Free Essay

Spaggetti + Rubber Bands

In:

Submitted By DONNEWMAN
Words 985
Pages 4
1
Elevating Production Efficiency, Part 2
Last time I introduced three concepts that affect our ability to get and hold

to stabilize — mesh tension: elongation, retensioning and workhardening. You'll re call we established a mesh
-
tension goal, to obtain and stabilize high tension, but turned right around and stated flatly that mesh is, by nature, unstable .
This month, we'll demonstrate how the three concepts just mentioned help resolve that contradi ctory state of affairs. But first, I've got to define some terms: What do we mean by stable? And why is mesh unstable?
Let's take the last first. Mesh is unstable for two reasons.
For one thing, it's elastic. We can illustrate this using something that arrives at your door everyday with your newspaper: a rubber band. If I stretch my rubber band out and let it go, it appears to snap back to its original shape. Mesh behaves in similar, but less exaggerat ed fashion. If stretched on a retensionable frame or stretching machine, it lengthens, but when pressure is released it appears to shrink back to original size. We call such dimensional recovery memory , because each object appears to remember it's original shape. Traditional thinking about mesh has assumed that mesh instability is due to its elastic properties alone. If we stretch our rubber band just enough to keep it from sagging, it is, of course, easily bent out of line or deflected. If we pull the band taut, you know what happens. It offers resistance to our downward pressure. This is the classic argument for high
-
tension screens: those more tightly strung exhibit less screen deflection when the squeegee displaces the mesh. As a result, there is far les s image distortion.
While there is nothing wrong with that argument (I intend to expand on it in a later installment), it's not the whole story behind mesh instability.
Don Newman, president of Stretch
Devices, is one of the industry's leading advocates of on
-
press production efficiency — primarily via the virtues of elevated screen tension. Here is the continuation of his comprehensive analysis of the subject, stripped of scientific jargon and mathematical formulae and revealed as a set of simple concepts. Workhardening:
You don't need an electron microsope to observe polymer reorientation - a platter of pasta will do. Under stress, som e bonds between noodlelike polymer chains break and mesh begins to elongate.
Additional
new bonds form between now
-
more
-
parallel polymers, creating a stronger mesh strand.
Spaghetti with Rubber Bands
2
A Slight Case of Amnesia
On more careful examination, we see that our rubber band's memory is not as perfect as we thought. Lay the rubber band out and measure its length. Now stretch it over an object large enough to highly tension the band, and leave it for a while. Later, when it's removed, your second measurement will r eveal the band is now longer. (Our s lengthened by a full quarter inch.)
We may observe the relationship between screen tension and elongation by making two marks on a piece of mesh prior to stretching it. A tension meter placed on the mesh after t ensioning will show the tension level falling as the mesh, like the rubber band, begins to relax. The marks will grow farther apart as the tension on the mesh drops. (Mesh, in fact.
Elongates approximately 1/64 inch per foot, per 7
-
Newton tension drop.)
When mesh "loses its memory" or elongates, increase in the stencil size is not just the temporary sort we observe when it is deflected by the squeegee, but permanent. And as the pencil marks grow farther apart, so do the elements of our stencil image.
So to answer our first question, high tension alone isn't enough. Mesh is stable only when elongation has (for all practical purposes) been defeated, creating an image area as rigid and unchanging as possible.
Unfortunately, mesh elongation is a much tougher nut to crack in practical terms. If, for example, we took an N300 mesh and cranked the tension up to 50 Newtons immediately, the elongation that would occur afterward as the mesh relaxed would create severe mis s - registration problems during printing. That doesn't mean you can't tension N300 mesh to
50 newtons, it just means you have to do it properly . But before we can properly put elongation in check, we've got to explore why it happens. As in last month's installment, we'll find our answers at the filament level.
Under a powerful microscope, what appears to us a solid, cylindrical object

the mesh filament

is actually a collection of millions of molecules arranged in chain
-
like structures called polymers. In a piece of new mesh. these are randomly (or only partially) oriented to one another, resembling a plate of spaghetti. The polymer chains are bonded to one another where they intersect. Those bonds give the mesh what strength it has and, in simple terms, that strength is limited by the number of bond sites between chains. When we stretch the mesh, it's like taking a fork and pulling the spaghetti from opposite ends. What happens?
First, the intersections between the spaghetti strands are disturbed. In like manner, by stretching mesh. W e begin to break bonds between the polymer chains. The mesh is actually coming apart at the seams

some of the bonds between polymer chains begin to disengage and the strand or filament begins to grow in length. The net result is a larger piece of m esh, overall, and explains why we experience that typical drop in tension after initial stretching.
Measurable memory loss:
Beore and after (below) shots confirm permanet elongation.
Becoming " forgetful ": Highly stressed over time, both rubber band and mesh filament relax or elongate and become incapable of full recovery to size.
Key word is " tends ": Like the rubber band, the individual me sh filament tends to recover it s original shape and size when deflected.

Similar Documents

Premium Essay

A Comparative Study of the Elastic Limit of Two Different Brands of Rubber Bands

...AUSTRALIAN INTERNATIONAL SCHOOL 2332 Chino Roces Ext., Fort Bonifacio, Taguig, Philippines A Comparative Study of the Elastic Limit of Two Different Brands of Rubber Bands A Research Paper Presented to the Faculty of Australian International School In Partial Fulfillment of RAISE Program (Research in Australian International School for Excellence) Ramos, Anna Dominique A. Grade 10 School Year 2013 – 2014 TABLE OF CONTENTS TITLE PAGE i APPROVAL SHEET ii ACKNOWLEDGEMENT iii ABSTRACT iv CHAPTERS I. THE PROBLEM AND BACKGROUND OF THE STUDY Introduction 1 Conceptual Framework 2 Statement of the Problem 2 Hypothesis of the Study 3 Assumption of the Study 3 Significance of the Study 3 Scope and Limitation 4 Definition of the Terms 4 II. REVIEW OF RELATED LITERATURE Conceptual Literature 5 Research Literature 6 III. METHODOLOGY Research Design 7 Materials 8 Procedures 8 IV. PRESENTATION, ANALYSIS, AND INTERPRETATION OF DATA Table 4.1 9 Figure 4.1 10 V. CONCLUSION AND RECOMMENDATIONS Major Findings 11 Conclusion 11 Recommendation 11 REFERENCES 12 APPENDICES 13 Approval Sheet In partial fulfillment of the RAISED (Research in Australian International...

Words: 2712 - Pages: 11

Free Essay

Heat Transfer Lab

...cars that have small front wheels and large back wheels. The reason for this is because during acceleration, the large wheels at the back can take advantage of the weight transfer during acceleration. The wheels at the front were made small due to aerodynamics. Since the small wheels at the front puts my car at a slope, air resistance will be greatly minimized. The second thing I had to consider was what type of rubber bands to use. I had access to large thin rubber bands, small thick rubbers bands and small thin rubber bands. I decided to use three small thick rubber bands tied together so that they acted like one giant rubber band. The reason I did this was because I felt that a small thick rubber band could store far more energy than a large thin rubber band or small thin rubber band if all of them were to be pulled 10 cm. If what I believed was true, then my car would make the maximum distance using the rubber bands I had. I also tied the three rubber bands together so that the energy would be increased. Three rubber bands allowed me to pull the rubber bands farther back without them snapping which also ultimately gave my car more stored energy, allowing it to travel farther and faster. The final thing I had to consider while designing the vehicle was how to make it climb the ramp without it slipping. I was using CD's as my large back wheels and plastic wheels as my small front wheels. I was sure they were going to slip on the smooth surface of the ramp. My solution...

Words: 373 - Pages: 2

Premium Essay

Force and Motion

...one end of a rubber band around the vertical rod on your table. 2. Stretch the rubber band to a length of 10cm beyond its unstretched length, L0. [pic] 3. Place two rubber bands around the rod and stretch them together a length of 10cm Question 1-1: How does the combined force of two rubber bands compare to the force of just one rubber band? 1. Repeat with 3, 4 5 rubber bands. Activity 1-2: Measuring Force with a Force Probe Open file L03A1-2a (Measuring Force.ds) Procedure: Note: The Force Probe is attached to a cart. 1. Zero the Force Probe. (With nothing attached to the Probe press "Start" on the control panel and observe the force. If it is not zero press the small button "TARE" on the side of the Sensor and confirm that the force measurement is now zero. If the Force Probe still registers a nonzero force contact your instructor.) 2. Measure the unstretched length of one rubber band and record here: Lo=________ 3. Place one end of the rubber band around the vertical rod on the stand. 4. Attach the Force Probe hook to the free end of the rubber band. See Figure 1A on the next page. [pic] Figure 1A 5. Click "Start" and pull the Force Probe horizontally to stretch the rubber band 10cm (10cm beyond the unstretched length). See Figure 1B below. [pic] Figure 1B 6. Select the statistics feature [pic] and determine the mean value of the force. Record the force in Newtons. _______N 7. Now stretch the rubber band 20cm. ...

Words: 1090 - Pages: 5