Process of Plastic casting

Casting, that is, pouring a liquid into a mold without pressure, can be done with several plastics. When there is not sufficient justification for making expensive dies, the casting process is generally used.

Open molds into which the liquid resin , mixed with a catalyst or hardener , is poured can be used for most plastics.

Centrifugal casting, using the same methods can be used with several plastics to such as polyesters.

Shell molding is some what similar to casting in that no pressure is used. In this process, powdered resin is placed in a heated mold until a layer fuses to the desired thickness. The unfused resin inside is taken out of the mold and the hollow molding removed. Sometimes the mold is rotated in what is called rotational molding.

Slush molding is similar to shell molding. It employs a thermoplastic-resin slurry or "slush" which is poured into a preheated mold. The heat causes slurry to set in a viscous layer of the desired wall thickness.

Both shell and slush molding produce thin-walled products such as toys and gloves.

Hand Lay-up is a simple method to form plastic materials into a required shape. Special shapes like boats , tanks and big-sized parts of plastics are produces by this method. First a mold of desirable shape is prepared. The mold is covered by fiber mats of desired thickness and plastic resin (such as epoxy) is draped over the mold and allowed to harden.

Comments

Advantages And Disadvantages Of Air Refrigeration System :

Advantages Of Air Refrigeration : 1. The refrigerant used namely air is cheap and easily available. 2. There is no danger of fire or toxic effects due to leakages. 3. The weight to ton of refrigeration ratio is less as compared to other systems. Disadvantages Of Air Refrigeration : 1. The quantity of Refrigerant used per ton of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content.

ELECTRICAL COMPARATORS AND ITS WORKING

Electrical Comparators are used as a means of detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a roll. The objec...

THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movemen...

Mechinopedia For Mechanical Engineers

Search This Blog