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Open Loop vs. Closed Loop Controls

In a recent webinar regarding Process Documentation, I received this question regarding process controls…
Frank
During an In-Mold Rheology Test, should we use the closed loop or open loop option on our machines? What is the difference?
note: For those new to the industry, open loop process control uses a set pressure to force polymer into the mold during first stage injection – adversely, closed loop control uses a velocity setpoint to control the filling of the mold.
My Response
Obviously, if your molding machine is limited to open-loop control, the you can perform the test, but make a note regarding the process control type. Since the screw speed drops as the mold fills, the apparent shear rate you calculate is less accurate since it represents an average of the shear the polymer encounters.
A more realistic representation of apparent shear rate the polymer encounters results from closed loop process control. This is because the screw speed is controlled, representing a more realistic picture of the shear thinning behavior of the polymer. This also gives you much more control over the process over the long term since changes in material viscosity will cause virtually no change to the first stage fill time.
Additional Thoughts
There are some advancements in machine technology which have little to no benefit, these include: back pressure profiling, hold pressure profiling, and cushion control. On the other hand, closed loop process control for 1st stage mold filling is one of the most beneficial advances in injection molding technology over the past 50 years.
For free multimedia presentations on process documentation including in-mold rheology, please visit: http://www.traininteractive.com/free/webinar/player/
-Andy

“Want Good Products… Train Good Employees”

A. Routsis Associates, Inc. was recently featured in a great Modern Plastics article by Clare Goldsberry entitled ‘Want good products and processes? Train good employees’.

In this article, she discusses how companies such as Molded Rubber & Plastics Corp. in Butler, WI improve their workforce through employee training at all levels from management to operator. In this article, Mike Dalton, director of quality for MRPC states “Basically, we’re only as good as our worst operator, so we carefully screen employees prior to hiring and then screen them during a 60-day training and orientation period.”
In this article, I discussed the right way for a company to train. “If they do the training the wrong way, it’s a disaster,” he says, noting that some companies give the training responsibility to the “lowest guy on the totem pole. Training has to be from the top down.”
Ultimately, the companies that succeed in both the upswings and downturns are those who have an in-house employee advancement system in-place to ensure your workforce is capable and flexible to handle the future. One, two, and three week seminars can help introduce new technologies to your employees, but real change occurs when learning is continuous, ongoing, and relevant to what your company does.
To read this article, visit:
-Andy

More Accurate Clamp Tonnage Calculations

I got this intriguing question the other day…

Jim
I am working with a part – family mould 1 + 1.  Here’s the scoop:

Projected Area:              89.75 “^2
Hold Pressure:               950 psi hyd.
Intensification ratio:        13.36
Plastic Pressure:           12,692 psi
Force:                           89.75 X 12,692 = 1,139,107 lbs.
Tonnage:                       1,139,107 lbs. / 2000 lbs./ton = 569T.

The tech guide says use 3.2T/in^2 = 3.2 X 89.75 = 287.2T.

The job was quoted in a 300T, and we run in 300T.

I would say, by theory, the plastic pressure based on hold pressure would be best calculation, but apparently not.  I don’t understand why.  It should be more accurate than the general rules of thumb that I’ve seen, based on part wall thickness etc. Any thoughts?
My Answer
The design guides are actually great references, since they generalize the pressure losses in the runner and sprue.
This would be a great time to perform a pressure loss study to really get detailed numbers.
In such a study, you perform a series of short shots through the air, sprue, gates, and then short shot the cavity.
The peak pressures at each stage in the process should be recorded.
So, for example…
air: 2000psi
sprue: 3000psi
gates: 4000psi
short shot: 12000psi
(12000psi) – (4000psi) = 8000psi pressure applied to mold cavity
In this simplified case, only 2/3 of the applied pressure, (8000psi) / (12000psi) = 2/3 reaches the mold cavity. In this case, I would apply this 2/3 factor to the overall packing pressure. So if the packing pressure was 6000psi, I would calculate the tonnage using (6000psi) x (2/3) = 4000psi. This would actually provide a fudge factor since some pressure is lost within the mold cavity due to a pressure differential across the mold cavity.
Additional Thoughts
In your case, you most likely have a restrictive nozzle, runner and/or gates, as well as an older machine which may not be transferring all the pressure to the front of the screw. Additionally, family molds tend to have large runner configurations in which the flow to one cavity is often restricted… resulting in even less pressure applied to one of the two mold cavities.
-Andy

Weld Lines vs. Meld Lines

In a recent conversation, I got this age old processing question…

George
Some of my techs call them weld lines, some call them meld lines… what is the difference?
My Response
Both Weld and Meld Lines occur where melt fronts intersect, yet they are different…
Weld Lines are formed when two melt fronts meet with little or no flow afterwards.
Meld Lines are formed when a melt front is divided by a mold feature, such as a core pin, and then meets afterward to continue flowing. Meld lines tend to be much stronger with better appearance.
The strength and appearance is directly affected by the molecular interaction and gas removal at the point of melt front intersection. 
Additional Thoughts
Many part designers will manipulate the part design and gate location to transform weld lines into meld lines. Additionally, in rare cases, you can adjust injection speeds to alter the formation of weld and meld lines.
-Andy

The Symptoms Of Machine Dependant Documentation

In a recent discussion with a brand new customer, when discussing common defects he posed this query…
Steve K.
Although we don’t really have one or two defects we get all the time, but we get a lot of different defects all the time. We run a lot of different molds, machines, and materials… and we change a lot of molds. Do you think that might have anything to do with it?
My Response
After a few pointed questions, Steve explained that his employees only documented the machine settings during start-up. Although this is very common in the injeciton molding industry, the process settings of one machine provide virtually no guidance when the mold is hung in a different machine. Even using the same barrel temperatures can result in melt tempertaure differences in excess of 50ºF! 
We went on to discuss the importance of machine independent documentation that includes part specific data such as part weight, fill time, coolant and melt tempertaures as well as the actual pressures applied to the polymer. We also strategized methods to get Steve’s employees to adjust their behaviors over the long term.
Additional Thoughts
Even though Steve had some good employee working for him, every time they put a mold into a machine, they put the data into the machine. When the part does not magically come out right… the have to process without a set of machine independant targets to aim for.
If the mold ran great in machine #4 last week you need to know specific details such as: Fill Time = 3.2 seconds, Melt Temperature = 427ºF, Final Part Weight = 23.5 grams. This information is invaluable when you put the mold in machine #3 next week.
If you or anyone you work with would like to learn more about process documentation… we have some FREE multimedia webinars on scientific molding which can be viewed at http://www.traininteractive.com/free/webinar/player/
-Andy

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