Hot Runner-Nozzle Heater

[Sammy101]

Dear Everyone,

Good day!

First I would like to thank everyone especially this site to where I learned the way of scientific molding.
Now, for my concern, we have molds in our customer that reportedly experiencing frequent busted nozzle. At least two to three times of having this problem for a month of production.
When we asked them, they mentioned that it is due to type of heater used that is prone to problem. The existing nozzle heater is an INCOE cartridge heater. Now we are planning to reply the whole runner system to make the nozzle heater with cable heater (tubular type). The modification is costly and we need justification that the problem is really because of the type of heater.
Honestly, I am not familiar with the hot runner system cause for in seven years in the industry, I only handled cold-runner type.

Your insight on this concern is highly appreciated.

Thank you very much every one!

[Suhas]

Hello Sammy,
Thanks for signing up here. In regards to your question, when you say nozzle, are you saying the one on the machine? Please give us a clearer pic and we can give you some suggestions. Yes, it is expensive. I have a feeling that you may just be Pressure limited.
Let us know.
Suhas

[Sammy101]

Suhas,

The nozzle I am referring is for the hot runner nozzle.
Does processing under pressure limited situation weighing

[Suhas]

If the process is using all the pressure the machine has then you are exerting all that pressure in the hot runner nozzle trying to fill the plastic in the last point to fill. Example, if you have a 20 inch long part with one gate on one end, by the time the plastic travels 20 inches not only has the plastic cooled down but because of fountain flow the flow channels is much smaller. Therefore you need a lot of pressure to fill the last say 1 inch. But now the plastic is still molten in the hot runner and so now you are putting all the machine pressure into the hot runner system and can damage it.
In my opinion, it is not the heater, it is the mold design, placement of gates and of course could be part design.
Here is what you do quick: Is the required injection pressure = the max machine pressure?? If yes, that is the problem.
Regards,
Suhas

[Jendalf]

Hi,
Suhas has right, we need usually more information. what type of materiál are you processing? what temperatures are set/get from the thermocouples for heaters (sometimes happend when you use wrong type of thermocouple you get different REAL temperatures even that required are shown at display). Also what is busted? freezed?

J.

[rickbatey]

Are the failures in the same location? If so that may be a combination of excessive injection pressure and the duration of time the manifold sees that pressure, poor mold or hot runner design, manifold supports. machining limitations, poor fit or expansion consideration of the hot runner when up to temperature. Are you using the correct temperature range in the manifold and barrel and not trying to push cold resin too far into the mold?
More info would help with a much more exact root cause.
Rick.

[Sammy101]

Dear Rick and Suhas,

The material is PP and the melt temp is set from 210 to 220 deg. C while the hot runner heater is set from Zone 1 to 7 with 366, 365, 351, 280, 327, 351 and 370 deg. C respectively.
The design of heater system was made by INCOE, so we have no further details of its effectiveness except in some of our molds, with INCOE hot runner also but we do not know if the design is similar, we barely experience the same problem.
For the max injection pressure, the max setting is set to 210 bar and actual is 160 bar so I think the process is not pressure limited.
The failure usually occured at zone 3, 5 and 6.
I just termed it 'busted' or broken (not working). I do not know if I used the term correctly for this problem.

In addition, I attached the picture of the drops layout.

The you so much guys.

106106

[rickbatey]

So what you're saying is that you have to replace the heaters because they've stopped heating? Were those temps in C? If so perhaps you should increase the barrel heats so you run the mold heats lower. Also do you use heat transfer paste on the heaters you replace? Not anti-seize as it will short heaters out when it gets hot due to the conductivity of the metal flakes. Lastly what are your water temp settings? Does the mold have any internal water leaks that could be grounding the heaters out? I've seen some molds leak only when under tonnage but be perfect when checked in the shop. Also be careful of the wire routing when replacing heaters and tape them up with high temp fabric tape made for the job.
Rick.

[Sammy101]

Dear Rick,

Yes. That is actually what is happening. We are also looking for the possibility of suggesting to change the setting to increase the barrel temp instead of the runner heaters.
Yes. The temperature is in Celsius. The setting of coolant for the core and cavity is set at around 25~28 deg.C (chilled water). As for the heat transfer paste, I am not sure if they are using such but I am glad you mentioned it. I will take a sneak peak on their PM process to see if they do.The last time I checked the condition, we have seen no leaking water in anywhere that may close to the heater but we are looking for the possibility of moist (from air) that may develop as heat and very cold medium interacts. Do you think this is reasonable to consider?

Thank you very much!

Sammy

[Jendalf]

First rise temperatures on barrell to 235-240 °C.
Second the temperatures on HR are too high. Set them +5-10°C higher than barell temperatures.

What is the closing/opening mechanism on HR nozzles? are they self closing, pneumatic or hydraulic controled?

[rickbatey]

Sammy the extreme high heat is shortening the life of the heaters but there is another method that really reduces heater life span, not using soft start when the mold heats are turned on!! Check and most likely you'll find out the cold start or evaporation mode is disabled to start the mold sooner. When cold, some moisture will be in a heater and if the amount of current is too high, that moisture when released can short the heater internally. I'm sure the thermal shock hurts the heater elements as well; room temp one moment and red hot the next. Also I think that the mold might suffer from setting it in the press without pre-heating the hot runner.
The reason I asked about the water is there may be too much surface contact between the cold section of the mold and the hot runner. That's another reason why the heats are so high as the manifold is losing so much heat to the mold steel. I worked on an early manifold system that used the gas inside to transfer the heat around the entire system. We had an issue with too much heat transfer so we made insulating crush rings out of alternating pieces of brass and stainless steel. No more issues. Last possibility: when pulling the mold for change over, depending on mold design and water fittings it could be easy to not drain and blow out the water lines, disconnect tyem while under pressure and the cold water spray, run, drip, or soak onto the hot runner.
Rick.

[Sammy101]

Rick,

That is quite an explanation but for that, I really got the point.
Tomorrow I will visit the plant for they reported another similar problem. I will apply all the information I have learned and see what will happen.

Thank you very much and I will make sure to get back for the update.

Sammy

[Sammy101]

Jendalf,

Sorry but I dot quote understand your second instruction, are your saying that I should increase the HR temp than of the barrel with +5/-10 deg. C variance?
I appreciate if you make it clearer.

I am not sure of the system for closing and opening. I will get back to you on this.

Thank you so much!

Sammy

[Jendalf]

If you setup the temperature on the barrel 240 °C the hot runners temperature should be around 250°C generally for PP. But def. not 380!

[Sammy101]

Jendalf,

Yes. I believe that also. Perhaps the heater actual power output is already below the input value. I will consider also checking the calibration status of the heater.

Thank you!

Sammy

[rickbatey]

There is another issue that may be at work here. The hot tips grow as they are heated and on occasion they grow too tall and create thin areas in the part, which makes fill the part difficult. Also some drops have tips inside them to control freeze off and stringing/gate vestige. At times the tip inside grows too much restricting the resin flow path. This can usually be determined by measuring the assembly at temp or by visual inspection while in the mold.
Rick.

[Jendalf]

Both are possible, but heat influence on thermal behavior of the functional units of the hot runners is more probable.

note (out of theme):
I´v challenged simmilar effect. Problem here was the pneumatic controled needle malfunction due to the low temp of cooling water. The cooling water that cool down the base of the needle (that is connected to the manifold) caused the tightening of its diameter around the needle. The pneumatic wasnt able to moove with the needle (due to the friction) and that casued me short shots. Quite interesting :)

J

[Sammy101]

Rick and Jendalf,

Please correct me if my understanding is not inline with what you wanted to explain, but this tip expansion basically restrict flows, correct?
Does it really causes the nozzle heater got shorted easily? Does build up pressure inside the hot runner passage affects the heater itself?

Sammy

[Jendalf]

Sammy,
nothing is 100% sure here, but the heat influence is one of the most probable option. Especially because of the high temperatures set on HR.
Another thing is that if the nozzle is selfclose the function unit inside can be weared or broken or it can contain the forreign element inside (like metal).

J

[rickbatey]

Sammy, the tip growth from heat restricts melt flow. To compensate for the loss, the heat has to be cranked up really high to get the resin to start flowing again.
Obviously there's an issue with the hot runner and tooling due to the fact that you know which zones the failure occurs. Be that too much cooling or what have you. The hot runner is an extension of the barrel so it should be close to the same heat range. Then only slight differences should be required to balance the cavities. It could be that the gate inserts are wrong and that is driving the crazy high heats.
Rick.

[Jendalf]

Yesterday i´v talked with Incoe representatives on the Trade fair. Generally he agreed what was said here but maybe from lack of experience he didn´t know what will happen with hot runners at high temperatures exactly (he didn´t try it yet :) )

[Sammy101]

Rick.

I think the friction has less chance for the cause but for the excessive cooling, I think we have some issue here; but I will try to consider verifying both idea to see where the problem is.
I will post the result once we have the conclusion. :)

Sammy

[Sammy101]

Jendalf,

That is nice to here from the INCOE themselves.
We will try to consider of lowering the heater temp from 300 deg.c and see what will be the possible effect on the heater itself.

Sammy.

[rickbatey]

Sammy-I meant flow restriction not friction. The tip grows into the orifice and blocks the flow. Now I've seen tips that were so small they shear heated the resin to the point of thermal degradation before. Each time the mold was injected the temps would climb over 50*F!! But that didn't cause heater failure.
Rick.

[Sammy101]

Rick,

Yes. I mistakenly accounted the friction to flow restriction.
By the way, this is another issue, but we observed in most cases, that our product under this mold (and heater) frequently experiencing voids on the product, especially near the gate or where the tip is located.
Do you believe this has something to do with the flow restriction or our issues with the tip or heater problem?

Sammy.

[Jendalf]

Any pictures? Voids can appear due high temperature but how does they look like?

J

[rickbatey]

Sammy I think that the high heats are causing the voids due to the heat or poor melt flow into the cavity. When you guys find the root cause and correct it, I think some folks will sleeping well for the first time in a long time.
Rick

[Sammy101]

Jendalf,

It is unfortunate that I do not have the picture now but the void is inside the plastic. Now the concern is, that the product is a case that we place acid, then later due to unwanted voids, the acid leaks later at the gate or the area near the gate.

Sammy

[Sammy101]

Rick,

We are also looking forward for the resolution of the problem and at last, you are correct, we can take some nap time. :)
I will take your advises.

Sammy

[Sammy101]

Dear Rick and Jendalf,

After I checked the condition of the mold and runner I found that it has no shut-off valve and just a free flow system. I attached the picture for reference.
Now, looking at the condition of the nozzle, which is an obsolete version of INCOE, I think the design is not as efficient as the latest design which leaves the opportunity to the heater to extend more heating to compensate the heat loss on the material. Though the outer cap of the nozzle assy is quite thick, still the heat loss due to contact with the colder plate is still bearing on the heater.
In addition to the concern, I found our that the this type of heater has a built-in thermocouple making it prone to frequent replacement if either the heater or the thermocouple is not working.

I will attach the hot runner assy in the next page since I think our page has a limit for the attachment.

Your insight with this findings is highly appreaciated and valued.

Sammy

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[Sammy101]

Rick and Jendalf,

Here is the after image when we open the hot runner system.

Sammy.

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[rickbatey]

Sammy- I wonder about the rust on the manifold because that's alarming for such a simple system. Lastly I hate that so many of the drops stuck in the mold base when the manifold was pulled. I would also expect a manifold that small to pull by hand w/o lifting eyes. Check the mold base for leaking plugs, water circuits, around the manifold. I bet there are some dripping water onto the manifold; at least that would explain heat settings and rust.
Rick.

[Sammy101]

Rick,

I will take that into consideration.
By the way, what do you thick of my findings with the condition of the heater system? I know it too outdated and perhaps the cause of too much heat-loose which forces the nozzle heater to work more?

Sammy

[rickbatey]

Sammy, I think one end of that manifold is way too rusted to be a coincidence. I bet there's a water leak internally or it gets wet when the lines are pulled off at change over. I think the water leak is rapidly killing the heaters and due to the set points required, I bet an active leak while in the press. It may only leak when under clamp force.
Rick.

[Jendalf]

Hi,
It would be really interesting to know how this case was solved and what was the rootcause.

after 4 years, we are older (of course) and maybe more experienced...

Sammy? :)
J

[iamplwirtz]

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