what is the characteristics of injection molding?
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Injuction molding usually has a higher initial cost because you have to manufacture the molds first from a substance that can take the increased heat and pressure. Injection molding is good for manufacturing a lot of the same thing. If you only want to make a few, you probably want to have a machine shop do it. Although your price-per-piece will increase, it is a heck of a lot cheaper than having the molds made. But, you can run about 1000-2000 pieces if you mold the parts if you are willing to fund the initial cost to make your molds.
Injection molding is used on thermoplastics. That is, plastics that melt. A mold is fabricated with at least two halves. The shape of the desired piece is cut into the steel of the mold. Realize that if a desired piece does not have sides that are not curved to some extent, the part cannot be removed from the mold after it has cooled and solidified. A plastic cup with a handle would require a moveable core. Assume that hte cup is to be molded as if laying down with the handle horizontal. The parting line for the two halves would be along the middle of the handle. To make the cup have an inside, a horizontal piece would have to be inserted duting the molding process, and removed before the finished piece could be removed from the mold. The details of the mold itself is that it generally has tiny grooves in the surfaces that come together so that the air in the mold can escape when the plastic is injected. The plastic generally enters through a sprue, relatively large at maybe a half inch in diameter. The actual desired piece is usually gated with a small hole to let the plastic in. Multiple cavities (for multiple pieces molded at one time) are common. the gates have a tree format to each of the cavities. The molds are usually cooled by water so that the hot plastic is cooled quickly and the mold can cycle a number of times per minute. High pressure is therefore necessary so that the plastic can fill the mold before it starts to solidify. Look at the bottom middle of a molded plastic cup and you can often see he place where the sprue was cut off the cup. Look at a toy store and you can often see the plastic parts of model kits still on the molding tree. The cavities are often made using rough machining to remove the major part of the material and then a process called called EDM (Electrical Discharge Machining). See ; http://www.milcowireedm.com/ for details.
You might look for details of machines at; http://www.kdcapital.com/Machines.cfm/IP/1_9_0_0