INJECTION MOLDING

Components of the Injection Molding Technique

• Injection Unit: a) melt the polymer.

b) to accumulate the melt in the screw chamber. c) to inject the melt into the cavity, d) to maintain the holding pressure during cooling.

• The Clamping Unit: a) to open and close the mold.

b) to hold the mold tightly to avoid flash during the filling and holding

• The Mold: a) Distribution of polymer melt into and trough out the cavities.

b) Shapes the part. c) Cools the melt and d) eject the finished product.

• Mold: The clamping unit is where the mold is attached. The mold can be closed or opened automatically and ejector pins are used to eject the molded object out of the mold. The filling of the mold is accomplished by the flow of the polymer melt under high injection pressure along the nozzle, runners and gates into the cavities.

• Different elements of the mold: 1.Distribution Channel: a) sprue, b)Gate, c)Runner 2.Mold Cavity 3.Ejection System: ejector pin

4.Cooling System: water is circulated to remove heat.

runner : leads plastic from sprue to cavity.

Sprue: leads plastic from nozzle to mold

Gate: control flow of plastic into the cavity,

• Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials.

• With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel.

• As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heated chamber, where it is melted.

• As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system.

• The mold remains cold so the plastic solidifies almost as soon as the mold is filled.

Injection Molding Cycle

Injection molding is a cyclic process:

1. Injection : The polymer is injected into the mold cavity.

2. Hold on time : Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage.

3. Screw back : At the same time the screw retracts and truns, feeding the next shot.

4. Cooling : The molding cools and resolidifies.

5. Mold Opening : Once the part is sufficiently cool, the mold opens and the part is ejected.

6. The mold closes and clamps in preparation for another cycle.

[ Hold Pressure: the pressure is applied at the end of the initial injection strock and is injected to complete the final filling of the mold and hold pressure till gate closure due to freeze off. ]

- The temperature must be high enough to melt the polymer but not too high to cause thermal degradation.

- Pressure must be high enough to avoid short shot (i.e. partially filled cavities) but not too high because we will have flash at the mold contact surface.

Good molding region

Injection Molding ‘Window’