Injection Molding
Many different processes are used to transform plastic granules, powders, plastic material moldable form, various forming methods. Most cases thermosetting materials require other methods usually heated soft state reshaped before cooling. Thermoses, other hand have yet been polymerized before processing, chemical reaction takes place during process,usually through heat, concept while studying plastics manufacturing processes polymers used.
Injection molding is by far the most widely used process of forming
thermoplastic materials. It is also one oldest. Currently injection molding accounts for 30% of all plastics resin consumption. Since raw material can be converted by a single procedure, injection molding is suitable for mass
production of plastics articles and automated one-step production of complex
geometries. In most cases, finishing is not necessary. Typical products include toys, automotive parts, household articles, and consumer electronics goods.
Since injection molding has a number of interdependent variables, it is a process of considerable complexity. The success of the injection molding operation is dependent not only in the proper setup of the machine hydraulics, barrel temperature variations, and changes in material viscosity. Increasing shot-to-shot repeatability of machine variables helps produce parts with tighter
tolerance, lowers the level of rejects, and increases product quality (i.e., appearance and serviceability).
The principal objective of any molding operation is the manufacturer of products: to a specific quality level, in the shortest time, and using repeatable and fully automatic cycle. Molders strive to reduce or eliminate rejected parts in molding production. For injection molding of high precision optical parts, or parts with a high added value such as appliance cases, the payoff of reduced is high.
Plastic granules are fed into the hopper and through an in the injection cylinder where they are carried forward by the rotating screw. The rotation of the screw forces the granules under high pressure against heated walls cylinder causing them to melt. As the pressure building up, the rotating screw is forced backward until enough plastic has accumulated to make the shot.
The injection ram (or screw)forces molten plastic from the barrel, through the nozzle, sprue and runner system, and finally into the mold cavities. Once the cavity is packed, pressure applied to the melt prevents molten plastic inside the cavity from back flowing out through the gate. The pressure must be applied until the gate solidifies. The process can be divided into two steps (packing and holding) or may be encompassed in one step(holding or second stage). During packing, melt forced into the cavity by the packing pressure compensates for shrinkage. With holding, the pressure merely prevents back flow of the polymer malt.
After the holding stage is completed, the cooling phase starts. During, the part is held in the mold for specified period. The duration of the cooling phase depends primarily on the material properties and the part thickness. Typically, the part temperature must cool below the material’s ejection temperature. While cooling the part, the machines plasticates melt for the next cycle.
The polymer is subjected to shearing action as well as the condition of the energy from heater bands. Once the short is made, plastication ceases. This should occur immediately before the cooling phase. Then the mold opens and the part is ejected.
When polymers are fabricated into useful articles they are referred to as plastics, rubbers, and fibers. Some polymers, for example, cotton and wool,occur naturally, but the great majority of commercial products are synthetic in original. A list of the names of the better known materials would include Bakelite, Dacron, Nylon, Celanese, Orlon, and Styron.
Previous to 1930 the use of synthetic polymers was not widespread. However, they should not be classified as new materials for many of them were known in the latter half of nineteenth century. The failure to develop them during this period was due, in part, to a lack of understanding of their properties, in particular, the problem of the structure of polymers was the subject of much fruitless controversy.
Two events of the twentieth century catapulted polymers into a position of worldwide importance. The first of these was the successful commercial production of the plastic now known as Bakelite. Its industrial usefulness was demonstrated in 1912 and in the next succeeding years. Today Bakelite is high on the list of important synthetic products. Before 1912 materials made from cellulose were available, but their manufacture never provided the incentive for new work in the polymer field such as occurred after the advent of Bakelite. The second event was concerned with fundamental studies of the nature polymers by Staudinger in Europe and Carohers, who worked with the DuPont company in Delaware. A greater part of the studies were made during 1920’s. Staudinger’s work was primarily fundamental. Carother’s achievements led to the development of our present huge plastics industry by causing an awakening of interest in polymer chemistry, an interest which is still strongly apparent today.
The Nature of Thermodynamics
Thermodynamics is one of the most important areas of engineering science used to explain how most things work, why some things do not the way that they were intended, and why others things just cannot possibly work at all. It is a key part of the science engineers use to design automotive engines, heat pumps, rocket motors, power stations, gas turbines, air conditioners, super-conducting transmission lines, solar heating systems, etc.
Thermodynamics centers about the notions of energy, the idea that energy is conserved is the first low of thermodynamics. It is starting point for the science of thermodynamics is entropy; entropy provides a means for determining if a process is possible.
Clear, a refrigerator, which is a physical system used in kitchen refrigerators, freezers, and air-conditioning units must obey not only the first law(energy conservation) but the second law as well.
To see why the second law is not violated by a refrigerator, we must be careful in our statement of law. The second law of thermodynamics says, in effect, that heat never flow spontaneously from a cooler to a hotter object.
Or, alternatively, heat can flow from a cooler to a hotter object only as a result of work done by an external agency. We now see the distinction between an everyday spontaneous process, such as the flow of heat from the inside to the outside of a refrigerator.
In the water-ice system, the exchange of energy takes place spontaneously and the flow of heat always proceeds from the water to the ice. The water gives up energy and becomes cooler while the ice receives energy and melts.
In a refrigerator, on the other hand, the exchange of energy is not spontaneous. Work provided by an external agency is necessary to overturn the natural flow of heat and cool the interior at the expense of further heating the warmer surroundings.