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What is Injection Molding...[continued]
Melt Preparation
In order for the plastic to be molded, it must first be converted from the pellet form above to a liquid like material that can flow into the desired shape in the mold. This process is accomplished by two methods.
The primary method is by shearing the pellets in a feed screw that is rotating in a heated barrel. The screw flights are tapered so that the distance between the root of the screw flights and the barrel gets smaller from the feed throat to the injection nozzle. The material is conveyed along the barrel through the turning of the screw and as the gap between the screw flights and the barrel wall gets smaller, it causes the pellets to be squeezed. This squeezing or shearing adds tremendous amounts of heat to melt the material (see Figure 1).
The second heating method involves heating the barrel. This is done by conductive heating from heater bands placed around the barrel.
As the material is conveyed forward along the screw, melted material is deposited in front of the screw tip in an area often referred to as the “shot”. With every turn of the screw, more material is deposited in the shot. This causes the material to compress and build pressure. As this pressure increases, the melt in the shot forces the screw backwards and causes the “shot” to grow to its final “shot size”. The entire process is dynamic in that it happens instantly as long as the screw is turning.
To ensure that the screw imparts adequate shear to the material, a pressure is applied behind the screw to counter some of the forces pushing the screw backwards. This is called the “back pressure”.
Feed screws are designed to make sure that there are no unmelted pellets mixed in with the final melt as this can cause problems with the injection process and with the quality of the finished parts. In addition, screws can be designed as needed for specific materials and wear properties.
By changing the rotational speed of the screw, the back pressure applied to the screw, and the temperature of the barrel, the melt properties can be adjusted to the application within a modest range. However, careful choice of the resin type and melt grade is important when attempting to target final part properties. (The melt grade, or Melt Index, is a measure of the “flowability” or “stiffness” of the plastic when it is melted).
Injection
Now that the material has been prepared, it needs to be injected into the mold. This can be done in one of two ways.
The first method uses another part of the molding machine called a plunger to push the melted plastic into the closed mold. The first molding machines were all built in this fashion where the melted material was delivered to the front of a plunger, which then injected the material into the mold.
While plunger machines are still made, for most machines, this has been replaced by reciprocating screws. This design has a screw which both rotates to shear the material and moves forward to inject the melt. To prevent any material from flowing back down the flights of the screw during injection, a sliding check ring is fitted on the end of the screw. During screw rotation, the check ring is forward and allows material to flow in front of the screw.
During injection, as the screw plunges forward the check ring dynamically retracts and mechanically restricts material from flowing backwards. The injection speed of the machine can range from very slow (10-50 mm/s) to very high (1500 mm/s). Faster injection speeds often require more force (and therefore more oil pressure) to inject the melted plastic into the mold. Sometimes, the required “injection pressure” depends strongly on the geometry of the part that is being filled.
Sometimes the required pressure is dictated by the “flowability” of the molten plastic. The easier the plastic flows, the less injection pressure is required. The reverse can be true also. Plastics that don't flow easier can require very high pressures during the injection process.
Forming, Cooling, Removing
In order to make the plastic part, a mold is produced out of metal that contains the reverse of the geometry of the finished product. As was stated earlier, the melt enters the mold at a very high pressure and therefore the mold must be held closed with at least the same pressure to prevent it from being forced open and plastic leaking out (called flashing). The part of the machine that provides this closing force is called the clamp. The clamp is sized in terms of how much pressure it can deliver in either US or Metric Tons.
In addition to the geometry of the finished part, the mold also contains channels to convey the melt from the machine to the shape or cavity being produced. These channels are called runners. In the simplest of molds, the runners are removed with the parts and are thrown out as scrap or ground up and reused in subsequent moldings. In more complex molds, the plastic in the conveying channels is kept molten. This eliminates the need to handle the additional material. This designed is referred to as a hot runner system and while it increases the price and complexity of the mold, the elimination of scrap can quickly pay for itself.
The mold is cooled by water (or other conducting fluids) to facilitate the cooling of the plastic. The coolant temperature can range from very cold to hot to the touch, but in all cases it is cooler than the melted material entering the mold. The rate at which the plastic is allowed to cool is determined by the type of resin used and the desired performance attributes of the finished parts.
Once the plastic has cooled enough to hold its shape, the clamp opens and the part is free to be removed, or ejected, from the mold. This is done by mechanical pins pushing the part out, compressed air blowing the part out, a robot removing the part, or in some cases, a person removing the part. Sometimes a combination of methods is used to remove the part.
Injection Molding Machine Design
Injection molding machines, like most machines, can be designed for various purposes, and, like most pieces of equipment, price often dictates quality. For the simplest of molding process, very simple and low-cost machines can be used. However, more advanced machines are needed as the process becomes more complex, or the number of cavities increases, or the overall cycle times get smaller.
The first molding machines were semi-automatic machines that had cycle times of over a minute. While some cycle times are still that high today, the trend is to be faster and faster. It is not uncommon to see high speed machines running cycle times of under 2.0 seconds. These types of applications place extreme demands on the mold and machine and require only the best built machines.
The very first molding machines were completely driven via hydraulic pumps. Today's machines range from “full-hydraulic” to “all-electric” control, as well as hybrid machines that use both hydraulic pumps and electric motors to perform machine movements. While the choice of which type of drive to use is best determined by the application and the parts being produced, it should be noted that the trend continues to be towards all-electric machines due to the energy savings that can be realized. However, certain applications such as those requiring high speed injection still lend themselves to using hydraulic functions. Many believe the future lies with hybrid technologies that combine the best of both technologies.
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