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Screw Bounce
Definition: Screw: a shaft with helical channel that coveys materials from the hopper to tip through a barrel. Bounce: 1) to spring back / rebound 2: spring or leap
What is screw bounce back? To begin we need to understand some basics on molding, and the injection sequence that occurs. The basics are that we form a charge/shot of material in front of the screw tip and upon receiving a signal we than inject the material into our mold. What happens is that pressure is applied to the rear of the screw which must be higher than the resistance to flow of the material into our mold. As we move forward the material compresses/ flows and not necessary in that order, or sequence.
As the material is injected into the tool pressure resistance increases and more pressure is required to keep it flowing and it also starts to pack out, this creates more compression on the shot volume in front of the screw. When we than reach the volume of material necessary to fill our cavity/cavities the machine typically switches to a hold or pack sequence to continue filling the material into our tool.
It is at this point if someone watches a machine work that they may see the screw bounce back. This is similar to a ball bouncing off the sidewalk or wall when hit or thrown. The force applied to the screw is applied to a material which compresses, (storing energy) the pressure applied initially is greater than that which is stored, and when taken off (switching to hold/pack) this energy pushes back the screw as it has more force now than the screw has on it on the rear. The bounce of the screw can also occur at the end of pack/hold too, but if occurring at this point in the sequence the gate is hopefully frozen and there is no issue with part quality, only runner quality. This is again an illustration of pressure equalization.
This can be observed in various ways, one is watching (if one can) the actual screw movement within the carriage unit, actually witnessing this bounce, another is to watch the computer screen and graphic position curve and seeing that the screw moves into a back position at switchover, and the other is to watch on the position numerical area within the controls and seeing that it goes to a point and then goes to a higher position before settling back down to allow position.
What does all this mean?
It means that the material is not necessary moving in a forward or filling way and that the machine is not in control at that moment. One way to exaggerate this is to take all pressure off of the pack or hold and allow the screw to fill the part. This usually produces the greatest bounce as there is no pressure to restrain the bounce back of the screw. The result is neutralizing the pressure in the mold, fill system and material in front of the screw tip. Similar to a non-gate seal we are allowing material to come back out of the cavity, or in this case also allowing the material to decompress or expand to nominal pressure, resulting in the screw being forced back -bounced back, this is at transfer position.
Is this detrimental to our part quality? That depends on the part and functionality of the part. This is a hesitation in the flow front, thus the molecules that were aliened and flowing into our part have now stopped, and in this stopped position they can now go back to their tangled ways, thus if the hesitation is long enough the parts may come out short as when reapplying pressure to the flow front it no longer wants to flow. This may also start to freeze off the gates as we have stopped flow. If there issues with part quality, if you have screw bounce at transfer, than it is something to be concerned about.
I must state that if we have filled the mold completely before transferring than this bounce back may not be detrimental to our parts. If one were to mold coffee can lids or any lid really with along flow path the solution to manufacturing a flat part is to fill as quick as one can, and not to pack at high pressure, or basically to fill and pack slightly on fill. This non pressure than allows the material to back flow out of the cavity thus equalizing pressure and shrinkage across our parts thus producing a flat or non-warped part. (Please note other issues can solution can be applied and this was only used as simple example)
One way to think of this is that we are filling the cavity in a uniform way and then stop and restart the fill process, the question than is does it harm the part in function, cosmetics or quality? Do you have occasional shorts or quality issues that you cannot explain? The only time I ran into a machine issue with this issue of screw bounce has been on old electric machines as the machine could not handle the shift in position of its screw and would go to a default error and then the entire machine would have to be rebooted.
The balance between fill, pack and hold should be understood, and machine functionality also need to be understood so as to produce parts under a repeatable controlled method.
SLSilvey
silveysplastics@hotmail.com | 
