What is an Injection Mold?
An injection mold is a tool that when all the parts are assembled, fits into an injection molding machine to manufacture identical plastic parts. Injection molds start with a drawing by an engineer of the part required for an application. Next an engineer with knowledge of the injection mold making process makes a mold drawing.
How an Injection Mold is Made
Making an injection mold takes a great deal of skill and some very expensive machinery. They are generally made out of tool steel by CNC machines. They are the negatives of the actual part required. There is also a runner system that allows the hot plastic to fill all the mold cavities in addition to channels that help cool or heat the mold thus controlling its temperature. About 10% of a typical injection molding cycle is dedicated to actually injecting the molten plastic. The remaining percentage is dedicated to cooling and solidification, packing and hold.
The simplest injection mold is a straight pull injection mold consisting of two halves. The “A” side and the “B” side. The front side or A side faces the cavity and is referred to as the cavity. The B side faces the cavity and is known as the core mold. However, not all injection molds follow that simple pattern.
Parts of an Injection Mold
The engineer designing the mold will decide if it requires an insert tool or free standing mold.
To make a free standing injection mold manufacturers buy pre-made mold bases from one of the several worldwide companies such as Misumi. DME or Hasco.
An injection mold will include cooling lines, a runner system, a sprue, cavities and gates. Injection molds also have threaded holes for eye bolts that are used in mounting the mold in a molding machine. Additionally, they have guide pins for lining the mold up on a work bench so that alignment of the A and B side is not necessary once the mold is in the machine.
Once a part solidifies it is ejected from the mold with ejection pin, pins and sleeves or using any one of the other methods including ejector plates, air ejection and more.
EDM (Electronic Discharge Machine)
But some parts have a cavity that cannot be readily milled. In this case an electrode made out of graphite or copper tungsten is turned on a CNC machine and then placed in the bath of an EDM (electronic discharge machine.) The EDM will erode the steel to form a cavity exactly the shape of the electrode. There are two types of EDM: sinker and wire. A sinker EDM is designed to make a cavity while the wire EDM uses a fine brass wire to cut simple or compound shapes.
The Runner System
The runner system allows the liquid plastic to flow into the cavities. There are three main sections to the runner system of an injection mold: the sprue, the runner and the gate.
A sprue is hooked up to the nozzle of the molding machine barrel. The molten plastic is delivered by the reciprocating screw to the nozzle and thus through the sprue. Then the material flows into the runners. Finally the molten plastic will fill each cavity through the gate. Gas may be released through a vent.
Injection Molding Machine
An injection molding machine is made up of a loader, a reciprocating screw, the barrel, the nozzle and a platen. During this process, polymer granules are first put into a dryer to remove moisture. Then the granules are moved into the barrel where they are heated to the melting point of the plastic being used. A large reciprocating screw runs through the barrel and moves the plastic along until it reaches the nozzle. It is injected through the nozzle into the mold which is installed on the platen. After the plastic cools, the mold opens and the plastic parts are knocked out by ejector pins. The cycle is usually repeated every 60-90 seconds.