Plastic molding method and mold design
Gas-assisted molding: Gas-assisted molding is not a new technology, but it has developed rapidly in recent years and some new methods have appeared. LPG-assisted injection is to inject a preheated special vaporizable liquid into the plastic melt, and the liquid is heated to vaporize and expand in the mold cavity, making it hollow and pushing the melt to the surface of the mold cavity. This method can be used for any thermoplastic. Vibrating gas-assisted injection is to apply vibration energy to plastic melt through the compressed gas of vibrating products, so as to control the microstructure of products and improve their properties. Some manufacturers can convert the gas used in gas-assisted molding into thinner products or produce large hollow products with higher quality and lower cost, but the key point is water leakage.
Push-pull molding die: two or more channels are arranged around the die cavity and connected with two or more reciprocating injection devices or pistons. Before the melt solidifies after injection, the screw or piston of the injection device moves the melt back and forth in the push-pull mold cavity. This technology is called dynamic pressure maintenance technology, and its purpose is to avoid the problem of large shrinkage when forming thick products by traditional molding methods. Thin shell products are formed by high pressure. Thin shell products are generally products with long flow ratio, and multi-point gate molds are often used. However, multi-point pouring will cause weld marks and affect the visual effect of some transparent products. Single-point casting is not easy to fill the mold cavity, so it can be formed by high pressure forming technology. For example, the cockpit of F 16 fighter is produced by this technology, and this technology has been used to produce PC car windshields. The injection pressure of high-pressure forming is generally above 200MPA, so the mold material should also have high strength and high Young's modulus rigidity. The key of high pressure forming is the control of mold temperature, and attention should be paid to the smooth exhaust of mold cavity. Otherwise, high-speed injection molding will lead to poor exhaust and scorch the plastic.
Hot runner mold: Hot runner technology is more and more used in multi-cavity mold, and its dynamic inlet technology is a bright spot in mold technology. That is to say, the injection time, injection pressure and other parameters can be set for each gate by adjusting the flow rate of plastic through the needle valve, so as to achieve balanced injection and obtain the best quality assurance. The needle valve can realize dynamic stepless motion by hydraulically driving the piston, and the position of the needle valve determines the injection flow and pressure. There is a pressure sensor in the channel, which can continuously record the pressure in the channel, and then control the position of the needle valve and adjust the melt pressure.
Mold for injection molding of molten core: This method is to put the fusible core made of low melting point alloy into the mold as an insert for injection molding. The product containing the fusible core is then heated to remove the fusible core. This molding method is used for hollow products with complex shapes, such as automobile oil pipes, air supply and exhaust pipes and other complex hollow plastic parts. Products molded by this mold also include: tennis racket handle, car water pump, centrifugal hot water pump, spacecraft oil pump and so on.
Injection/compression molding mold: Injection/compression molding can produce low stress. For products with good optical properties, the process flow is as follows: mold closing (but the movable mold and the fixed mold are not completely closed, leaving a gap for later compression), melt injection, secondary mold closing (that is, compression makes the melt dense in the mold), cooling, mold opening and demoulding. In mold design, it should be noted that the mold is not completely closed at the initial stage of mold closing, so in order to prevent overflow during injection, the structure to prevent overflow should be designed in the mold.
Laminated mold: arranging multiple cavities on the clamping side, instead of arranging multiple cavities on the same plane, can give full play to the plasticizing ability of injection machine. This kind of mold is generally used in hot runner mold, which can greatly improve the efficiency.
Injection mold for layered products: The injection mold for layered products has the characteristics of * * * extrusion molding and injection molding, and can realize multi-layer combination of materials with any thickness on the products, with the thickness of each layer as small as 0. 1 ~ 10 mm and the number of layers as many as several thousand. This kind of mold is actually a combination of injection mold and multi-stage extrusion mold.
Die Slip Forming (DSI): This method can be used to form hollow products or composite products of various materials. The process is as follows: mold closing (for hollow products, the two cavities are at different positions at this time), respectively injecting, moving the mold to the two cavities, and injecting the resin combined with the two cavities in the middle. Compared with blow-molded products, the products formed by this method have good surface accuracy, high dimensional accuracy, uniform wall thickness and design.
Aluminum mold: A prominent point of plastic manufacturing technology is the application of aluminum-gold materials. The service life of aluminum alloy plastic mold developed by Corus company can reach more than 300,000 times, and PechineyRhenalu company can use its MI-600 aluminum material to make plastic, and the service life can reach more than 500,000 times.
die making
High-speed milling: At present, high-speed cutting has entered the field of precision machining, and the positioning accuracy has been improved to {+25UM}. The rotational accuracy of hydrostatic bearing high-speed motorized spindle is below 0.2um, and the spindle speed of machine tool can reach 100.000r/min. The high-speed motorized spindle with aerostatic bearing rotates up to 200 degrees. The fast feed speed of 00r/min can reach 30 ~ 60m/min, and the feed speed can even reach 60 ~ 120m/min if large guide rail and ball screw, high-speed servo motor, linear motor and precision linear guide rail are used. Tool changing time is reduced to 12s, and roughness Ra.
Laser welding: Laser welding equipment can be used to repair the mold or clad the metal layer to increase the wear resistance of the mold, and the hardness of the mold surface after laser cladding can reach 62HRC. The time of micro-welding is only 10-9 seconds, so heat can be prevented from being transferred to the adjacent area of the solder joint. The common laser welding process is adopted. When working, the temperature at the distance of 15mm from the solder joint can reach 150~200C, while the temperature is only 36C when using micro-welding technology. This will not cause changes in the metallographic structure and properties of the material, nor will it cause problems such as warping deformation or cracking.
EDM milling: Also known as EDM generative machining technology. It uses a simple tubular electrode rotating at high speed to process two-dimensional or three-dimensional contours, so it is no longer necessary to manufacture electrodes with complex shapes.
Three-dimensional micromachining (DEM) technology: DEM technology overcomes the shortcomings of LIGA technology, such as long processing cycle and high price, and integrates three main processes: deep etching, micro electroforming and micro replication. It can produce molds for micro parts such as gears with a thickness of only100μ m..
Integrated technology of three-dimensional cavity precision forming and mirror EDM: by adding solid fine powder into common kerosene working solution, the spacing between polished electrodes is increased, the electro-gas effect is reduced, the dispersion of discharge channels is increased, the chip removal is good, the discharge is stable, the machining efficiency is improved, and the machined surface roughness is effectively reduced. At the same time, the mixed powder working solution can also form a coating with high hardness on the surface of the die workpiece, which improves the hardness and wear resistance of the die cavity surface.
Mould surface treatment and strengthening
In order to improve the life of the mold, in addition to the conventional heat treatment methods, the following are some commonly used mold surface treatment and strengthening technologies.
Chemical treatment: Its development trend is from single element infiltration to multi-element, to multi-element infiltration and composite infiltration, and from general diffusion and diffusion to chemical vapor deposition (PVD) and physical chemical vapor deposition (PCVD).
Ion permeation, ion implantation, etc.
Laser surface treatment: 1 Use laser beam to obtain extremely high heating speed and realize surface quenching of metal materials. Very fine high-carbon martensite crystals are obtained on the surface, and the hardness is 15% ~ 20% higher than that of the conventional quenching layer, while the central structure will not change. 2. The high-performance hardfacing layer is obtained by laser surface remelting or surface alloying. For example, after alloying with CrWMn composite powder, its volume wear is110 of quenched CrWMn, and its service life is increased by 14 times. 3. Laser melting treatment is to melt the metal surface by using a laser beam with high energy density, and to carry out tissue quenching, so that a layer of chilled structure of liquid metal is formed on the metal surface. Because the surface layer is heated and cooled very quickly, the obtained structure is very fine. If the cooling rate is high enough through the external medium, the crystallization process can be inhibited and amorphous state can be formed, so it is also called laser melting-amorphous treatment and laser glazing.
Rare earth element surface strengthening: this can improve the surface structure and physical, chemical and mechanical properties of steel, increase the infiltration rate by 25% ~ 30%, and shorten the treatment time by more than 1/3. Common rare earth carbonitriding, rare earth carbonitriding, rare earth boron carbonitriding, rare earth boron aluminizing and so on.
Electroless plating: it is reduction precipitation of Ni-P, Ni-B, etc. Ni-P and Ni-B alloy coatings were obtained by chemical tester in the solution of metal surface. On the metal surface. In order to improve the mechanical properties, candle resistance and technological properties of metals, it is also called autocatalytic reduction plating and electroless plating.
Nano-surface treatment: solid surface materials are based on low-dimensional non-equilibrium materials such as nano-materials, and on specific processing techniques, which strengthen or give new functions to solid surfaces. (1) nano-composite coating is formed by adding zero-dimensional or one-dimensional nano-particle powder materials into traditional electromigration solution. Nano-materials can also be used for wear-resistant composite coatings. For example, adding N-ZrO _ 2 nano-powder material to NI-W-B amorphous composite coating can improve the oxidation performance of the coating at 550-850℃, the corrosion resistance of the coating is improved by 2-3 times, and the wear life and hardness are also obviously improved. (2) Nano-structured coatings have been significantly improved in strength, toughness, corrosion resistance, wear resistance, thermal fatigue resistance, etc., and a coating can have the above properties at the same time.
Rapid prototyping and rapid prototyping
Rapid prototyping and rapid tooling is an important technology in new product development. In the past, people always thought that rapid tooling was limited to small batch trial production. However, in recent years, rapid tooling is also making efforts to manufacture semi-durable metal molds in medium or even large batches.
The spray forming process is to form a metal spray layer on the surface of the prototype, and then reinforce and remove the spray layer to obtain a metal mold. Using high melting point spraying materials, the surface hardness of the die can reach 63HRC.
The methods of direct rapid manufacturing (DRMT) of metal molds mainly include selective laser sintering (SLS) with laser as heat source, laser melting deposition (LENS), deposition with plasma arc as heat source (PDM), spray forming three-dimensional printing (3DP), metal sheet LOM technology and so on. The precision of SLS forming is improved. The shrinkage decreased from 1% to below 0.2%. Although the density and mechanical properties of parts manufactured by lens are greatly improved compared with SLS method, there is still a porosity of about 5%. It is only suitable for manufacturing simple geometric parts or molds, and the surface quality will not be too high due to the adhesion of unmelted particles.
In shape deposition manufacturing (SDM), welding materials (filaments) are melted by welding principle, and ultra-high temperature droplets are deposited layer by layer by thermal spraying principle, so as to realize interlayer metallization combination, which can avoid the problems of low precision, poor surface quality and low comprehensive mechanical properties caused by RT technology based on lamination principle.
Rapid tooling technology is produced and developed in the competition with traditional machining technology, but the milling speed is as high as 100.000r/min. Surface accuracy's excellent high-speed milling technology has become the biggest competitor of DRMT technology.