New here with a head full of questions

[Valentin Leung]

Hello there!

I thought that it would be good to present myself and my background before asking questions around. Valentin Leung, I am a Chinese native French working in Hong Kong for a US boss (boy, that sounds complicated).

I have been a Product Design Engineer in a molding company, we are mostly doing thin packaging. I was literally fed up of waiting during mold trials, so I decided to look into molding process, so after working in plastic design for so long, I crossed that bridge and ended up trying to process my own molds. That's about me.

I have crossed upon my boss notes about scientific molding and was thinking to learn a bit more. But, it's just notes. Before he and I decide to fly me over the US to learn more in a seminar and on hands sessions, I decided to order the book from Suhas. While waiting for delivery, I still have millions of questions.

We have plenty of mold processors here... but they can't articulate what and WHY they are doing the way they do in a way that I can understand fully. Every time they are called in, they do their *MAGIC* and tune the process in and I am dazzled. I don't think I have control of anything, hence my quest to understand more.

I am using hydraulic machines from Welltec, I believe it's servomotor driving hydraulic pumps, I can do recovery and clamp motion in parallel, tonnage ranges from 260t to 450t.

I still have a few questions:
-Why is there a difference between the velocity I set and the Actual Injection Velocity? Can't I just shoot at max speed? or the max speed is for the movement of the screw without plastic inside the barrel? What's the purpose to be sold a so-called high speed machine if we can't never reach it? I know some will tell me that a Camaro might have a 180mph speed gauge but it can't never reach it and it's sold everyday, but when I go in injection machine trade show, they are everywhere with their 800mm/s, 1000mm/s.

-During the viscosity vs injection rate curve data logging, I noticed that if I shoot too quick, I have a cushion that is different to the VP I set, a bit further away. I naively believe it's normal and I think it's because of the screw's inertia vs the plastic viscosity (am I right?). Then the question is: just after the VP, what happens? during that period of movement because of inertia, is it to be controlled within the holding/pack phase? is it part of it? Does the time I set in the holding start to count as soon as it reaches VP? or it happens erratically without control whenever the machine decide that it has control over the flow? Should I just stay in the area closer to the left on the viscosity to reduce the inertia and get more control over the flow?

-I read here and there that the optimal flow settings is on the right side of the viscosity vs injection rate. Maybe 20-30mm/s of the max so that I can allow for variation for that. Is it right? My boss says that during his training, they were saying that it's just at the bend, after the curve turns from near vertical. I am highly skeptical though.

-What governs choice of parameters for the holding/pack phase? What I am currently doing is that I try to stay in control until the actual flow from the machine reaches 0. So I set the injection pressure and Flow at the same values as the VP point and progressively lower them while adding enough time until the flow comes under 10mm/s. In parallel, I look at the part quality and further increase pack time if I see sinks on it that is able to tuned with the pack. During injection, I need to keep the pressure high enough so that the machine stays in control, do I need to do the same thing during the holding/pack or the machine will just bluntly reach the set Pressure and be done with it, obviously it is not from the curve the control gives me, but then? why is it different?

-Right now, I haven't touch the metering side... I think I should, is the metering just to prepare the following shot? can it change some characteristics of the melt?.

Some of these questions might be answered in the book, but its target delivery is Oct. 24th...

Thanks for enlightening me.

[brentb]

Wow! Wait to the 24th
and,

KEEP ON MOLDING!

brent

[JayDub]

Too many questions for me to tackle, but I’ll try a couple of related ones to get you started.

First, from the viscosity curve, your injection speed should be selected so as to stay in the relatively flat region of the curve (a little bit away from the bend). This gives you a process that is relatively insensitive to machine variation.

Now, if your screw velocity (injection speed) is not reaching its set-point, your process is pressure limited. Moving the screw forces the melt to flow and pressure is just resistance to flow. So either you are exceeding the capability of the machine (there is too much resistance for the flow you want) or your high pressure limit is just set too low. If you are hitting maximum machine pressure, you will need to reduce resistance to flow – for instance by increasing the size of the sprue and runners. Also make sure you are using an appropriately sized nozzle tip orifice.

[Suhas]

Hello Valentin,

I will try but you will have more questions and so Yes, it is best that you wait for the book. Most of these are answered directly or indirectly there. Adding to JayDub and being brief,

Viscosity Curve: A little after the knee (or bend as you say). By the way you do not have to do this study.: http://fimmtech.com/index.php?id=6&subid=53

Pack and Hold: Just remember for now it is for compensating the shrinkage and keeping the plastic from flowing back out of the cavity.

Metering: Yes, change in screw speeds and back pressure will absolutely change the melt quality.

You will be enlightened soon :) :) As the Gods will tell you - Patience is a virtue.

Regards,
Suhas

[Valentin Leung]

Wow! Wait to the 24th
and,

KEEP ON MOLDING!

brent

Thanks, now, that I showed the molding floor I can process a tool... people think that they can rely on me for whatever topic downstairs. At least, it gives me opportunities to learn and practice.


Too many questions for me to tackle, but I’ll try a couple of related ones to get you started.

First, from the viscosity curve, your injection speed should be selected so as to stay in the relatively flat region of the curve (a little bit away from the bend). This gives you a process that is relatively insensitive to machine variation.

Now, if your screw velocity (injection speed) is not reaching its set-point, your process is pressure limited. Moving the screw forces the melt to flow and pressure is just resistance to flow. So either you are exceeding the capability of the machine (there is too much resistance for the flow you want) or your high pressure limit is just set too low. If you are hitting maximum machine pressure, you will need to reduce resistance to flow – for instance by increasing the size of the sprue and runners. Also make sure you are using an appropriately sized nozzle tip orifice.

Thanks for the confirmation.
As I don't have unlimited headroom on the pressure side, I think it's just that I can't push the material any faster, so thanks, working in thin packaging does not really allow me to push any extra plastic in the tool.


Hello Valentin,

I will try but you will have more questions and so Yes, it is best that you wait for the book. Most of these are answered directly or indirectly there. Adding to JayDub and being brief,

Viscosity Curve: A little after the knee (or bend as you say). By the way you do not have to do this study.: http://fimmtech.com/index.php?id=6&subid=53

Pack and Hold: Just remember for now it is for compensating the shrinkage and keeping the plastic from flowing back out of the cavity.

Metering: Yes, change in screw speeds and back pressure will absolutely change the melt quality.

You will be enlightened soon :) :) As the Gods will tell you - Patience is a virtue.

Regards,
Suhas

Thanks for the link, I see that the tools I have are pressure limited. So, I'll shoot as fast as I need.
For the pack and hold, we are using hot runner + valve gates, so if I see that the screw is still moving forward, it should mean that the plastic is flowing into the cavity.
Metering, I'll have to investigate that.

Patience patience... I should have paid Amazon for their Shipping Express premium.
Now, I am going to mold something to clear out my mind.

I saw your e-mail, going to take some time understanding everything in the book, and if we see value flying me over, we will obviously contact you for that.
Thanks for your help and enlightenment.

[Valentin Leung]

Me again,
I just finished the book, took me more time than I thought. Anyway, I still have questions not resolved.
For my feedback on the book, it's well written although I think I spot a few typos here and there (I say I think as English is far from being my mother tongue). It was easy to read and understand, I still long for more as it was written to cover a whole range of different molding configuration and things that happens at other shop my not happen in my shop. But what I liked is that it emphasized the importance of understanding the nature of plastic and it's specifications which I never took time to think about in the past. So I am positive on the book, except in terms of price, it's quite expensive, but I am living in another living expenses scale.

At first I thought I understood what "pressure limited" meant, but I was wrong. Here is a Fill graph from my machine, so pressure and flow during FILL.
Let me explain:
71
The tool is a cover part in a thin packaging assembly, 0.65-0.75mm thickness 6 cavities hot runner with valve gates. My machine is a 450t, max values for speed is 150mm/s and pressure is 160 bar.
You can see that the set flow is way higher than the actual flow. You can see that the purple curve, actual flow is much lower, I am already contacting the machine manufacturer to understand why it is different from the set value. Why on Earth would the machine output command is different from the set one?
If the white curve is following the purple curve, I am ok, but the yellow curve is what I ask the machine to do and it's a lot higher, I still have pressure available under my feet, the screw should be able to kick-in. I did not display the actual pressure but from my memory, it is the same as the green curve which is the set pressure. So there should be no difference.
Do you see similar effect on your side? is the set velocities never reached?

My other issue was what about after FILLING, does the machine kick-in the HOLDING phase directly?
Also, in the filling I always tried to keep the set pressure above the hydraulic pressure feedback (keep the green curve above the blue curve) so that the machine stays in control of the hydraulic pressure, if it's below, I think that the valve will endlessly try to adjust itself and it means that I am not in control. But during PACKING? other than influence on the dimension? Is it driven only dimension? Should I care about the holding graph then? Should I keep the set hydraulic pressure above the feedback?

If you have any feedback on that I am all ears.

Thanks

[Suhas]

Hello Valentin,
Thanks for some feedback. Here are my answers in general.
1. The Book: I have no control over the price. In fact, it does not belong to me. I too have to buy the book when I give it to my seminar attendees. I get a small discount and may be can treat you to a beer some day :) .
2. Although the basic working is the same, all the machines have different working features. In some machines, you will need to add a value for 'Inj Time' for the screw to move, in some you will not. It will depend on the machine manufacturer. So my suggestion is to ask the company that makes the machines how these work. In Arburgs, if you set the hold pressure to 900 and the second one to 500 for 2 seconds, then it will drop in a smooth slope from 900 to 500. In case of an Engel machine, it will be a sudden drop after 2 seconds (just another example).
3. Similarly, every mold is different. You must again do all the studies to see what is the over all picture. I am not sure if anyone can clearly and emphatically answer your questions since we are not at the machine. There is still a lot of experience based knowledge processing out there and I have a lot of respect for that. I am a big advocate of having cushion in the process. I think it is an ABSOLUTE necessity. I just finished a seminar and one of the persons told me that they mold lenses and they always have zero cushion! As soon as they add cushion, the lens properties go into the dumps. I have no answer for him (yet).
4. The book gives you a foundation, does not give you the solutions and explanations to everything out there. I wish I had time to write more. Again, I get zero money to write, I get 10% when it sells.
Hope you are not disappointed.
Regards,
Suhas

[Valentin Leung]

Thanks for the beer.
I know it's difficult to compare things when we have different machines, different control sets, different material. During the week-end I came up to the conclusion that during FILL, the machine is trying to reach a certain set speed value given the pressure limits I set (if it does not reach my pressure limit, it will continue to raise its speed). During HOLD, it's the reverse, I am controlling a certain pressure value given the speed limits. The machine will try to adjust the pressure as long as material is flowing into the tool. That's basically how I see our machines working.

Anyway, this afternoon, I'll spend time on a 4 cavities with cold runner (something I learnt to hate) and another 6 with hot runner and I'll confirm my HOLD control thoughts.

For the cushion, yes, I am overly watching that, I make sure that it stays the same with same hydraulic pressure and flow curves in my control panel, if I hit 0, my curves for graph will reach a peak at the end of the shot and I would hear a big "THUMP" coming from the screw, next thing you hear is the boss screaming across the molding floor. For lenses manufacturing, I don't have this expertise, but I would start by asking myself what does bottoming out the screw brings to the process? can this effect be created by something else that bottoming out the screw on the barrel? can we add a sort of bumper/stopper on the machine that stop the screw movement keeping a small amount of cushion? Would this give the same results? I don't think any tool need to bottom out the screw, I don't think he is shooting lenses right at the tip of the nozzle and from the mold perspective I think the runner is some form of cushion (if it does not stays in the barrel, it goes in the runner), what is the impact of the quantity of runner there? can we split some and move it back into the barrel to leave cushion and protect the machine? (I have been taught by the machine owner... that screw bottoming is bad, real bad, it can be deadly on my paycheck)

I also found out something during my processing, it's what I call the residency time, during recovery, I am using a low to medium speed and delaying it as much as possible so that the residency time of the plastic at the front of the screw is the shortest possible while not impacting my cycle time. I successfully eliminate some gate burn marks and lower deformation. That came up after I was hardly thinking of what the plastic needs as you teach, so I really thank you for that.

[Jorge Villegas]

Me again,
I just finished the book, took me more time than I thought. Anyway, I still have questions not resolved.
For my feedback on the book, it's well written although I think I spot a few typos here and there (I say I think as English is far from being my mother tongue). It was easy to read and understand, I still long for more as it was written to cover a whole range of different molding configuration and things that happens at other shop my not happen in my shop. But what I liked is that it emphasized the importance of understanding the nature of plastic and it's specifications which I never took time to think about in the past. So I am positive on the book, except in terms of price, it's quite expensive, but I am living in another living expenses scale.

At first I thought I understood what "pressure limited" meant, but I was wrong. Here is a Fill graph from my machine, so pressure and flow during FILL.
Let me explain:
71
The tool is a cover part in a thin packaging assembly, 0.65-0.75mm thickness 6 cavities hot runner with valve gates. My machine is a 450t, max values for speed is 150mm/s and pressure is 160 bar.
You can see that the set flow is way higher than the actual flow. You can see that the purple curve, actual flow is much lower, I am already contacting the machine manufacturer to understand why it is different from the set value. Why on Earth would the machine output command is different from the set one?
If the white curve is following the purple curve, I am ok, but the yellow curve is what I ask the machine to do and it's a lot higher, I still have pressure available under my feet, the screw should be able to kick-in. I did not display the actual pressure but from my memory, it is the same as the green curve which is the set pressure. So there should be no difference.
Do you see similar effect on your side? is the set velocities never reached?

My other issue was what about after FILLING, does the machine kick-in the HOLDING phase directly?
Also, in the filling I always tried to keep the set pressure above the hydraulic pressure feedback (keep the green curve above the blue curve) so that the machine stays in control of the hydraulic pressure, if it's below, I think that the valve will endlessly try to adjust itself and it means that I am not in control. But during PACKING? other than influence on the dimension? Is it driven only dimension? Should I care about the holding graph then? Should I keep the set hydraulic pressure above the feedback?

If you have any feedback on that I am all ears.

Thanks


Hello. I was watching your curve and the first thing that came inton my mind was, ( are the curves using the same scale? set and real). do you have any info on the type of valves on your machine? I could help you understand the problem but I qould need more info.

[Valentin Leung]

Yes they are using the same scale, I can't change it, speed scale is on the left, pressure on the right.