.
The biggest feature of the structural foaming method is that it can be injection-molded without the need for additional equipment. An ordinary injection molding machine can be used for injection molding. However, the high-pressure structural foam injection molding machine using the cavity expansion method has a secondary increase compared with ordinary injection molding machines. Mold pressure device.
In recent years, structural foam injection molding technology has been widely developed, and there are many molding methods, but they can be divided into three types: low-pressure foaming method and high-pressure foaming method (Note: The low pressure and high pressure refers to the mold here. Cavity pressure); two-component foaming method.
Low pressure foaming
The difference between low-pressure foam injection molding and ordinary injection molding is that the cavity pressure of its mold is low, about 2~7Mpa, and ordinary injection molding is between 30~60Mpa. Low-pressure foam injection molding generally adopts the lack of injection method, that is to say a certain amount (not The plastic melt (containing the foaming agent) filled in the mold cavity is injected into the mold cavity. The gas that is decomposed by the foaming agent expands the plastic and fills the mold cavity. The low-pressure foam injection molding on the ordinary injection molding machine generally involves chemical emission. The foam is mixed with the plastic and plasticized in the barrel. The self-locking nozzle must be used.
At the time of injection, due to the rapid diffusion of gas, the surface of the product will be rough. Therefore, the injection speed of the injection molding machine is fast enough. Generally, the supercharger is used to increase the injection speed and injection volume, and the injection action is completed in an instant.
High pressure foaming
High-pressure foaming injection mold cavity pressure between 7 ~ 15Mpa, using full injection method, that is an injection volume is exactly equal to the volume of the mold cavity. In order to make the parts get foam expansion, you can use forced expansion of the cavity, or Part of the plastic out of the cavity. Generally used more cavity expansion method.
Compared with ordinary injection molding machines, the injection molding machine adopting the enlarged cavity method has increased the secondary mold clamping pressure device. When the molten mixture of plastic and foaming agent is injected into the mold cavity, it is delayed for a period of time, and then the mold clamping mechanism is The movable plate moves backwards for a short distance, so that the moving mold and the fixed mold of the mold are slightly separated, the cavity is enlarged, and the plastic in the cavity begins to foam and expand.
After cooling, the product forms a dense skin on its surface. Since the foaming and expansion of the plastic melt is controlled by the moving platen, it is also possible to control the thickness of the dense skin of the product. The movement of the moving platen can be an overall movement. It can be partly moved to make partial foaming, so as to obtain products with different densities. The high-pressure foaming method has high requirements for the manufacturing precision of the mold, high mold cost, and has a secondary mold-locking pressure maintenance requirement for the injection molding machine.
Two-component foaming
Two-component foam injection molding is a special high-pressure structural foam injection molding method. It uses a special two-component injection molding machine. This injection molding machine has two injection molding devices: One set of cores for injection molding products, one for injection molding. The surface layer of the product. During the injection molding, the skin material is injected first, and then the core material mixed with 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 and the cavity is completely filled. After the frit fills the cavity, a small amount of foam material is not injected. The frit seals the gate. After the product is removed, the gate is removed and the lighter product with a non-foamed dense skin and core foam is obtained.
Microcellular foam molding
Microcellular foaming is a physical foaming method. The cell diameter of conventional foams is generally greater than 50mm, and the cell density (the number of cells per unit volume) is less than 106 cells/cm3. When these large-sized cells are stressed It often becomes the birthplace of initial cracks and reduces the mechanical properties of the material.
Injection molding is one of the main molding methods of microporous plastic products. After the plastic raw material is added to the barrel of the injection molding machine, the screw plastic force is applied under the screw shearing force and the heating of the heating coil, and the blowing agent is directly injected into the end of the melting section of the injection screw. The melt is evenly mixed and then high pressure and high speed are injected into the mold cavity.
A sudden pressure drop in the cavity causes a large amount of supersaturated gas in the melt to separate out, foam, swell, and form to form a microporous plastic product. In addition, a nucleus can be formed by changing the temperature, and the pressure can be changed. Compared with the method, it is easier to control, but it is not suitable for gas-insensitive plastics.
The development of microcellular plastics injection molding technology is relatively difficult because the size of cells is much smaller than that of conventional foams. To obtain good microcellular plastic products, it is necessary to ensure that supercritical fluids enter the melt in the barrel. Accurate measurement requires that the plastic melt must be fully mixed, homogenized, and dispersed to form a homogeneous mixture, to ensure that the nucleation point in the melt must be more than 109/cm3, to control the nucleation bubble expansion, etc. in time. The requirements of the equipment itself and the injection molding process parameters are very high.
The characteristics of using this technology are:
Product weight reduction by approximately 50%;
Injection pressure reduced by about 30~50%;
Reduced clamping force by 20%;
The cycle period is reduced by 10~15%;
A gas input device with a metering control device is used.
Conclusion:
Due to the presence of air bubbles, foamed plastics are lightweight, save material, can absorb shock loads, perform heat insulation and sound insulation, have higher specific strength, and can save material, reduce energy consumption, and reduce costs. Can vigorously promote the application.
In addition to the advantages of the above-mentioned general foams, microcellular foamed plastics also have more excellent mechanical properties. Therefore, the range of applications for foamed plastics is very wide, especially in the production of automobiles, aircrafts and various transport equipment and other fields. Application value. It can be expected that foam injection molding, especially microcellular foam injection molding, will be a hot spot for the processing of plastic processing machinery and products.