At present, most of the appearance parts of household appliances are obtained by injection molding. In the process of injection molding, defects such as weld marks, gas marks, and deformation are prone to occur; high-gloss traceless molds can solve the above defects. Let's take a look at the ten major elements of high-gloss traceless injection mold design. 1. The principle of high-gloss traceless injection molding 1. High temperature mold molding has high temperature requirements (generally around 80℃-130℃). After the injection molding enters the pressure holding, cooling water is used to reduce the mold temperature to 60-70℃. Pressure holding molding at a higher mold temperature is conducive to eliminating defects such as weld lines, flow marks, and internal stress of the product. Therefore, the mold needs to be heated during operation. In order to prevent heat loss, an insulation board is usually added to the fixed mold side. 2. The surface of the mold cavity is extremely bright (generally mirror grade 2 or higher) The products produced by the high-gloss mold can be directly used for installation (assembly) without any surface treatment. Therefore, it has high requirements for mold steel and plastic materials. 3. There are many hot nozzles in the hot runner system. Each hot nozzle must have a sealing needle and an independent airway. It is individually controlled by solenoid valves and time relays to achieve time-sharing glue feeding, so as to achieve the purpose of controlling or even eliminating weld marks. The control method is complex. 4. Heating method. There are usually two ways to heat the mold: steam (hot water) heating and electric heating rod (tube) heating. The steam (hot water) heating method is to input steam (hot water) into the mold through a specific temperature controller during the injection process, so that the mold can be heated quickly; after the injection, the mold is cooled with cold water to cool the mold quickly. The electric heating method is the same as the water heating temperature controller in principle, but the heat source is different. Electric heating is a secondary energy source, and water heating is a tertiary energy source. In principle, electric heating has less energy loss, high utilization rate, and good energy-saving benefits. It is easy to use, so it is wise to use electric heating if it is a flat (surface) product. Figure: Steam heating Figure: Heating rod heating
II. Mold materials
1. Mold materials for product surface with normal requirements can be: NK80 (Datong, Japan), etc.; 2. Materials for high gloss requirements can be: S136H (Sweden), CEANA1 (Japan), etc.; 3. NK80 does not need quenching treatment; S136H should be quenched to 52 degrees after rough processing; CEANA1 itself has 42 degrees and does not need quenching treatment (it is recommended to use this steel because it does not affect subsequent processing or modification); 4. There are also good choices among the German Gritz brand: CPM40/GEST80
Figure High-gloss mold
Three mold water channel design 1. Water channel aperture size design The water channel uses a 5-6mm aperture; the water nozzle uses 1/8 or 3/8 thread (mold side), and the other side uses 3/4 British thread (old-fashioned connection method); the pipe material is stainless steel pipe; now we have changed to one inlet and one outlet, and the diversion port is best made in the mold. The interface uses a DN25 connection, so that the heat energy loss is small, the operation is convenient, and the interface is convenient. 2. Product surface design The water channel is generally 5-6mm away from the product surface; a larger one will affect the mold heating time, and a smaller one will affect the strength of the mold. The water channel parallel to the product surface needs to be evenly arranged (15mm equal distance distribution in the center of the original material) Thermocouples should be designed in the middle of the two water channels, with a depth of more than 50mm and a maximum of no more than 100mm, depending on the mold structure and flexible control. Each set of mold PT100 is one-to-one, to maintain its accuracy, it must be inserted into the mold cavity and fixed. Use a lead wire to connect to the outside of the mold, and then connect to the temperature controller socket. 3. Mold water channel joint design The mold water channel joint must be designed at the upper and lower sides or the rear side of the mold; the operation side (standing side) is not allowed to have water channel inlet and outlet or water pipe arrangement to avoid pipe rupture and injury to production personnel. Remember! 4. Mold inlet and outlet nozzle design The mold inlet and outlet nozzle adopts a splitter design, and the hydrothermal mold temperature controller system has only one inlet and one outlet interface to reduce excessive water pipe connections and reduce unnecessary heat energy loss; and achieve the purpose of energy conservation and energy saving. And the surface of the corrugated pipe is wrapped with thermal insulation tape to play a role in heat preservation and energy conservation. 5. Construction holes of the mold The construction holes of the mold (unwanted holes) should be blocked with plugs to ensure that there is no air leakage or water leakage. The method is to first use copper plugs, and then use tapered throat teeth and high temperature resistant glue to seal; high-gloss molds are more particular about the arrangement of cooling water channels (hydrothermal mold water channels are shared). Good water channel arrangement can not only greatly improve injection molding efficiency, but also play an important role in improving product quality. The water channel of the high-gloss mold must not only be uniform but also sufficient (there must be enough). In this way, the mold temperature rises quickly; at the same time, the mold core is directly transported out of the mold core without using a sealing ring, which can prevent the mold from operating at high temperature for a long time, causing the sealing ring to age, and can also reduce the maintenance cost of many molds. It is worth mentioning that the water pipe of the high-gloss mold must be made of high-temperature resistant material (250℃) bellows. High-pressure 1.6Mpa bellows to prevent the water pipe from bursting under high temperature and high pressure. For round products, circular water transportation is used; for long strip products, parallel water transportation channels are used. For products with large height differences, a water well is used; for special-shaped products, a three-dimensional water transportation method consistent with the product appearance is used.
4. Mold insulation system 1. Mold core design The four sides of the mold fixed mold core or the movable mold core must be hollowed out; there must be a certain gap between the mold frame and the core (depending on the thermal expansion coefficient of the mold material, 1mm on one side). Prevent the mold frame from expanding to reduce the contact surface between the mold core and the mold frame, so as to minimize the heat loss; the mold core and the mold frame are locked by an oblique wedge or other similar methods, and the front end is made of dust resin or other materials (such as asbestos board) with obvious heat insulation effect. 2. Mold frame design The detailed structure of the mold frame and the core, the cooling water of the mold frame is very important. In order to prevent the heat energy in the mold core from being transferred to the mold frame, a circle of water should be arranged up and down near the guide column. 3. Guide sleeve design The moving part of the guide sleeve should be made of graphite material as much as possible or the front end of the guide column should be avoided. It is enough to ensure a length of 25mm at the fitting part;
V. Mold gate design The mold gate design should reduce the weld mark as much as possible, and facilitate exhaust and reduce shear. For molds using water-heated temperature controllers, the gate size should be larger and a large gate should be used as much as possible for glue injection. Without affecting the product function and molding efficiency, the gate should be shortened as much as possible in length, depth and width. 1. The gate is too small If the gate is too small, it is easy to cause appearance defects such as insufficient filling (short shot), shrinkage depression, weld line, and molding shrinkage will increase. 2. The gate is too large If the gate is too large, excessive residual stress will be generated around the gate, resulting in deformation or rupture of the product, and the gate removal process is difficult. It is best to use one gate unless the flow ratio exceeds the actual limit. The flow length curve of the resin will provide the flow length of the material under a certain molding condition. Multiple gates often produce weld lines and weld marks. In addition to long and narrow products, the use of a single gate will ensure a more consistent distribution of materials, temperature, and pressure holding for better matching effects. 6. Mold exhaust should be spaced 10mm apart around the product as much as possible, and exhaust grooves should be evenly distributed with a depth of 0.15mm; the middle veneer of the product also needs exhaust design.
7. Mold parting surface matching Because the temperature of the high-gloss mold has a large drop, the veneer matching requirements are higher, and the veneer area should be reduced at the same time. 10mm matching around the parting surface is enough.
Eight Heating rod (tube) high-gloss mold design 1. There should be electric heating rods (tubes) on the upper and lower sides of the gate. The cooling water hole is generally 6mm (larger is better); the center distance between the two water holes is 15-20mm; the distance between the heating rod wall and the product surface is 5mm, and the center distance between the two heating rods is 20mm; the distance between the cooling water and the heating rod wall is 6-8mm. If conditions permit, it is best to intersperse and arrange with the electric heating rod. 2. The water transport in the inner mold cavity can be sealed with a high-temperature resistant sealing ring or a hard seal. 3. The diameter of the heating rod is 4.92mm, and the mold is designed to be 5mm. Before assembling the heating rod, use a 5mm ejector to grind the edge to remove the burrs of the heating rod. 4. The mold inlet and outlet nozzles use the same manifold design (cooling water) as the steam heating mold, because the electric heating mold control system has only one inlet and one outlet water pipeline.
9. Requirements of high-gloss molds for products. High-gloss molds have very strict requirements on product structure. The brighter the product, the more sensitive it is to the refraction effect of light. Any surface defects will be discovered quickly. Therefore, how to solve the shrinkage problem is the primary problem of high-gloss products. Generally, the thickness of the rib position of the product does not shrink if it does not exceed 0.6mm times the thickness of the main glue position, or the shrinkage is small and not easy to be discovered, which can be ignored. However, for high-gloss products, such requirements are far from enough. The thickness of the rib position of the product needs to be reduced to no more than 1 times the thickness of the main glue position, and the screw column position must also be made into a crater-type inclined roof structure.
10. Selection of plastic materials for high-gloss molds. Currently, commonly used high-gloss plastic materials are generally ABS+PMMA and ABS+PC, PMMA, ASA, etc. As a commonly used casing material, ABS+PC products are better than HIPS in impact resistance, surface gloss and hardness, so when producing high-gloss products, high-gloss ABS materials are usually selected. If weather resistance is required, ASA may be selected, and PMMA alloy materials may be selected in terms of hardness. Let's talk about ABS materials in detail below.
1. How to control the melt viscosity of ABS? ABS is an amorphous polymer with no obvious melting point. Due to its wide variety of grades, appropriate process parameters should be formulated according to the grades during the injection molding process. Generally, it can be molded at a temperature above 160°C and below 270°C. During the molding process, ABS has good thermal stability, a wide range of options, and is not prone to degradation or decomposition. In addition, the melt viscosity of ABS is moderate, and its fluidity is better than that of polystyrene (PS) and polycarbonate (PC). In addition, the cooling and solidification speed of the melt is relatively fast, and it can generally be cold-solidified within 5 to 15 seconds. 2. How to control the water absorption rate of ABS? The fluidity of ABS is related to both the injection temperature and the injection pressure, among which the injection pressure is slightly more sensitive. For this reason, during the molding process, the injection pressure can be used to reduce its melt viscosity and improve the filling performance. ABS has different water absorption and adhesion properties due to different components. Its surface adhesion and water absorption rate are between 0.2% and 0.5%, and sometimes between 0.3% and 0.8%. In order to obtain a more ideal product, it is dried before molding to reduce the water content to below 0.1%. Otherwise, there will be defects such as bubbles and silver threads on the surface of the product. Usually, 1% metal powder needs to be added to plastic materials to improve the high-gloss metal effect.
