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High gloss mold technology and application

Article source:Shanghai Hongxue Mold Technology Co., Ltd. Published time:2018-12-05 18:15:44
1. Features of high-gloss mold
The high-gloss mold controls the mold temperature through the mold temperature machine, and the hot runner, solenoid valve and time relay control the time-division glue to produce high-end products with defects such as bright surface, no shrinkage and no weld mark.
High-gloss molds generally have the following characteristics:
1. The mold molding temperature is high (generally 80-90 degrees)
The pressure-holding molding at a high mold temperature is advantageous for eliminating defects such as weld lines, flow marks, and internal stress of the product. Therefore, the mold needs to be heat treated during operation, and in order to prevent heat loss, a resin heat insulation board is usually added on the fixed mold side.
2. The cavity surface is extremely bright (generally mirror level 2 or higher)
The products produced by Gaoguang Mould can be directly used for installation without any surface treatment. Therefore, it has high requirements for mold steel and plastic materials.
3. Hot runner system has more hot nozzles (generally 6-8 nozzles, sometimes more)
Each hot nozzle must be equipped with a sealing needle and has an independent air passage. It is separately controlled by a solenoid valve and a time relay to realize time-divisional feeding, thereby achieving the purpose of controlling or even eliminating the welding mark.
2. Highlights of high gloss mold design
For the characteristics of high-gloss molds, the details that should be highly valued when designing high-gloss molds are as follows:
1. High-gloss mold requirements for product structure
Gaoguang Mould has strict requirements on product structure. It is well known that the brighter the product is, the more sensitive it is to the refraction of light, and the slightly flawed surface will soon be discovered. Therefore, how to solve the shrinkage problem is the primary problem of high-gloss products. According to experience statistics, the thickness of the general product ribs does not exceed 0.6 times the thickness of the main body glue position, so it will not shrink, or the shrinkage is less likely to be found, which can be ignored. However, for high-gloss products, such requirements are far from enough. The thickness of the product ribs should be reduced to not more than 0.4 times the thickness of the main body glue. For screw positions, special treatment must be done.
2. Hot runner system of high light mold
Hot runner systems are critical to high gloss molds. Whether it is possible to inject a product without weld lines, the quality of the hot runner and the adjustment of the machine are decisive factors. The characteristics of the high-gloss mold hot runner system: the hot nozzle of the high-gloss mold must have a sealing needle, and each hot nozzle must have a separate air passage (not allowing multiple air passages to be connected in series), so that the solenoid valve and time can be passed. The relay is used to separately control the feeding time and the amount of glue entering each hot nozzle, and prepare for the injection molding machine.
3. Cooling and heating system for high-gloss mold
When the plastic material is injected into the mold cavity, the higher the surface temperature of the mold cavity in a reasonable temperature range, the better the surface quality of the product and the lower the internal stress of the product. Therefore, in order to obtain good surface quality of the product, it is a good way to increase the surface temperature of the cavity (however, increasing the temperature of the mold will reduce the efficiency of injection molding and requires comprehensive consideration). Generally, the high-gloss mold is heated by a mold temperature machine during injection molding, so that the plastic is kept under pressure and lowered in the mold at a high temperature, thereby eliminating defects such as flow marks and weld lines, and achieving a mirror effect.
The high-gloss mold pays more attention to the arrangement of the cooling water channel. Good water channel layout not only greatly improves injection efficiency, but also plays an important role in improving product quality. The water channel of the high-gloss mold should not only be uniform, but also must be sufficient, so that the required temperature can be easily reached by the mold temperature machine. At the same time, the long water pipe is used to directly transport the water from the core without using a sealing ring, which can prevent the mold from working at high temperature for a long time, resulting in aging of the sealing ring and reducing the maintenance cost of many molds. It is worth mentioning that the water pipe of Gaoguang Mould must use high temperature resistant tubing to prevent the water pipe from bursting under high temperature and high pressure.
4. Material selection of high gloss mold
Factors affecting the surface quality of the product In addition to the cavity temperature, the choice of mold steel and plastic materials is also crucial. The selection of high-gloss mold steel and the choice of plastic materials are introduced below.
1 mold steel selection
The performance and life of the mold are directly related to the steel used in the mold. Plastic mold parts are subject to external influences due to different working conditions. Therefore, in addition to some basic requirements for the performance of steel, there must be some emphasis. For example, large-scale high-gloss plastic mold cavity steels, in addition to good machinability, electrical discharge processability and weldability, also need to have excellent polishing properties and high hardness. This not only can form a bright and beautiful surface, but also can reduce the wear of the plastic on the surface of the cavity, which not only prolongs the service life of the mold, but also reduces the injection pressure and protects the injection molding machine. At present, commonly used high performance mirror plastic mold steels are: POLMAX, LKM838H, LKM818H, S-STAR (A), NAK80, STAVAXS136, STAVAXS136H, OPTIMAX, X13T6W (236), X13T6W (236H) and so on.
2 choice of plastic materials
At present, the plastic materials used in the front casing of the television manufacturing are generally HIPS and ABS. As a common two-shell material, although the cost of ABS is larger, the molded product is better than HIPS in terms of impact resistance, surface gloss and hardness, so in the production of high-gloss products, high light is usually used. ABS material.
ABS is an amorphous polymer with no apparent melting point. Due to the wide variety of grades, appropriate process parameters should be formulated according to the grades in the injection molding process, generally above 160 °C, below 270 °C. In the molding process, ABS has good thermal stability, a wide range of options, and is not easy to degrade or decompose, and the melt viscosity of ABS is moderate, and its fluidity is better than that of polyethylene (PS), polycarbonate, etc. And the melt cooling and solidification speed is relatively fast, generally it can be cooled in 5-15 seconds.
The fluidity of ABS is related to the injection temperature and injection pressure, and the injection pressure is slightly more sensitive. For this reason, in the molding process, it is possible to start from the injection pressure to lower the melt viscosity and improve the filling performance. ABS has different properties of water absorption and water adhesion due to different components. The surface adhesion water 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, Drying before molding, the water content is reduced to less than 0.1%, otherwise, bubbles on the surface of the workpiece will appear, such as bubbles and silver.
5. Inclined top mechanism of high-gloss mold
The sloping top mechanism is often adopted in our high-gloss mold design, especially in the high-light mold design of the LCD front case.
In the front, we mentioned in the requirements of the product structure of Gaoguang Mould, in order to prevent shrinkage, the screw column must be processed, which requires us to use the inclined top mechanism, in order to make the display and the shell good contact. Usually, the surface of the shell surface will be pre-deformed. Therefore, it must be handled carefully when optimizing the product and designing the inclined roof, so that when the product is ejected, neither the rubber will be scrapped nor the product will be whitened. Form a qualified product.
It is also worth noting that the slanting mechanism is not suitable for large core distances, but in high-gloss dies, it is often encountered with a slanting mechanism that requires a large core distance. The special guiding rod is used to overcome the frictional force of the inclined top seat, and the direction of the stress of the inclined top rod is changed, so that the inclined top rod is not subjected to the torque during the ejection and resetting, and therefore, a large inclination can be achieved.
6. High-light mold exhaust system
In a sense, the injection mold is also a displacement device, that is, the plastic melt enters the mold cavity and simultaneously displaces the air in the mold cavity, and these gases must be discharged in time, otherwise, many hazards will be caused. In order to achieve high light effects, the exhaust system of the high-gloss mold must be reasonable and sufficient. Generally, the high-light mold exhaust system is set up in the following ways:
1Opening the exhaust slot
For molds for forming large and medium-sized plastic parts, the amount of gas to be removed is large, and a venting groove must be opened. The venting groove is usually opened on the pusher (the parting surface and the side of the core) and the biliary surface. The position of the venting groove is preferably at the end of the melt flow, and the size of the venting groove can be smoothly discharged by gas. The principle is not flashing, the width is 3-5mm, the length is 3-10mm, and thereafter it can be deepened to 0.8-1.5mm, and the spacing of the grooves is kept at about 50mm.
2 using the slot gap exhaust
For the combined fixed mold or core, it can be exhausted by the split gap, and if necessary, the high pressure air gun can be used for backflushing to clean the blocked air passage.
3 using the thimble, the cylinder exhaust
In the absence of flash, the gap between the thimble, the cylinder and the core is intentionally increased to complete the venting.
4 pouring system end exhaust
The exhaust of the gating system is often the most easily overlooked, which is a very important part.
7. High-gloss mold thermocouple system
Thermocouple is the most widely used temperature device in temperature measurement. It belongs to contact temperature measurement in temperature measurement. Its main features are wide temperature measurement range, stable performance, simple structure, good dynamic response and better remote transmission. The -20mA electrical signal facilitates automatic control and centralized control. The thermocouple's temperature measurement principle is based on the thermoelectric effect. Many high-demand molds are equipped with thermocouples (temperature detectors) for monitoring mold temperatures.
The working principle of the probe line in the high-light mold: the high-light mold is heated by the mold temperature machine. When the mold temperature is higher than the set value, the contactor of the temperature probe line is powered off, and at the same time, the mold temperature machine stops the mold. Heating; when the mold temperature is lower than the set value, the contactor of the temperature probe is closed, and the mold temperature machine heats the mold again, so that the mold temperature is repeatedly controlled, so that the mold is always working within the temperature range we desire, which is beneficial to Improve the quality of the product.
Thermocouples have positive and negative polarity. When wiring, be sure not to reverse the connection. Otherwise, the temperature control table cannot read a few, and it is easy to be mistaken for the temperature control table failure. When installing a thermocouple, the position of the temperature measurement point should be as close as possible to the glue position as the structure allows, in order to truly reflect the temperature state of the plastic.