Unbalanced shot 1

[sizzer33]

Hi everyone,

Some advice needed on one of my tool.

Here's some info on the part as i don't think i'm allowed to put a picture of it:-

Part shape: Mouse top key. Its curve, arc of circle around 90 deg.
Thickness: 2mm all the way with 2 clips and 2 bosses.
Gating: Sub to pin. (submarine gate to quarter of ejector pin). GAte is on one end of the part on the long side.
Gate dimension: its big, i think at least 1.5mm diameter. Its not exactly round, more like a pie shape as its quarter of an ejector pin.
Mould cavity: 4
Mould type: Hot sprue to cold runner.

THE PROBLEM: We are having problem with 50% and 90% fill shot. 2 parts are running slower than the other.

We've checked the mould temperature between cavity and its about 4 deg Celcius difference.

Since gating is so big, i presume we don't have a problem with gate shutting off.

We're testing the tool on a brand new german machine in suppliers side and made a hold time study, its very good result. So shouldn't be a problem with the torpedo. Fully filled parts have a good part weight ratio between cavities,less than 3%.

The cavity side of the tool is polish to around A2. We had some venting problems in the middle of the part so we increase some vents on the lifters(use to make the clips). PArt surface is shiny as ever.

Please advice on any other cause for this uneven flow.

Many thanks.

 

[patprimmer]

Is the machine holding a cushion.

A sub gate, even a big one can still freeze of fairly quickly.

I presume by hold time study, you increased hold time and weighed parts progressively until there was no further increase in weight. If not do that.

Have you tried increased hold pressure, increased injection speed, later switch over time, increased melt temperature, increased nozzle temperature, better insulation of the nozzle from the back of the mould, higher back pressure, does the machine have enough capacity, how long and thick is the peg from the gate to the part, do the vents vent to atmosphere, what is the length to thickness ratio of the part from the gate to the farthest point in the cavity, are the 4 parts identical mirror images or are some different parts.

I am a bit confused where you say all parts are within 3%, but also say some parts are 50% & 90% full.




Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

I too am bit confused but try this:

Start with short shots - so you fill around 1/4 of the cavities - compare the size of the (short) part in the cavities. You may find that they start off same size and as shot is increased, parts get more different. Imho, if this occurs, gate sizes are different. You could try swapping the sub gate pins over to see if this transfers the problem to another cavity. (or just get a rat tail diamond file and gradually open up the sub gates until all is well. (This is a job which we always do ourselves as moulders as toolmakers have little chance of getting everything exact - a bit of "art" is required over engineering!)

Caveat: That injection speed is max possible without dieseling, and all process parameters are correct.

Also you have indicated that venting was an issue - it may still be - try some packing tape on the tool shut face next to the short cavities.

Varying fill can of course be attributed to machine settings (too many to mention here, but too low back pressure is an easy one to adjust)

Cheers

Harry

 

[Pud]

"We're testing the tool on a brand new german machine"

Forgot to mention: It doesn't matter how old the machine is - if set up incorrectly, you will never get a job off it!!

H

 

[patprimmer]

Like Pud said and also it does not matter where the machine was made so long as the parameters are set correctly.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sizzer33]

Hi Pud, PatPrimmer,

Hold time study is exactly what you mentioned Patprimmer. We done that. Parts weight looks fine and stable.

3% means we measure the weight of 5 shots and the part weight differs by only 3%. 50% and 90% means we fill the part at short shot 50%, and then 90% to see whether the material are flowing evenly to all cavities. This is where we fail.

There's good cushion at around 3.5mm. All 4 parts are identical to each other.

We already tried to play with the gate size, our mould supplier increased the size of the gate for the 2 slower part. NO changes to the part.

One thing i need to ask, does surface finish on a tool affect the flow?.. Currently the surface finish is said to be SPI A2, but we really didn't check this.

Cheers.

 

[Pud]

Surface finish should not affect flow - the material flow front should not "slide" against the cavity walls - if it does you can get get a poor finish (sharkskin effect):

http://tinyurl.com/6p7exw

Quote: "All 4 parts are identical to each other"

If the parts are ok what seems to be the problem?


Cheers

Harry

 

[patprimmer]

If you are saying that you repeatedly get 2 parts full, 1 part at90% full and 1 part at 50% full, but the entire shot repeats with 3% each time, then you are not holding a cushion, even if the machine indicates you are.

Your problem is not surface finish, it will be gate size or runner size and length or venting or temperatures or shot size or injection speed or restrictive flow path.

Increase shot and use a bigger machine if necessary. Put tape on the parting line and pack until it flashes the two cavities that completely fill. Give it maximum pressure and as much speed as it will take without burns etc. drop the speed and pressure when at about 90% of required shot size, but leave enough speed and pressure to flash it slightly.

What material

What barrel temperatures.

What mould temperature.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bvanhiel]

I don't think that's what he's saying, Pat.

I believe he's saying that when they try to short shot at 50% and at 90% that the parts vary greatly cavity to cavity. When they run full shots everything is within 3% on part weight.

I'm not a processing guy, but as a part designer I don't care what the 50% shot looks like. If the parts look good when they're fully packed out your done.

-b

 

[Pud]

Quote bvanheil:

"If the parts look good when they're fully packed out your done."

As a processor too - quite! (Our customers don't care either!)


Harry

 

[patprimmer]

If that is what the OP means, we all really just wasted a lot of time on this. I guess I thought of that as one meaning and dismissed it as being to silly to contemplate.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[degete]

you're sure about the gate diameter of all 4 cavities?
Did you measure with a cilindrical gauge pin the gate diameters?
Please do this first

 

[mjh368]

noyone asked what the runner design looked like?

geometrically balanced?

 

[dwightdixon]

We do not know how many cavities he has and whether the tool is geometrically balanced. I would presume this knowledgeable molder would have made sure of this by this time. You might consider using 'melt flipper" technology to rheologically balance the tool. Many people say it works but, I think there is a licensing fee. Small electrical heaters or reducing the cooling in the runners feeding the slow cavities might be another consideration. If the customer thinks the parts are good and you do not add to the cycle time to fill the slow cavities, then the whole thing may be a moot issue.

 

[sizzer33]

Hi everyone,

Thanks for the good advise.

Some of our process guys would like to try out "melt flipper" way. I think mainly problem now should be in runner and controlling the flow by putting some turbulence on the faster flowing cavities.. We have check the gating size for all cavities and it doens't vary too much.

bvanhiel: What you stated is correct. Thats exactly the problem we're facing.. Unfortunately, cavity balancing is part of our customers and our factory approval process.

 

[dgowans]

So what you're saying is that short shots appear quite unbalanced but fully packed parts weigh within 3% of each other and are otherwise acceptable?

I'd take another look at gate sizes and venting to verify that there is consistency among all cavities rather than explore technology that might require a licensing fee. I understand the desire for balanced flow, but to go through this exercise to satisfy the requirement that short shots from each cavity be identical seems ridiculous.

 

[Pud]

Agree wholeheartedly with dgowans.

Just get a round needle diamond file - give it a twiddle in the "short" gates and retry. Keep twiddling until you get what you need. If your criteria is exact filling then gate sizes are immaterial?

Or just try some packing tape on the tool faces adjacent to the short cavities to check venting...

Harry

 

[patprimmer]

Like Pud said.

You do not need a toolmaker, you do not need to remove the mould from the machine. Just lightly file the gate on the shortest shot, take another shot, file a bit more etc etc until it is right. It really is that simple and you don't need any fancy technology and you don't need to measure anything but shot weight.

If you have not already tried the tape on the parting line trick, I give up.

Once again, It is not rocket science, it is real basic stuff.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mjh368]

yes the gate file thing will work at some point(we are talking about a 40% imbalance, which we have yet to hear what this equals in grams??)

well this all does seem moot if the final product is good balance can have a large influence on quality down the road.

If everything wears the same which cavs flash first?
Where will you see the short shot first??
When you replace tooling do you want to specify different gate sizes?


tolerances, volume,cycle and internal shop skill all factor in here.

I would rather pay up front and figure it out (runner design, flipper, etc) then just bore out the gate to who knows where and deal with the same problem down the road.

 

[Pud]

Quote:

"If everything wears the same which cavs flash first?
Where will you see the short shot first??
When you replace tooling do you want to specify different gate sizes?"

In 25 years at the game I have never seen a tool wear at the split line. Seen plenty gates, ejector pins, guide pillars etc worn though. Also seen tools damaged at split line due to incorrect moulding too.

"Replacement tooling" is getting rarer and rarer as product lifecycles reduce. When we have tools made I expect them to become obsolete rather than worn out.

And I have also been balancing gates for the same time (toolmakers take too long!) and and yet to have a problem.

Just my experience - you are welcome to do what you want with it - but the more you mess about for some intangible, unquantifiable benefit, your competition is up and running making money while you are not.

My last post in this thread - horses and water come to mind...

Cheers

Harry