Plastic- Case Study 4 Actual Case Study End Flashcards
In cold rolling what does and does not affect the flow stress of the material?
Strain rate doesn’t affect the flow stress. But the material will work harden due to it being strained. Means the material exiting the rolls has a flow stress higher than the material entering the rolls
What is the mean flow stress of the material in cold rolling?
The average of the exit and entry flow stresses
What is rolling pressure and how is it distributed?
The rolling load divided by the area of contact. Roll pressure vs length of contact (L) graph starts at flow stress of material at entrance, almost straight diagonal up to neutral point (curved top) then back down to flow stress of material at exit (higher). Neutral point is a bit to the right of middle due to work hardening during rolling
What are back and front tensions?
Tension in the plane of the sheet applied to material as it either enters or exits the roll gap.
Back tension: produced by controlling the speed of the uncoiled relative to the roll speed.
Front tension: created by controlling he speed of the wind up reel.
They can both significantly reduce the rolling load (and pressure)
How does tension affect the roll pressure distribution?
Front tension: follows normal curve up from starting flow stress but peaks earlier so N shifted left, then comes back down to a lower value than normal exit flow stress.
Back tension: starts lower than normal starting flow stress and peaks later (to right) but still lower and joins normal curve to end flow stress.
Back tension about twice as effective as front tension in reducing rolling load.
Other effects of tension
Less wear on the rolls.
Improved flatness of the product across the width
What is mill spring and what is it determined by?
Where as the arterial is rolled the force exerted by the material causes the rolls to separate elastically, leading to an exit thickness greater than the original roll separation. This is mill spring.
Determined by the overall stiffness of the rolling mill
How do roll mill spring curves work?
Load vs plate thickness. See slide 30. Have one straight diagonal line up from x=roll gap for the elastic deformation of the rolling mill. Have a concave curve down for plastic deformation of plate and the rolling load as a function of final thickness hf (down to initial thickness h0). Plastic curve from F=χ.w.rt(R.(h0-hf)).σflow. Elastic line obtained from mill stiffness. Point of intersection of lines is final plate thickness.
Experimental steps to finding the mill stiffness
Take series of slabs of varying thickness.
Pass them through rolling mill with known roll gap.
Measure rolling load and final thickness of slabs.
Calculate mill spring for each slab (hf-roll gap).
Plot rolling load against mill spring.
Slope of line gives mill stiffness.
Emill=F/(hf-hmill) or F=Emill.(hf-hmill)
How to find the roll gap (hmill) for a particular hf
Equate both F=χ.w.rt(R.(h0-hf)).σflow
and F=Emill.(hf-hmill).
Rearrange to make hmill the subject
hmill=hf-χ.w.rt(R.(h0-hf)).σflow/Emill
Sub the numbers in from the case study
Mill stiffness seems to be 5
For 5mm thick plate need hmill at 4.72mm
Why do we get variation in the final thickness of the plate?
Looking at the rolling load equation, any variation in rolling load leads to a change in final thickness. If rolling load increases then mill spring will increase and so will final thickness. If friction increases increases then thickness will increases. Increased initial plate thickness or flow stress (both can vary according to remit by +/-0.5mm or +/-10MPa) increases final thickness.
Why will the level of friction vary?
In reality lubrication control is extremely difficult to maintain
Ideal situation for mill spring curve
Friction, flow stress and entrance thickness are all normal (140MPa and 160MPa and 6mm). Then the intercept of the two lines on the mill spring curve is at 5mm height and within acceptable region from 4.95 to 5.05mm. Elastic curve up from 4.72mm roll gap and plastic curve down to 6mm initial plate thickness.
Mill spring curved at the extremes
If entrance thickness is 5.5mm then plastic curve is left and lower so intersection is just less than 4.95mm so out out of specification.
If entrance thickness 6.5mm then plastic curve right and higher so intersection just over 5.05mm so out of specification.
Means roll mill control is required
Process control that could be added to the rolling operation
One method is to measure the thickness of the plate as it exits the roll. If thickness starts to increase then roll gap could be automatically reduced and vice versa.
Or could apply a front or back tension to plate during rolling. If thickness increased then tension could be increased (effect of decreasing rolling load and reducing mill spring) and vice versa.
