Andy Routsis: September 2009 Archives

Although I am not familiar with studies to validate this specific conclusion, but you can find studies on many of the aspects which contribute to this general rule of thumb. I can give a good argument to support both the 80% and 20% limits. Even more important... is the fact that you can easily generate your own data to validate/test any of these arguments with your specific processes.

80% - This rule of thumb is provided to give a buffer to allow for process variation for a couple compounding reasons... (1) A good cushion should be between 5-10% of the overall shot size. (2) Many machines require 2-5% of the shot size to decompress the screw after recovery. (3) The check ring will typically vary 2-5% during fill resulting in a similar variation in cushion size. When you add these variations up, you need a 10-20% buffer to help ensure you can properly fill the part.

20% - The typical general purpose screw contains approximately 1-2 shots of material within the flights of the screw. This means that a process running at 50% capacity will have an estimated barrel residence time between (2) and (4) * (cycle time). Likewise, a machine running at 20% capacity has an approximate residence time between (5) and (10) * (cycle time). If you bring this to the extreme, a process running at 5% capacity could have a barrel residence between 2000% and 4000% of the cycle time!

These are rules of thumb, and therefor there are always exceptions. With the use of Accurate process controls and short travel check rings, you may be able to violate the 80% rule. I never recommend violating the 20% rule as it is likely to affect the part quality, process stability, and it will waste a large amount of energy.

Unlike most manufacturing processes, the compressibility and shear thinning characteristics of plastics cause inherent variability in the process.

-Andy

There are many aspects which must be considered when reviewing the transfer from 1st to 2nd stage. Most of these can be seen on your machine's pressure curve.

Under-Damping:

- As the injection pressure graph transitions from first to second stage, the curve should not dip far below the specified packing pressure. If this occurs, the molding machine is ‘under damped’ and is not capable of transferring from velocity control to pressure control without losing significant pressure. Under damping often results in sinks and short shots due to insufficient packing pressure. This only occurs when molding with hydraulic machines, and indicates a faulty hydraulic valve.

Over Damping:

- If the pressure curve transfers between stages gradually, the molding machine is ‘over damped’. This indicates that additional material is being forced into the mold at a pressure higher than the set point. An ‘over damped’ molding machine will often result in flash or overpacking due to the fact that the hydraulic valve can not transition into pressure control adequately. The controller may need to reprogrammed or a hydraulic valve may need replacement to resolve ‘over damping’.

Overshoot:

- Similar to over-damping, many machines have heavy injection units which are difficult to slow down when the machine transfers from 1st to 2nd stage. This can often cause inconsistent fill. Although most electric molding machines have accurate servo motor controls which prevent this, many hydraulic molding machines are very susceptible to this situation. On these machines, it is a great idea to use one or two transitional speeds to prevent this condition.

Fluttering:

- Fluttering or erratic changes within the injection pressure curve during transfer indicate faulty hydraulic valves or electric servo motors. These fluctuations, as with the other conditions above, affect shot-to-shot consistency and should be improved to the best of the machine's capabilities.

You should always evaluate the capabilities of your molding machines with rigorous testing. This is another reason why machine-independent process documentation is so important... process outputs tend to ignore the idiosyncratic behavior of any individual machine.

-Andy

Just because something provides good information does not always mean it is an effective training method. Reading does not provide particularly high retention rates: meaning that you’re likely to forget most of what you’ve read!

Studies have shown that most people can only remember about 20% of what they read. This retention drops even lower when the reader is unfamiliar with the topic, making it particularly ineffective when training new hires or inexperienced personnel.

Certain factors can increase retention of what’s being read, such as an emotional connection with the information. As much as we’d all like employees to be passionate about plastics, it is unrealistic to expect them to react emotionally to what is usually pretty dry reading material.

I always recommend plants have such periodicals to their employees since these periodicals can be great tools when trying to introduce someone to a new featured technology.

-Andy

As the shear rate, or flow rate, of the polymer increases... the viscosity decreases. This rheological behavior is unique to polymers and is called ‘shear thinning’.

When graphing this, viscosity is plotted on the vertical, ‘Y axis’ and shear rate is plotted on the horizontal, ‘X axis’. Shear thinning will appear as a steep decline in the viscosity of the polymer as the shear rate increases.

Once most of the shear thinning occurs the polymer’s viscosity starts to level out. After this point, the viscosity will remain relatively consistent - resulting in a more stable process. For this reason, you should process on the right hand side of the curve.

You can learn more about this test and other aspects of scientific molding if you participate in our free online webinars.

-Andy