How to Select Plastic Auxiliary Equipment — Heating and Cooling

Water Chiller

How to select water chiller

How to select a water chiller? In fact, a mould is a heat exchanger. Heat transfers from the melted plastics to the mould, and then from the mould to the chilled water, which is the cooling medium circulating continuously. Finally, only a little transfer into air and mold plate of injection molding machine. As we know, most of molding cycle is used for cooling, which may take up 80% of total time. So it is absolutely necessary to control the cooling time in minimum level. For example, a molding cycle is generally 20 seconds. If we adopt the chilled water generated by water chiller instead of water stored in water tower, the molding cycle will be reduced to 16 seconds. Although the initially cost for water chiller is high, it may enhance the output by 20% which is profitable in long-term production. Therefore, how to choose energy of chilled water? According to the above we know that it is related to specific heat of molding materials, melting temperature, weight and de-molding temperature.

The computational formula of energy of chilled water needed is: Q=C×M×△T×S
Q stands for the energy of chilled water needed, kcal/h
C stands for specific heat of raw material, kcal/kg℃
M stand for weight of raw material, kg/h
△T stands for the temperature difference of melting temperature and de-molding temperature.
S stands for safety coefficient (usually 1.35-2.0), when one water chiller equip with one molding machine, we generally select 1.35. When one water chiller equip with many molding machines, we generally select 2.0. For example, if you select an air-cooled water chiller, S should be larger too.
For example: A mould produces PP products, the output capacity is hourly 50kg, what is the energy needed? What type of water chiller is suitable?
Q=50×0.48×200×1.35=6480(kcal/h);
The energy needed is hourly 6480kcal/h, so you can select water chiller with standard power of 3HP or so. But this is an example, and complete data is hard to get in real production. According to our long-term planning and experience, △T=200℃ is an average number got by long-term statistics for many common products.
If there is hot runner on mould, the energy of it should be taken to calculate, and the unit is KW. When calculating, the unit of KW should be converted to kcal/h, 1KW=860kcal/h. If a factory has sufficient water, which has low temperature and cost, it is unnecessary to use water chiller. But it is unrealistic unless the factory is located near lake with low water temperature. If using deep wells to supply water whose temperature and flow can meet requirement, it will cost too high. The method is suitable for experimental facility, but it is also unrealistic for factories.

Temperature Difference of Chilled Water

The cooling water (chilled water) temperature of mould is usually limited by processing materials and form of products, such as PE light-wall beaker, mould required the chilled water temperature below 0℃ above. But in most cases, the requirement is above 5℃ which can be provided by water chillers, and low-temperature water chillers can supply cooling water with temperature below 5℃or 0℃. (Requires brine or antifreeze)

The temperature difference of chilled water between inlet and out inlet on mould is set according to requirement of products. In most cases, it is most suitable to set the temperature difference at 3℃~5℃, but 1℃~2℃ is available. The lower the temperature difference is, the heavier the chilled water flow is needed to take out the same amount of heat. On the contrary, the required flow is less. For example, if temperature difference is 5℃, chilled water should be 60L. If temperature difference is 2℃, chilled water needed is150L.

Flow of Chilled Water

Chilled water flow of mould connects directly with heat quantity to be removed by mould and temperature difference of chilled water flowing into and out of mould. For example, if the heat quantity of 6480kcal/h should be taken away form mould and the temperature difference is 3℃, how large is the flow required? The flow of chilled water Q=6480÷3÷60=36（L/min）.

Treatment of the Chilled water quality

Softening of water, in the process of using water chiller, is a problem should not be neglected. The PH value of water also needs to be observed continuously, and 7 is the optimal value. Corrosion phenomenon will occur as PH value is higher than 7. If we don’t take an action, fouling will appear in evaporator and mould and insulates heat and even make efficiency of energy conversation reduce by 30%. Obviously, it is necessary to make water softened. The most effective method is to equip an electric water softener, which is made by ion exchange principle. Water softeners which directly connect to pipeline of circulating water are selected according to different flow. It doesn’t cost too much and detergent may be added into circulating water periodically.

Flow and Pressure of Water Chiller

The pressure of cooling water used for cooling of injection mould is selected within 0.1~0.2Mpa (lift head is 10~20m). Common water chillers can not meet the requirement of water supply by adopting proper pump.

The relationship between flow and diameter of pipes

Cooling of Hydraulic Oil

Usually hydraulic oil was cooled by water tower. It is not only the optimal method, but also very economical according to production cost. Unless there is special requirement for temperature, it can be cooled by chilled water. When cooled by cooling water, we should consider the motor power. When do it like that, we should adopt the motor with power that is 0.6 time more than that of the chiller.

Insulation for Pipeline of Chilled Water

Chilled water pipeline must be insulated, because it avoids not only serious waste of refrigerating effect, but also dew forms at the ektexine of the pipeline. For example: if chilled water temperature is 10℃ and ambient temperature is 30℃, the thermal radiation of metal pipeline, with length of 25m and surface area of 25m2 , can reaches 750kcal/h which is almost 10% of that generated by compressor of 3HPand 6% of that generated by compressor of 5HP.

Water chiller was usually connected with mould by reinforcing hose, because this type of hose can insulate heat. But if the length of it is over 5m, the thermal insulation should be considered properly.

Attached list: Different molding materials with their molding temperature and heat specific heat capacity.

Heater

The heater mainly applies to heat up the mould and maintain temperature, and they can be used in other similar applications. The selection of it is the same with water chiller, which is a refrigeration process but the heater works in a heating process. It is related to specific heat of materials, injection temperature, weight of mould, heating time of achieving a desired temperature and ambient temperature.

Heat energy needed for heating a mould up to a desired temperature: Q=C×M×△T×S/T
Q stands for desired heat energy kcal/h.
C stands for mould specific heat kcal/kg℃.
M stands for mould weight kg.
△T stands for temperature difference between ambient temperature and mould temperature.
T stands for ideal time of heating up to a desired temperature.
S stands for safety coefficient, usually take 1.5~2.0.
For example: A customer wants to have a mould heated up to a desired temperature in 0.5hr, the weight of it is 300kg and injection temperature is 80℃. What type of heater is needed?
Q=C×M×△T×S/T=0.114x300x(80-30)x1.5/0.5=5130Kcal/hr=5.96KW
So STM-607W is available.