Choosing The Correct Union White Plastisol
For Underbasing Applications

* Overview

With so many white inks to choose from on the market today, it can become very confusing and frustrating when trying to choose the correct white plastisol to use as an underbase for flash curing applications. Is there one "miracle white" that does it all or is it necessary for printers to inventory more than one white ink? This paper will honestly answer those questions as well as deal and rate the best choices of Union Ink products to use as underbase whites. Squeegee, mesh, printing, flash curing and curing parameters will also be discussed.

* What is Underbasing?

Underbasing is a process where a white ink is printed on a dark colored shirt, placed underneath a drying unit called a flash cure dryer which brings the ink film to a minimum gel temperature and allows colors to be printed on top of the white underbase ink. Because this process basically turns the substrate you are printing on from a dark color to a white, it allows you to produce vibrant prints while utilizing standard opacity inks such as Union Ultrasoft, Mixopake, Autoline or Tru-Tone series inks.

* What Union White Inks Are The Best Choices For 100% Cotton Dark Shirts?

The best choice for underbasing 100% cotton dark shirts is PAFC-1020 Fast Flash White. This ink has been specifically formulated for underbasing. It's creamy, easy-printing consistency provides a smooth surface for overprint colors. PAFC-1020 has a low after flash tack which is important if you cannot afford to sacrifice a print head as a cool down station (see section titled "Printing Parameters" for more information about cool down stations. Fast Flash White is not designed as a stand alone white (such as MIXO-1000, PADM -1020C, PLHT-1055 or PADM-1057) and will not produce satisfactory results when used as a standard plastisol or as a mixing white.

PADM-1020C Maxopake White also provides excellent results for this application. This ink has a creamy, easy to print consistency and flash cures quickly without any after flash tack. It also provides a smooth surface for overprint colors. If a printer wants to have one ink for both an underbase and standard white ink for 100% cotton PADM-1020C is the best choice.

MIXO-1000 Mixopake White works well for printers wanting one ink for 100% cotton also. The only disadvantage of MIXO-1000 is a print head on automatic presses may be needed for a cool down station because of a slight after flash tack..

Even though PLHT-1055 Lightning White will provide excellent results for this application it is not recommended for 100% cotton garments. Cotton garments are not prone to the inherent bleeding problems of polyester fabrics and the extra cost of a low bleed ink for this application is unnecessary.

* What Inks Are The Best Choices For Polyester or Polyester Blend Garments?

Because shirts containing any amounts of polyester are prone to dye migration (bleeding) a specially formulated ink must be used on this substrate. This ink is manufactured with special ingredients to prevent the dyes of the shirt from migrating through the ink film. Consult the Union Ink Catalog section "General Information About Plastisol Inks" for more information on the causes and prevention of dye migration.

The best choice for underbasing polyester blended dark garments is PAFC-1050 Fast Flash Low-Bleed White. PAFC-1050 is specially formulated to minimize dye migration, provides a smooth printing surface for overprint colors and flash cures tack free very quickly helping to eliminate the need for a cool down station. As in the Fast Flast White for cotton, PAFC-1050 does not provide satisfactory results when used as a standard plastisol or as a mixing white.

Even though PLHT-1055 Lightning Low-Bleed White and PADM-1057 Maxopake Low-Bleed White may also be used as underbase whites they are not necessarily the best choice for this application. Low-bleed whites are typically formulated with special resins, fillers, and plasticizers designed to stop dye migration. Some even contain a small amount of blowing agent (commonly used to manufacture puff inks) in order to increase opacity and maximize their bleed resistance. These ingredients can cause an uneven printing surface for your subsequent colors if the ink deposit is too thick and requires a longer flash time. However, many printers are successfully using the PLHT-1055 Lightning White as an underbase through 160 and higher mesh count screens. Lightning White's creamy, easy to print viscosity allows printers to put a thin ink film on top of the garment by printing off contact. This thin ink film provides a smoother surface for overprint colors as well as a faster flash time than coarser mesh underbase screens. The superior bleed resistant of Lightning White also helps blocks dye migration as long as the entire ink film reaches 300 degrees Fahrenheit during the curing process.

* Mesh Specifications For Underbase Whites*

MIXO 1000, PAFC-1020, 1050, PADM 1020, 1057 and PLHT-1055

Simple block areas: 86T-110T (Manual Press) 125T (Automatic Press).
Detailed images: 110T (Manual Press) 140T (Automatic Press).
Fine line halftones: 125T-140T (Manual Press) 160T-200T (Automatic Press).

* Mesh Specification For Overprint Colors*

Simple block areas: 200T (Manual Press) 260T (Automatic Press).
Detailed images: 230T (Manual Press) 305T (Automatic Press).
Fine line halftones: 260T (Manual Press) 350T (Automatic Press).
*These are guidelines only and may need to be varied depending upon the amount of detail and the number of colors in the design

* Printing Specifications

Underbase screens on your automatic or manual press should be set off-contact in order for the ink to clear the screen cleanly. It may be necessary to double stroke the underbase to clear the ink from the screen cleanly and provide the smoothest surface for the overprint colors.

Squeegees should be set at a slight angle and with just enough pressure applied to clear the ink from the screen. It is recommended printers using automatic presses back off the squeegee pressure until the ink does not print and then slowly increase the pressure until the desired underbase print is achieved. Remember, you are trying to print the ink on top of the garment, not drive the ink through the garment. If the print starts appearing on your platen you are probably using too much pressure.

If possible, overprint colors should be printed from the smallest to largest area of coverage in order to minimize pick-up on the backs of the screens. Squeegee pressure is important in this stage because you are now printing on a piece of non-absorbant vinyl which is not as forgiving as an absorbent garment. Too much pressure will cause the colors to smear. To alleviate this problem either use a lighter pressure during the print stroke or utilize finer mesh counts for overprint colors.

* Flash Curing the Underbase

Your flash cure unit must be large enough to not only cover the design but should be at least 4" larger than the design you are flash curing. Some of the smaller 16 inch square units can barely cure a 14 inch design much less the oversized prints in demand by customers today.

The flash cure unit should be positioned above the design so you can get a partially cured print in a matter of 3-5 seconds while not burning the garment. Plastisols will reach what is called the gel temperature at approximately 240 degrees Fahrenheit. This can be measured with either a temperature strip or temperature probe available from Union Ink Company. Caution: Be careful not to fully cure the underbase ink. An overprint color may not adhere to a fully cured underbase and flake off after the shirt is washed. Another indication of overcuring the underbase is if little pinholes of white can be seen through the overprint color. If so, lower your flash cure temperature or raise the height of the flash cure unit.

After flash curing the underbase white, the ink may be appear sticky even though it is fully gelled. This is known as "after flash tack" and usually occurs in inks not specifically formulated for flash curing. This occurs more frequently on automatic presses because of the production speeds. This tack goes away in a few seconds but slows down production in the meantime.

To eliminate this problem a cool down station may be necessary or fans may be needed between print heads to cool down the underbase print. For example, on a six color press the underbase would be printed on head #1, flash cured on head #2, head #3 would be empty and used as the cool down station. The overprint colors would be printed on heads #4, #5, and 6. Because of the cool down station you will be limited to three colors. This is why the best choice is to use an ink specifically designed for underbasing where after flash tack is not a factor.

On manual presses with rotary table platens "after flash tack" is usually not a factor because the underbase has had sufficient time to cool and lose the "after flash tack" before the print comes back around for the overprint colors.

For designs with more than two or three colors it will be very difficult to achieve an acceptable print without adding a second or even a third flash to the printing sequence. As each new color is printed, a little bit of the previous color is picked up on the backs of the screens resulting in mottled looking and uneven color intensity.

*Disclaimer

Customer testing is required and should be mandatory with this product or any new product or process before running production. Our technical advice and recommendations given verbally, in writing, or by trials are believed to be correct. They are not binding also with regard to the possible rights of third parties and do not exempt you from your task of examining the suitability of our products for the intended use. We cannot accept any responsibility for application and processing methods which are beyond our control, nor can we accept responsibility for misuse by you of the products or use by you of the products outside the specified written instructions given with the products. User must protect sensitive skin, exposed wounds and eyes from contact with products.

* For More Information

This paper was written and produced for your information by the staff of Union Ink Company. For more information about this product, process or any other Union Ink product please call 1-800-526-0455.