Most agree that the goal in molding is to produce identical parts, a difficult task indeed due to the hundreds of process variables.
Let’s discuss one that many complain about: material viscosity variations. Then let’s see what we can do to attenuate or normalize this variable from a processing point of view.
The viscosity of a resin can change for a number of reasons:
1. Moisture content. Certain hygroscopic resins, like polycarbonate and PET, will suffer chain degradation due to hydrolysis if not dried properly. It’s not just splay you need to worry about; the amount of residual water can greatly influence the viscosity of the material.
2. Lot changes. Though certified to have the same specifications, even the same melt-flow rate, resin lots can differ in viscosity when processed on an injection press. In fact, even though they have the same average molecular weight, the molecular-weight or chain-length distribution can be different.
3. Screw-rotate time. Since most of the energy to melt the plastic comes from the mechanics of the screw rotation and backpressure, changes in screw-rotate time cause viscosity variations.
4. Additive type and percent. Usage of color, fillers, mold release, flow aids, antistats, antioxidants, regrind, etc. can have a huge impact on viscosity. How well the mix is blended can also play a role.
5. Resin temperature. Higher temperatures result in lower viscosity, lower temperatures in higher viscosity. Nothing new here, but this can vary during processing due to hot-runner oscillations, heater-band function, etc.
6. Mold temperature. The resin behaves as if the viscosity changed if you have a hotter or cooler mold.
7. Injection velocity. Plastics are shear-sensitive and change viscosity dramatically with changes in injection speed. This is the “viscosity curve” and nearly all long-chain molecules exhibit this behavior.
It would be interesting to know which of the above has the largest influence on viscosity. If I had to pick one, it would be the injection velocity, or shear rate or fill time. Even though the resin can start out at a different viscosity, if driven to the same shear rate you can attenuate some of the viscosity variations and get a more consistent process. Simply stated—run to run, shot to shot, summer to winter—keep fill time the same and your process will be more consistent. Pick a fill time for the “life” of the mold.
So if you concur that fill time needs to be kept constant for all production runs, how do you accomplish this? Certainly there are differences of opinion in the industry. Some processors feel it is their job to adjust for these changes. However, is it really possible, plausible, or the right strategy to expect an operator to stand at the machine adjusting it as different variations arise? I prefer the strategy where the machine automatically adjusts, something like a car on cruise control. It can be done during first-stage filling, provided you set up the machine correctly.
Do not run pressure-limited, and make sure the machine is properly load-compensated. We will concentrate on load compensation for this article and assume you’re running under velocity-control conditions—that is, the machine’s first-stage injection has an “appropriate” amount of extra power (pressure, usually not maximum pressure), so that velocity is controlled by allowing the pressure to vary in response to viscosity changes. This will allow the machine to keep fill time the same, providing it is “load-compensated.”
What does “load-compensated” mean? Just that no matter what the load (viscosity), the machine will use the appropriate pressure to drive the distance from the start of injection to the stroke cutoff position in the same amount of time, just like on cruise control. So whether you have polymer, water, air, or ice cream in front the screw, you will get the same fill time. Testing this is simple: Shoot plastic into the mold and, using the identical stroke, drive air into the mold. That is a huge difference in “load,” and your machine should provide an identical fill time for both air and plastic.
CONDUCTING THE EXPERIMENT
With all safeties in place, get the machine up to operating temperatures and a steady-state condition. With second-stage hold and/or pack pressures reduced to near zero, adjust your shot to provide a 90% to 99% full part by volume, not by weight. It is critical that the machine is not pressure-limited and operating with an appropriate “Delta P” and that the part be visibly short. Ideally, you are using about 75% of the barrel. If using under 25% of barrel volume, results may not be of value.
Under semi-automatic or automatic conditions, mold a part or two. Record the fill time to hundredths of a second and the peak pressure during injection; it may or may not be the pressure at transfer—you have to know your machine. Ensure you get peak pressure. Now set up the machine to shoot air into the mold instead of plastic. In manual mode, bring the screw to the zero position, pushing all of the plastic in front of the screw out through the nozzle. You do not have empty the entire barrel, just ensure there is as little resin in front of the screw as possible.
Now retract the screw, without rotation, to the exact same start point as your shot with plastic. If the start point with plastic was 4.35 in., make sure this “air” shot is starting at 4.35 in. too. In semi-automatic mold, shoot air into the mold, again noting fill time and peak pressure. Without getting into equations, the fill times should be nearly identical; a general rule of thumb would be ± 0.04 sec for normal fill times. Results can be influenced by length of stroke, amount of pressure difference and long or very short fill times. The bottom line is that both shots, plastic and air, should have the same fill time and a large difference in peak pressures. Do this at four different velocities.
If the machine passes this test, you can be assured it will provide consistent fill times as the resin viscosity varies for any reason. Make this a requirement for the annual maintenance check of the machine.