Deflection using strain gages?

[Colnago]

Hi Guys,

I'm wanting to measure deflection in an injection mould tool. I'm planning on using strain gages to measure the strain in the tool under injection pressure. Is there a way I can use these strain readings to calculate the deflection in the plates?

Thankyou for any help. :)

 

[Nihilist]

I can tell you it's possible but I'm unable to help you, sorry. Strain is directly related to deflection by Poisson's ratio.

As it happens I'm actually interested in any information about strain gauges. What types are available, where can they be purchased, expected cost etc. I work with welded structures up to 8m, with a tight budget.

 

[Studiot]

Have you considered a photelastic coating?

 

[Vadar2012]

I've done this before, but my memory is a little rusty. I think you just use the strain gauge formula to find the strain, which is the ratio of the original length to the displacement. It's a lot easier if you know the GF of the strain gauge though.

 

[Colnago]

The Gage factor is 2.14.

I understand that strain is just simply change in length over original length. However this only tells me how much the gage he's elongated and not the distance the gage has deflected downwards. I know there must be a way to do this, I've searched the internet trying to find a way but with no luck.

As for photoelastic coating I don't know a lot about it. Do you know of a good source so I can study it further?

Thanks for the replies.

 

[Studiot]

How about a sketch to explain further the directions of forces involved and deflections desired and also what is accessible and what is covered and therefore less accessible?

 

[Vadar2012]

Ok I'll give you an example for a cantilever. You can find the deflection at the position of the strain gauge using this formula:

epsilon = (3*deflection*d)/(2*L^2)

epsilon = Strain
d = beam thickness
L = Beam length

You find the strain from the GF equation, then plug it into the forumla above.

 

[Colnago]

Apologies for the amateurish drawing. Basically this is the corss section of the tool. A square mould cavity in the middle of the tool will deflect the cavity plate downwards. I am trying to calculte how much this deflection is.

Thanks Vader, that exactly the type of thing I'm looking for. Can you tell me where you found it or if you know of another one for my setup.

 

[Studiot]

Thank you, the diagram is fine.

You have a basically two dimensional stress situation to consider in your mould plate.
Since it is a plate there will be deflection in two directions and a single strain gauge will not suffice.

You should therefore bond a strain rosette to the under surface of the plate in a central position to obtain maximum strain and therefore sensitivity.

This will allow you to deduce the principal strain and stress directions and the curvature of the plate due to strain.

Knowledge of the curvature will in turn lead to the vertical deflection.

Why do you not simply use a dial gauge mounted internally in the empty mould chamber and load up the plate?
You can get digital gauges that will provide a remote readout since you can't view inside the chamber.

 

[Colnago]

Hi Studiot. The reason I can't use a dial clock is that the iejection plate moves up and down in that empty space. I have fitted some small buttons to protect the strain gauges but there would be no room for any bigger measuring device like a dial clock and still have ejection working correctly.

I'm planning on using two strain gauges both in quarter bridge at 90 degress from each other directly under the cavity.

Knowledge of the curvature will in turn lead to the vertical deflection.

This is the bit I don't understand, how can I relate the curvature to a deflection. The equipment I have will give me a reading in micro strain, I just don't know what to do with this in order to find the actual plate deflection. Sorry if I am being dumb here, but I really don't know a great deal about it.

 

[Studiot]

Why do you need the ejection plate fitted if you don't have anything to eject?

 

[Colnago]

Why do you need the ejection plate fitted if you don't have anything to eject?

I'm still going to be filling the cavity with plastic and making parts to create the same load conditions on the plate as in production. So I need the ejection to work to remove the plastic parts without having to strip the tool down and remove it that way as it'll take far too long.

 

[Studiot]

You would still have to calibrate your strain gauges.

What I suggest is to incrementally load the plate with a dial gauge etc instead of the workpiece.

This will give you a load/deflection graph for your plate.

You could do this with the strain gauges in place as to calibrate against known deflection, then change the dial gauge for the work piece and just measure the strain gauge output.

 

[Vadar2012]

Why can't you just model it as a simple fixed-fixed beam? Get the strain from the gauge reading. Convert this to a stress and hence a force (easy if you know all dimensions and material properties). From that you can just plug the force into the appropriate beam deflection equation. This is extremely simplified, but would be a decent approximation.

If this is something you want to try. I don't mind helping.

 

[Studiot]

Because it's not a beam it's a plate.

 

[Vadar2012]

Because it's not a beam it's a plate.

Yeah, but if the dimensions are sufficient it wouldn't be too bad for an approximation. Or just use the plate equations... But yeah, dial gauge would at least be accurate.