Saturday, March 3, 2007

Injection Molding Basic Knowledge

So, what is injection molding?
To put is basically, injection molding is the process of forcing melted plastic in to a mold cavity. Once the plastic has cooled, the part can be ejected. It is useful when the parts are too complex or cost prohibitive to machine. With this process, many parts can be made at the same time, out of the same mold

Injection Molding is the process of forcing melted plastic in to a mold cavity. Once the plastic has cooled, the part can be ejected. Injection molding is often used in mass-production and prototyping. Injection molding is a relatively new way to manufacture parts. The first injection molding machines were built in the 1930's.

Injection molding is a plastic-forming process used in the production of most (about 70%) of plastic parts. Other plastic-forming processes include blow molding, pressure-forming, and thermo-forming. Injection molding is generally used in the high-speed manufacture of low-cost, high-volume parts, like videocassette cases, plastic cups, printer parts, refrigerator parts, automotive parts, and other electronic parts like casing, gear.

The process of injection molding begins with a barrel full of hot, liquid plastic. The plastic is rammed at high pressure into a mold. Once the plastic fills the mold, it is allowed to cool and solidify. The finished part is then extracted (usually automatically) from the mold. You will learn more about the injection molding process in next week's lab. This week, we will concentrate on mold design and analysis.

The mold defines the shape of the part, as well as the path by which the molten plastic flows from the barrel. A simple mold has several features:
  • Fixed and Moving Platens - These are rectangular blocks of aluminum or steel into which the shape of the part is cut.
  • Cavity - When the fixed and moving platens are touching, the space formed by the cut-out portions, called the cavity, defines the shape of the part.
  • Sprue - The sprue is a hole cut into the center of the fixed platen. Molten plastic flows from the sprue to fill the cavity.
  • Runners - Runners are channels cut into the platens that direct molten plastic from the sprue to the gates.
  • Gates - Gates are small openings between runners and cavities. These are the points at which plastic enters the cavity. They are generally small so that the finished part may be easily broken away from the useless sprue and runner material

Generally step process in the injection molding process are mould close - injection carriage forward - inject plastic - metering - carriage retract - mould open - eject part
There are six major steps in the injection molding process

1. Clamping
An injection molding machine constists of three basic parts; the mold plus the clamping and injection units. The clamping unit is what holds the mold under pressure during the injection and cooling. Basically, it holds the two halves of the injection mold together.

2. Injection
During the injection phase, plastic material, usually in the form of pellets, are loaded into a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorized screw that mixes the molten pellets and forces them to end of the cylinder. Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mold through a sprue, while the pressure and speed are controled by the screw. Note: some injection molding machines use a ram instead of a screw. · basically the screw extends from the hopper to the injection chamber.

3. Dwelling
The dwelling phase consists of a pause in the injection process. The molten plastic has been injected into the mold and the pressure is applied to make sure all of the mold cavities are filled.

4. Cooling
The plastic is allowed to cool to its solid form within the mold. It usually uses cooling process in mold base, the cooling process was made compact with injection machine so its can do automatically by setting in injection machine

5. Mold Opening
The clamping unit is opened, which separates the two halves of the mold in two plates model, but in three plate model, mold will separates in three halves, include cutting runner process.

6. Ejection
An ejecting rod and plate eject the finished piece from the mold. The un-used sprues and runners can be recycled for use again in future molds. Ejection of parts usually doing by ejector system, ejector system consist of ejector rod, spring, ejector plate and ejector backing plates.

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