drop test impact 1

[dclapp]

I am thinking of making a molded plastic part out of polyurethane and need to make sure the enclosure I am building will substain a drop on the ground from 6 feet in the air. The area of concern for the plastic I am rating (a thermoplastic) is the IZOD Impact rating of 0.51 ft*lb/in for Notched impact test. I have the other mechanical properties of the material if necessary. Box characteristics: 6 x 6 inches, 5 lb with 1/4 inch walls.

Any suggestions would be useful!

Thanks

DC

 

[Haf]

dclapp,

After reading your post, I'm not sure what your question is. Are you trying to calculate the force of impact and the associated stresses? If so, see "reaction force of a dropped object," thread404-36240.

Haf

 

[Biggadike]

Some general comments:

1/ Polyurethane is very tough - is there a danger that your plastic will survive but the contents will be broken?

2/ I had some success with designing a Zytel part to resist impact recently. The trick was not to put too many features into the section which could act as stress raisers. If you see an area (say, a corner) which you think will be weak, don't jusr stick a rib across it. All that will happen is a failure next to a rib. Stick to smooth flowing shapes with constant section. Very slight curves are better than straight lines as they bow along their length disipating energy.

3/ Is your criteria realistic. 6 foot drops are rare?

4/ This will be almost impossible to test until the real part is made so overdesign is the key. I know some FEA bod will always try and convince you they have all the answers. Use FEA to find obvious problems but until FEA can model and predict residual stress due to mould flow and weld line weaknesses then it will tend to be over-optimistic.
Use Mouldflow to check for welds at point needing to be strong.

 

[dclapp]

Thanks for your inputs.

The issue I have is I do not have a good reference point for comparing differant types of polyurethane. The specs for the "machine cast" thermoplastic for instance is similar to ABS plastic except for the IZOD or Impact rating of 0.51 ft*lb/in (for ABS, lexan etc it is 10 or so). You are right that most polyurethanes are tough, but my question is whether or not this type is tought enough to suvive for my drop test- before I pay for making the molds!

Thanks for your suggestion of rounded edges...I think that is a good idea...

Can you provide more information on MouldFlow? Can I assume it is a FEA modeling tool

 

[dclapp]

One more comment!!...

I made an attempt (I am not mechanical engineer!) to calculate the forces involves as follows:

KE = mgh = 5 lb * 6 = 30 ft*lb

if I assume that the we have a concrete floor and all the force is put back into the box over some range of compression, say 1/8th of an inch, then the force involved on the impact point is
F = 30 ft*lb/ (.01 ft) = ~ 3000 lb;

Now how do I go about applying this to my box so that I can calculate an impact stress? What really is an inpact point and how does that relate to the Impact rating in units of ft*lb/in?

thanks,

DC

 

[tomwalz]

If you try really hard you may find a supplier who knows what they are doing and can help with this. Phone calls seem to work best as almost no one seems to answers emails asking for technical help.

Tom