Extra Requirement for Molds

Additional Requirement for Molds

images(1) Pressure air. Some molds require air pressure for their operation. In general, the designer should be aware that compressed air, especially in large volumes, can be very expensive, especially if it is left to blow for any length of time.

  • Blow downs(air jets or air curtains) are often used to assist the products to rapidly clear the molding area. There are several commercial air jets on the market with low consumption of pressure air. Their initial cost is paid back rapidly by savings from wasted air volume.
  •    Air-operated actuators.The air volume used is usually small, compared with a blow down. There could be problems with controlling the speed and uniform motion of air actuators, but they are simple and inexpensive.
  • Air required for air ejection,which is usually activated on demand, for a very short time. Most of the time, the actuation time is controlled from the machine control panel. The designer must make sure that the intended machine is equipped with sufficient controls and hardware (timers, valves, and large enough supply lines). It may be even necessary to add pump capacity, for the added volume of air that will be required for the planned mold. If much air is needed for short blasts, one or several suitable accumulators could be installed near or even on the mold.

Where pressure air comes into contact with the molded products, for example, in blow downs or in air ejection, the air must be filtered from any oil residues, water (always present in air lines), and so on, before reaching the outlets in or at the mold, to prevent contamination of the products if they are used for food or pharmaceutical purposes. (Unfortunately, most air actuators require lubricated air, unless their seals are selected for dry air.) A low-pressure, high-volume blower with its air intake from the shop environment, or better yet, from within an enclosure built around the molding machine when special “clean room” requirements are specified, is a preferred solution to ensure that there is no oil or water contamination in the air as it comes into contact with the plastic products. In many cases, such blower can be directly mounted on the top of the mold. Another advantage is that the power consumption of this type blower is low, on the order of 0.2kW (1/4hp) or less, and does not require timing or valving.

AirCylinder_F1(2) Auxiliary hydraulic supply. For some operations, compressed air may be not suitable. (a) Air cylinders are often jerky in their operation, especially with long strokes. (b) In cases where several air cylinders actuate one large mold member, the forces can be uneven and the member can jam. (c) In most molding shops the compressed air pressure is fairly low, usually about 600kPa (80psi), and rarely 900kPa (120psi), so large air actuators are needed to produce large forces. It could be difficult to accommodate sufficiently large cylinders within the available mold space, or even outside the mold. In all these cases, the much more powerful hydraulic cylinders would be an alternative. The hydraulic pressure could be taken from the machine system with a pressure reducing valve, and by providing the necessary safety measures to protect against the very high pressures in that system. A preferred method, however, is to use an auxiliary power supply, usually at a system pressure of about 3,500 kPa (500psi). This is much safer and requires much less expensive hardware (valves, hoses,etc.) than that for higher pressure. The motion of hydraulic operators is smooth and the speed can be well controlled.Two points of caution, though. Hydraulic oil (with some special, expensive, exceptions) is highly flammable and there is always the danger of leaks, especially if the leaks were to occur near heated areas of the mold, for example, near a hot runner system. Also, products used in the food or pharmaceutical industry could be contaminated by the oil; this is usually specified as not allowed.

CIM-cooling-channel(3)    Cooling water supply. This is a very important area of concern. There is not much sense in designing the mold with very sophisticated cooling circuitry if the cooling water supply is insufficient in temperature,volume,and pressure. An individual chiller unit may be the answer if the plant supply is too small or has not enough pressure. It is also important that the coolant is clean, that is, with a minimum of minerals or dirt,and is not corrosive. Dirty coolant could gradually plug the water circuits or coat the channel walls with a poor heat conducting layer of dirt and lime,thus reducing the cooling efficiency,and could require frequent cleaning of the coolant channels if the mold is expected to maintain high productivity. Corrosive action of the coolant could attack and eat away the mold steels; rust creates insulating layers similar to lime and dirt deposits. It is always good policy for the designer to check with the molder to ensure that there are no such problems with the water supply, and to specify that only clean,noncorrosive coolant is used with the mold.

PDR_1198-crop(4)    Electric power and controls. The electric power supply in North America and in most developed countries is usually sufficiently stable and uninterrupted, except during natural catastrophes, and of not much concern to the designer. This is not the case in developing countries, where power interruptions occur frequently; the effects of such interruptions on the operation of a mold may cause concern. Typically, in the case of a power failure, a machine using a cold runner mold will just stop,but can resume work as soon as the plastic is again up to molding temperature. However,in a hot runner mold the melt will freeze in the manifold and nozzles and it may take much more time to restart (in small molds between 15 and 30 minutes). The expected savings through using a hot-runner mold may become an illusion. The controls (breakers, heat controllers) available to operate a mold on a specific machine must be discussed with the molder when designing a mold that will require additional heat controls; typically, such controls are required for hot runner molds. For safety reasons,heaters in molds are rated at 230VAC or less,and the power consumption may be from as low as 40 W per heater,such as in some nozzle heaters, and up to several thousand watts in hot runner manifold heaters.

Since heaters are often bundled in parallel and operated by designated controls, it is important to ensure that adequately sized circuit breakers and so on are available; some can be controlled with time-percentage controllers or variable (voltage) transformers, whereas some will need thermocouples and heat controllers.

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