Getting it all in...
Definition: Packing: the continuing slow flow of material melt into a mold cavity after it has filled, but before the gate has frozen.
As the name describes this is when the part is packed out. Packing is the function in the injection phase which yields the finished part size, shape of the part. Fill to a 95-98% full part is desirable and fill is what influences many of the cosmetics of the part itself.
Packing / holding? In the discussion here these two terms are to be used separately. Please note that on many presses they are used interchangeably in setting pressures, times and velocities. As described above packing is to pack out to achieve a part size shape via dimensions. Holding is to hold pressure onto the screw and thus the plastics through the runner system and gate area so as to maintain the pressure on the cavity after packing. Another description would be that packing forces the amount of material into the cavity to give a desired part shape/weight and or a volume of material into said cavities (determined by screw movement/cushion) while holding maintains the position of the screw, after that said position and cushion is achieved.
The part in most cases shouldn't be filled during pack, as that was accomplished in filling, but in truth many times the pack is both filling the end of the part and then packing it out. This may be due to part shape, cosmetic issues, mold issues or other issues and thus the processors is now having to compromise how to manufacture a part so as to achieve the desired part out of the tool.
Pressure is the main portion of the pack, ( if molded correctly) as it is only finishing the final phase completing the pressurization of the mold cavities so as to compensate for shrinkage, and final 5% or less of fill. This than indicates that speed of pack may or may not be of importance, but again how much is being packed, how full is the part.
In packing of the part, the final 5% or less of material is pushed into the mold cavities. As the screw moves forward it than reaches a point of complete fill. The pack than maintains and prevents sinking and or short shots, yields the finished part size and uses both a bit of speed and pressure to accomplish the task at hand.
Packing pressure is dependent on material used, pressure loss throughout the system, gate size and size requirements. The material used typically can be classified as amorphous and semi-crystalline and a basic rule of thumb is that amorphous grades need only 30 to 50% of the injection pressure to pack out while semi crystalline need may be as much as 100% or more of the injection pressure due to the characteristics of these materials. In fact packing pressure is relegated to finished part size, shrinkage, and how much pressure was loss in getting into and across the cavity.
The speed of pack pressure can be set on some machines while in others it is set by the manufacture. This can be a concern if one is trying to pack correctly, or take care of a flashing issue and or short shot issue. The speed of pack is also a great concern if the part is not filled to the 95% level minimum as now the pack has to fill the part the final amount and while pressure is a number one area of pack the speed now comes into play.
Packing pressure is typically applied by time, though it may also be applied by position. What is meant is that on most presses there is only a time function for the pack pressure, usually called out in seconds. This is most commonly seen on machines where only injection forward time is called out and or where pack and hold area are one. In our definition of pack we might on some machines be able to pack with pressure until a position is reach on the screw stroke and then to change to what is referred to as a hold pressure.
With a valve gated mold, it is common to switch by time but also by position of screw. Thus when the screw reaches a set position the valve gate closes; sealing off the cavity with the pressure at it set point, this than optimizes the process.
Variations in pack pressures rate are used. These changes can be to start low and then go higher and or to start high and drop off. In the Low to high what may be accomplished is to prevent a bit of flashing within the tool as one is not hitting the bottom of the cavity and allowing the material to thicken up/setup a bit before applying all the pressure necessary to pack it out to size. This than creates a skin thick enough that hopefully well expand but also capture material so it does not flash the mold.
In a high to low the pack is putting the material into the cavity and then allowing it to relax a bit by using less pressure, usually the screw has reached a position and then is maintain or allowed to float back a bit.
To be continued:
SLSILVEY
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