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1st to 2nd Stage Response Time

I was asked this great question the other day…

eMailer
I heard something about machine response (reaction time during switching from fill to hold).

How does this response influence my process?
My Response
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.
Additional Thoughts
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

Reading Materials are Not Training Materials

I was recently asked about the effectiveness of reading materials…

Chris
We receive several monthly periodicals at our plant, and my manager asked me why these cannot be used for training… I know this won’t work, but I don’t know how to explain this to him.

Can you help me understand why this is so?

My Response
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.
Additional Thoughts
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

Process Inputs vs. Process Outputs

In a recent webinar on the topic of Scientific Molding, I received this question regarding process outputs…
Vishu
In a scientific injection molding process, can we change process inputs while maintaining the same outputs?
My Response
There are many cases where a change in the inputs may have little or no affect on the actual process… For example, if the process was pressure limited, a change in the injection speed will have no affect on the process. This is why the process inputs are significantly less important than the resulting process outputs.
Additional Thoughts
To illustrate this with another example… if your process has too much screw speed and back pressure, a ten degree drop in barrel temperature may not have the desired effect on the melt temperature. If you concentrated on the melt tempertaure instead, you would know how each change directly affects the melt temperature.  
Both of these scenerios relate specifically to why documentation of process outputs is critical. For free multimedia presentations on scientific molding, please visit: http://www.traininteractive.com/free/webinar/player/
-Andy

Minimum Mold Sizing

Someone was preparing for some in-house training, and asked me this important question…
Marcel
We are selecting a mold for training. I have a mold in house, however the size of the mold base is 8″ x 8″. I know the manual suggests a mold base minimum of 17″ x 17″ under full tonnage. Can this mold be used for training if it is not under full tonnage?
My Response
Although every situation is different… as a general rule, you do not want to use a mold which uses less than 2/3 of the space within the tie bars. For example, if the horizontal spacing between the tie bars is 18 inches and the vertical spacing between the tie bars is 12 inches… then the minimum mold size should be 12w x 8h.
If you go below this value, the force is will be concentrated on the center of the platen. The factor is not the amount of tonnage being used, but the centralization of force. Since the platens are cast, they do can be deformed very easily… a mold which is too small will quickly increase the concavity of the stationary platen, and possibly damage the movable platen as well.
Additional Thoughts
A bolster plate can also be added to the mold to provide additional support and distribute the force more evenly across the platen. This plate bolts to the clamping plate and extends beyond the mold base; thus increasing its effective size. In some cases, bolster plates can be as much as nine inches thick and can extend beyond the mold base by as much as twelve inches.
-Andy

Interpreting Viscosity Curves

As a response to the Fill Time, Viscosity & Molding Economics blog I received this question…

E-Mailer
With respect to a in-mold rheology test, why should we choose to process on the right-hand side of the curve?
My Response
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.
Additional Thoughts
You can learn more about this test and other aspects of scientific molding if you participate in our free online webinars.
-Andy

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