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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

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

Using Hold Pressure Velocity

I received this question from a frequent blog contributor, and believe it makes a good discussion on decoupled processes…

Milan
On some of our machines, we can adjust the hold velocity. How should this feature be used when molding a decoupled II process.
My Response
Before I respond, I want to relate the general attributes of this process:
  • A process that uses one injection speed to fill – whenever possible
  • The mold fills 95 to 98 percent full during first stage
  • All cavities are short shot during first stage
  • First stage fill is velocity-controlled and not pressure limited
  • Second stage pack is pressure-controller and not velocity limited
  • Process uses only 20 to 80 percent of the machine’s available shot size
  • The final cushion is approximately 10 percent of the overall shot size

In essence, the purpose of a DII process is to ‘decouple’, or separate, velocity controlled fill from pressure controlled pack. To do this, you should either turn the velocity control off so the pressure control takes prominence.

Additional Thoughts
Always make sure you understand the specific controls on your machines. In some cases, the 2nd stage velocity is actually a limit rather than an actual setting. In this case, you should set the speed near the maximum so that 2nd stage pack does not become velocity-limited.
-Andy

Fill Time, Viscosity & Molding Economics

A friend emailed me this great question the other day regarding the relationship between fill speed and pressure…
JM
The pressure required to fill a particular mold went up from 950 psi at 1 in/s to 1050 psi at 3 in/s. 

I understand the higher pressure needed to hit the higher speeds, but shouldn’t this be offset by the significantly reduced viscosity?

Does it hold true that anytime you increase the fill velocity, you see an increase in fill pressure… all things being equal?
My Response
The pressure required to fill will increase because the viscosity change will not completely outweigh the pressure losses… The overall energy consumption does drop considerably during fill since the pressure to fill is being appllied for only one third the amount of time!
It is very likely that a rheology curve would demonstrate that the 1 in/s fill is on the left hand side of the shear thinning transition region and the 3 in/s is on the right hand side of this region.
Think of it in vehicular terms… Your 1 in/s is like pushing a large pickup (higher resistance to movement) at 20 miles per hour, while your 3 in/s is like pushing a compact car (lower ristance to movement) at 60 miles per hour. It takes more gas per minute to move the smaller car at the higher rate, but it takes less time and you consume less gas overall getting to your final destination.
Additional Thoughts
Always keep in mind…the reduced viscosity due to shear thinning will actually save you money… making your processes much more economical since the overall energy consumption to fill the mold as well as the time are reduced.
Additionally, the drop in viscosity will also reduce the pressure required to pack out the mold cavity during 2nd stage. In the long run, all these changes can make a big difference in the productivity and efficiency of your facility.
-Andy