Must-Know Facts When Screen Printing on Polycarbonate Films

By Jim Hingst

Screen printing polycarbonate films is different and more challenging than printing other films. Jim Hingst offers tips to ensure printing success.

Compared to other plastics, such as polyesters, polycarbonate films are much easier to print.  Polycarbonate has a naturally high dyne level, which means that you can achieve better ink adhesion. Unlike other plastic films, such as polyester,  polycarbonate films require no surface pretreatment before printing. And, since polycarbonate withstands higher curing temperatures than polyester, the stability of these films allows the printer to maintain better registration. Still printing polycarbonate films is not without its problems. To minimize the occurrence of these problems, follow these tried and true helpful tips.

1.  Read and Heed. 

In selecting materials, read the manufacturers tech bulletins and follow their recommendations. The manufacturers conduct  extensive tests of the compatibility of the primary components of graphic panels: the ink, the polycarbonate film and the transfer adhesive.

2. Test, Don’t Guess.  

The ink manufacturers can provide you with ink recommendations and instructions on processing.  Nevertheless, to ensure compatibility of the ink with the film and the transfer adhesive, the screen printer must conduct his own testing.  Incompatibly of materials is a major cause of print failures. Prior to production always qualify the raw materials: the inks, the film and the transfer adhesive. All three components have to work together.  The ink may have good adhesion to the film.  But after you apply transfer tape, the chemicals in the adhesive  can weaken the anchorage of the ink.

3. Cool It!

After printing some sample sheets of polycarbonate, allow the ink to cool before performing ink adhesion test. Any type of testing conducted prior to production, should closely approximate shop conditions.  Testing should include checking the adhesion of the ink to the substrate; intercoat adhesion (the bond of one layer of ink to another) and compatibility of the tape with the ink.

4. Cross Hatch Test. 

To test ink adhesion, run a cross hatch test. The cross hatch test consists of scoring the printed ink many times with an Xacto knife and then scoring the ink again over the first set of lines at 90° angle .  Using a plastic squeegee, an aggressive tape, such as 3M Brand #610  clear tape, is rubbed over the scored cutlines.   The tape is then pulled off 180° against itself, in one quick motion. If any of the ink comes off, the adhesion of the ink to the substrate is insufficient.

5. Stick with a Winner. 

Once you find a combination that works for you,  stick with that winning formula.  Details of every job should be well documented so that nothing is left to chance when duplicating a reorder. Repeatability of results depends largely on reproducing all of the variables involved in printing.  In addition to screen tension, record key information on your production order, such as mesh, ink formulation and cure rate. 

6. Shelf Life. 

Although polycarbonate films typically have a much longer shelf life than other plastics, ink adhesion can change as the plastic ages.   To extend the shelf life of polycarbonate film,  store all of materials in a temperature and humidity controlled environment. A printer can print on one lot polycarbonate  and have had great ink adhesion, only to print the same job on the remaining sheets with poor results. This is just another reason that it is important that printers should ‘Test, Don’t Guess’. 

The $64 question is: what causes ink adhesion problems in printing  on older polycarbonate film?  While some have suggested moisture absorption as the culprit, the most likely candidate is exposure  to UV light and oxygen.  As the polycarbonate absorbs UV light, it excites the photons of the atoms, which in turn creates free radicals. Free radicals are atoms with a few extra electrons. These extra electrons make free radicals highly reactive, or what I like to think of as being chemically promiscuous.   These free radicals chemically react with any impurities in the polycarbonate film,  such as sodium. The result of this reaction is degradation of the polycarbonate. This is why the film can yellow and become brittle.

Degradation can occur with or without oxygen. With it, though, the problem is much worse. Here’s why. The oxygen reacts with the polycarbonate breaking the molecular chain especially where one carbon atom bonds with another carbon atom or with an oxygen atom. Without oxygen, degradation still occurs because UV radiation initiates cross linking.  This process is called photo-oxidation.  The by-products of these chemical reactions are the creation of H2, H2O, CO and CO2. 

As  polycarbonate oxidizes, the surface breaks down.  As the outer layer or skin of the film degrades, it becomes very weak and molecular debris accumulates on the surface.  When older polycarbonate film is screen printed, the loose debris forms a barrier between the unaffected  part of the film and the ink.  Because the ink adheres to this debris, ink adhesion is poor.

7. Product Exposure.

Prior to production, always investigate the environment to which the graphic is exposed, the durability requirements of the job and the layout. Any pre-production testing should check the compatibility of  the polycarbonate with any chemicals that it will likely come in contact with during daily use, including acids, petrochemicals, solvents and cleaning chemicals.

8. Solvent inks. 

Both solvent-based or UV screen printing inks adhere to polycarbonate well. Solvent based inks generally bond well because the solvent in the ink  chemically bites into the film.  Although problems with solvent inks are rare, a poorly cured ink could cause poor  ink adhesion. Some solvent ink systems may take some time for the solvents to evaporate fully and the ink to achieve full adhesion.

The key to success in printing with solvent inks is to completely dry the inks so there are no residual solvents. Force drying of the inks at temperatures between 130° and 150° F is the preferred method. Some potential problems that can arise from insufficient drying, are  poor ink adhesion, stress cracking and sheet curl.

9. UV Screen Print Inks.  

When printing with UV inks, use a  monofilament polyester mesh with a high mesh counts, in the range of 355 to 390. Using a fine mesh screens will ensure the reproduction of fine detail and a thin deposit of ink.  Other variables that can affect the thickness of the ink deposit include stencil thickness, ink viscosity and squeegee durometer.

When printing with UV inks, thinner deposits of ink are also better, because the UV light can more easily penetrate and more thoroughly cure the layer of ink. If the ink deposit is too thick, the UV light will not fully penetrate the ink. This can mean that the layer of ink that is making contact with the substrate is not cured or is not fully cured. Usually this uncured or under-cured ink is too soft to bond or adhere to the substrate. 

10. Thumbnail test. 

You can often tell whether the ink is properly cured by simply scratching the surface of the ink with your thumbnail.  Although thumbnail testing is not scientific, a properly cured ink will resist scratching.  

11. Thumb Twist Test. 

Just press your thumb against the ink and give it a twist.  If the ink is not thoroughly cured, a hardened layer of ink can slip over an uncured layer of ink. A properly cured ink also should not feel tacky. Instead it should feel dry to the touch.

12. Curing of UV Inks. 

Over curing a UV ink can cause as many problems as under curing ink.  As the exposure of the UV ink increases, the ink film becomes harder. Overexposure of the ink can embrittle it, which could lead to some fracturing of the surface of the ink.

The initial layers of ink often receive additional exposures of UV light as the subsequent ink layers are printed and cured. With each exposure, the initial layers harden.  As the first layers of ink harden, it becomes more difficult for the subsequent layers of ink to bond to them. Poor intercoat adhesion can cause delamination of the graphic overlay from the substrate to which it is adhered.

Different types of UV curing units produce UV light of varying wavelengths. Some wavelengths of light can yellow and oxidize polycarbonate films. Overexposure to UV light can also harden and oxidize the surface of polycarbonate film. This   oxidation of the film’s surface can inhibit the ink’s ability to bond to the film.  Do not, however, under-cure the ink layers in an attempt to prevent oxidation from occurring. Each layer of ink must be properly cured for good ink adhesion. Consult the ink manufacturer or your distributor for recommendations regarding which type of curing unit is best for your application.

Poor ink adhesion to polycarbonate films often results from one of three reasons. First, the ink and the film may not be compatible.  Second, the ink may be under-cured. And third, the surface of the polycarbonate may be degraded.

In the curing process, exposure to UV light can cause the outer surface of the polycarbonate films to degrade.  This commonly occurs when polycarbonate films are exposed to shorter wavelengths of UV light, in the range of 200 to 260 nanometers (nm).  Degradation of this outer surface of the film can cause ink adhesion problems. 

Several variables affect the curing of UV inks, which include the type of curing lamps, the age of the lamps, the conveyor belt speed and the thickness of the ink deposit.

 

Some colors absorb light better than others.  For this reason, different ink colors can require different or varying degrees of UV energy to properly cure. Black, for example, often requires higher energy to cure, because the density of the pigment can block the light from penetrating all the way through the ink layer. Ink manufacturers try adjusting their ink formulations to compensate for this, so that the different colors cure at the same rate.  

13. Intercoat Adhesion. 

Not only must the ink bond to the base polycarbonate film, multiple layers of ink must bond together.  The ability of one layer of ink to bond to another layer of ink is commonly referred to as “intercoat adhesion”.  In performing a cross hatch test, if one layer of ink splits from another, it could indicate that the ink layers are poorly adhering to each other.

14. Dealing With Static. 

Many plastics, such as polycarbonate films, are prone to pick up a static charge.  Static in the polycarbonate sheet can attract dirt, which can result in printing defects.  Static electricity can also cause a printing problem called spider webbing.   Air conditioning and humidity control helps in minimizing static.

Keep humidity in your plant between 15% to 20%.  Some ink manufacturers recommend much higher ambient humidity, not only for the print area of your shop, but also the storage area.  Air filters, deionized air and good housekeeping practices can also reduce contamination.

The type of static eliminator that blows deionized air  on the sheet has worked well for many printers.  This type of unit is also used frequently by flexo and offset printers.  This type of unit shoots negatively charges particles onto the sheet as the films passes underneath a bar.  By neutralizing the static charge, you can prevent spider webbing.  Controlling static is especially critical when printing panels with small copy and sharply defined edges. 

Some inks can also build a static charge. To help dissipate the static, some ink companies recommend adding a drop or two of dishwashing liquid to the ink. Before trying any of these remedies, be sure to carefully read any pertinent technical literature or contact a representative of the ink company.

15. Laminating the Transfer Adhesive. 

Before laminating the transfer adhesive, shearing or die cutting the polycarbonate film, a  good practice is to wait 24 hours after printing, whether you use either solvent inks or UV inks. This waiting period allows the ink system to completely cure. 

16. Die Cutting Recommendations. 

Steel rule die cutting is usually the easiest part of the job. Polycarbonate  films can be more easily die cut, with less fracturing of the edges than other types of plastic films, such as polyesters.  The toughest part in die cutting is specifying the right die for the job. In ordering steel rule dies, good communication with your die maker will ensure that you will receive tooling that will satisfy your requirements. The most critical information is the type and thickness of the material to be cut; whether or not the film will be laminated with a transfer tape; the quantity to be cut; how often will the die be used; the rule type; the tolerances for the job; and the delivery date required. You also need to communicate if you want the die filled with foam rubber to eject the cut parts from the tooling. 

For films thinner than 0.375 mm, we recommend using a 2 point or 0.7 mm rule.  For thicker films, use a 3 point or 1.0 mm rule. Using rubber strips on either side of the die rule, aids the ejection of the part.

A number of different rules, such as the center bevel rule, the facet bevel rule, and the side bevel, can be used in cutting polycarbonate films.  In most cases, you should order a single bevel blade with the bevel to the scrap side.  Usually a rule with a bevel on both sides tends to distort during die cutting. The only time to order a double bevel blade is when you are  cutting really thick sheet, such as 60 mils or thicker.

The unique physical properties of polycarbonate make it more difficult to cut than other plastics. Rather than shattering, polycarbonate absorbs the impact of the die and tends to “push back” when pressure is applied. As the plastic pushes back, it can distort the rules, as well as distorting the part being cut.  To aid in cutting polycarbonate films, a long ground bevel edge on the rule will minimize the resistance of the plastic film.

Die cutting polycarbonate becomes more difficult as the plastic film becomes thicker.  Cutting 15 mil and 20 mil polycarbonate can be especially challenging, compared to cutting 10 mil product.  When films are laminated with a transfer adhesive, the cushioning effect of the adhesive also adds to die cutting difficulty.

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About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.

© 2015 Jim Hingst

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