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Measuring Perforation Strength

Strength

The strength of a perforation or its resistance to tearing is at best equal to the strength of the paper and at worst equal to 0. The strength is also dependent on the type and weight of the material being measured.  There are gages available which will measure the force needed to pull a perforation apart.
It is the relative strength which I will discuss below and how to make an educated guess what perforation wheel to use. Between the two extremes, 100 % of the material holding the perforation together ( tear strength of the material ) and 0 % of the material holding the perforation together we need a way to understand what factors influence our choices of tools.  Because a perforation is a series of cuts separated by a series uncut areas the strength of the perforation is a function of the percentage of cut to uncut areas, or a percentage of material left to hold the perf together.

To calculate the % hold you use the ratio of uncut material to the total material for a given length.
Say for example 1" length.

% Hold = uncut material in a 1" length
                       1" length

% Hold for a 4 cuts/inch perforation using a .030" tie is:

% Hold = ( 4 x .030" ) = .120" = 12 % 
                     1"              1"
We can do this for a series of standard perforator wheels/blades/knives all using a .030" tie.  Keep in mind that the tie is the un cut portion of paper or material.  Below is a sketch showing what the perforation looks like and the calculated percentage of material next to it.

Meaden What Perforation looks like

If we do this calculation for the standard cuts per inch using different widths of ties we see the relationship between cuts per inch and tie width.

Meaden Wheel Hold VS. Cut

 

 

 

 

What is a perforator wheel and how is it used?

     A crush type perforator wheel is a slitter wheel/blade/knife with slots cut in the outside edge all the way around the outside of the wheel. Below is a sketch of a perforator wheel/blade/knife with 12 slots cut in the outside edge.

Meaden Perf Wheel

     The cutting action takes place when a piece of material, usually paper or plastic, is pinched between this wheel and a hardened steel anvil.  The purpose is to create a place on the material which is partially separated. This allows the material to be easily separated by pulling along the line of the perforation.  This allows us to make forms or labels which can be kept in one piece some of the time and then a section can be torn away at a later date.  Below the picture shows what the line made by the above wheel would look like on a piece of paper.

http://www.meadenprinting.com/images/wheels/perf_2.gif

     There are a number of features on the perf wheel/blade/knife that can be changed in order to vary the way the perforation comes out on the material, and customize the perforation to suit its application.  
First of all is the frequency and size of the cuts.  The section of the material which is cut is called the cut and the section of the material that remains is called the tie.  The length of the cut and length of the tie can be changed in order to make the material tear away more or less easily. Below is a picture showing a blow up of the cuts and ties.

Meaden Perforation Diagram

     Normally a perforator wheel/blade/knife is specified by calling out the number of cuts per inch it will produce on a piece of material.  For ease of manufacturing the standard slot width( tie width ) in the wheel is .030" in the USA and 1mm ( .040" ) in other places.  The strength of the perforation is dependent on the amount total amount of uncut material.  The perforations strength can be increased by leaving more material uncut to hold the perforation together.  This can be done by either increasing the size of the tie and leaving the number of cuts per inch the same or by increasing the number of cuts per inch and leaving the tie width the same.  It is more economical not to purchase special tie widths.

     Standard perforation wheels/blades/knives normally are available with 2 to 16 cuts per inch, and tie widths can vary from .020" to .060" with .030" being standard.

 
 
 

 

 

 

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Meaden Precision Printing

March 2014

We have been supplying the printing, converting, and packaging industry with products since the late 1950's.  Our products include, but are not limited to- standard punches & dies, special shaped punches & dies, crimp-locks, and perforated, slitter & score wheels / blades / knives.  We manufacture 95% of the printing items in our plant and we are also a distributor for SchoberTechnologies & Sisprint.