WHAT IS THE INJECTION MOLDING
The injection molding process is a semi that involves injecting a polymer or ceramic in the molten state (or rubberized) in a closed mold under pressure and cold, through a small hole called a gate. In that mold material solidifies, beginning to crystallize in semi-crystalline polymers. The final piece or part is obtained by opening the mold cavity and remove the molded part.
Injection molding is a popular technique for the manufacture of very different. Only in the U.S. plastics industry has grown at an annual rate of 12% over the past 25 years, and the main processing is plastic injection molding, followed by extrusion. An example of products manufactured by this technique are the famous LEGO interconnectable blocks and Playmobil toys and a lot of car parts, components for aircraft and spacecraft.
Polymers have succeeded in substituting 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. It pollutes the environment not directly, does not emit gases or aqueous waste with low levels of noise. However, not all plastics can be recycled and some that can be recycled are deposited in the environment, causing environmental damage.
The popularity of this method is explained with the versatility of parts that can be produced, the speed of manufacturing, design scalable from rapid prototyping processes, high levels of production and low costs, high or low depending on the cost automation of the part geometries very difficult that would be impossible by other techniques, castings require little or no finish as they are finished with the 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, use screws with different characteristics depending on the desired polymer melt. The study of the melting process of a polymer in the injection unit must consider three thermodynamic conditions:
1. The processing temperature of the polymer.
2. Polymer heat capacity Cp [cal / g ° C].
3. The latent heat of fusion, if the polymer is crystalline.
The fusion process involves an increase in the heat of the polymer, resulting in increased temperature and friction between the barrel and the spindle. The friction and shear are essential for efficient fusion, since polymers are not good conductors of heat. An increase in temperature decreases the viscosity of the polymer melt, the same happens with increasing cutting speed. Thus both parameters should be adjusted during the process. There are also standards for each polymer metals in order to prevent corrosion or degradation. With some exceptions, such as PVC, most plastics can be used on the same machines.
The injection unit is originally a machine with a single screw extrusion, having the barrel heaters and temperature sensors to maintain a constant programmed. The depth of the channel and the spindle gradually decreases (or drastic, special applications) from the feeding zone to the area of dosage. Thus, the pressure in the barrel increases gradually. The mechanical stress, shear and compression added heat to melt the polymer system and more efficiently than if it had only heat, which is the primary reason that uses a screw and not an autoclave to obtain the cast.
A substantial difference with respect to the extrusion process is the existence of an extra part called backup camera. This is where the polymer melt is accumulated to be injected. This camera acts like a piston, the entire unit acts as the piston pushes the material. Because of this, some end up underutilized spindle, so long guns are recommended for efficient mixing processes. Both extrusion and injection must take into account the relationships of PVT (pressure, volume, temperature), which help understand how it behaves in a polymer melt.
Clamping unit 
It's a mechanical or hydraulic press with a clamping force large enough to counteract the force exerted by the polymer melt to be injected into the mold. Localized forces can generate pressures of hundreds of MPa, which are only found on earth naturally only in the deepest points of the ocean.
If the closing force is insufficient, the escape by the binding material of the mold, thereby causing the mold shop to open. It is common to use the projected area of a piece (area representing the unit perpendicular to the total closure of the cavity) to determine the clamping force obtained, other potential gaps or holes in the piece.
Schematic of a commercial mold panel, which only lacks the cavity for the desired piece
The mold (also called a tool) is the most important part of the injection machine, as is the space where the piece is generated, to produce a different product, simply changing the mold, being an interchangeable piece that is screwed into unit closure. There are two major types of mold, one in which the injected plastic and another in which injected metal.
The mold parts are:
* Chamber: is the volume in which the part is molded.
* Channels or ducts: they are conduits through which the molten polymer flows due to the injection pressure. The feeder canal is filled through the nozzle, the following channels are called troughs and finally found the gate.
* Cooling Channels: These are channels through which coolant circulates (most commonly water) to regulate the temperature of the mold. Its design is complex and specific to each piece and mold, this, given that cooling should be as homogeneous as possible throughout the cavity and on the fixed and the mobile part, that in order to avoid the effects of contraction . Note that at the time of designing a mold, the cooling system is the latest to be designed.
* Ejector bars: when opening the mold, these bars eject the molded part out of the cavity, and can sometimes be assisted by a robot to perform this operation.
THE PRINCIPLE OF MOLDING
Injection molding is one of the plastic processing technologies most popular, accounting for a relatively simple to manufacture components with highly complex geometric shapes. This requires a machine that includes an injection mold. In the latter, is made a cavity whose shape and size are identical to the part that is desired. The cavity is filled with molten plastic, which solidifies, maintaining the molded shape.
The three-dimensional polymers retain their shape when cooled below its Tg, and therefore also of its melting temperature for semi-crystalline polymers. Amorphous polymers, as useful temperature is lower than its Tg, are in a thermodynamic state of pseudoequilibrio. In this state, the rotation and relaxation (unwinding of the chains) of the polymer are highly impaired. It is for this reason that in the absence of effort, three-dimensional shape is retained. The semi-crystalline polymers have also the property of form crystals. These crystals provide dimensional stability to the molecule, which is also-in-thermodynamically stable crystalline region. The entropy of plastic molecules decreases dramatically due to the ordering of molecules in crystals.
COLOR IN PART
LEGO pieces in different colors injection molded
The coloration of the parties to shape is a critical step, since the beauty of the hand, the identification and optical functions depend on this process. There are basically three ways to colorize a part in the processes of injection:
* 1. Use colored plastic that is needed (pre-colored).
* 2. Use a natural colored plastic pigment mixed with powder or liquid dye.
* 3. Use a natural-colored plastic and mixed with color concentrate.
The choice is cheaper and more efficient use of the color concentrate (English Masterbatch), which is designed with flow characteristics and viscosity index of the polymer line to be processed. With color concentrates can change from one color to another quickly, easily and cleanly. The powdered pigments present greater problems in coloring the color concentrates and these more than the pre-colored, however, are the most expensive pre-colored and have a higher thermal history. The most common processing issues regarding the color of a piece are: color lines more or less intense, spots, flashes, and orange peel.
Colors can be either opaque and, if the polymer is transparent, translucent colors are allowed. It is important that the supplier of color concentrates is aware of the final application of the party, to use dyes and pigments do not migrate to the surface. In polyolefins colors should not be used because they migrate, a common misconception in the industry because they are cheap, although these savings lowers the quality of part and may result in a claim by the client.
The colors in the end may be translucent, solid, pastel, metallic, pearlescent, phosphorescent, fluorescent, etc.. However, polymers such as ABS are harder to color as polyethylene by high temperature and yellowing process.
An expert in designing a color must have an impressive visual ability, since their eyes are trained to recognize colors with minor differences, which requires a natural ability and vast experience. It should be noted also color theory, since the pigments are subtractive and additive light is in addition if, as lens color is a piece of metal, glass, liquid, paper or polymer different from the final polymer, it is possible that under different light is not less than the final color of the target. Therefore must decide what will be the light in which the colors should be observed. For people who are not experts in identifying color, colorimeters are very useful, although their level of confidence does not equal 100%. An untrained person may see two different colors as equal as the same and two different, due to errors in the angle with respect to the incidence of light, distance from each other objective, ambient light, etc..
To inject a polymer, specifically a thermoplastic, it is necessary to know its glass transition temperature (Tg) and melting temperature of the crystalline region (Tm), if a semi-crystalline polymer.
The operating temperature of each thermoplastic is not standardized, and varies by vendor. It is therefore necessary to request a Spec Sheet is where both the rate of flow as the temperature, which is also a range of temperatures, and the degradation temperature, which gives an interval within which to work material efficiently.
Some modern techniques include the injection of multicomponent, ie one part containing two polymers linked together or polymer additives with different colors and separated into layers. In this technique two polymers can be injected in the same room. There are two methods for achieving this: one is with two injection units, and another with an injection unit composed. A polymer is immersed in the other, or a color is immersed in the other, to save costs: This technique is called sandwich or sandwich injection. Another possibility is to inject cells forming agents or foaming agents that reduce the density of the molded part.
The injection may contain metal implants, ceramic or plastic. These are manually or automatically placed into the mold, over which the polymer is injected, by geometrical means, avoid their separation on cooling.
In the molding chemical reaction extruder is not used, but liquid components to be administered. These resins can be enabled or activated by uniting the different fluids. A typical example of polymer injected by this process is polyurethane and polyurea. Generally, temperatures in this process are much lower than the temperatures of the injection screw.
Injection of thermoset rubber and consists of an injection process that includes all the necessary ingredients for cured or vulcanized, but at low temperatures in the canyon. This should cause little friction in the material to prevent overheating and premature reaction, thus changing the desired reaction kinetics. The reaction ends up right in the mold, which is not necessary to cool.
The injection of polymers with modern equipment and conducting polymers semiconductors require much less care than in traditional semiconductor processing silicon and germanium. The clean room is not necessary and the process can be done with care similar to the injection of medical equipment.
The injection of composite materials such as wood-plastic polymer or natural fibers, carbon fiber and nanoparticles have a particular problem, because the screw tends to break, cut or agglomerated particles, therefore, present a double challenge: on the one part must be dispersed and distributed (as any pigment), while they should remain as stable as possible. Nanoparticles are generally crowded, reflecting a loss of mechanical properties and not an increase, because stress is a direct function of the joint area particle-polymer.
Plastic Injection Molding