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Two Channel Method

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4.3.2.3.2 Simultaneous Injection
Two channel technique:
The two channel method, developed by Battenfeld in the mid-1970s, includes a phase of simultaneous injection. The process sequence can vary but a typical example would be:
• Injection of skin to a pre-set switch point
• Injection of core material begins so both skin and core flow together
• Injection of just core
• Injection of just skin
• Part packing and cooling followed by ejection.
This co-injection moulding process entails the injection of molten plastic for the skin layer into the mould cavity. After a certain pre-set time, usually in the region of 0.1-0.3 seconds, a second plastic that will make up the core is injected and, for a period, there is simultaneous injection of …show more content…
This can be seen by looking at Figure below and comparing the pressure profiles and screw velocity of the simultaneous and single channel methods. In the sequential method, the period of stagnation after injection of skin (A) but before injection of core (B) can be clearly seen, resulting in a drop of cavity pressure and a period where there is no movement of material in the screw. This demonstrates many of the limitations of sequential injection discussed earlier. This pressure drop is less apparent in the simultaneous …show more content…
A typical duration is 25% of the injection time for the skin component. Mould filling dynamics dictate that optimum distribution of core material is obtained if the skin viscosity is kept slightly lower than that of the core. This is due to rheological factors that will be explained in the next section. Separate operation of two injection units makes it easier to control skin thickness in various parts of a moulding. This is because, by separate control of the velocity profile of the skin and core, the skin thickness can be adjusted in various parts of the moulding. Extra amounts of skin material can also be injected during holding time to seal the gate area. This also ensures that the nozzle is clear of core material and prepared for the next shot.
Again, due to the dynamics of mould filling, material entering the tool and cooling on the walls near the gate can get re-melted and flushed away due to frictional heat generated by the incoming molten flowing melt. This can lead to variations of skin thickness and leave the skin near to the gate region much thinner than that on the rest of the moulding. This effect is generally more pronounced on the opposite side of the gate due to the higher shear experienced in this region. In order to overcome this, the three channel technique was

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