Injection Molding _001

5/19/2014 injectionmoldingexpertise.com

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Home Process Equipment The Mold Material TroubleShooting

The Basic Injection Molding Process.

The injection molding process is the process where we melt a thermoplastic

polymer, give it the desired shape and cool it down to solidify the material

and produce a useable part. The steps are to melt our plastic, inject it

under high pressure in a mold, cool it down and remove it from the mold.

Melt. Shape. Cool Eject!

The injection molding process can be divided in 4 seperate phases:

1. The Injection Phase

2. Holding or Packing Pressure Phase

3. Cooling & Plasticizing Phase

4. Ejection Phase

What happens exactly during those four phases of the molding cycle?

At the start of each new molding cycle the screw is in the back position and the

material is kept at a temperature suitable for the processed material by the

heater bands around the barrel. The screw is at such a position that there is

sufficient material in front of the screw to fill the tool cavity for a single shot

plus a small amount to be able to apply the pressure in the holding phase.

Material suppliers provide you with a general guideline how to set up your barrel

temperatures with a given material. It is very important to realize that the

actual melt temperature can significantly differ from the barrel temperature,

by as much as 50 C based on my personal experience. This is due to the fact

that other parameters will also influence the actual melt temperature as is

further explained on the Melt Temperature and How It Is Achieved Page

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Injectionmoldingexpertise.com 2012

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At the start of the injection phase a pressure is applied to the screw moving it

forwards. The check or back flow valve at the tip of the screw closes as a result

of this movement and enables the screw to act as a plunger to push the molten

material (the melt) in the mold cavity.

The speed of the screw moving forward can be controlled by controlling the

speed of the screw or by maintaining a certain barrel pressure. Most injection

molding machines allow you to vary the speed or pressure during injection

creating an injection speed profile.

The injection phase ends when we have filled the 95 99 % of cavity of the

mold and then the process moves to the next phase: holding pressure. The point

at which we move from the injection phase to the holding pressure phase is

called the Switch Over Point, abbreviated SOP.

In order to be able to still apply pressure to the melt in the tool it is required

that there remains a certain amount of material in front of the screw tip; this

is called the cushion.

The purpose of the holding pressure phase is to replace the volume lost during

the cooling down and solidifying of the melt. The holding pressure phase can last

as long as the gate remains open and molten material can still enter the cavity.

In the cavity the material will cool down and exhibit thermal shrinkage. In case

of semi crystalline materials shrinkage due to crystallization, will be additional to

the thermal shrinkage, more detailed information can be found here.

From the holding pressure phase we move into the cooling phase. Apart from

cooling the part down sufficiently to be able to eject it from the tool

undamaged, during the cooling the machine is prepared to deliver molten

material for the next cycle.

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During the cooling time the material for the next shot is prepared called

plasticization. The screw is rotated at a pre-set speed and pushes the material

forward. As the nozzle of the injection unit is in contact with the tool

containing the solidified part flow of the material is prevented, pressure will

build up and the screw will be pushed backwards. The check valve will open,

allowing material to flow in front of the screw tip. A back pressure is applied

and acts as a counterforce to the screw that is pushed back to improve the

melting behavior by increasing friction and improves the degassing. More

detailed information on screw speed, back pressure and plasticization can be

found here.

The cooling and plasticization phase ends when the screw reaches its pre-set

position and the cooling time has lapsed and the cycle moves into the ejection

phase.

The clamping force can be lifted and the tool can be opened to allow the part to

be released from the tool. Ejector pins etc. usually aid with the release of the

part from the tool. When the tool is closed and the clamping force is reinstated

the machine is ready for the new cycle.

As you may gather from the description of the basic process, there are many

parameters an operator has at his disposal to influence the process. Please

review this list that mentions and explains the most common parameters used

in setting the process.

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Injection Molding _001