Role Of Utility Equipments In A Molding Shop

ROLE OF UTILITIES IN A MOLDING SHOP UTILITIES: Irrespective of the discipline of operations, “Utilities” or “service equipments” form the back bone of a manufacturing system as these equipments contribute to the proper functioning of the main machinery. In a molding industry, supporting equipments that assists the process are: • Raw material handling equipments. (Hopper Loader, Blenders, Dozers, Dehumidifiers, Hopper dryers, Ovens etc.,) • Recovery Equipments. (Grinders, Granulators etc.,) • Mold Temperature Controllers & Hot runner Controllers • Part Handling equipments (Robots, conveyors, sprue separators etc.,) Similarly, the service line equipments that support the system of operation are: • Cooling towers as to supply cooling water recycled. • Softeners as to maintain the quality of the cooling medium • Chillers for supply of chilled water to molds and Mold sweat preventor • Air compressors for supply of air • Hoists for handling of molds, machine assembly etc., Why we need a cooling system? In every manufacturing process, repeatability is the key factor for productivity. Consistency of the process parameters is essential for the repeatability. These process parameters are influenced by various factors and temperature (of oil, mold) is one of the major factors that influence the process parameters. As to maintain a consistent temperature, a cooling system is essential. We control the temperature of Hydraulic oil (used in the machine), hopper throat area and molds in our process. The effects due to lack of sufficient cooling system: If the temperature of hydraulic oil is not maintained (recommended value is 47°C to 50°C), pressure differences would occur because of drop in viscosity resulting in inconsistency of process parameters. Over heating of oil would also damage the oil condition, the seals, result in leak and loss of oil etc. Similarly, if the hopper throat is kept hot, the plastic granules enter into the barrel begins to soften at this stage itself. They stick to each other, prevent free flow of granules. This will increase doze time, lead to material bridging. If the mold temperature is not maintained, the dimensional consistency of the part produced cannot be controlled. The increase in mold temperature leads to heating of machine platens, which, in turn, lead to increase in tonnage than the set value (only in toggle machines). How a cooling system works: Water from a sump is pumped into a system where it will extract heat. The heated water is forced through a cooling tower, cooled and brought back to the sump for another cycle. During the process, there will be loss and this loss has to be compensated from time and again. Gopal

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ROLE OF UTILITIES IN A MOLDING SHOP

Heat Exchanger: is a device that transfers heat through a conducting wall from one fluid to another. The heated water is cooled further for recycling. COOLING TOWERS: Principle of Operation: In an induced draught type cooling towers, the principle of adiabatic expansion is adapted to cool the water. The cooling tower houses multiple nozzles that have small orifices as openings, PVC fills or wooden logs stacked in zigzag manner as to create a turbulent flow and a fan to blow air outside. When the water (that carries the heat from the system) is sprinkled through the small nozzle openings, the temperature reduces. As the air is thrown out of the tower by the fan, the water evaporates. This evaporation removes the heat from the remaining water. This results in the cooling of water. The cooled water is again taken to sump for another cycle. Though this forms a close-loop, there is a loss of water due to evaporation. This loss is directly proportional to the difference in temperature (ΔT). This loss of water is to be made up at regular intervals. However, in this process, one cannot reduce the water temperature less than the wet-bulb temperature. The wet bulb temperature is read when a wet wick is covered over the bulb dipped in a distilled water container whereas the dry bulb temperature is read as it is. Reasons for not getting desired cooling: The fan in the cooling tower is not working The sprinkle-nozzles are clogged or broken. Scale formation on the wall of heat exchangers resulting in water not taking away the heat. In-sufficient water flow rate. Water flow has to be turbulent for efficient heat transfer. Laminar flow will not extract heat from the system. How scaling is formed: We have seen that evaporation takes place while cooling. Water contains variety of salts in it. As the water evaporates, the pure water goes as vapor and the salts are left over and get sediment in the cooling tower itself. Similarly, on the heat exchanger walls, the alkaline contents in the water form a layer of scale in the process of extracting heat. Blow-down and Water treatment: The salt sediment collected in the bottom of the cooling tower has to be removed to prevent clogging of water lines and hence the water from the bottom of the cooling tower reservoir is removed and so the sediments. This is termed as blow-down.

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ROLE OF UTILITIES IN A MOLDING SHOP To avoid frequent scaling water is treated with scaling inhibitors, corrosion inhibitors and treated for hardness control. Softeners are used for this. CHILLERS: We have seen that with cooling towers, we cannot reduce the water temperature lesser than the wet bulb temperature. However, our process requires more cooling and perforce we use chillers. Chillers are of two types namely Water-cooled and Air-cooled. In our facility, the impac chillers and Flying tiger chillers are water cooled ones and the Nova-Frigo are air-cooled ones. How it works: The chilling medium or refrigerant is compressed and the gas is transformed into vapor with high pressure. This is passed through condensers (water or air) where the vapor is transformed into liquid. This liquid at high pressure is passed through an expansion valve. Due to Joule-Kelvin effect, the temperature drops drastically and this chilled liquid is passed into exchangers and this takes away the heat from the water. In the process, the liquid is heated and transforms into gas that is compressed again for further cycles. A closed-loop circuit gives better efficiency in a chilling system. The temperature required for mold cooling should be 10° to 15°C. A chilling system requires 3% more capacity to reduce every 1° of water temperature below 10°C. Mold Sweat Preventor: Here the atmospheric air is sucked in, cooled by means of chilled water dehumidified, then further cooled by refrigeration and warmed by the condensation process and blown. The functioning of this is similar to a chiller. General problems with chillers: The compressor would trip due to two major problems, High and Low pressure. If the cooling medium (water or air) itself is hot or lack of the cooling medium, the compressor would trip due to High pressure. If the cooling medium is cooler more than the required range or lack of gas at the suction point of compressor, the compressor would trip due to low pressure. Calculating the size of the system and flow rate: The amount of heat to be removed depends on the quantity of the plastics produced. The table provides guidelines on ton requirement for every kg of plastics produced for different materials. POLYMERS Kg/ hour / ton HDPE 13.6 LDPE 15.9 PP 16 ABS 22.75 HIPS 24 Hot runner molds would require chilling capacity of 0.15tons/Kw of heater load. Gopal

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ROLE OF UTILITIES IN A MOLDING SHOP

Optimum heat transfer would take place if the flow were turbulent. To ensure turbulent flow the flow rate should be of 150 liters /min. Cumulative flow requirement of heat exchangers, hopper throats and molds are to be calculated. To overcome the resistance in the pipe lines, the water pressure at the generating point should be at least 5kg/cm². If the pressure requirement at the end point is more, we can use booster pumps. WATER QUALITY: Despite having all the equipments working in good order, one may not get the desired results if the quality of the water is not good. Water can classified into two categories namely Scaling and Corrosive. More the water is harder, more the potential for scaling and more the water is softer heavier the corrosion in the lines. One has to strike a balance between the two by monitoring the key parameters. Those key parameters are pH value, Hardness (total Magnesium and calcium carbonate content, total alkaline content), Total dissolved salts (TDS) and iron content. The hardness and TDS is measured in PPM (parts per million). The pH value of the water can be maintained by dozing acids. The ideal range is 7.5 to 8.5. The hardness of the water can be maintained by operating the softeners. By adding sodium chloride (common salt), the magnesium and Calcium ions are replaced with sodium ions thus turning the water softer. However, in this process the TDS remains unaltered. The ideal range of hardness would be 50 to 100ppm. The TDS can be reduced only by changing the basic nature of the water. Water harvesting and re-charging ground water table is the permanent solution for this. However, TDS ranging less than 1000ppm is normal. As we have read earlier, we have to strike a balance between harness and TDS levels. On ideal conditions, one would maintain the ranges of pH 7.5 -8.5; Hardness <50ppm; TDS<1900ppm. AIR COMPRESSORS: Compressors are required for raw material handling equipment, part handling equipment, pneumatic ejection in molds, movement of safety gates, maintenance of molds and machine and though not advised cleaning. Compressors are of two types – Reciprocating and Rotary Screw compressors. The compressed air can be stored in a receiver and from there the air can be distributed. This also facilitates removal of moisture or water that has condensed in the receiver. Flow rate is necessary to add up the compressed air requirement in terms of m³/hr for the shop floor. As to compensate the leakage and expansion the air

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ROLE OF UTILITIES IN A MOLDING SHOP capacity has to be calculated at 25% more. Similarly, pressure has to be maintained at 6 kg/cm² and hence at the generation point the pressure should be 8.5 kg/cm². Hopper Dryers, Ovens and Dehumidifiers: Some of the polymers processed are hygroscopic in nature that means they can absorb moisture from the atmosphere. When these materials are processed with moisture content, they would result in poor surface finishes such as silver streaks, diesel effect and splashing. This also will have effect on the impact and tensile strength of the part produced. Hence it is necessary to remove the moisture from the material. Nylon, Polycarbonate, Polysulphone, ABS are some materials that are hygroscopic by nature. Preheating of material improves the plasticizing rate of a machine. The atmospheric air is a mixture of various gases and water vapor. The presence of water vapor in the air is termed as Humidity. The amount of vapor contained in the air depends upon the temperature and pressure. The factors that influence drying of plastics are  Air temperature  Initial moisture content  Pellet size  Dew point In an oven, there will be number of trays with tiny holes on the bottom to facilitate hot air to be blown. The blower will be mounted on the rear side with heaters in the sideward. The requisite temperature can be set on the temperature controller. As the blower pumps air that is heated by the heaters, the plastics is dried. Hopper dryers are nothing but a hopper with passage for the hot air to be blown inside. The principle of operation is similar to oven. Heater with a temperature controller and the air-duct is provided on the side of the dryer. Higher air temperature means lesser relative humidity. However, the dew point would not change with higher air temperature. Thus the moisture cannot be removed from the air. As the moisture equilibrium content of the plastic pellets is limited, the degree of dryness is also limited. Thus smaller pellets dry faster than the larger ones. WHY DEHUMIDIFIER? The air surrounding the pellets should have lesser dryness as to pull out the moisture from the pellet to atmosphere. A dryer only blows hot air onto the pellets and the moisture is not effectively removed. Effective drying requires desiccant dehumidifier, which functions on the principle of physical adsorption. Desiccant, a highly hygroscopic material is porous in nature and can absorb moisture without any change in its size. When a heated air is passed through a desiccant, the dryness of the air is improved. Gopal

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ROLE OF UTILITIES IN A MOLDING SHOP

The desiccant can get saturated beyond which it cannot absorb moisture. However, by regenerating the same, it can be re-used. If the desiccant is exposed to a flow of air stream at very high temperature, the moisture from the desiccant is driven out and it is ready for another cycle of adsorption. In a dehumidifying unit, there will be two sets of desiccant cartridges. While one will be absorbing moisture the other will be in regeneration process and the dehumidification of material will not have hindrances. HOT RUNNER CONTROLLER: To facilitate free flow of molten plastics into a vast area and reduce wastage on runners, hot runners are used in molds. Heaters will be provided on the runner passage and they will be controlled by controllers from outside. Though this similar to the temperature controllers used on the machines, this is special because it drives heaters of smaller sizes and accuracy is the main criteria (or else the material will decompose). There will be 6-8 zones mounted on racks with a mother board connecting all these cards. A conventional controller will have analog temperature controllers with feedback through a thermocouple and dimmer-stat (auto transformer) where there is no feedback available on the right temperature of the surface where the heater is mounted. Courtesy, technology advances, now days we have PID based (Proportionate Integrated and Derivative) temperature controllers that are driven by microprocessors. In this type, we can choose closed-loop with thermocouple, use in percentages and in pulse mode. Based on the circuit and feedback the card will drive the triac to energize the heater. A fast acting glass fuse (faster than HRC fuses and MCBs) are used in this circuit as to prevent damage to heater or triacs. MOLD TEMPERATURE CONTROLLERS: The temperature of the mold is maintained at a desired value by balancing the heat exchange (between melt and by water from cooling channels). A stipulated temperature of a mold minimizes the initial rejections during startup. As we have seen earlier, a rise in mold surface temperature leads to dimensional inconsistency of the part produced and tonnage variations in toggle locking machine. The heating medium in MTCs can be Oil or Water. Oil is preferred the temperature is to be maintained more than 60°C. We use MTC with water circulation.

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