Injection Molding

Injection Molding (Moulding) Machine

The clamping end with moving platen takes half of the mold.  This opens and closes the mold and supplies sufficient force to keep the mold closed when molten plastic is injected under pressure.


The injection unit takes plastic raw material granules, heats them until molten, and injects them into the mold.  Machines have a barrel that contains a screw, the barrel has heater bands around the outside which raises the temperature to the correct level to melt the plastic.

The rotating screw forces the plastic along the barrel.  This in turn forces the screw back as the molten material collects at the end of the screw.  When the right amount of material for the next shot has accumulated the screw stops rotating.  The screw then acts like a plunger moving forward and forcing the molten plastic into the mold.

Structure of Injection Molding (Moulding) Machine

Injection Molding (Moulding) Process

The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes. This process completely different with Extrusion process. Basic of this process is filling melted PVC into a mold and get finished product by cooling.Plastic material in the form of granules is melted until soft enough to be injected under pressure to fill a mold.  The result is that the shape is exactly copied.  Once the plastic molding has cooled sufficiently to harden the mold opens releasing the part.  The whole injection molding process then repeats. You can get a idea by following given  image.

Clamping

 Prior to the injection of the material into the mold, the two halves of the mold must first be securely closed by the clamping unit. Each half of the mold is attached to the injection molding machine and one half is allowed to slide. The hydraulically powered clamping unit pushes the mold halves together and exerts sufficient force to keep the mold securely closed while the material is injected. The time required to close and clamp the mold is dependent upon the machine - larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.

Injection

 The raw plastic material, usually in the form of pellets, is fed into the injection molding machine, and advanced towards the mold by the injection unit. During this process, the material is melted by heat and pressure. The molten plastic is then injected into the mold very quickly and the buildup of pressure packs and holds the material. The amount of material that is injected is referred to as the shot. The injection time is difficult to calculate accurately due to the complex and changing flow of the molten plastic into the mold. However, the injection time can be estimated by the shot volume, injection pressure, and injection power.


Dwelling

after the molten plastic has been injected into the mold pressure is applied to ensure all cavities are filled.


Cooling

The molten plastic that is inside the mold begins to cool as soon as it makes contact with the interior mold surfaces. As the plastic cools, it will solidify into the shape of the desired part. However, during cooling some shrinkage of the part may occur. The packing of material in the injection stage allows additional material to flow into the mold and reduce the amount of visible shrinkage. The mold can not be opened until the required cooling time has elapsed. The cooling time can be estimated from several thermodynamic properties of the plastic and the maximum wall thickness of the part.


Ejection

After sufficient time has passed, the cooled part may be ejected from the mold by the ejection system, which is attached to the rear half of the mold. When the mold is opened, a mechanism is used to push the part out of the mold. Force must be applied to eject the part because during cooling the part shrinks and adheres to the mold. In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material. The time that is required to open the mold and eject the part can be estimated from the dry cycle time of the machine and should include time for the part to fall free of the mold. Once the part is ejected, the mold can be clamped shut for the next shot to be injected.

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