Perforations allow a document to be separated into smaller portions of the whole and they allow a document to be folded easily (similar to the function of a score). In terms of printing specifications, perforations are classified according to bursting strength or tpi, which refers to "ties per inch" or "teeth per inch."

Burst and Tear Strength

• The burst strength is a measurement of the pressure (as measured with a burst strength gauge) that is required separate a document at the perforated location(s).
  
  
• The tear strength is the resistance that a perforation offers in preventing a document from separating at the perforated location(s). This also correlates with the physical effort that is necessary to separate the document at the perforated location(s).Tear strength can be categorized as "light release," which is easily torn; "medium release," which provides moderate tear resistance; or "stiff release," which provides the greatest resistance to detachment even after the document is folded at the perforation and is subjected to rough handling.

The purpose and function of a perforation often determines whether the perforation is to be an easy release variety (3 to 6 TPI) or a stiff release variety (10 to 18 TPI). The weight and thickness of the paper stock also affects the burst strength.

Ties Per Inch/Teeth Per Inch

The ties (or tabs) are the areas along the length of the perforation that are not cut and serve to "tie" together the sections of a document on either side of the perforation. The ties alternate with the cut areas to form the perforation. The cut areas are referred to as teeth. Perforations contain approximately the same number of ties per inch as teeth per inch, but the widths of the ties and teeth are usually not the same, which allows for easier detachment of the document sections separated by the perforation.

Perforations can be applied to paper stocks and other substrates either while the job is being printed (online) or they can be applied before or after the job is printed (off-line). There are many types of conventional offset presses that are capable of applying a perforation online and some digital press manufacturers offer attachments that can perform a number of online finishing processes, including perfing. Many types of off-line perfing equipment are available as well. Perforations can be applied to paper either parallel to the grain (paper fibers) of the paper or perpendicular to the grain. Parallel and perpendicular perforations can be applied at the same time and both types of perforations can be applied as full perfs (the full length or width of the printed piece) or as partial perfs (perfs that are not the full length or width of the document). Additionally, a combination of full and partial perforations can be applied to the paper stock depending on the capabilities of the equipment used.

Applying Perforations Online

Perforations Applied Parallel to the Grain

Long grain paper is most often used for printing. Long grain refers to paper fibers that are parallel to the longest dimension of the paper. Because this is also the dimension of the paper that is parallel with its travel through the press, any perforation that is applied parallel to the grain would also be parallel to the direction the paper travels through the press.

A small, segmented, metal wheel is most often used for applying a parallel perforation. The segmented wheel rolls along the paper and applies the required cuts per inch as the paper is pulled through the press. Multiple parallel perforations can be applied at the same time when using the segmented wheels and the positions of the perforations on the paper can be in almost any parallel configuration. Since the printing and perfing of the substrate are accomplished in one operation, considerable savings in labor and cost can be achieved.

Perforations Applied Perpendicular to the Grain

For perforations that must be applied perpendicular to the grain and/or to the direction the paper travels through the press, hardened steel perforation rules are used. The steel rules are inserted into slots in the cylinders of the press, which allows the rules to stamp the perforation into the paper as the press cylinders revolve. Like perf wheels, the steel perf rules are available in various ties per inch. Compared with parallel perfing, there are usually more limitations as to the number and placement of perpendicular perforations, so it is best to consult the print supplier to learn of the possible limitations.

Applying Perforations Off-line

There are occasions when perforations are added as an off-line process, before or after the product has been printed. Generally, products that have a perforation applied after printing consist of smaller quantities and have been printed on sheet-fed presses. Larger quantities are more economical to print and perf in one operation using equipment such as a conventional web-fed offset press that is equipped for the application of perforations or a digital press with finishing attachments.

Applying Perforations with Off-line Rotary Equipment

Many of products that are perfed off-line are sent through rotary equipment that can apply only full-length perforations. Perf wheels, similar to those used for online perfing, are used on the off-line equipment. The perf wheels apply full perforations either parallel or perpendicular to the grain of the paper, depending on the direction the paper is sent through the equipment. It is important to remember that long grain paper is easier to feed into the equipment than short grain paper.

When both parallel and perpendicular perforations are required, the paper is sent through the equipment twice. To help save time, some products can be manufactured with several printed copies on one large sheet so that when the sheet is sent through the perfing equipment, the perforations are applied to all of the copies on the large sheet at the same time. The sheets are then cut into individual finished pieces. The only drawback of this technique is that multiple documents can be damage at one time if there is a feeding problem or if the perforation is not applied in the correct position.

Applying Perforations with Off-line Flatbed Equipment

Complicated perforation configurations, such as a perforation bordering a business card that can be detached from a larger document, can be applied with a perf cutting method that is similar to die cutting. Perforation rules, containing the specific number of ties per inch that are suitable for a particular application, are constructed into an outlined shape that is identical to the shape of the area requiring the perforation. A platen (flatbed unit) press is often used for this type of application. When the perforation blades are pressed into the substrate using the platen press, the required area of the document is perforated. The document remains intact until it is time to detach the perforated area or shape.

Preperfed Paper for Digitally Printed Applications

Some paper is perforated before it is printed. The print supplier might produce their own preperfed paper or they can purchase it from a converter. This type of paper is especially useful for digitally printed applications that require a perforation or perforations in standard positions on the paper, such as for documents printed on 8½ x 11 sheets that must be folded for insertion into a #10 envelope. Billing statements are an example of an application that often incorporates perforations for folding purposes and for easy detachment of the statement portion that must be returned with the payment.

An Application in which the Paper Was Preperfed Before Printing

Perforation Selection Criteria

The perforation patterns shown on the right are general representations of the cut and uncut dimensions of various perforations. The length of both the cut and uncut sections can be shorter or longer depending on the special requirements of the perforated application. The patterns shown are merely standard dimensions and can be changed as required.

• 3 to 4 TPI: When one section of a document needs to detach easily from another section, 3 and 4 tie perforations are good choices. They are often applied to business forms that contain stubs used to join multiple part forms. The light perforation allows the parts of the form to be detached easily. 3 and 4 tie perforations are not recommended for a document that must remain intact for long periods because the ties are not strong enough for that purpose.
  
  
• 5 to 6 TPI: Because there are more ties per inch, 5 and 6 tie perforations are stronger than the 4 TPI, but they are too loose to be used for documents that must remain intact for lengthy periods. They are popular choices for multiple part business forms.
  
  
• 7 to 10 TPI: 7 TPI through 10 TPI perforations are stronger than the 5 or 6 TPI and help to keep a document intact until it is ready to be detached. They are most often used as a perforation within a document that allows the user to remove a section of the document.
  
  
• 11 to 14 TPI: Perforations of 11 to 14 TPI are referred to as statement perforations. They are most often used as a perforation within a document that allows the user to remove a section of the document. For example, the top or bottom section of a "statement of account" form may be detached and sent back with a payment. The perforations are durable so it is best to fold the perforated section of the document to weaken the perforation, which allows for easier detachment.
  
  
• 15 to 18 TPI: 15 to 18 TPI are stronger than statement perforations and must be folded to weaken the perforation so that it bursts more easily. They are often used for magazine inserts and mailers.
  
  
• Letteredge or Invisible Perforation: As the name implies, this perforation is nearly invisible and leaves a clean, smooth edge after the perforation is detached. The most common ties per inch for letterhead perforations are 50, 66, and 72. Unless a specific TPI is requested, most print providers use one TPI as their standard for all applications requiring a letteredge perf. A letteredge perforation does not work well on paper stock heavier than 24 lb. bond because it is too difficult to control the depth of the perforation. Using a stock that is too heavy or thick may result in a perforation that does not detach properly or it may result in a document that falls apart easily at the perforation. If you are unsure of which TPI to use, you should discuss the options with your print supplier.

Selection Tips

• There may be special requirements that should be considered for the perforation, such as applying a perforation that will not burst with repeated or excessive handling of the application.
  
  
• Carefully consider any additional processing that will be required on the paper stock, such as imprinting or folding, and how the end user will detach the perforated section.
  
  
• Consider the function of the perforation, such as a perforation used for folding or for bursting.
  
  
• Consider the properties of the paper stock that is to be perforated:
  
  
  • Weight or thickness of the stock.
  • Stock elasticity or "spring back" of the material.
  • Content of the stock, such as plastic, label substrate, or recycled material (recycled stock has shorter fibers that can influence the effectiveness of the perforation).
  • The type of coating on the stock (which may cause the perforation to become lighter in depth than what was anticipated and therefore less likely to detach as desired).
    
    

• Consider the bursting strength that is required and the tear strength desired.