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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.

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Rules for a DIII Process

A frequent blogger asked this question the other day…
MJ
I know, the rules for DII process as follows:
 
•  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
 
Do you have something like this for DIII process?
My Response
As with above, the following items are also rules of thumb, and are not set in stone.
•  A process that uses one injection speed to fill – whenever possible 
•  The mold fills 80-90 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 velocity-controlled and not pressure limited
•  Second stage transfers at approximately 95-98 percent full
•  Second stage transfer is controlled using post-gate cavity pressure
•  Third stage hold 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
Additional Thoughts
A DIII process requires cavity pressure transfer capabilities, and should only be used if you and your technicians have a high level of competence. Such a process gives you excellent control, but may be overkill for many applications with good processing windows.
-Andy

The Affect Of Screw Diameter On Shear Rate

One of our customers brought up this question…
Jim M.
How does shear rate vary for the same fill rate going from a small screw to a large screw?  The volumetric flow rate being the same of course, but is there a difference in shear rate caused by the barrel / screw size differences?
My Response
With respect to shear during first stage injection, virtually all the shear occurs in the nozzle, runner, gates, and mold cavity. The diameter of the barrel, in comparison, is inconsequential.
If you used the same rotational speed (RPM) with a larger diameter screw, then you would create more shear during recovery since the circumferential speed would increase. For this reason… you should try to match the recovery times to help balance the shear caused during screw recovery.
Additional Thoughts
What is usually overlooked is the ID and length of the nozzle which may change from machine to machine as well as the resulting melt temperature due to recovery… These oversights often make people think it’s the barrel that is causing the difference.
-Andy

The Costs Of Not Training

This is a short excerpt from our executive audiobook entitled ‘The Science of Training’…

Excerpt
There are clearly some expenses associated with setting up and launching a well-planned training initiative. This is undeniable. But have you stopped to think about how much you’re spending by NOT training your employees? Or maybe you are trying to train your employees without a structured training plan. In either case, you are likely losing thousands of dollars each month that could easily be saved by having a competent, versatile, and productive workforce.
You can listen to it here:

Max Velocity Settings for Thin Wall Molding

A blog reader submitted an entry this morning…

Jean Francois
We are using a decoupled process. During fill, the pressure is set to a max pressure and should not to be reached. However, when molding thin wall parts with long flow paths, it is common to reach the maximum machine capacity pressure before reaching the expected velocity.
Question: In this case what is the best recommended process: 
1) Lower the programmed velocity to a point where requested pressure in lower than the machine capacity (with 10 to 15% margin)? 
2) Keep high programmed velocity with the maximum pressure set at the maximum of the machine capability.
My guess is solution 1) should be the best because the process will be more stable. The drawback of this solution is that we are not using the max velocity that the machine can really provide.
If I choose solution 2) any variation of melt viscosity will change the process…
So what is the best?
My Response
Your choice is correct… but I should clarify the reasoning.
In establishing a robust injection molding process, the purpose is not to use the fastest velocity possible, but to use a high velocity which provides a shear rate which is above the point of shear thinning. You should perform an In-Mold Rheology Worksheet to determine where shear thinning 
occurs in your process.
If necessary, you can try processing at a velocity consuming approximately 90% of max pressure if it provides the best appearance or performance. Just ensure that you closely monitor the fill time consistency, possibly placing a lower limit alarm on your process.
Additional Thoughts
You should always avoid a pressure limited process. If necessary, you can avoid pressure limiting the process by reducing the injection speed through the use of profiling.
-Andy 

Correcting Outdated Process Philosophies

An employee confronted his management with this proclamation he found on a website… I was asked to explain why this was an incorrect philosophy for a 21st century processor. 
Outdated Philosophy
Plastic is compressible.  We must take the injected shot past its ability to compress so that it acts like a hydraulic (non-compressible) fluid to completely fill the mould.   As the cavity fills, there is no measurable pressure because it is pushing air out.  Once filled, the plastic compresses.  At some point in time it is so compressed it is no longer a compressible fluid but responds as if it were a hydraulic fluid.  It is at this point that the cavity is fully pressurized and the process switches from fill to hold, just before the spike on the curve.  The screw is slowing down, but the pressure is now packing.  The only way to hold pressure on the part is to maintain a cushion.  The smaller the cushion is, the higher the amount of pressure we can apply on the melt.  However, if it bottoms out, there will be no pressure on the melt.  If the cushion cannot be maintained, the amount of pressure on the melt will be inconsistent.
My Thoughts
This is the philosophy behind the older, pressure controlled, machines. This process theory focuses on, and requires, a fully pressure limited process.
Aside from the reciprocating screw, velocity-control is the most important advancement in the injection molding of thermoplastic polymers… The theory above requires that you neglect this feature completely.
In 99% of the cases, such a process will be significantly less reliable and more machine dependent than a robust, velocity-controlled process with a short shot during 1st stage fill.
Additional Thoughts
There are always opposing positions… but you would be hard-pressed to find any successful consultant or educator who would subscribe to the theory espoused above.
-Andy

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