NBR air trap problem

[JULIAN NGUYEN]

I am trying to produce NBR (medium ACN)anti-fatigue mats which hardness is about 50 shore A. However there are many scars on its surface. Would anyone here help me to get it resolved ? Thank you very much
My formulation
Nitrile 100
Caco3 40
Carbon black N330 10
DOP oil 10
6PPD 1.5
Paraffin wax 2
Zinc Oxide 5
Acid Stearic 1
Sulfur 1.5
MBTS 0.8
Btw, When i used NR, this issue doesn’t appear

 

[GrahamBennett]

Julian,

First question is, which way up in the mould is the pattern? Are the raised areas in the top half of the mould or the bottom?

forumtowers.com

 

[JULIAN NGUYEN]

It has pattern on both sides. The other side’s pattern is smaller and rarely the scars appears.
The pattern on my first post is on the top half
Please find the attached picture for its both side patterns

 

[GrahamBennett]

Thanks for your reply.

Do you apply "bumps" to the press as it closes?
By bumps I mean rapidly opening and closing the platens a number of times to allow any trapped air to escape before full pressure is applied to the mould.

forumtowers.com

 

[Compositepro]

You could try adding some talc or similar powder to your mold release. The surface texture will create a path for trapped air to escape, or at least spread out over the mold surface. More pressure will also help.

The air is trapped due to the tack of the rubber as it spreads over the mold. Degree of mixing and type of rubber will affect that.

 

[JULIAN NGUYEN]

Thank you for your support Graham
I apply bumps several times when The mold is closing but there is still the same issue. It has never happened when i use NR compound

 

[JULIAN NGUYEN]

Thank for a good advice Compositepro
Can you please explain more in detail what is a good mixing method for NBR. I am using NBR 3345

 

[tom1953]

Your viscosity at molding temp may be less than optimum for good molding. If you have a higher viscosity grade of NBR (e.g., 3360), try that.

 

[Ahmad Uzair]

How was the blank material being put inside the mould? Maybe if you can make the blank thick could help this issue. Bumping 5 time, Gap 1.5.

“I have no special talents. I am only passionately curious.”-Albert Einstein

 

[Luis.Tormento]

scarring can be caused by air retention or low NBR compound or high viscosith raw material.
If air is trapped, increase the viscosity of the NBR (47-52 Mooney 100C recommended), if there is a lack of pepetization on high viscosity rubber , add 4 phr of zinc stearate when processing the rubber.

 

[Oksitocin]

We have had similar problem with the NBR - we solved it with changing the base polymer - instead of PERBUNAN 3345 we used KRYNAC 2850 and the results were surprisingly good.

 

[GrahamBennett]

Changing from NBR3345 to NBR2850 reduces the oil resistance so BE CAREFUL! I'm inclined to agree with Tom1953 - use a higher viscosity base polymer such as NBR3560 or even NBR3570.

http://www.over50sforum.com

 

[Oksitocin]

Yes we know that the oil resistance is reduced - but this was not very important for the final product in our case, tests have been made and the results were ok. Thanks for the tip.