Take a Closer Look at Pad Printing

By Michael DeCesare

12 January, 2006

© 2006 Pentex Print Master Industries Inc.

 

How do you achieve good pad printing results?

With 30 years in the pad printing business we think that it is important to stop and review the things we think we know as sometimes we can surprise ourselves and learn that we have developed a few bad habits over the years. Changing technologies and chemistry have allowed the pad printing process to mature to a point where it has become an essential part of manufacturing and product promotion. The pad printing process has evolved to become widely accepted particularly in the plastics industry due to special features which allow easy printing on unconventional shapes which are impossible to print with other printing techniques.

 

A good pad printing job can be described as an optimum ink transfer that is achieved by means of a sharp balance between surface tensions of the corresponding surfaces; that is pad, ink, cliché (printing plate) and substrate which turns into high definition printing. In order to achieve a commercially acceptable print, there are several factors which need to be addressed before beginning: they are Artwork, Printing Plates, Printing Pads, the Substrate, the Ink, and the printing environment.

 

New Lazex Computer to Matte Film with Lazex Emulsion Enhancer Spray

Good quality art is the definite starting point because the reproductions you print can only be as good or as bad as the original artwork. Not only does the artwork need to be high quality, but it must be printed on the proper film in the proper format.  Use the Lazex Matte film and a high quality 1200 DPI laser printer to create your film.  It is not recommended that you use Vellum or Clear Glossy film because they do not give you the highest quality image reproduction on the printing plate.

 

Using a glossy film on polymer plates is not recommended for several reasons, glossy films have a high surface tension which traps air between the film and the plate, and this will cause marks to be left on the plate when washing out the plate.  Gloss Finish film has a higher static charge that attracts dust and will create small marks that will transfer onto the printed part during the print process.  The Lazex Matte Finish Film which is designed for use on polymer plates does not create a static attraction and will prevent pin-holes and water marks on your printing plates. 

 

The film must be printed POSITIVE, EMULSION DOWN. Inspect the film for any marks, pin holes or unwanted defects before proceeding. If you are satisfied with the quality of the artwork, you now need to enhance the density of the film. This is achieved by spraying the matte Lazex film with the Lazex Toner Enhancer Spray. The side of the film that the laser printer prints onto is called the EMULSION side; this is the side that needs to be sprayed. After spraying, allow the film to dry for several minutes before proceeding. The image on the film should be noticeably darker than it was; it is vitally important that the image be as dark as possible.

 

Processing Polymer Printing Plates (Clichés)

Printing plates hold the "Master Image" in the printing process; all prints are generated from the printing plate. Using a high quality printing plate is essential. Plates can be from one of two categories, steel plates or polymer plates. Steel plates are used for print runs over 100,000 parts, polymer plates are used for shorter runs. Both types of plate will reproduce high quality prints. For simplicity, we will use polymer printing plates for our discussion.

 

First and Second Exposures:

Step One – with Lazex Film Emulsion down.  The printing plate must be exposed to Ultra-Violet Light and the Film Art in order to create the image on the plate. Place the emulsion side of the plate against the emulsion side of the film, you now place these face down on your UV exposure unit and expose the plate to the UV light. The film will mask or protect the areas of the plate where the image is to be located; the exposed areas will begin to harden when exposed to the UV light. The longer the exposure time, the shallower the depth of the relief area of the image.  The exposure time is dependant on the type of film being used, the source of UV light and the distance the plate is from the UV light source.  Using a Pentex PTX-702 UV Exposure unit and the Lazex Matte Film, we recommend an exposure time of 70 seconds.

 

Step Two – 300 Line Gravure Screen for Normal Images and 150 Line Gravure Screen for larger open images.  Using a gravure screen is necessary to prevent ink being pulled out of the relief area of the plate by the ink cup (or doctor blade).  The gravure screen creates a dot pattern that will act as a bridge for the ink cup and prevent it from dipping into the relief area.  Choosing the right gravure screen depends on the type of plate and the size of artwork being printed.  We recommend that a 300 line gravure screen is used on a polymer plate for most types of print but a 150 line gravure screen may be used on prints that have larger open areas.  We recommend an exposure time of 35 seconds using a Pentex PTX-702 Exposure Unit.

 

After the exposure is completed the plate is ready for the wash out process.  Polymer printing plates can be either WATER WASH (Pentex PTX-52W) or ALCOHOL WASH (Pentex PTX-49A) printing plates, the type of plate determines what processing method is used, Water is used for WATER WASH plates and an Alcohol solution is used for ALCOHOL WASH plates. The WATER WASH plates have a lower shore hardness (softer) surface and should be used on OPEN INK RESERVOIR PAD PRINTERS; the ALCOHOL WASH plates have a harder surface and can be used on both OPEN INK RESERVOIR and SEALED INK CUP PAD PRINTERS. When the processing is completed, Polymer plates must be dried for a period of time determined by the manufacturer, the PTX series of polymer plates must be dried for 30 minutes.  Failure to dry the plates for the proper period of time will result in a plate with an uneven and unpredictable relief area and you will not be able to get a proper ink pickup from the plate. The final step is to harden the printing plate by exposing the plate to UV light for 3-4 minutes - this will harden the whole plate including the relief area that was masked by the film during the first exposure.

 

Choosing Your Printing Pads

Printing pads are made of silicone and are used to "Pick Up" or "Transfer" the print from the printing plate and "Deposit" the print onto the substrate (part) that is being printed. Printing pads are available in a variety of shapes and sizes and are made of different silicone types depending on the characteristics that are required. Choosing the right shape of printing pad is determined by the shape of the product being printed and by the image to be printed. For instance, a radically shaped pad works very well on a flat surface.  The hardness (durometer) or type of silicone is chosen by determining the shape and strength of the surface to be printed on. For flat and smaller, tighter concave surfaces, a harder pad (60+ durometer) would be required. On rounded objects a softer (less than 60 durometer) pad may be considered. Choosing the correct pad is critical. Using the wrong pad can cause pinholes or other defects in the print. The printing pad should be replaced on a regular basis as the pad will age causing the silicone to loose its effectiveness to release the print, it will suffer wear and tear as part of the printing process and may even become damaged as a regular part of printing.

 


Understanding the Substrate

The substrate is the item being printed. Understanding the surface will allow you to determine how the surface needs to be prepared. Surface preparation consists of one or both of the following operations: cleaning and pre-treating. All parts must be cleaned as finger prints, dust, and any other surface contaminant will not allow the print to be deposited properly onto the surface of the part. Substrates that have a high surface tension such as Polypropylene, Metals, Chromed and Lacquered Surfaces need to be pre-treated after they are cleaned. Pre-treating can be done by one of three methods: Chemical Wiping (InkFlexx MP2000-104 Pre-Wipe), Flame Treating, and Corona Treating. The method you use would be determined by the quantity of parts and your budget.

 

InkFlexx – One Ink Formulated for Printing on All Substrates

Once the parts have been prepared, we can concentrate on our ink. The InkFlexx MP2000 series printing inks will have the greatest impact on the quality of print and adhesion of ink onto the substrate.

 

With the wide variety of substrates, it is important to know the composition of the material that you are printing on because the ink will interact differently with different substrates. Using a premium quality pad printing ink is essential. Many inks are simply reformulated screen printing inks which do not have the opacity that is required for pad printing. Remember that pad printing puts down a print that is approximately 20 microns thin where screen printing lays down double that thickness.  With only half the amount of ink required for screen printing, pad printing inks must deliver equally opaque color reproduction. Why would you use an ink that is not designed for the job? 

 

InkFlexx inks were the first pad printing inks formulated specifically for pad printing and for use on a wide range of substrates, (rather than the usual hodge podge of ink types used for each different substrate). InkFlexx has simplified the pad printing process by eliminating errors caused by selecting the wrong ink. It has also meant a reduction in ink costs as only one ink needs to be inventoried, (not three to four different types of inks as other suppliers would have you do).  Just think… one ink!

 

Extensive research has brought forth the InkFlexx line of pad printing inks which doubles as a superior screen printing ink when high quality detail printing and opacity are required. It may be used as received or the pigmentation may be reduced with extenders to produce equally excellent results.

 

Some important characteristics of InkFlexx Inks are:

 

1.     Optimum Ink Transfer – Achieved by means of a sharp balance between surface tensions of the corresponding surfaces: Pad, Ink, Cliché and Substrate. The result is a high definition print in either spot color or process color printing.

2.     The high opacity of the InkFlexx Inks allows printing of high relief jobs and printing of light colors on dark substrates with a single hit.

3.     Quick drying properties allow for high speed printing of multiple colors while still maintaining high gloss characteristics.

4.     Very good mechanical and chemical resistance exhibited by the basic ink may be further enhanced by the addition of the available additives.

5.     Wide range of standard colors which are also used as mixing colors.

6.     Very good adhesion to a wide variety of materials including, ABS, Acrylic, PBT, PET, PET G, Polyamides, Polycarbonates, Thermo-rigid Polyester, Polystyrene, LD-PE, HD-PE, Polypropylene, Rigid and Flexible PVC, SAN, Glass, Stainless Steel, Aluminum, Chrome Plate, Iron, Tin, Wood, Lacquered, Galvanised and Powder Coated surfaces.

 

When preparing InkFlexx you must know whether or not the ink is to be used as a single or a two-component ink mixture. A two-component ink mixture is an ink that is using a hardener (also referred to as a catalyst or an epoxy ink). The ink mixture should always be measured by weight using a digital scale accurate to at least 0.1 grams. Always follow the InkFlexx formulation guide recommendations on the amount of thinners, hardeners, and/or any additives that are required. Using amounts other that what is recommended may not give the desired result. When inks are mixed in the wrong proportions, you may experience problems with transfer, opacity, sharpness, and adhesion. Also, cross-linking products from other ink lines is not recommended as thinners, hardeners, and additives are designed to work with the ink in a certain way, and using another product will affect the chemistry and you may have undesirable effects.

 

The Printing Environment

Where you do your pad printing is an important consideration.  You want to locate your pad printing in an area that has a controlled temperature environment, is clean, and is well lit.

 

The pad printing process works through the evaporation of the thinners from the time the ink is picked up from the printing plate to the time that it is deposited onto the substrate.  One of the most important factors in controlling this variable is the ambient temperature of the print environment.  The printing room should be air conditioned and maintained at a constant temperature year round.

 

This room should be kept clean, you do not want dust to get onto the print.  Keeping pad printing machines close to working machinery such as injection molding machine is not recommended.

 

Having a well lit environment is important not just for the operator's comfort, but is also used as a quality control tool.  Colors will look different under different lighting conditions; the ambient lighting should be as close to daylight conditions as possible.

 

Information about Pentex

With over 30 years of service to the pad and screen printing industries, Pentex has the expertise and experience to help you achieve your print production goals. There are several important reasons why Pentex should be your supplier and a partner in helping you achieve success. At Pentex, we are always looking for ways to improve our machines and supplies to make your pad and screen printing operations more efficient. From our pad printers to our high-tech DiscMaster series CD screen printers, no idea is too small for our engineers. New ideas are always being explored to further improve our product line to satisfy our customers' growing needs. When an off the shelf solution is not what you are looking for, our design and engineering experience will allow you to draw on over 30 years of real world solutions. We will work with your production team to ensure maximum productivity at every stage of your printing operation. No detail is too small, no task is too large. At Pentex, we are proud of the reputation our machines have earned in the last quarter century. We are equally proud of our reputation for providing great service to our customers.

 

You can contact us by email at Pentex@inkflexx.com or visit our website at www.inkflexx.com.