How to avoid the CONTRACTION VOIDS??? 1

[pontvi]

Could anyone help me to improve my injection process?
We are talking about a thick part, made of nylon with glass fiber. Once injected, in the surface it seems to be well compacted but in the interior there are "bubbles".

At first sight I tought they were air traps, but reading injection manuals and courses, now, I think they are CONTRACTION VOIDS.

If anyone know how to improve this problem, it would be very good to me. It's a hot runner injection part. It's injected with 6 gates in the core (I mean the thickest part of the piece), but I'm not able to eliminate these voids.

Thanks in advance.

 

[Pud]

Shrinkage voids are designed in. Avoidance techniques are:

Slow injection may help.
Avoid gating directly into a big, open space. This causes jetting which also contributes to voids. Inject onto a corepin or component wall.
Long, high pressure hold times combined with high tool temp - may/may not work.

How thick is the section you are feeding on? six gates seem a bit excessive as a first impression - a single big gate is often preferable as it will stay "open" longer, enabling longer hold time to be effective before gate freeze-off.

Sometimes, GF nylon and voids are inseparable - it just happens (see first sentence).

Voids also reduce impact strength significantly due to the sharp edges produced (notches).

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[pontvi]

Thanks for your information.

Well, thickness of the part vary in height. I mean, top part thickness is 58 mm, in the middle is 30 mm and bottom is 20 mm.

Real problem is not the impact strenght. Real problem for me, is that this part is mechanized, and in this moment appears some bubbles/contraction voids.

Our thinking is that maybe varying the way of injection we can avoid this contraction voids or maybe move them to other part of the piece.

Right now, 6 injection points are in the thicker part of the piece, in the interior, in a 45 mm diameter. And it's in the same area where contraction voids appear.

Any other idea? Do you need more info?

Thanks in advance.

 

[patprimmer]

To eliminate voids in any hydroscopic material:-

Make sure the granules are dry.

Increase effective pack time and pressure. This is achieved by increased hold time and a larger gate. It is the minimum dimension of the gate that is critical as that is what controls freeze off time. There is no point to holding up once any point in the feed system is frozen off.

If you are using nylon 6.6, nylon 6 may provide a better moulding with regards to voids.

Harry is correct re aiming the stream from the gate into a wall so it does not jet or stream from the gate.

A very hot mould will produce sinks rather than voids if it is under packed as the surface stays soft longer, thus allowing it to suck in as a result of contraction.

We need details of runner, nozzle and gates and heat system to the hot runner to form an informed opinion.

Are there also voids in the runners.

45mm sounds unbelievably thick and would indicate you need a gate with a minimum of at least 15mm thickness on the smallest dimension.

Sometimes an extrusion grade base resin helps with voids as you can pack harder without flash.

Plenty of clamp pressure (at least 3 tonnes per square inch of projected area is required.

A good ring check valve that holds a cushion is required. When moulding GF nylons, these require regular replacement.

The barrel bore must also be in good condition in the ring check seal area for the ring check to seal.

If the hot runner extends to a heated gate, the heat must be sufficient to avoid freezing before the part is fully packed.

At 45mm section thickness, I would expect a hold time of several minutes is required.

You can check effective hold time by starting low, and progressively increasing time and weighing parts until it is seen that the part weight does not increase with increased hold time.

The hold pressure should be such that it is just short of flashing the mould or is just showing the slightest signs of flash.

Blowing agents can also remove contraction voids but replace them with thousands of small well dispersed bubbles. This foam will be stronger than contraction voids. The aesthetics of foam vs void is subjective.

45mm really is unbelievable. Can you post a dimensioned drawing or sketch with gate and runner and nozzle positions and sizes shown.

I hope it is not a hot tip gate system.






Regards
Pat
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[pontvi]

Dear both,

Here you have an example of piece. It's not the exact shape, but very similar.

More info about hot runner type, Ø of runners, and size of nozzle I will provide in two hours.

We have two possibilities: contraction voids or air traps... Not sure about one of them exactly...

Your response is highly appreciated.

Thanks in advance.

 

[patprimmer]

The gates are extremely undersized.

To tell the difference between air traps and voids.

Air traps will normally appear burnt as air trapped then heated by exposure to injection pressure will become hot enough to severely degrade the surrounding polymer.

If you heat the plastic adjacent to the bubble to a point where the plastic softens, if it is air it will blow out but if it is a void it will suck in.

Regards
Pat
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[pontvi]

Dear Pat,

More info about:

1. Hot Runner Nozzle Size: 3,2 mm.
2. Hot Runner From Nozzle to Split into 6 Core: 25 mm. with 6 degrees opening. I mean, an increasing cone.
3. Each of the 6 runners to gates have 5,2 mm. that goes in cone shape to 2 mm. gates.
4. Hot Runner Type:
5. Weight of the part: ± 300 gr.
6. Two cavities.
7. Gate System is submarine gates from runner.

Your response is highly appreciated.

We are working on increasing gates size.
Also, improve closing areas of the mould to enable us to inject faster with more presure. In this moment, closing areas are not optimized and the mould opens if we push hard and fast.

Thanks in advance.

Yours sincerely,

 

[patprimmer]

I forgot earlier, the mould needs to be well vented with vents to atmosphere.

The vents should have a short shallow land then be greatly increased in depth so that they do not easily block with mould deposits.

I would NEVER recommend submarine gates for this moulding.

Regards
Pat
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[Pud]

45mm really is unbelievable

Believe, Pat - have faith...

Imho, you have absolutely no chance of packing that part with a) those sub gates and b) those runner diameters. Both will freeze off before the staggeringly thick section can be filled.

I'm not sure what the problem is? If it is because they can be seen (through 20mm???) why not make the parts black/any opaque colour?

As Pat suggested, blowing agent may help but will give not very good surface finish.

Good Luck

Harry

http://www.tynevalleyplastics.co.uk

 

[pontvi]

Well, the real problem is that these parts goes mechanized after the injection process.

When you take a part recently injected you have no problems, but once they are mechanized bubbles/voids appear... That's the problem.

The part is black already.

We are looking to re-size the runners.
Hot Runner Nozzle Ø to 3,7 (max recommended by contructor)
Increase Ø Runners to 6,5
Increase Ø Gates to at least 3 mm.

Also, we will check vents (air exits), however I believe that problem is contraction voids, not air traps...

Also, we will check moulding closing to improve it, to reach more injection speed and more injection pressure avoiding flash/burr.

Any other idea???

Thanks in advance guys. you're really helping me.

 

[patprimmer]

I don't wish to apear rude about your english, but your use of the word mechanised seems wrong here and I don't know your meaning.

Do yo mean that the part is machined in a lathe, mill or drill and that cuts into voids.

Regards
Pat
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[Pud]

Pat,

I have assumed that there are post moulding machining ops which reveal these voids (e.g. Drilling/tapping/machining a face etc. A bit like metal casting (remember that? stuff like aluminium/cast iron!)
I do think your suggestion of having a very hot tool and trying to get the shrinkage on the exterior of the part may be the quickest/dirtiest solution.

The hot runner also appears to be a hot sprue bush, feeding onto a runner system, rather than a "proper" hot tip (I guess).

My personal opinion is that the op is on to a loser in this case due to part design rather than moulding conditions/tool design (although "possibly", the hot sprue bush fed directly onto the part might have been a better choice as sub gates for this section should not have been considered if voids are likely to a problem).

I have shown this thread to our sales guy who has spent the last 20 years selling nylons from EMS Chemie. He is of the same opinion - maybe 6-8mm section thickness max. It appears that voids are a common feature (note: not "fault") in GF nylons - he did suggest that a GF/Mineral filled grade of material might be better (60% total filler!)

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

I am still thinking 15mm gates, and the rest at a guess, 20 to 25mm runners and a tapered spru bush from 20 to say 25mm dia nand a nozzle of 15mm dia

If the spru is electrically heated or a so called straight it could be say 15mm and parallel.

As Harry says, it should be a direct spru into the thickest part.

It could also be a very thick diaphragm gate where the existing runners are. If that were a 15mm thick diaphragm it would need to be machined out, but as you machine the parts anyway, that might not be such an issue.

Moisture conditioning will also be a unique challenge at that thickness.

Is it type 6 or 6.6 or some other nylon. That will make a difference.

If it is type 6.6 does it need to be, and why.

Harry is correct. Mineral filler will help if you can accept the loss of other properties. A blend of mineral and glass might also work.

Regards
Pat
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[pontvi]

Good Morning both,

Well, sorry for my english. It's no t really good.
I mean that the part has a post-process after injection and the part is machined in a lathe.

And, I have been reviewing the part, and I'm so sorry about my mistake, max. thickness is 10 mm. and min. thickness is 5 mm. It's a circular part, with an inner Ø.

Please, advise about ideas.

Thanks in advance.

 

[patprimmer]

Well that does make an enormous difference, 45mm down to 10m

3mm gate should suffice.

You may get improvements up to a 6mm gate.

At 6mm the part is pretty much optimised. Bigger still is unlike to have any further benefit.

Regards
Pat
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[Pud]

Wot Pat sez.

http://www.tynevalleyplastics.co.uk

 

[pontvi]

Other question to both,

Any idea to improve Air Traps?
Taking into account that we cannot increase vents as we can get flash.

Thanks. Your help has been much appreciated.

I will inform about improvements once the mould is modified and trials done.

Yours sincrely,

 

[Pud]

pontvi: One step at a time... Do not rush these things or you will never know what actually cured it if the problem occurs again in the future!

H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Like Harry said.

After that, possibly a diaphragm gate as you do post machining anyway.

Gate position and design and position of vents is how you avoid gas traps. Slow injection speed can also help, but apart from venting, GF nylon should be injected as fast as possible. The change over point between speed control of the injection stroke and pressure control of the hold up or packing stage is critical.

Also when trying to avoid bubbles, you must be absolutely sure the material really is truly dry.

You must have a ring check valve that will hold a stable cushion during hold up time.

Air traps also normally involve a burn mark. Clear bubbles are contraction voids.

I will put it another way.

Diesel fuel is a hydrocarbon.

Nylon is mostly made up of hydrocarbon, well kind of.

diesel fuel self ignites when mixed with air at 18:1 compression ratio over atmospheric, ie 18 bar.

Injection pressures in the cavity at maximum pack are something like 1500 bar. The potential to burn in the presence of air is obviously extreme.

Regards
Pat
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[pontvi]

Good afternoon guys,

Could you get me more info about mineral fillers? Maybe recommend some specific one (brand, type).

Thanks in advance.