Choice of Plastic material and joining method

[DavidFinAZ]

I'm a ChemE who spent an entire career in semiconductors. Now, I'm trying to invent something but don't know much about plastics. My immediate concern is creating a water-tight seal between some plastic parts of a functional prototype.

I have two flat plastic rings of the same size (2"OD x 1.5"ID x 1/8"THK). I want to glue and/or weld a thin plastic film, 2-4 mils thick, between the two rings. The prototype rings will be manufactured via 3-D printing. In production, I expect that the rings will be injection molded.

If I use an adhesive to join the plastic parts, it will be either, Sikaflex 291 (polyurethane) or DowSil HM2500 (silicone hot melt). If I choose to weld, I don't know if the method will be ultrasonic, RF or laser welding.

QUESTIONS

1) If I choose to use either of the above adhesives, can someone recommend a preferred plastic material for the rings and thin film? (Note: There is no constraint on the choice of plastic for any of the parts EXCEPT that all materials MUST be NSF 61 compliant.)

2) Is there a preferred combination of plastic materials & plastic welding method (i.e., a method that will create the most consistent, reliable seal)?

3) There may be some pleating (i.e. accidental folding which would increase the film thickness in some areas) of the plastic film immediately prior to welding. Would all of the above welding methods (ultrasonic, RF or laser) be able to accommodate this variation in the film thickness without any problems?

 

[rickfischer51]

It would be good to know more about this. Its not possible to make an intelligent material recommendation with what you've told us. We need to know end use, loading, environment, temperature, etc to recommend a material. I find it odd that you have zeroed in on the adhesive but are completely open to the plastic. Having said that, a couple observations. First, olefins (polyethylene and polypropylene) are very difficult to adhesively bond because they are non-polar. Not impossible, but there are better choices. Second, RF welding only works with polymers with a dipole. PVC and urethanes are easily RF welded. PET nylons and I think ABS can be, but not as easily. Ultrasonic works on many thermoplastics, but I think high filler content can cause problems. Tried welding GF nylon once, didnt work well. Also, you want to prototype with the materials you will use in production, not whats available for 3D printing, because they may nodbond or weld the same.



Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[DavidFinAZ]

To rickfischer51,

Thanks for you comments.

What I have described is part of a test fixture to test the effect of water pressure on the seal between the glued/welded materials. Actually, I really only need to glue/weld the thin film to one of the rings. However, I am concerned that at some pressure level, the water will create a sheer stress on the glued/welded joint such that it will cause the thin film to lift off (i.e., "peel off") from the plastic ring. (NOTE: The thin film will not be subject to the high water pressure because there will be excess of the film such that the water pressure will be restrained by the walls of the test structure vessel.)

With the above in mind, I am evaluating two options:
1) Glue/weld the film to one plastic ring and mechanically reinforce the glued/welded joint with an o-ring, or
2) Glue/weld the plastic rings on both sides of the the plastic film to create a "pseudo" solid piece of plastic with the plastic film extending out of the glued/welded joint.

The joint will be exposed to up to 120 psi of water pressure. Water temperature = 32F - 60F.

I chose the adhesives first because there aren't many adhesives that are NSF 61 compliant. I'm somewhat confident that the DowSil will create a viable seal, but it is more difficult to work with because it is a hot melt silicone and I don't have automated equipment (at this time) to handle the different parts. The SikaFlex adhesive will allow me more time to move the parts into correct orientation after the adhesive is applied.

So that leaves me to seek help on the choice of materials as well as an alternative to gluing (i.e. welding).

FYI, it's hard to remember to include every aspect of the issue when you're posting on these boards, but I'm experienced to know about the differences between materials for prototypes and product parts. If there is suggested material that I cannot 3-D print, I realize that I can have it machined. Remember, my main goal is to obtain proof of concept for the seal design.

I appreciate any suggestions that you can give...

 

[hendersdc]

David,

Make sure your 3D printing process is suitable for your test. For example, FDM is porous and will leak. I would suggest machined parts for your test fixture to ensure any failures are actually seal failures. Plenty of rapid prototype shops out there that offer quick turn CNC parts that aren't much more expensive than 3D printing.