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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.

Check Ring Drift vs. Repeatability

In a recent e-mail, I received this question from one of our customers…
Jim M.
Regarding cushion, regardless of dynamic check ring tests (e.g. get 1.3%) if the machine will not hold a cushion, should the culprit be repaired?
For example, the gate freeze test shows 7 seconds & the hold time is set to 8s.   When I transfer over to pack stage, the screw drifts about .020″ during that time.  Is that an OK scenario, or is zero drift the answer?  If I put the pack time at 20s, the part still looks good, but screw keeps slowly moving & doesn’t stop.
My Response
Every check ring drifts. Tests such as the Dynamic Check Ring Repeatability Test test consistency during fill. A 1.3% variation implies that your check ring is wearing very evenly and your process should remain consistent. If the amount of drift makes you uncomfortable, it may be time to inspect and/or replace. But, a long as the drift is consistent, you should be able to maintain a robust process.
Additional Thoughts
There are a variety of check ring designs available to the molding industry… and all of them leak. A properly functioning check ring will leak consistently. If you are running a material with a very low viscosity, you may want to ensure the check ring you are using is appropriate to the applicaton and properly sized for your barrel.
-Andy

The 80-20 Rule For Available Shot Size

I received this common question via e-mail the other day…

MJ
I heard that it’s good to have shot size from 20 to 80 percent of the machine’s available shot size. Has this conclusion was tested, or has somebody published a white paper study?
My Response
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.
Additional Thoughts
Unlike most manufacturing processes, the compressibility and shear thinning characteristics of plastics cause inherent variability in the process.
-Andy

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