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An In-Depth Look at Distortion Printing

Some of the most impactful display graphics owe much of their appeal to the painstaking process of distortion printing. This article uses an actual job to describe the demanding workflow.

Take a walk through any large grocery store, mega-mart or shopping mall, and you are likely to see examples of printed and formed plastic used as key components in P-O-P displays and other types of product merchandising. Distortion forming is the name of the process used to create these displays, and the P-O-P industry has relied on it for decades. Liquor sales, sporting goods, fast-food, video games, and even Hollywood blockbusters—all have benefitted from the use of distortion forming. 

Distortion forming is the process of printing an image onto a sheet of plastic and strategically vacuum-forming the sheet on a mold so that specific areas of the printed image appear in corresponding areas of the resulting three-dimensional part. Computer software has made the process quicker and more repeatable, but overall manufacturing techniques have changed little over the years. To understand how the process works, we need to start in the middle. Let’s begin with a basic overview of vacuum forming.

Vacuum forming
Joliet Pattern, Inc. produces distortion-formed parts via screen printing and cut-sheet vacuum forming. In this method, individual pre-printed sheets (as opposed to roll stock), are delivered to the vacuum-form press. 

During the vacuum-forming process, a thermoplastic sheet is clamped tightly in a frame and heated until it becomes soft and pliable. At this point, known as the glass transition temperature (Tg), the sheet actually begins to sag (Figure 1). The sagging sheet is brought in contact with the mold, which contains minute holes (vac-holes) drilled through its outer surface, to a cavity beneath the surface. Vacuum is applied to the cavity, which, via the vac-holes, evacuates air trapped between the mold and the pliable sheet coming in contact with its surface. Surrounding air pressure forces the compliant sheet to conform to the mold’s every detail (Figure 2). After forming on the mold, the sheet is left in place to cool. Coolant, running through copper tubes embedded in the mold, carries heat away from the sheet, accelerating its cooling time. As the sheet cools, it re-hardens and retains the shape imparted to it by the mold. The sheet is pulled off of the tool and removed from the frame, completing the forming cycle. 
 

If an image is printed on the sheet prior to vacuum-forming, that image will stretch and distort with the sheet as it becomes malleable. To combat this distortion and to register specific image areas to targeted locations on the mold, the image must be manipulated or counter-distorted in prepress. A properly distorted image, while still unformed, may look more like road-kill than the finished 3-D part it becomes. This image manipulation is the core of distortion forming and will make or break a project.

A real-world example
Joliet Pattern recently completed a signage project for the Kentucky State Lottery that involved 400 two-sided, distortion-formed outdoor signs. Each sign consisted of two identical, 32-sq-in., vac-formed panels (front and back), bounded by an aluminum frame (Figure 3).
 

Substrate selection  A clear substrate is preferred for a project like this, because it allows us to print second-surface, which affords the screen ink a layer of protection from the elements and cleaning solvents. We chose polyethylene terephthalate glycol (PETG), a clear thermoplastic, for several reasons. It is strong and rigid, has fairly high impact strength, weathers well, UV-ink adhesion is rock-solid, and it vac-forms easily and predictably. Each sign panel must be rigid enough to be self-supporting, so we chose 0.10 in. as a starting gauge—thick enough to give the finished panel adequate mass, yet thin enough to allow mold detail to show through the substrate.

Tooling  We fabricate a production mold using the traditional method, starting with a male pattern made of wood. In this case, the use of male tooling places the finished sign panel’s front surface away from the tool, so the substrate’s naturally glossy surface will not be affected by contact with the mold’s surface.
 

To start the wood pattern, we digitally print a full-size paper proof of the customer-supplied artwork. We sketch areas on the proof to indicate raised plateaus on the finished piece. Individual plateau heights are also indicated. This hand-drawn map is sent to the pattern shop, along with a digital art file for pattern fabrication. Individual shapes matching the full-size map are cut out of wood and assembled. All vertical faces of the pattern are angled-in (draft) a minimum of 3° towards the center of the pattern. Patterns with draft angles less than 3° create potential problems during the mold’s casting process. Similarly, molds with draft angles less than 3° may provide sticking points that, during the vac-form cycle, impede the cooled part as it is pulled off of the tool.

The mold for our Kentucky Lottery sign panel is a sand-cast aluminum plate, mounted to a 4-in.-deep, wooden vacuum box. After the wood pattern is mounted to an oversized piece of plywood and painted with wood sealer, it is moved to the foundry, where it is pressed into a container of fine sand (called a flask). The casting sand is compacted around the pattern, which is then carefully removed, leaving a mirror-image cavity of the pattern behind. To keep the casting hollow, a plug (called a core) is inserted into the cavity to prevent it from filling up with aluminum and becoming solid.
 

We heat aluminum in a kiln while preparing the flask. The molten metal is poured into the cavity prepared in the flask. After the aluminum cools, the flask, which is constructed in sections, is disassembled, the sand knocked away, and the rough casting is ready for finishing.
 

We polish and sand the casting’s rough surface until it is smooth, much of which is done by hand. Our Kentucky Lottery sign-panel casting is basically a large, square plate with several raised plateaus arranged over its surface. This is a shallow part—about 1.5 in. from lowest to highest point. Sanding and polishing this mold is fairly easy. Next, tiny vacuum holes are drilled through the casting. In general, the holes are about 0.025 in. in diameter. They’re small enough to prevent hot plastic from drawing into them during production, which would create dimples on the plastic’s surface.
 

Heat transfers to the mold each time a sheet is formed. The mold becomes so hot after a small number of cycles (the forming and cooling of each sheet) that it will no longer adequately cool the plastic sheets that form on it. Not only is the plastic’s cooling time affected, but image location on the tool—its registration—also becomes uncontrollable. We use many inches of continuous 0.25-in. copper tubing in a serpentine pattern on the inner surface of the mold to prevent this build-up of heat. We insert thin wire segments into the vac-holes to prevent them from clogging during the next step.
We use a spray-metal process, much like blowing hot solder off of the tip of a soldering iron, to cover the copper tubing completely, thereby permanently bonding it to the casting in the thermally conductive metal. Once in production, temperature-controlled water will be circulated through the copper to cool the tool—just like the way antifreeze cools a car engine. The thin wire segments are then removed from the vac-holes, opening them up through the spray-metal inner layer of the casting.
 

The last step in mold fabrication is construction of the vacuum chamber beneath the casting. A 3-in.-deep box, sized to match the casting’s length and width, is constructed using thick lumber sidewalls and a birch-plywood-bottom panel. A 1.5-in.-diameter hole is drilled through the bottom panel and matching pipe-flange attached panel’s outside, creating a vacuum port. Two brass couplers are attached to a sidewall, to which the two ends of the coolant tube are plumbed. These are the coolant’s inlet and outlet. Internal joints are sealed with silicone, and then the casting is sealed and screwed to the completed vacuum box. In the final step, we check the vacuum box and the coolant line for leaks.
 

Image location and manipulation  We ordered the PETG stock at the time we began work on the tooling so the sheets would be ready and waiting for the completed mold. Ordering custom-run sheet is very important to the success of a distortion-forming project. For example, when heated in a vac-form press, extruded sheets stretch more across their length (the direction of extrusion), than their width (the transverse direction of the extruder). This differential can cause a lack of consistency from sheet to sheet. One way to minimize this issue, providing the extruder can accommodate it, is to call the sheet’s longer dimension its width—thus, its shorter dimension becomes its length. The sheet stock for our project is square, so we have a different problem—we must make sure each sheet is oriented the same way (relative to its direction of extrusion) from screen printing through forming.
 

We’ve asked our PETG supplier to add a scribe line to each sheet—another advantage to ordering a custom-run material. As the sheet is being extruded in a continuous line, a pen or stylus is mounted above the line, about a quarter of an inch in from one side and allowed to mark the stock as it flows past the instrument. After the material is cut into sheets, each one is marked with a scribe line that indicates the sheet’s extrusion direction. The scribe line becomes an orientation guide for our square sheet, delineating length from width and top from bottom.
 

The new mold is mounted in the vac-form press and we’re ready to begin. From this point forward, consistency is the prime objective. First, we must determine where on the sheet to print the image. Because of the way the sheet will sag during its heat cycle in the vac-form press, simply positioning the image in the middle of the sheet may not be adequate. Using Adobe Illustrator, we’ll create a new art file, sized to match the dimensions of the PETG sheet. Next, we create a grid with horizontal and vertical lines spaced 0.25 in. apart. The grid will cover the same area as the mold, which is about 2 in. less than the sheet size in each direction. The grid file is downloaded to the screen department, where it is used to create one stencil. The grid image is printed on four PETG sheets, with white ink for maximum visibility. We used Sericol’s 3-D series UV ink for this job because it’s designed for vacuum-forming applications. One gridded sheet is retained in our graphics/prepress department. Exact guide points used on the screen press are marked on the remaining sheets, which are then sent to the vac-form department.
 

At the vac-form press, the guide points are transferred from the gridded sheets to the pneumatic clamps surrounding the mold. This ensures that both machines will guide to the same three points relative to the sheet. 
 

We then bring the mold up to operating temperature by cycling scrap material until the coolant (water) being circulated through the mold’s cooling tubes reaches its target temperature—about 170°F, depending on the substrate. We keep the water within a 5-8° range by using a temperature-controlling machine called a chiller. We also manage external influences that contribute to misaligned images, such as changes in ambient room temperature, humidity, and even stray breezes created by fans in the vicinity of the vac-form press. These factors must be noted at the beginning of the process and kept as consistent as possible throughout the production run.
 

The gridded sheets are vacuum formed as soon as the mold reaches operating temperature. After cooling, the formed sheets are sent back to the prepress department so the image can be properly located on the sheet. One of the sheets is sent off-site to aid in steel-rule-die fabrication. 
 

A profile view of the Kentucky Lottery mold shows it has a 1-in.-wide frame that’s raised about 1 in. above the plane of the perimeter cut line. Inside the frame, a white background is recessed about 0.25 in., and three distinct plateaus—each raised 0.25 in. above the white background—showcase the main graphical components of the art. The newly formed grid sheet is, in effect, a 3-D topographical map of this layout. Strategic x-y coordinates on the 3-D map, such corners of the frame, or key points of the plateaus, may be back-plotted to the original, flat grid sheet retained in the prepress department. The more coordinates that are transferred from the 3-D map to the 2-D flat sheet, the more accurate the registration of the printed image to the mold. 
 

Next, we send the combined grid with logo image file to our Lüscher direct-to-screen system so that we can create the first generation stencil. The logo image is composed of four colors (black, blue-PMS 300, green-PMS 375 and a double-hit flood-coated white background), so we’ll image and expose five stencils. We use a 350-thread/in. mesh for the black, blue, and green. We double-hit the white background to boost its opacity. We use 330- and 380-thread/in. mesh for the two screens dedicated to the white ink. The black screen combines the black components of the logo and the grid.
 

We’ll print six sheets or so for this first attempt. We haven’t removed the guide stops from our screen press, so these sheets—and all subsequent sheets—guide to the same stops as the original grid. Once printed, the sheets are sent to the vac-form press, which has been pre-heated in anticipation of their arrival. 
 

We cycle several scrap sheets through the forming press to simulate conditions of the production run, and the six first-generation sheets are cycled without pause. Generally, the last sheet of the group is sent back to the prepress department for image adjustment. We then compare the newly formed, first-generation part to the Illustrator file and look for areas of the image that do not fit the plateaus properly. 
 

Adjusting the image file becomes a matter of opposite movements. The grid, which was printed atop the logo image, helps to confirm the amount of movement necessary to counter misalignments, but it is not exact. Some guess work is involved. We address the misalignments and send the revised file to the screenroom again for the production of the second-generation screen set (Figure 4). This time, however, we’ll reclaim and remake only the black, blue, and green screens; the white flood-coat screens are unaffected by the distortion process and can be used again.
 

As before, five or six sheets of the second-generation image are printed and vacuum formed. We might be able to correct slight image misalignments at the forming pressby making small adjustments to the sheet’s position in the clamp frames, but going back to the art file to make corrections becomes necessary when substantial movement is required. Some distortion projects, due to image or forming complexity, may take six, seven, or even eight generations of image manipulations to correct. Ultimately, the Kentucky Lottery sign required five moves.
 

Adjustments to the image become too fine somewhere around generation three or four for the grid to be helpful, so we drop it from subsequent generations. Each generation normally takes about six to eight hours to complete from file manipulation to vacuum forming. A complex distortion-forming job may take as long as five or six days to complete.
 

A distortion job requires constant operator attention, no matter how careful we are to manage variables. Ultimately, the variables make for higher scrap rates projects, but a properly managed, dedicated run of extruded sheet helps keep rejects to a minimum. Joliet Pattern completed the Kentucky Lottery job, including installation, within 10 weeks of receiving client approval.
 

Have a comment about this article? E-mail it to the editors at screen@stmediagroup.com.

Andy Wood and Rick Turner
Andy Wood is owner and president of Crest Hill, IL-based Joliet Pattern, Inc. Rick Turner is the company’s senior project manager.

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