1. gas (water) assisted injection molding
Since the advent of reciprocating screw injection molding machine, gas-assisted injection molding is one of the most important developments of injection molding technology. It uses high-pressure gas to produce hollow cross-section in injection molding parts, which reduces residual internal stress of products, eliminates surface shrinkage marks of products and reduces materials, showing incomparable advantages of traditional injection molding. The process of gas-assisted injection mainly includes three stages: the initial stage is melt injection. At this stage, plastic melt is injected into the cavity, which is the same as traditional injection molding, but the melt only fills 60%-95% of the cavity, and the specific injection amount varies from product to product. The second stage is gas injection. At this stage, high-pressure inert gas is injected into the melt core, and the melt front continues to flow forward under the drive of gas pressure until the whole cavity is filled. In gas-assisted injection molding, the melt flow distance is obviously shortened and the melt injection pressure can be greatly reduced. Gas can enter the workpiece directly from the cavity from the main channel or through the gas injection element. Because gas always permeates in the direction with the lowest resistance (high temperature and low viscosity), it is necessary to specially design gas channels in the mold. The third stage is gas pressure maintaining. At this stage, the parts are cooled while maintaining the gas pressure. Further, the pressure transmission characteristics of gas are used to press the parts evenly outward, and the volume shrinkage (secondary infiltration) caused by melt cooling and solidification is supplemented by gas expansion to ensure that the outer surface of the product is close to the die wall.
Gas-assisted technology makes it possible to use injection molding for many parts that cannot be injected by traditional technology. It has been widely used in almost all fields of plastic parts such as automobiles, household appliances, furniture, electronic devices, daily necessities, office automation equipment, building materials, etc. As a challenging new technology, it opens up a new application field for plastic molding. Gas-assisted technology is particularly suitable for manufacturing injection molded products in the following aspects:
1) tubular and rod-shaped products: such as handles, hooks, chair handrails, shower heads, etc. The hollow structure can be used without affecting the function and performance of the product; Greatly saves raw materials, shortens cooling time and production cycle.
2) Large flat products: such as automobile dashboard, inner grille, parabolic satellite antenna for foreign military and commercial machines, etc. By setting the air passage in the part, the rigidity and surface quality of the product can be significantly improved, the warping deformation and surface depression can be reduced, and the clamping force can be greatly reduced, so that larger parts can be molded with smaller equipment.
3) Complex structural products with thick and thin walls: such as TV sets, computers, printer shells, internal supports and external decorations. This kind of products can't be molded at one time by traditional injection molding process. The adoption of gas conveying technology improves the freedom of mold design and is beneficial to the integration of parts. For example, the number of internal brackets and external decorative pieces required for Panasonic 74cm TV case is reduced from 17 in conventional injection molding technology to 18, which can greatly shorten the assembly time.
Water-assisted injection molding is a new technology developed by IKV company on the basis of gas-assisted injection molding technology. It uses water instead of nitrogen to assist the flow of the museum, and finally uses compressed air to squeeze water out of the parts. Compared with gas-assisted injection molding, water-assisted injection molding can obviously shorten the molding time and reduce the wall thickness of products. It can be applied to any thermoplastic, including those with low molecular weight and easy to blow through, and can produce rod-shaped or tubular hollow products with large diameter (above 40mm). For example, for parts with a diameter of 10mm, the production cycle can be reduced from 60s to 10s (wall thickness L-00 s). For parts with a diameter of 30mm, the production cycle can be reduced from 180s to 40s (with a wall thickness of 2.5 ~ 30 mm).
IKV and Ferromatik Milacron are currently perfecting the prototype, and other gas-assisted injection molding manufacturers such as Baitenfeld and Engel have recently joined the development team. Water-assisted injection molding is mainly used to produce important medium conduits with smooth inner surfaces. Its quality and economic benefit are not as good as gas-assisted injection technology.
2. Mold sliding injection molding
Mold sliding injection molding is a two-step injection molding method developed by Japan Steel, which is mainly used to manufacture hollow products. The principle is that the hollow product is divided into two parts and injected into semi-finished products respectively. Then, slide the semi-finished product and the mold to the matching position, and close the mold for the second time. Then the plastic melt is injected into the joint of the two parts of the product (twice injection), and finally a complete hollow product is obtained. Compared with blow-molded products, this product has the advantages of good surface accuracy, high dimensional accuracy, uniform wall thickness and great design freedom. Compared with the traditional secondary molding method (such as ultrasonic welding), the mold sliding injection molding method has the advantages that the semi-finished product does not need to be taken out of the mold, so the problem that the semi-finished product is cooled outside the mold and the shape accuracy of the product is reduced can be avoided; This can also avoid the problem that the welding strength decreases due to local stress in the secondary welding method.
3. Melt core injection molding
When injection molding plastic parts that are difficult to demould in structure, such as hollow plastic parts with complex shapes such as automobile oil pipes, intake and exhaust pipes, are generally divided into two halves and then assembled, resulting in poor sealing performance of plastic parts. With the more and more applications of this kind of plastic parts, people introduce the melting core molding process similar to lost wax casting into injection molding, forming the so-called melting core injection molding method.
The basic principle of molten core injection molding is: first, cast a fusible core with low melting point alloy, then put the fusible core as a part into the mold for injection molding, take the part containing the core out of the mold cavity after cooling, and then heat it to melt the core. In order to shorten the melting time of the core and reduce the deformation and shrinkage of plastic parts. Generally, oil and induction coil are heated at the same time. Induction heating melts the fusible core from the inside out, and oil heating melts the alloy surface layer remaining on the inner surface of plastic parts.
Fused core injection molding is especially suitable for complex, hollow and unsuitable composite products. Compared with blow molding and gas-assisted injection molding, this molding method can make full use of existing injection molding machines, although it requires more molds and equipment for casting fusible cores and equipment for melting cores, and has greater molding freedom.
In molten core injection molding, parts are manufactured around the core. The core part is dismantled immediately after it is made, which seems to be similar to the practice of traditional basic industries, and it is not novel. However, the key issue lies in the core material. Traditional materials cannot be used as the core of plastic processing. First of all, it is not hard enough to keep its shape in the molding process, especially it can't withstand the impact of pressure and melting. More importantly, the accuracy will never meet the requirements of plastic products. Therefore, the key is to find a suitable core material. At present, tin bismuth and tin lead alloys with low melting point are commonly used.
Fused core injection molding has developed into a special branch of injection molding. With the demand for polymer materials in the automotive industry, some parts have been mass-produced. For example, the handle of a tennis racket is the first mass-produced fused core injection product. The all-plastic multi-head integrated intake pipe of automobile engine has been widely used; Other new applications include: automobile water pump, water pump propeller, centrifugal hot water pump, spacecraft oil pump, etc.
4。 Controlled low pressure injection molding
The traditional injection molding process can be divided into the filling process of controlling the melt inlet speed and the pressure maintaining process of controlling the melt inlet pressure to supplement the cooling shrinkage of plastics. During the filling process, the entrance velocity of the melt is constant. With the filling process, the flow resistance of the melt in the mold cavity gradually increases, so the inlet pressure of the melt is easy to increase, and the inlet pressure has a high peak at the end of filling. Due to the high pressure in the cavity, not only will the molten material overflow, mold expansion and other undesirable phenomena be caused, but also greater internal stress will be generated in the plastic parts. Plastic parts are easy to warp and deform after demoulding, which makes it difficult to meet higher requirements for shape accuracy and size accuracy of plastic parts, and it is also easy to crack during use.
In order to reduce or avoid the internal stress caused by excessive cavity pressure during the filling process of plastic, the deformation of plastic parts should be limited to a lower range, and the plastic parts should be filled at the lowest pressure required for filling, thus reducing the pressure in the cavity. The main difference between controlled low-pressure injection molding and traditional injection molding is that the filling stage of traditional injection molding controls the injection rate, while the filling stage of low-pressure injection molding controls the injection pressure. In the process of low-pressure injection, the inlet pressure of the cavity is constant, but the injection rate changes. At the beginning, the injection is carried out at high speed, and with the extension of injection time, the injection rate gradually decreases, which can greatly eliminate the internal stress of plastic parts and ensure the accuracy of plastic parts. In the process of high-speed injection, the shear viscous heat generated by high-speed melt flow can increase the melt temperature and reduce the melt viscosity, making it possible for the melt to fill the cavity at low pressure. Because low-pressure injection is based on constant pressure to fill the melt, low-pressure injection machine has its own unique oil pressure system.
In order to realize low pressure and high speed molding, it is necessary to improve the injection system of traditional injection molding machine. At present, a multi-cavity hydraulic injection system has been developed abroad, and its main functions are:
1) The maximum injection pressure can be changed in multiple stages under the same oil pressure;
2) High-speed injection can be performed at low injection pressure.
Because the basic principle of low-pressure injection molding is the same as that of general injection molding, the mold structures used in the two molding methods are exactly the same. The low-pressure filling in low-pressure injection molding can avoid the fracture or damage of the small core and improve the service life of the mold. On the other hand, low-pressure injection molding has little wear on the mold, and the requirements for temperature control and exhaust of the mold are not very high. Simple zinc-aluminum alloy injection mold can be used, which can not only reduce the production cost, but also quickly produce small-batch precision plastic parts, meeting the needs of multi-variety and small-batch production in the market at present.
5。 injection compression moulding
This molding process was developed for molding the surface of optical lenses. The molding process is as follows: the mold is closed for the first time, but the moving mold and the fixed mold are not completely closed, leaving a certain compression gap, and then the melt is injected into the cavity; After the melt injection is completed, a special clamping piston is used to realize two intersecting molds. In the process of completely closing the mold, the melt in the cavity flows again and is compacted.
Compared with general injection molding, injection-compression molding has the following characteristics:
1) melt injection is carried out under the condition that the mold cavity is not completely closed, so the runner area is large, the flow resistance is small, and the required injection pressure is also small.
2) Melt shrinkage is compensated by applying pressure to the outside of the cavity to reduce the size of the cavity (the cavity directly compresses the melt), so that the pressure distribution in the cavity is uniform.
Therefore, injection compression molding can reduce or eliminate the molecular orientation and internal stress caused by filling and packing, improve the uniformity of product materials and dimensional stability of products, and reduce the residual stress of plastic parts. Injection compression molding technology has been widely used to mold plastic optical lenses. High-precision plastic parts, such as optical discs and thin-walled plastic parts, are difficult to inject. In addition, the application of injection compression molding in glass fiber reinforced resin molding is becoming more and more common.
6. Shear controlled orientation injection molding
The essence of shear-oriented injection molding is to exert dynamic pressure on the melt through the gate, so that the polymer melt in the mold cavity produces vibration and shear flow, and under its action, the molecular chains or fibers in different melt layers solidify in the product, thus controlling the internal structure and micro-morphology of the product and achieving the purpose of controlling the mechanical properties and appearance quality of the product. There are two ways to introduce vibration into the mold cavity: screw and auxiliary device to increase vibration.
1) spiral vibration
The working principle of screw vibration is to provide pulsating oil pressure to the injection cylinder to make the injection screw move back and forth to realize vibration. The vibration generated by the injection screw acts on the melt and is transmitted to the mold cavity through the polymer pavilion, so that the melt in the mold cavity vibrates, and this vibration can last until the mold is closed. This device is relatively simple and can be realized by using the control system of injection molding machine or by reforming the hydraulic and electrical control system of injection molding machine.
2) The auxiliary device vibrates, that is, the auxiliary device vibrates by installing a vibrating device between the mold and the nozzle of the injection molding machine. In the injection stage, like general injection molding, usually the melt only passes through one gate, and the piston of this gate retreats to keep the flow channel unobstructed, and the other piston cuts off the other flow channel; After the cavity is full, the two pressure maintaining pistons start to vibrate at the same frequency driven by independent hydraulic system, but the phase difference is180o. Through the reciprocating motion of the two pistons, the vibration is transmitted to the cavity, so that the melt in the cavity is cooled and the vibration shear flow is generated at the same time. Experiments show that this process is helpful to eliminate the common defects of products (such as shrinkage cavity, crack, surface depression, etc.). ) and improve the weld strength; The orientation of molecules or fibers can be controlled by shear control orientation molding technology, and products with higher strength than ordinary injection molding products can be obtained by reasonably setting the position and number of gates.
In the process of shear-controlled orientation injection molding, after polymer melt is injected into the mold cavity, a solidified layer begins to appear in the mold cavity. Because the velocity gradient near the solidification layer is the largest, the melt is strongly sheared and the orientation degree is the largest. The velocity gradient near the central layer is small and the shear effect is small, so the orientation degree is also small. Vibration is introduced in the process of holding pressure, so that the polymer melt in the die cavity is cooled and sheared by vibration, and the orientation produced by vibration shearing forms an orientation layer with a certain thickness due to the cooling effect of the die. Compared with non-vibration, the thickness of orientation layer produced by vibration shear flow is much larger than that of ordinary injection, which is why the mechanical properties of products can be improved by introducing vibration shear flow into mold cavity. In addition, due to the periodic compression, pressurization and decompression expansion caused by vibration, large internal shear heat can be generated in the thin-walled part, and the cooling of these parts can be delayed, so that the shrinkage of the thick-walled part can be fully supplemented from the gate, and the defects such as shrinkage cavity and depression can be effectively prevented.
7。 Push-pull injection molding
This molding method can eliminate melt seams, voids, cracks and micropores in plastic parts, and can control the arrangement of reinforcing fibers. It adopts two injection units, namely the main injection unit and the auxiliary injection unit, and a double-winding mold. When working, the main injection unit pushes the melt to overflow the mold cavity through the bypass. Excess material enters the auxiliary injection unit through another gate, and the auxiliary injection screw retreats to receive excess melt in the mold cavity; Then, the auxiliary injection screw moves forward to inject the melt into the mold cavity, and the main injection unit receives the excess melt in the mold cavity. The main injection unit and the auxiliary injection unit push and pull repeatedly in this way, forming the vibration shear flow of the melt in the mold cavity. When the melt near the die wall solidifies, the melt in the core flows in vibration shear, and when the melt near the die wall solidifies, the melt in the core is oriented and gradually solidified under the action of vibration shear, forming a highly oriented product. The molding of general products needs about 10 cycles, up to 40 cycles.
The cycle of push-pull injection molding is longer than that of ordinary injection molding, but because the material cools and solidifies during push-pull movement, the pressure holding stage is not very important for controlling shrinkage and warpage. In push-pull injection molding, the injection stage and the holding stage are combined into one. The results of push-pull injection molding of glass fiber reinforced LCP by this injection process show that compared with conventional injection molding, the tensile strength and flexural elastic modulus of the material can be increased by 420% and 270% respectively.
8。 Layered injection molding
Layered injection molding is a molding process with the characteristics of both extrusion molding and injection molding, which can produce very thin layered state in complex parts at will. In layered injection molding, two different resins are injected at the same time, so that each melt is gradually layered in the * * * extrusion die through a multi-stage * * extrusion die, the thickness of each layer becomes thinner, the number of layers increases, and finally, the layered form obtained through the above process is preserved, that is, the two kinds of tree fingers do not exist in a disordered * * mixed state in the thickness direction of the product, but are compounded and superimposed together. It is reported that the thickness of each layer that can be molded by layered injection is 0. 1- 10pm. Melaleuca products. Because of its layered structure, it retains the characteristics of each component material, and can give full play to the material properties better than the traditional mixture, so that its products have outstanding advantages in blocking gas penetration, solvent resistance, transparency and so on.
9。 Injection molding of microcellular foam
In the traditional structural foam injection molding, chemical foaming agent is usually used. Due to its low foaming pressure, the produced parts are limited in wall thickness and shape. The application of supercritical inert gas in microcellular foam injection molding is limited. The injection molding of microcellular foam uses supercritical inert gas (CO2, N2) as physical foaming agent. The process flow is divided into four steps:
1) gas dissolution: the supercritical liquid of inert gas is injected into the polymer melt through the injector installed on the structure to form a uniform polymer/gas system;
2) Nucleation: During the mold filling process, gas is precipitated from the polymer due to the pressure drop, forming a large number of uniform gas nuclei;
3) Bubble growth: gas grows under precise temperature and pressure control;
4) shaping: when the bubbles grow to a certain size, cooling and shaping.
Microcellular foaming is very different from general physical foaming. First of all, a large number of inert gases such as CO2 and N2 need to be dissolved in the polymer during microcellular foaming processing, so that the gas can be saturated in the polymer. It is impossible to obtain such a high gas concentration in the polymer-gas homogeneous system by general physical foaming processing methods. Secondly, the nucleation number of microcellular foaming is much higher than that of general physical foaming, and the thermodynamic state is gradually changing, which easily leads to the defects of large cells and uneven cell size distribution in the product. The thermodynamic state of microcellular plastics changes rapidly during the molding process, and its nucleation rate and bubble number greatly exceed those of general physical foaming molding.
Compared with ordinary foam molding, microcellular foam molding has many advantages. First, the bubbles formed by it are small in diameter, which can produce thin-walled (1mm) products that are difficult to produce because of the large micropores of ordinary foam plastics; Secondly, the pores of microcellular foam materials are closed-cell structures, which can be used to block packaging products; Thirdly, carbon dioxide or N2 is used in the production process, so there is no environmental pollution problem.
On the basis of MIT's concept of microcellular foam, American Trexel Company industrialized microcellular foam injection molding technology and formed the patented technology of MuCell. The main advantages of MuCell technology in injection molding are endothermic reaction, low melt viscosity and low temperature of melt and mold, thus reducing the molding cycle, material consumption, injection pressure and clamping force of products. And uniquely, this technology can be used for injection molding of thin-walled products and other products that cannot be foamed by foaming technology. MuCell's breakthrough in injection molding technology has provided great capacity for the production of injection products that other injection molding processes did not have before, and opened up new ways for new product design, process optimization and product cost reduction. Injection molding products using MuCell technology are being applied in many industrial fields, including automobile, medicine, electronics, food packaging and other industries.