Injection molding has been one of the main molding methods for plastic products in the past, present and future, and is currently developing in the direction of energy saving, precision molding, noise reduction and high automation. Various problems affecting the molding process and product performance are gradually being solved, and injection molding machines and molds are also more designed and manufactured using CAD/CAM/CAE technology. The equipment level will be further trended towards serialization, standardization, scientific and generalization. Some new processing technologies and equipment, such as gas-assisted injection molding technology and hot runner mold technology, have also appeared to improve the molding process and product performance.
Gas-assisted injection molding technology
Gas-assisted injection molding technology dates back to 1971 when Americans attempted to use a gas-filled injection molded hollow heel to obtain a patent. In 1983, the British derived the "Cinpres" to control the internal pressure molding process from the structural foam molding machine room decoration materials. The process was quickly adopted as a new process at the 1986 German International Plastics Machinery Exhibition and was called the future technology of the plastics processing industry. Since its practical application in the 1980s, it has revolutionized the traditional injection molding process and has attracted more and more attention. At present, almost all thermoplastics and some thermosetting plastics used for injection molding can adopt gas-assisted injection molding technology. This technology has significant advantages in saving raw materials, preventing sink marks, shortening cooling time, improving surface quality, reducing stress and deformation in products, reducing clamping force, improving production efficiency, and reducing production costs. It breaks through the limitations of traditional injection molding and can be flexibly applied to the molding of a variety of parts.
For example, the use of the filling stage to inject gas can save raw materials, reduce costs, and shorten the cooling time. It can produce columnar parts (such as clothes racks, ax handles, beach chair legs, etc.); it can reduce the clamping force and reduce The advantages of warping deformation and improving the surface quality of the workpiece can produce flat-shaped parts with built-in air passages (such as automobile dashboards, household appliances, large furniture, etc.); Advantages of molding, processing of thick, thin-walled integrated parts (such as car window chutes, office and stationery) and so on. The disadvantages of gas-assisted injection molding technology are: the cost of special equipment is high, the vent hole may cause the surface quality of the workpiece, the clamping force required for processing some parts is large, and the gas-assisted injection molding mold The design and manufacture require additional consideration of airway settings and the like. In addition, many control parameters have been added to the process, such as gas pressure, melt injection volume, delay between melt-gas injection, gas holding time, etc., for lack of gas-assisted injection molding, mold design and manufacturing. Experienced engineers and technicians have certain difficulties.
The basic working principle of gas-assisted injection molding is to inject molten plastic into the mold cavity by the action of gas, and use the expansion of the pressurized gas in the plastic melt to form a hollow section when the shape is intact. Since the melt temperature near the surface of the mold is low and the viscosity is high, the melt temperature at the center of the mold cavity is high, and the dryness is low, so that the gas forms a cavity in a thick portion of the plastic part (such as a rib). The surface molten plastic is continuously filled toward the end of the mold by the gas to form a part to be formed.
The main process of gas-assisted injection molding can be divided into three stages: melt injection, that is, polymer melt injection into the mold cavity. This process is the same as traditional injection molding, generally 60% to 97% of the cavity filled with the cavity. Product injection); gas injection, that is, injection of high-pressure high-purity nitrogen into the melt core, the melt flow front continues to flow forward under high-pressure gas turbulence until it fills the entire cavity; gas holding pressure, that is, maintaining gas pressure In the case of cooling the workpiece, during the cooling process, the gas is pressed from the inside to the outside to ensure that the outer surface of the product is close to the mold wall, and the volumetric shrinkage caused by the solidification of the melt is replenished from the inside through the secondary penetration of the gas. The gas is released and recycled for recycling, the mold cavity is opened, and the workpiece is taken out.
The equipment required for gas-assisted injection molding is to use a conventional injection molding machine with a gas injection unit. The gas injection device is composed of a gas pressure preparation system, an air nozzle and a special air pressure control system. The gas from the industrial cylinder is pre-compressed through the second plunger cylinder, the maximum capacity of the gas contained in the cylinder can fully meet the pre-inflation requirement, and the gas is compressed in the second plunger cylinder. The high pressure required to inflate. The entire gas preparation process does not have any adverse effect on the injection cycle, as it occurs simultaneously with the metering process in injection molding and does not take up the injection time of the equipment.
Hot runner technology
Hot runner technology is an advanced technology applied to plastic injection molding runner systems. The hot runner element is equivalent to extending the spray of the injection molding machine to the feed port of the mold cavity, and heating and precise temperature control by the electric heating element, so that the plastic in the flow channel system is always in a molten state during the injection molding process, so that the injection pressure is generated. Minimal loss, injection molding without condensate. Theoretical research and practice at home and abroad have shown that the use of hot runner technology in injection molding molds plays an important role in improving the quality, material saving, energy saving and production efficiency of plastic products. Therefore, hot runner injection molding will gradually replace ordinary flow injection molding.
The working principle of the mold is as follows: the molten material in the injection molding machine is injected into the mold cavity through the hot runner nozzle. After the plastic is cooled and shaped, the movable mold is opened, and the push rod of the injection molding machine drives the unloading plate to remove the plastic product from the punch. .
The hot runner technology system is mainly composed of a hot runner component, an electric heating component and a temperature controller. The structural form of the hot runner element is mainly divided into a thermal spray element for a single cavity mold and a hot runner assembly for multi-cavity or single-cavity multi-point feed. The hot runner assembly typically consists of a hot runner plate and a hot nozzle. Commonly used thermal nozzles are available in external thermal nozzles and internal thermal nozzles. The external heating nozzle heating coil is outside the nozzle, and the thermocouple is installed in the middle of the nozzle. The structure is simple, the temperature control is accurate, the flow passage is smooth and there is no resistance, and the melt pressure loss is small. The ceramic heat insulation ring makes the heat conduction only 7% of the steel, and the contact surface with the mold is only 6% of the whole, and the heat loss is small. The use of a dot gate allows the residual stress inside the article to be extremely low. Wide range of use, can be used for plastics with poor fluidity. The electric heating element of the internal heat type nozzle is installed in the torpedo body, and the molten resin passes through the flow path outside the torpedo, and the heating efficiency is high. With a point gate, the tip of the torpedo remains at the center of the gate, which facilitates the cutting of the gate after injection and allows the residual stress inside the product to be extremely low. Adaptable to a wide range, but the structure is complex. There is also a valve nozzle that prevents runny and is suitable for high flow resins. The hot runner plate is generally a steel plate heated by a rod-shaped electric heating element, and a flow passage is opened therebetween, and the molten resin enters the mold cavity through the passage and the hot nozzle. The hot runner plate is reliable in operation, has small melt pressure loss, and is easy to replace the electric heating element.
