Injection Molding Machine (IMM): Basic Functions

Injection molding process is a versatile process. Many changes has been made in injection molding machines and injection molding process. Variety of injection molding machine are available today. Fully manual, semi-automatic, fully automatic injection molding machine are available today. Fully automatic machine are further divided into categories like hydraulic injection molding machines, All-electric injection molding machines, and hybrid Injections molding machines.

The basic function of all injection molding machines are same either it’s manual and fully automated.

Injection molding is a cyclic process. The cycle of injection molding machine include following steps:

• Melting
• Mold closing
• Injection
• Cooling
• Mold opening
• Ejection

Injection molding is a process in which plastic resin/chips are transformed into molded plastic part. Injection molding process starts from feeding plastic into hopper. From hopper plastic goes into the barrel and a rotating screw with flights on it conveys and melts that plastic. Screw on injection molding machine is divided into three sections or zones. 1st section is feeding zone 2nd is called compression zone and 3rd last one is called metering zone. In some new IMMs there is a new mixing zone. Which increases the mixing efficiency of screw. Screw flight depth decreases from feeding zone to metering zone. At the end of metering zone a NRV (Check valve) is used to prevent back flow of molten plastic.

Molten plastic is stored in between nozzle and check valve. Reciprocating screw (Have ability to move back and forth) of injection molding pushes that molten plastic into mold cavities. In mold plastic part are formed and cooled.

Molds of injection molding are different in shape and fabrication material is also different. One side is core and second side cavity.

After the molded part cooling it is ejected.

Some common defects in Injection Molding

Common defects in injection molding process are flash, Short Shot, Gas or burn marks, Lines or streaks and bubbles etc.

Extra material at parting line is called flash. Flash may be due to high injection pressure and high holding pressure. Flash may also occur because of low clamping force.

Short shot means injected plastic is less than required to fill the cavity completely. It may occur due to low injection pressure and may be due to some other reasons like poor air venting in the mold.