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Understanding Dimensional consistency:
Shrinkage and Warpage
Dimensional in-consistency and warpage are on account of shrinkage behaviour of the plastic melt in mould. Plastic melt shrinks volumetrically- in all three dimensions. If shrinkages are equal in all the three directions, then part size reduce with out any distortion. In reality, it does not happen that way. Shrinkages along the direction of flow is more that that in transverse direction. This is called area shrinkage and it is with constrains. Wall thickness reduces due to shrinkage with out any constrain. This unequal shrinkage causes distortion of the part that is called warpage. Linear shrinkage can be influenced by restrains in the mould, Crystallinity and orientation.
Shrinkage and Crystallinity
Crystalline content increases with lowering of cooling rate and decreases with faster cooling rate. Higher crystallinity means higher shrinkage. It means that part dimensions could be controlled by controlling cooling rate in different areas of the mould when wall thickness is not constant. Cooling rate could be increased by using Ba-Cu insert in the mould.
Shrinkage and Process
Melt follows its PVT characteristics described by Wander Walls equaltion ie. Relationship of p,v and T. Here specific volume reduces with reduction of temperature at constant pressure. At filling phase, melt compresses 10-15% and hence it relaxes at switch-over point. This causes the smooth change over from filling to (Hold-on/ follow-up) pressure phase.
Hold-on pressure and hold-on time influences the shrinkage. High pressure and time reduces the shrinkage. Lower pressure and time increases the shrinkage of the part.
Melt must spread with uniform velocity in all direction from the gate. Velocity and freezing rate should be such that it should be possible to maintain flow up to the boundary of the part. Gate should freeze last as hot melt flowing during filling phase keeps the gate region hotter that the end of flow region. Faster freezing of part may not allow full part to be filled, even if just filled it may shrink more. Slower freezing of part would extend the cooling time. Gate must freeze after adequate melt moved from nozzle to the gate to compensate for shrinking volume of the part.
The problem occurs when part has varying wall thickness. Thinner section freeze faster than thicker section. Hence cooling design should take care of such problems by placing cooling channel should be closer towards thicker section and cooling channel at farther distance from the thinner section. Thicker section of the part can have Ba-Cu insert. Ba-Cu insert would cool faster because of it's high thermal conductivity.
Shrinkage can vary from gate to the end of flow path (boundary) if melt does not freeze uniformly. It is less at the gate and more at the end of flow. This can cause distortion.
Shrinkage and Orientation
Alignment of polymer chains are stretched in parallel -oriented- with shearing. With high cooling rate this orientation can be trapped in the moulded part. This can result in stressed area in the moulded part. With low cooling rate the stretched chain gets chance to relax and hence shrink to natural chain. Hence shrinkage in the direction of orientation is more.
In a wall section, it is observed that, the frozen layer in contact with core and cavity is formed with least shear-least orientation. Next layer of melt is subjected to more shear stress. It freezes the moment flow stops, thereby trapping the orientation. The next -central layer is subjected to less shear and cooling rate is also less here. This gives more time for oriented chains to relax in the melt. Hence orientation is less in the central layer. It means that wall thickness consist of layers of different tensions (stress). If the core and cavity surface temperatures differ, then, it is bound to distort.
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