Adhesion of Self Skinning PU RIM to thermoplastics 1

[ornerynorsk]

Hello Everyone,

We've been using RIM molded (low pressure) self-skinned parts of polyurethane for a number of years, and have had success in overmolding and encapsulating metal armatures in many of our products. Now we are working on a project which will use an injection molded backer plate in which we cannot rely on any mechanical feature to adhere the PU to the plastic plate. Adhesion must take place in-mold, either through natural chemical bonding or through the assistance of a surface treatment or coating to the substrate. The geometry is very flat and shallow, not enough room for undercuts or other features to aid in mechanically gripping the PU overmold.

Our molder (US based) is very capable and willing to try new processes, but this is the first time they have encountered such a project, as well. We've found a few resopurces online and from the material vendors, but nothing of enough substance to point us in a definitive approach to this.

Our preferred material choice for the backer plate will be unfilled nylon or ABS, but this is certainly not set in stone.

Just wondering if any forum members have had successful experience in this area?

Thanks in advance.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[KENAT]

We have a vacuum molded 'enclosure' that we bond a PU RIM foam liner into as a separate operation.

This work is completely done as the vendor though so I don't have the adhesive details.

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[ornerynorsk]

Thanks for that, Kenat. We've been investigating that route, as well, but we are trying to avoid the 2nd op of joining 2 pre-molded components. We've got our molder running several "educated" off-hand tests. I'll post results as we get them.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

ornery

My thoughts are that it will be relatively easy to do tests.

I know non foamed injection moulded PU sticks to nylon when making PU tyred nylon castor wheels, but the PU is moulded with the first shot and the nylon with the second so the nylon having a higher melting temp melts the PU enough to get a weld at the interface. I don't know if this helps you though.

I think you will need to test it at a very basic level. If that shows promise, retest at a level closer to your real product situation.

My first rough test would be along the lines of, get a plaque of nylon or ABS or whatever.

Use an existing mould for which the plaque could fit in the cavity with plenty of clearance inside the closed mould.

Use BluTac or double sides tape to hold the plaque in place while the mould closes and a shot of the PU is taken. Remove from mould and see if it stuck.

You might want to heat the plaque in an oven first to about what you would expect the first stage of the moulding to be when the second shot is taken.

Regards
Pat
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[ornerynorsk]

Thanks for that, Pat. I learned something new! I never realized that casters were done with the PU being the first shot.

The rough testing was along the lines of our thinking as well. Yesterday we were able to try some things, and I have listed the basic results below:

Plain, untreated ABS - Peels easily, nominal adhesion but not enough for practical purposes.
Plain, untreated unfilled toughened Type 6 nylon - same as untreated ABS.

Both types of thermoplastics, surfaces thoroughly wiped with laquer thinner. No discernible difference between this and the untreated.

Grit blasted, unfilled toughened Type 6 nylon - much better adhesion but will still peel. ABS got overlooked on this round of testing.

Both types of thermoplastics, flame treated - (neutral flame passed over surface quickly, not enough heat to cause any localized melting, similar to "de-haring" fine flash) We experienced a very good level of adhesion, difficult to peel. Will not peel as an entire piece but rather tears off in small chunks.

To date we have not tried any "primers" or coatings. We are going to test further with the flame treating.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[ornerynorsk]

Strike that, didn't mean "de-haring", but rather "de-hairing". We've had no problems with large rabbits :>)

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

I think Sikkens makes some good primers for nylon and two pack PU paints. That might be a good starting point for primers.

Regards
Pat
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