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The injection molding process begins with the gravity feeding of polyolefin pellets from a hopper into the plasticating/injection unit of the molding machine. Heat and pressure are applied to the polyolefin resin, causing it to melt and flow. The melt is injected under high pressure into the mold. Pressure is maintained on the material in the cavity until it cools and solidifies. When the part temperatures have been reduced sufficiently below the material’s distortion temperature, the mold opens and the part is ejected.
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The complete process is called a molding cycle. The period between the start of the injection of the melt into the mold cavity and the opening of the mold is called the clamp close time. The total injection cycle time consists of the clamp close time plus the time required to open the mold, eject the part, and close the mold again.
There are four basic components to an injection molding machine:8296
1. Injection unit/plasticator
2. Clamp unit
3. Injection mold
4. Control system
Injection units
Plunger injection units (Figure 15) were the first types used for injection molding, but their use today is quite limited.
The reciprocating screw injection molder is the most common molding machine in use today for molding polyolefins.The injection unit (Figure 16) mixes, plasticates and injects a thermoplastic melt into a closed mold. The reciprocating screw accomplishes this in the following manner:
1. The injection cycle starts with the screw in the forward position.
2. Initially, the screw begins to rotate in the heated barrel. Resin pellets are forced by this action to move forward through the channels of the screw.
3. As the pellets move forward, they are tumbled, mixed and gradually compressed together as the screw channels become shallower. The section of the screw nearest the hopper is called the feed section, in which no compression takes place.
4. As the pellets travel down the barrel, they are heated by friction and the heat conducted from the external electric heater bands.
The friction is caused by the pellets sliding against themselves and the inner wall of the barrel and the screw surface. The heat from the friction and conduction cause the pellets to melt. The majority of the melting occurs in the transition section of the screw, where compression of the polymer is taking place as the root diameter of the screw is increased.
5. Next, the melted polymer is further mixed and homogenized in the metering section of the screw. In the metering section of the screw, the root diameter has reached its maximum, and no further compression takes place.
6. The polymer melt flows in front of the screw tip and the pressure produced by the build-up of polymer in front of the screw causes the screw to be pushed
backward in the barrel as it continues to rotate.
7. The screw stops turning when the volume of melt produced ahead of the screw tip is sufficient to completely fill the mold cavity and runner system (the channels leading to the mold cavity). This amount of material is called the shot size and the period during which the screw rotates is called the screw recovery time.
8. The screw is then forced forward, injecting the melt into the mold. This is called the injection stage.
In order to compensate for material shrinkage in the cavity due to cooling, an excess amount of material is generally held in front of the screw at the end of the injection stroke. This extra material is called the cushion and, during the packing phase, some of the cushion material continues to be slowly injected into the cavity to compensate for the volume lost due to the shrinkage of the material in the mold and the compressibility of the plastic.