TPE surface tension water drainage issue

[PATERLEY]

Hello All,
I designed a TPE sink strainer. The original design and prototypes were urethane casted with a "looks like feels like" resin, but was not TPE. The initial prototype did not have draft, but had the same number of drainage holes, air vents, etc. For injection molding, 3 degrees of draft was added to the bottom holes, about 50 of them, but the design otherwise is the same. The draft is from the top side down, so the holes are larger toward the basket of the strainer.

What we are encountering now is some increased surface tension with water drainage. It will drain when water is running, however there will be about 1mm of water left in the bottom, and it wont completely drain due to the surface tension. I am not sure whether it is due to the draft, or the material, or both.

The initial design without the draft did not have this problem.

At this point, we can't really change the tool much without significant cost. I am sure larger holes, might solve the problem, but it's not really an option right now.

My question is, all things being equal, would adding an A-1 finish theoretically reduce the surface tension and aid in drainage? Or do you think it would be negligible. We are currently at a B-1.

As it stands, the difference is so subtle that softened water will drain completely, but hard water has trouble with that last mm. Probably not a big deal in the long run, but we have come this far, and want the strainer to work perfectly.

I appreciate your input. Happy to add any details needed.

Cheers,
Pat

 

[btrueblood]

Worth a try, surface texture can/does impact surface tension. You might also try spraying them with a hydrophobic like silicone oil, just to see what happens. It kinda sounds like you need bigger holes, though.

 

[PATERLEY]

Thanks for your reply! We were trying to maximize drainage, while not making the holes too large so scraps can pass through. The spray is an interesting idea, we will see if that works. We will probably try the A-1 polish. At this point, to enlarge the holes would involve retooling 1/2 of the mold. It is just interesting that 3 degrees of draft over .08 of an inch could change the dynamics enough to see a difference in drainage. I am sure the TPE also makes a difference, though.

Thanks again.

 

[Compositepro]

Surface tension and surface energy refer to basically the same thing except that the term surface energy is used for solids and surface tension is used for liquids. If water is being held in your strainer above the holes it would be due to too low of a surface energy of the plastic, causing water to bead-up into beads too large to pass your holes. This could be due to mold release (internal or external). Silicone oil would make the problem worse. You can probably find additives for your plastic to fix your problem.

 

[PATERLEY]

Thanks Compositepro,
If I am understanding you correctly, the material is too *hydrophobic* and less wettable, so what I want is more wettability, so there is better water distribution and thus drainage. I know all of these ideas are interconnected but not necessarily interchangeable. I am just trying to decide whether a polish would make the problem better, or worse.

Thanks,
Pat

 

[Compositepro]

Super-hydrophobic materials have a rough texture so that there is less contact area with any liquid. You want the opposite, but surface texture is much less important than surface energy.

 

[Pud]

Try moulding some in a TPU. If the prototypes worked then it's the material rather than design.


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Compositepro]

Plasma or flame treatment can also be used to increase surface energy. You can use a hand held propane torch to do an experiment to show the effect of increasing the surface energy. The technique is demonstrated in this link:

https://www.youtube.com/watch?v=UxZl3FqFFdY