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With the rapid development of the plastic processing industry, the use of hot runner molds has become more and more widespread. The main advantages of using hot runner molds for injection molding are: no nozzle recycle, saving raw materials; shortening the molding cycle, improving productivity; ensuring the molding quality of plastic parts and realizing production automation.
In the field of bottled water production, bottle types range from 300 to 1500ML. At present, mineral water bottles with specifications below 600ML are optimized for injection-blowing products: the threaded mouth is clear and the cap is easy to screw; the parting line is small; there is no cutting mouth, etc. . Whether it is the "one-step method" or the "two-step method", the billet must be injected.
Using traditional cold runner injection molds to inject pipe blanks, especially one-mold multi-cavity molds, the main and sub-runner occupies a large proportion of the material, resulting in a lot of recycled materials. If discarded, it will cause huge waste. Smashing and mixing may bring Many problems such as reduced transparency, mixed impurities, and increased brittleness of preforms are potential hazards to food hygiene and packaging quality, and it is difficult to automate the production process.
Using hot runner molds, as long as the process parameters are adjusted correctly to meet the process requirements, there is almost no recycle material, the material utilization rate will reach more than 99%, and the work site is clean and easy to achieve automation.
However, to realize the wide application of hot runner molds in the field of PET bottle manufacturing, the following conditions must be met from many aspects:
1. The rationality of the mold structure
For a multi-cavity mold, the structure is roughly divided into three parts: main runner, heating plate and hot runner nozzle. Among them, the hot runner nozzle is the most basic part. The molten PET enters the cavity through the nozzle through the main runner in the heating plate, which is directly related to the quality of the tube blank. Therefore, the following principles must be adhered to in the mold structure:
1.1 Maintain the thermal balance of the nozzle so that the resin is always in a molten state during the injection process. For this reason, it is necessary to make the contact surface between the nozzle and the template as small as possible.
1.2. Ensure tightness. Such as the "face" between the nozzle and the runner plate, the joints between the parts of the nozzle must be sealed to prevent the molten resin from overflowing under the injection pressure.
1.3. When the cooled nozzle reheats and starts to work, the condensate at the gate is not easy to melt, and it is often "melted". Therefore, the length of the nozzle should be designed reasonably and the nozzle should be made of a material with good heat transfer such as beryllium copper.
1.4. The nozzle is often clogged, so it is necessary to ensure that it is easy to disassemble.
2. The rationality of gate design
The structure of the gate has a great influence on the quality of the tube blank and the forming process. Practice has proved that the use of a tapered gate is the best, with a smaller end diameter, faster heat dissipation, and easy condensation during the pressure holding stage.
In addition, the size should be appropriate, too large is not good for the heat balance, and the condensation is slow; too small will easily lead to "white turbidity" at the bottom of the bottle.
3. Matching injection molding machinery
For a multi-cavity hot runner mold, the pressure loss cannot be ignored. The injection molding machine is required to provide stable injection pressure and adjustable injection speed, as well as control back pressure. In addition, it can also provide heating power for the hot runner.
4. Mature process conditions
4.1 Effective mold temperature control: mold cavity and mold core can achieve rapid cooling, cooling water is required, and the water hardness is appropriate, anti-scaling and anti-blocking.
4.2. During the injection molding process, the screw can be "reversed" (pressure relief) to ensure that the bottom of the blank is formed well. This is particularly important for the "one-step method", otherwise the bottle will easily leak the bottom.
4.3. Choose a suitable heating temperature, the temperature of the hot runner is 2-4℃ lower than the temperature of the barrel.
4.4. The cooling, injection, holding time and even the opening and closing time of the mold must be well coordinated with the injection and pre-molding speed, so that the cycle can be minimized.
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