THAT IS THE INJECTION MOLDING
Injection molding is a semi-continuous process which involves injecting a polymer melt or ceramic (or rubberized) pressure in a closed mold and cold through a small hole called a gate. That mold material solidifies, beginning to crystallize in semicrystalline polymers. The part or end part is obtained by opening the mold cavity and remove the molding.
Injection molding is a popular technique for the manufacture of very different items. In the United States alone, the plastics industry has grown at a rate of 12% annually for the past 25 years, and the main process is the transformation of plastic injection molding, followed by extrusion. An example of products manufactured by this technique are the celebrity interconnectable LEGO blocks and Playmobil toys, as well as a large number of automotive components, components for aircraft and spacecraft.
Polymers have managed to substitute other materials such as wood, metals, natural fibers, ceramics and even precious stones; Injection molding is a more environmentally friendly process compared to paper manufacturing, logging or chrome. Not polluting the environment directly, it does not emit gases or aqueous wastes with low noise levels. However, not all plastics can be recycled and some can be recycled are deposited in the environment, causing damage to the environment.
The popularity of this method is explained with the versatility of parts that can be manufactured, rapid manufacturing, scalable design from rapid prototyping processes, high production and low costs, high or low automation as the cost of the part geometries very complicated than would be impossible by other techniques, castings require little or no finishing are then finished with desired surface roughness, color and transparency or opacity, good dimensional tolerance of castings with or without inserts and with different colors.
The injection unit
The most important parts of the machine are:
The main function of the injection unit is to melt, mix and inject the polymer. To achieve this spindles of different characteristics are used as the polymer to be melt. The study of the process of fusion of a polymer injection unit must consider three thermodynamic conditions:
1. The processing temperature of the polymer.
2. The polymer heat capacity Cp [cal / g ° C].
3. The latent heat of fusion, if the polymer is semicrystalline.
The fusion process involves an increase in the heat of the polymer, resulting from an increase in temperature and friction between the barrel and screw. Friction and shear forces are critical to an efficient fusion, since polymers are not good conductors of heat. An increase in temperature decreases the viscosity of the molten polymer; the same happens with increasing cutting speed. Thus both parameters should be adjusted during the process. There are also standards for each polymer in order to prevent corrosion or degradation metals. With some exceptions, such as the PVC-, most plastics can be used on the same machines.
The injection unit is originally an extruder with a single screw having a barrel heaters and sensors to maintain a constant set temperature. The depth between the channel and the spindle decreases gradually (or drastic, in special applications) from the feed zone to the metering zone. Thus, the pressure in the barrel increases gradually. The mechanical stress of cutting and compression system and adding heat to melt the polymer more efficiently if it would only heat, being the main reason why a screw is used and not an autoclave for the cast.
A substantial difference from the extrusion process is the existence of an extra part called backup camera. This is where the molten polymer to be injected is accumulated. This chamber acts as a piston; entire unit works as the piston pushes the material. Because of this, a part of the spindle ends being underused, so long guns are recommended for efficient mixing processes. Both injection and extrusion must consider relations PVT (pressure, volume, temperature), to help understand how a polymer melt behave