Compression moulding NBR - Quality issue

[Elastomatt]

I am currently having a quality problem with an NBR product in a compression tool.
The product is a largish flat faced round seal, however there are air traps/flowmarks forming on and just off the flat sealing face on the internal face of the product only. The customer has a spec for marks on the critical & non-critical areas and these marks are cause for rejects.

My question is what can be done to remove or reduce these marks. The tool has been vented (to air) but the defects location are inconsistent and appear to get trapped before they can reach a vent.

The extrusion blank has a smooth outer surface and there isn't any porosity in the end product (although there is in the blank). Blank weight seems to be enough as there is a reasonable amount of flash in the dump groove when demoulding. Blank is also shaped to the profile.

There are some short bumps at the start of the moulding process (after 13secs for 2secs).

Any advice or resource to check out would be appreciated!!

Thanks in advance, Elastomatt

 

[Compositepro]

It sounds like your extruded blank is not an appropriate shape for your part. During the compression molding process the blank should flow in a way that air is pushed out to the vents and part lines. If the blank has a cup shaped surface that is pressed against a flat mold surface, air will be trapped and compressed, causing a pit or burn mark.

 

[GrumpyPlasticBastard]

Cut up the blank, several small pieces. First verify there are no voids or gas pockets in the blank.

Now put the cut up blank into the mold and run a cycle.

Let me know how it looks.

Sometimes... pre-formed blanks have gas in them that can't escape during molding. Other times a simple change in geometry (cut up) can assist in venting.



----
BFL Dreamworks
"If we don't have an answer we'll get one"

 

[Elastomatt]

Thanks guys, I will get the blank shape reviewed and trial the cutting and moulding.
I will report back after the next trial run.

Elastomatt

 

[tom1953]

Any chance it's a mold release issue?

 

[GrumpyPlasticBastard]

After consulting with a compression molding expert... we think the porosity is your issue. Little tiny voids full of gas.



----
BFL Dreamworks
"If we don't have an answer we'll get one"

 

[Elastomatt]

Thanks GPB,

I have had a look at the blank forming line and the extruser doesn't have a vacuum. Would this help remove the porosity in the blan?

 

[GrahamBennett]

Seems to me most avenues have been addressed already but no one has mentioned using a compression press fitted with a vacuum hood. Also, have you made sure the mould split lines are as close to the mould top plate as possible? you'ld be surprised at the number of times I've not seen this during the preparation of the mould design drwgs.

If you are not seeing porosity within the moulded part then porosity in the blanks is not a problem.

 

[GrumpyPlasticBastard]

Elastomatt:

It might help.

----
BFL Dreamworks
"If we don't have an answer we'll get one"

 

[Elastomatt]

Thanks Graham and GPB.

We have a trial scheduled for the end of next month, press doesn't have a vacuum box/hood option. Gasket is quite large as is the section.

I'm going to follow the production through from blank production and preparation through to moulding.

THe tool is a 2 plate mould with a split line low on the ID and just off the top on the OD. Position of split lines is dictated by the profile (according to the tooling guys).