Fill, Pack, Hold, and is it uniform?
Definition: Injection: the act of injecting. Inject: to force into a passage, cavity
An overview of the injection portion of a cycle for a molding machine is in most cases the following:
Step one: Fill the part
Step two: Pack the part
Step three: Maintain pressure on screw/ram until gate is frozen
Step four: the possible use of screw delay
Filling the part:
When in fact one looks at filling the cavity /cavities with plastics it is hoped that they all fill uniformly, but the important statement here is to fill the cavities. It is hope for that this means 90 plus percent full minimum but ideally 95 to 98% full so as to allow consistency in packing. It does not mean 50 percent full.
Why, this is because in filling the cavity, there is a certain orientation of molecules due to shear, which relates to flow in the cavity and because fill and pack are completely different objectives. To accomplish proper filling the press must have the pressure and speed capable of performing the task, on each mold used. On today's machine control there are set points for both speed, and pressure. In addition there are usually available various segments in which this speed and pressure can be changed, via linear positions on the screw stroke.
The questions which must be answered are which speed and pressure to set, how full to make the part and what percentage of the overall barrel is being used.
Speed can be evaluated by performing a shear viscosity curve, or as is the case in many shops, the speed is set just below where the material shows signs of burning, or possibly the blush forming at a gate area. Pressure is set at minimum to achieve the speed desired, but with enough extra to compensate for plastic material variation. The pressure available on fill may be excessive since the part is only being filled to a short shot. Caution: if a multicavity tool or one of complex and or delicate construction one must be cautious with pressure, using the safeties which can be employed to prevent over pressurization.
This than brings up how is one to transfer from filling to the packing phase. On most machines today, there are multiple forms of transfer. These are position, pressure, and time. The following points are directed at using above 25% of barrel capacity and below 75% of barrel capacity. Outside of these boundaries may require a different thought process in the determination of the switch over requirements and some experimentation to find the best method.
Position: this yields the most accurate method of a volumetric change since one is looking at a set point on the injection phase for the machine to switch from filling to packing. On an electric machine it is much like a stop sign in the road, very precise since the screw well at that point switch over. If a hydraulic machine than it has a bit of carryover, not so much a stop, but a roll through not quiet stopping fully but slowing down while shifting to packing. (a boat in water) By using position one can envision a syringe and the injection of a fluid, it is a volume and once established can be maintained and transferred to any capable press.
Pressure: can yield good results when a said pressure is reached, the machine shall transfer to the packing, but this result is dependent on where the signal is coming from. Is it off the machine system, or is there a pressure switch in the mold. If in the mold than the question is which cavity, which position in the cavity, near the gate or end of fill. In the mold is the best placement, since it is measuring the pressure inside the cavity. While on the press is measuring a force used to push the plastic material into the cavities, this plastic must flow through, nozzle, sprue, runner and gates all of which may and do influences a pressure drop in the material flow. Further a lot change and or temperature change within the barrel and or mold may affect this resistance and the end result of how full the cavity is when using pressure from the machine as a signal to switch.
Time: as transfer point is still being used in some shops, and as far as method allows variation of the amount of material into the cavity. In transferring by time, than any change that may be made in the process from lot, temperature, pressure and or speed changes well have a direct effect on the volume of plastic used to fill the part. To illustrate this think just of temperature, if the heats are raised the material may flow easier, thus it pushes easier and flows further when raised and would in effect possibly flash the mold. If the temperature is turned down than it becomes stiffer, does not flow as easy and thus less is injected into the cavity in the same period of time. The same is true for pressures and speeds when the machine is set to transfer by time. ***time is one of the variables that may aid in repeatability when the shot size is less than 10% of barrel capacity.
How much material should be injected via fill into the cavity is dependent on the part, wall thickness and flow length. Another issue may be the cosmetics of the part. It is the experience of the author that somewhere between 90 to 98% full is achievable in about 85% of the cases. The ideal goal for filling should be the 95% to 98% range, which than allows pack to function properly.
In many thin wall parts 100% is injected into the part on fill, and this is due to the fact that if there is a hesitation in switching to pack, the material freezes off. Please understand that thin wall molding is another specific area of molding requiring knowledge, and a different thought process for these challenges.
A simple test to perform if short shots are capable of being produced on the tool is to take pack, hold pressures and their speeds off the set up (on some presses this is time too) and produce parts only with fill. This than would produce parts that are short (hopefully), and as this is your transfer point the weight of this entire short shot would be your weight at transfer.
The test is to run 10 shots each at the following conditions:
- Transferring by position
- Transferring by pressure
- Transferring by time
At each of these tests, monitor shot weight, fill time, (#3 would be injection time) and pressure at transfer. Save and mark the individual shots as a comparison as to what happens shot to shot and from one process variable to another.
What may be noted is the transfer by position yields a fairly consistent shot weight, while pressure and time may be more variable. It must be stated that repeatability is what is necessary for a consistent molding process and this starts in the fill of the part, both in the volume of material injected into the cavity and time it takes to do so. If one is using pressure transducers in the cavity than this can trump the other two. A point here is which one yields the best results for a given tool, machine and material combination?
The additional benefit of performing such a test is the fill volume per cavity. What percentage full are they by weight to a full part from that cavity and then what percentage to each other. Remember we wish to fill to 95 to 98% full in all cavities, has this shown an area of concern?
To be continued.
SLSILVEY
28092013.01
