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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

Scientific Molding For High Speed Applications

In a recent webinar regarding Scientific Molding, I received this question…
Ralph
Is scientific molding applicable to high speed packaging machine where injection fill time and hold time are around 0.5s with an overall cycle time of 3sec?
My Response
Yes, you should always approach processing logically and document the process by its outputs… especially in your case, where any change in your process is virtually imperceptible to the human eye. Such changes can only be identified through the accurate measurement and proper documentation.
Additional Thoughts
Although every process seems unique, all injection molding processes operate under similar principles. There is a 1st stage fill time, 2nd stage pack time, final part weight, melt temperature, etc. All these factors are imperative in ensuring a process can be consistently repeated. For free multimedia presentations on scientific molding, please visit: http://www.traininteractive.com/free/webinar/player/
-Andy

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

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