EVA injection molding 1

[Guanoolic]

I'm a fresh Materials Engineer and I just started to work in a shoe soles factory that works with compression molding and injection molding. I'm in the injection area and last week my boss told me that he ordered a new machine for injecting EVA, it's supposed to arrive in January.
I started searching information about the material and it's process, but I just found relevant information about EVA foam sheets.
I was wondering if anyone could help me find some information on this.

Thanks for your time on this

 

[NunoH]

Hi Guano.

Did you use to find info on Material websites, while doing your degree?
There are a few that are useful.
For general info on EVA check:

http://www.azom.com/search.asp?q=ev...REPORTS|AZOM-JOBS|AZOM-EXPERTS|AZOM-EQUIPMENT

Look for material suppliers, as their info at times is quite useful.

i assume that if you boss ordered an injection machine for EVA he must know about it. so perhaps yuo should ask him for info.

according to what i've just read EVA used in soles is crosslinked. I dont know how do they crosslink it. if chemically or radiation.
if other people dont reply to your post, try the Plastics Engineering forum, as well.

Good luck!



"Ask DNA"

 

[patprimmer]

You cold try contacting DuPont. They were only second on my google search. I stopped readingthere as I suspectyou really could do it yourself. I used Ethylene Vinyl Acetae instead of EVA for the search.

Also try Evatane. That gives Arkema

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Guanoolic]

NunoH,

I forgot to mention that I had some problems finding EVA with low density (arround 0.2/0.3 g/cm3) but I found some suppliers here in Europe, and probably going to work with them.

My main problem is to find information about the processing of injection molding of EVA foams, since the foam effect should give us some problems on dimensional control and molds design.

My boss ordered the machine without knowing the process, just because he thinks it's a good option for the company's markets in the future.

About the crosslinking, as far as I could understand the crosslinking takes place with the injection temperature, so it would be a radiation/thermal conductivity effect.


New Postpatprimmer,

As expected through my search I found DuPont products (all EVA resins have density +0.9 g/cm3) and the same problem with Arkema .

And of course I used Ethylene Vinyl Acetae instead of EVA for the search too

I found this forum after a couple of days of searching and I registered and just posted because I couldn't find information, I just thought I could find here someone that could help me with their personal knowledge or their way of searching being different from mine, but one thing for sure I wasn't expecting, a fast answer for someone..


I just need to thank both of you for your promptness on this topic, and apologise for the lack of information on my first post.

 

[patprimmer]

EVA foam is simply EVA with an SG in the 0.9s and a blowing agent.

If producing sheet, the blowing can be done by direct inclusion of gas, but in injection moulding it is normally by using a chemical that is just stable at melt temperatures to not start producing gas until it is in the mould.

I think for EVA, a carbonate and citric acid might work.

For a real rough job I have even seen a guy deliberately add water to generate steam in the barrel.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[NunoH]

Your company must be doing well or your boss is south european, as he ordered a machine already.

I thought you didn't want foam products.
I don't really know much about injection of foams. Just that, as Pat mentioned that they generally take a blowing agent.
It's funny the water story.

Hmmm, Pat is your suggestion something similar as flour with vinegar in a 1.5l plastic bottle? :]

I would suggest for you to get info on EVA and, separately info on injection moulding for foam products.
you can find some stuff in books.google.com . I would also suggest your local Uni library.




"Ask DNA"

 

[Guanoolic]

About the foaming agents, I also read about it and the story of the water. There is the option of buying EVA with the foaming agent in it, one less thing for me to worry about.

The info I really have trouble finding is about processing of EVA foams by injetion molding.

I know that to make low density products I'll have to have a smaller mold so when the part is ejected the foaming effect takes place and the part gets the wanted dimensions.

But how smaller has the mold to be? Does it have any particular design, different from other injection molds? How can I assure that the expansion while foaming is isotropic? Are the processing parameters similar to other injections?
There are a bunch of questions like thise that I could answer if I had some information, but I don't :S

For example I uploaded a image I found from a mold use to produce a tire in EVA, in my opinion footwear or shoes soles must be more complex to work with and my lack of information is scaring me. :S

Image from the mold:

http://files.engineering.com/getfil...961-4c6f-9faf-053014931aa6&file=Tire_mold.JPG

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"

 

[patprimmer]

To mould foamed EVA, you use the correct sized mould and under fill it the amount you want the material to expand to.

If you want to halve the SG of the EVA you only half fill the mould then let the blowing agent expand to fill the mould. if you want to drop the SG to 25% of the original SG you only 1/4 fill the mould.

You leave the part in the mould until the plastic freezes then eject it. If you are aiming for low density it will still expand more after ejection, but how much is at best a guess aided by experience.

You mould it like a low temperature PE. The temp depends on the VA content. Environmental stress crack resistant PE often contains a few % of VA but they are still called PE not EVA. I think from old memories they really only start referring to it as EVA if it is over about 10% VA when it gets to be more VA than PE, you can actually mould it by rubbing it between your hands until it melts. These high VA materials are only used as an additive as a material with a melting point at 50 deg C or less is pretty seless in it's own right.

The higher the VA the softer the grade, and the lower the melting point. The soft grades cycle VERY slow an need real cold moulds.

The softer the grade, the more it sticks to the mould and it also is more difficult to eject without punching the ejector mechanism through the part. A vapour honed surface releases easier than a smooth finish when moulding rubber type materials.

If the part keep swelling from the thick sections after ejection, cycle times might be reduced by dropping the parts into freezing cold water.

The final size and apparent density will depend on a lot of factors. Far to many to cover here. The raw material suppliers will be able to provide comprehensive user guides.

It may be wise to make a prototype mould to experiment with final size vs mould size and moulding conditions.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Pud]

Guanoolic,

The foaming process takes place during the melt stage and if you are lucky, during the injection phase due to material shear heating through the feed point (gate).

Materials usually exhibit shrinkage when moulded and tools are made slightly bigger to compensate. With elastomers (especially blown ones), size can be altered to some extent by holding pressures as the material is compressible in the mould (i.e. the part can be bigger than the tool).

Great if you can buy stuff with blowing agents already in, takes out one variability at least. This is likely not to sodium bicarbonate but some other stuff which will have a narrower operating temp range and be more consistent. Nitrosoamine class agents are often used in synthetic rubbers.

The manufacturers of the material will have all the info you need regarding shrinkage, processing data etc, etc. Usually they have a "man in white coat" who will come and assist you when you first use the material. If you can find two sources, even better as they will both be keen for your business!

Cheers

Harry

 

[Guanoolic]

patprimmer,

I guess I'll really have to wait for the machine and the molds to get to know the foaming effect better.

Just got a data sheet from a supplier and I'm waiting for more information other supplier, one of the points missing in the data sheet is the VA content, so I asked for mor information.

According to the data sheet, injection should be arround 100ºC and the mold should be arround 180ºC and after 5/7min in the mould the cross-linking takes place.
When the mold is opened the expansion is up to 5 times the starting mold volume.. I guess the option you told wont work, because if I inject this material with f.e. half the mould, after 5/7min I don't know if the cross-linking that is taking place will give me a homogeneous material.. maybe I really need to have small molds and after injecting open it and wait for it to foam.

Pud,

As far as I could understand from the data sheet the idea is to avoid the foaming before the material get to the mold, so I would say that low temperatures and low shear should be applied.

For sure that the supplier's technician will visit us to help and probably the one from the machine too, but you are right I the more information sources I get the better I'll work, and I need to show the boss good work

G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"