How to choose the hot runner for the injection mold
Hot runner technology, with its advantages of low scrap rats, shorter the molding cycle times, has become an important direction of development of plastic injection molding process. In western countries, hot runner system are widely employed in injection molding process, in China mold making industry, hot runner system is also a trend since china injection molders face labor cost and raw materials increase.
So how to choose the right hot runner system to maximum the advantages of it? There are a few aspects that we need to take into consideration to determine the most proper hot runner system to use.
Hot runner injection system pressure drop can not be ignored. Many hot runner system users have misunderstandings: the plastic melt inside of the hot runner is always hot, so hot runner injection pressure drop is much smaller than the cold runner. In fact, to meet the needs of hot runner design, length of melt flow in the hot runner system will be greatly increased, so hot runner injection system pressure drop is big and can’t be ignored.
In actual case, it happens a lot when it’s difficult to inject due to the big injection pressure drop, so for some of the plastic resin, like PC, POM which had poor melt-flow performance, and some big parts which required long melt flow length to get fully filled, we recommend to do mold flow simulation in advance. Actual design and determined should be made base on data output.
One major difference between the hot runner systems is the way to heat the melt. A heater inside the heating system is mounted directly on the melt channel, material gets heated inside. External heating system heat the material externally, the raw material is allowed to flow through the runner without any barriers. External heating provides a more reasonable melt shear force curves.
There are many injection mold gate types available. Factors to consider include: allowing the gate marks, gate location and injection of material types. Whether a non-crystalline, or crystalline thermoplastic elastomers, have a different type of gate is limited to a specific material. Understanding of the injection mold gates and materials help to make the appropriate choice.
Standard or custom made
Usually ,the hot runner suppliers provides solutions of custom made hot runner systems and standard hot runner systems. the user should try to choose a standard hot runner systems, including standard length and standard-sized nozzles, runner boards, gate inserts and so on. This is not only because the standard hot runner systems have low prices, short delivery time, but also because the interchangeable parts are available, easy to do maintenance.
Types of plastics processing
When selecting hot runner systems for injection molds, we must consider the plastic resin to proceed. For example, if we process glass reinforced plastic (such as glass reinforced nylon materials, etc.), the gate inserts much be able to provide good wear resistance; If the plastics resin to proceed is easy to get discomposed (PVC), we should use external heat system runner to avoid flow melt dead corners. If we proceed resin with poor melt flow property such as PC, a larger size nozzle and larger size cross-sectional runner need to considered in priority.
In order to ensure optimum performance, analysis for processing conditions of specific resin is necessary, therefore, not only to consider the weight of the product, but also to calculate the appropriate melt channel dimensions, including melt flow index, injection time, injection molding cycle and melt temperature and so on. Proper runner size is critical to the performance of the hot runner, otherwise it will cause the degradation of plastics injection molded parts or cause uneven or incomplete filling and so on. To determine the optimal size of the melt runner, you need to consider the pressure drop, residence time, temperature, shear rate, frequency and so on.
Multi-zone temperature control
If the user requires a large size, complex hot runner systems, or to proceed the temperature sensitive plastic resin with narrow processing parameters, a multi-zone temperature controller system should be employed. It allows users to adjust the temperature locally. An ideal system should have a uniform hot runner temperature distribution, but in actual case, they might be different due to different factors: the quality of the hot runner heater assembly, heat lost on the joint between the mold hot runner system and the mold.
There are many brands of hot runner system in the market, price, quality, delivery time, services are all need to be considered to choose the right one, Aco mold use different brands to meet customers’ satisfaction, frequently to use hot runner brands includes Mold-master, husky, syventive, INCOE.D-M-E, etc.
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