The key aspect of high-gloss injection molding is the mold temperature control system. Unlike general injection molding, the main difference lies in the control of mold temperature rather than the requirements for injection molding machines. The mold temperature control system for high-gloss injection molding is commonly referred to as a high-gloss mold temperature controller. This system works in tandem with general injection molding machines to synchronize actions during the filling, pressure holding, cooling, and opening and closing of injection molding.
The temperature control system’s key technology is the heating method of the mold surface, and the high-gloss mold surface mainly obtains heat through the following ways:
1. Heating method based on heat conduction: Heat is conducted to the mold surface through internal pipes of the mold using oil, water, steam, and electric heating elements.
2. Heating method based on thermal radiation: Heat is obtained through direct radiation of solar energy, laser beam, electron beam, infrared light, flame, gas, and other mold surfaces.
3. Heating the mold surface through its own thermal field: This can be achieved through resistance, electromagnetic induction heating, etc.
Currently, practical heating systems include an oil temperature machine for high-temperature oil heat transfer, a high-pressure water temperature machine for high temperature and high-pressure water heat transfer, a steam mold temperature machine for steam heat transfer, electric heating mold temperature machine for electric heat pipe heat transfer, as well as electromagnetic induction heating system and infrared radiation heating system.
(l) Oil temperature machine for high-temperature oil heat transfer
The mold is designed with uniform heating or cooling channels, achieved through an oil heating system. The oil heating system allows for preheating the mold as well as cooling during the injection process, with a maximum temperature of 350°C. However, the low thermal conductivity of the oil results in low efficiency, and the oil and gas generated can affect the quality of high-gloss molding. Despite these drawbacks, the enterprise commonly uses oil temperature machines and has significant experience with their usage.
(2) High-pressure water temperature machine for high temperature and high-pressure water heat transfer
The mold is designed with well-balanced pipes on the inside, and different temperatures of water are used at different stages. During heating, high temperature and superhot water are used, while during cooling, low-temperature cooling water is used to adjust the temperature of the mold surface. Pressurized water can raise the temperature to 140-180 °C quickly. Aode’s GWS system is the top choice for manufacturers of high-temperature and high-pressure water temperature control systems because it allows for the recycling of hot water, resulting in low operating costs. It is currently the most widely used system in the domestic market and is considered the best alternative to steam.
(3) Steam mold temperature machine for steam heat transfer
The mold is designed with balanced pipes to allow for the introduction of steam during heating and the switch to low-temperature water during cooling. This process helps achieve optimal mold surface temperature. However, using high-temperature and high-pressure steam heating systems can lead to high operating costs as it requires installing boiler equipment and laying pipelines. In addition, due to the fact that steam is not recyclable in the production process, it has a longer relative heating time compared to water. Reaching a mold surface temperature of 150°C requires approximately 300°C of steam.
(4) Electric heating mold temperature machine for heat transfer of electric heating pipes
Resistance heating elements such as electric heating plates, frames, and rings use electric heating pipes, with the electric heating pipe being the most commonly used. It consists of a metal tube shell (typically stainless steel or copper) with a spiral electric heating alloy wire (made of nickel-chromium or iron-chromium alloy) evenly distributed along the central axis of the pipe. The void is filled and compacted with magnesia, which has good insulation and thermal conductivity, and the two ends of the pipe are sealed with silica gel. Electric heating elements are used to heat air, solids, and various liquids.
Currently, the heating system of directly installed electric heaters in molds is expensive, and mold design patents need to be paid for. However, electric heating pipes heat up quickly, and the temperature range can be controlled up to 350°C. With this system, the mold temperature can be heated to 300°C in 15 seconds and then cooled to 20°C in 15 seconds. This system is suitable for smaller products, but due to the higher temperature of the heating wire directly heating, the relative die life is shortened.
(5) High-frequency electromagnetic induction heating system increases the temperature of the workpiece according to the principle of electromagnetic induction.
The skin effect causes the strongest eddy currents to form on the surface of the machining parts, while they are weaker inside and approach zero at the core. As a result, this method can only heat the surface of the workpiece to a limited depth, making the heating area small and the heating rate fast – exceeding 14 °C/s. For example, a system developed by Chung Yuan University in Taiwan has achieved a temperature rate of over 20 °C/s. Once the surface heating is completed, it can be combined with rapid low-temperature cooling equipment to achieve rapid heating and cooling of the mold surface, enabling variable mold temperature control.
(6) Infrared radiation heating system Researchers are developing a method that uses infrared radiation to heat the cavity directly.
The heat transfer form associated with infrared is radiation heat transfer. This method transmits energy through electromagnetic waves, doesn’t require a heat transfer medium, and possesses a certain penetration ability. Compared to other methods, it offers advantages such as energy saving, safety, simple equipment, and ease of promotion. However, due to the weak absorption capacity of the bright metal’s flame, the heating speed could be faster.
(7) Gas receipt system
The injection of high-temperature gas into the mold cavity before the filling stage can rapidly and precisely increase the mold surface temperature to about 200°C. This high-temperature area near the mold surface prevents compatibility issues due to severe temperature differences. This technology requires minimal modifications to existing molds and has low manufacturing costs, but demands high sealing requirements.
However, there are still some challenges with the temperature control system. Practical heating methods such as steam and high-temperature water heating are limited, and high-gloss injection molding requires a separate mold temperature control system used in conjunction with the injection molding machine. Furthermore, the equipment and operating costs are high. The goal is to develop and implement economically viable large-scale production of variable mold temperature control technology without affecting the molding cycle. Future research and development are needed, particularly in practical, low-cost rapid heating methods and integrated high-gloss injection molding machines.
High-gloss injection molding is a common method used by injection molding enterprises, which produces glossy products. By increasing the interface temperature of the melt flow front and contact point of the die surface, intricate mold parts can be easily replicated. By combining high-gloss surface molds with special engineering plastics, high-gloss injection molding products can be achieved in a single step. This lathe process is also known as rapid thermal cycle injection molding (RHCM) due to the rapid heating and cooling, variable mold temperature, dynamic mold temperature, and alternating cold and hot mold temperature control technology. It is also referred to as spray-free injection molding, no-weld mark, and no-trace injection molding for eliminating the need for post-processing.
The heating methods include steam, electric, hot water, high oil temperature, and induction heating mold temperature control technology. Mold temperature control machines are available in various types such as steam, superheated, electric, water, oil, and electromagnetic induction mold temperature machines.
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Post time: Sep-02-2024