Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


SUV plastic under body tray

SUV plastic under body tray

SUV plastic under body tray

What material would you suggest for a SUV plastic under body tray (aero-cover under the engine).
It has to pass tough off road durability testing.

RE: SUV plastic under body tray

How about Noryl GTX from GE Polymers.  This has been used for exterior body panels such as fenders and door outer panels that must withstand impacts.  You can learn more at:


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: SUV plastic under body tray

What about good old Polypropylene or epdm modified PP, as used in bumper covers


RE: SUV plastic under body tray


Modern bumper covers are not very damage tolerant.  It seems that the design criteria are minimum cost (obvious) and thermal expansion just enough to tolerate paint baking.  Maybe that is pessimistic - what do you think?

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: SUV plastic under body tray

Does the panel have to resist deformation to prevent damage to components or is it supported and just prevents sticks from entering the mechanicals etc?

RE: SUV plastic under body tray

Strength will depend very much on design, especially in the area of attachment points.

One dissadvantage of PP in bumpers is that they need to be painted off line (as the PP will not withstand the temperatures in a normal paint oven), or at least had to when I was involved about 10 years ago, but this I expect would not need painting.

Another dissadvantage of any plastic, but especially PP is it's high co-efficent of expansion, especially when compared to steel. This means that bumpers must be designed to hide the extra expansion vs the steel body. As I guess this underbody cover will only go from chassis rail to chassis rail, and is mostly out of sight, I expect that this would not matter in this case.

I am presumeing that this is aero, not protective, as I would probably advise steel for a skid plate.

If it is a skid plate, and was to be plastic, I wouldn't consider anything other than super tough or glass filled nylon. Nylon would also be my second choice to PP for an aero only component, that is if the environment was to severe for PP.

If you carefully choose the right grade of PP/EPDM for your trials, you should be able to pick one with similar shrinkage to nylon 6, so the mould could work for either material, giveing the luxury of alternate materials to test in the field.

Obviously, this is all pretty well off the cuff nonsence without a lot more detail about the design, function and environment of use.

I would prefer to refer to Noryl GTX as PPO/PA alloy as this is suposed to be a technical forum, not a marketing tool.

As I expect that ultimate dimensional stability, and paintability are not an issue here, I see no advantage in PPO/PA alloy over PA.


RE: SUV plastic under body tray

Good comments from pat.  Just for clarity, I have no link with GE Plastics.  I mentioned them specifically because I don't know of anyone else that makes PPO/PA alloys.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: SUV plastic under body tray

I think GE have a patent on the combination, as they still seem to be the only people offering it.


I looked up your profile, and realised you were not a plastics salesman, but a quite well qualified materials engineer, with a focus more towards metals.

I also noticed that you are a very active participant, who seems to give more than he takes by the stats I saw, so my appologies if a slight was percieved.

I do have a bit of a comercial thing against GE, as over here, they take a very gung ho approach and polute any presentations at technical seminars with very heavy commercial bias, so any implied slight was at them, not you.

My appologies for any offence implied.

My last involvement in an auto bumper application was to try to get a boxed in nylon beam filled with PU foam specified as bumper combined structure and cover. Its main features were weight reduction, reduced assembly processes and online paintability, but it worked out to expensive compared to a pp cover over a spot welded steel reinforcing beam


RE: SUV plastic under body tray


I know GE has patents on PPE (in fact, PPO is a trademark for their version of PPE) and PPE alloys.

Thanks for the "well qualified" compliment.  I am qualified to design/engineer with polymers, but I do have a focus more towards metals.  One "complaint" I have regarding Eng-Tips is that the polymer side has fewer "good" questions and fewer experts.

No slight was perceived.  Similar comments have been made regarding GE Plastics by many people.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: SUV plastic under body tray

We are using PP (3mm) right now and it is getting the sh1t beaten out of it.
There is an all aero-cover version and a skid plate version which still recieves a short aero-cover.

Both versions have to pass some serious off road durability testing, that results in some hard contact, although at present I'm not sure if the initial failure is the fasteners.

I have spoke with a GE rep who suggested most of the materials listed in the comments above. The problem being the delta cost per pound compared to the PP we are using now.

There does not seem to be an in between choice, from 45 cents a pound we are going to $1.60 > $2 a pound.

All the bumper covers (fascias) we produce are in TPO.

Thanks for your thoughts.

RE: SUV plastic under body tray

Try talking to DuPont. At least here, their people are a bit more objective

Nylon would certainly be my second choice, filled or unfilled, depending on the mode of failure.

If you already have a mould, try some extrusion grade 6.6 in the existing mould. With a really hot mould, it might shrink almost as much as PP co-polymer.

The extrusion grade will give increased toughness without looseing shrinkage, so the parts just might still fit.

What is the mode of failure of the existing PP parts

e-mail photos of new and failed parts or original drawings of parts to patprimmer@swiftdsl.com.au if you can. I will then have a much better idea of your problem.

Are the parts tearing off from the bolt holes, wearing out, or cracking and breaking.


RE: SUV plastic under body tray

The failure mode is the million dollar question. We are only getting photos of parts after they have been ripped off the vehicle. If may be that the fasteners are failing. But we have seen hard contact before over a course that represents pot holes (when we get hard contact the part is acting like a skid plate and at that point plastic will lose). The parts we see are ripped, scuffed and generally mangled. The vehicle durability folks are being tasked with providing us with the failure mode this week. So I should have a better idea in the next few days. Luckily we are only using a prototype tool at present so we have some time to cut the production tool to the correct shrink.

RE: SUV plastic under body tray

The prototype tool is music to my ears.

Almost without exception, I was called in to fix problems, after all else hadfailed, and the mould was made and shrinkage thereby locked in.

I will wait until I see damaged parts, prefferably at various stages of damage.


RE: SUV plastic under body tray

It has not been mentioned but this would NOT be a good application for polycarbonate. It would be too chemical sensative. Espeacily if there was any stress caused by instalation. A part like this will see exposure to oil and many other fluids in service. Brake cleaner would be one of the worst for PC. This may not show up in your durability testing if nothing leaks and you have clean mechanics.

RE: SUV plastic under body tray

How about a filled PP, such as talc, glass or long fiber? should give you the best of both worlds. Say between 15 to 30%

assuming you dont need to paint it due to location.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close