Sericol Plastics, in all their forms, have transformed our lives in ways unimagined when wood and metal were the only fabrication materials available. Plastics can have a high tensile strength, but still remain easy to bend and shape. Low temperatures can be used to thermally manipulate plastic and form curves, angles, or entirely new shapes. Plastics also are easily cut and machined, and they can be decorated with a variety of print methods. In the 100 years since they first became a commercial reality, plastics have had a positive impact on technological development, and they now find application in virtually every facet of our lives.
Decoration of plastics to make them attractive and/or functional has also been an area of progress over the years. Screen printing has played an important role in the development of plastics technology, and advances in plastics themselves have led to the evolution of new screen-printing ink systems.
Among the most common applications for plastics are those that involve changing flat sheets of material into three-dimensional objects through a process known as vacuum thermoforming. This article will introduce the basic components and procedures involved in vacuum thermoforming plastics and explain how screen-printed graphics and inks are incorporated in the formed objects. Particular attention will be paid to UV-curable ink systems and the benefits and constraints they impose on plastic decoration for thermoforming.
The process of vacuum thermoforming plastics developed into a commercial reality at the end of the 1950s. Originally intended as a low-cost method for creating pre-production proofs and prototypes, the process has grown over the last 50 years to become an accepted system for producing large quantities of commercial-grade products. Today, thermoformed products feature high-quality decoration and intricate shapes that belie the overall simplicity of the vacuum-thermoforming process (Figure 1).
Early examples of formed articles were often left undecorated after forming or occasionally painted or decorated with transfers. Such decoration was always applied after the forming process, due, in part, to the fact that no suitable processes or materials were commercially available to decorate the flat material prior to forming.
However, as the thermoforming process evolved, so did the screen-printing process and screen-ink technology. By the early 1970s, solvent-based screen inks were available that could withstand the temperature and stresses of thermoforming. These inks saw continued development and increased use for the next two decades. In the past few years, however, the focus of ink development for thermoforming applications has moved from solvent-based formulations to UV-curable inks. Once thought to be unusable in forming applications, UV inks have come to revolutionize the production of vacuum-thermoformed products.
Plastics for thermoforming
Not all plastics are created equal, and some are better suited to thermoforming than others. For the purposes of this discussion, we’ll categorize plastics into two main groups: thermoset and thermoplastic.
Thermoset plastics undergo a chemical reaction when exposed to heat, pressure, catalysts, UV energy, and/or other conditions. The exposure leaves the plastic infusible and permanently formed. Thermoplastics are of the most interest to companies engaged in thermoforming because these materials have the ability to repeatedly soften when heated and harden when cooled.
Thermoplastic materials themselves fall into two categories: amorphous and crystalline. Amorphous thermoplastics have a less critical forming temperature than crystalline thermoplastics and are therefore preferred by thermoformers. Typical amorphous plastics are ABS, acrylics, and polystyrene, all of which are commonly printed by the average graphics screen shop. Typical crystalline thermoplastics are polyethylene and polypropylene, which rapidly change state when heated and can quickly become molten, making them more difficult to process.
Plastics soften when they are heated. The ideal temperature to which they should be heated for forming is known as the glass transition temperature (Tg), which varies from material to material. At the Tg for any given plastic, the material is easy to bend, form, and work into new shapes. If this temperature is exceeded, the material starts to behave more like a liquid than a solid, and controlling its shape becomes difficult. Amorphous thermoplastics have much wider latitude between their Tg and the temperature at which they become liquid, which makes them easier to process.
The forming cycle begins by preheating the plastic sheet until it reaches its Tg and attains a pliable state. The actual temperature at the forming stage is much higher than–often close to double–the Tg, but applied for only a short time. The actual forming temperature and duration is dependant on the type of plastic, as well as the thickness of the material. As material thickness increases, so do the times for both preheating and forming. Table 1 outlines Tg values and forming temperatures recommended for several common thermoforming plastics.
|Table 1 Processing Temperatures for Commonly Thermoformed Plastics|
|Material||Tg (°F)||Mold temp. (°F)||Form temp. (°F)|
|ABS||200||180||200 – 350|
|Acrylic||220||180||280 – 370|
|Polystyrene||200||180||300 – 350|
|Polycarbonate||300||250||350 – 400|
|PETG||190||180||350 – 500|
The selection of a particular plastic material for producing a formed item is based on a number of factors that are influenced, but not limited, by the end use of the item. The material selected can have a considerable bearing on the decorating options available.
Frequently, the forming and decorating functions are handled by separate companies, and the decorating stage is considered long after the material has been selected and the final shape of the item has been decided by the thermoformer. For screen printers, it’s imperative to be an integral part of the process early in the planning and design stages. To reduce the likelihood of incompatibilities between forming and decorating requirements, an increasing number of screen printers are adding thermoforming capabilities to their own operations.
Some examples of commonly formed plastics and the applications for which they are suited are listed in Table 2. As this table shows, the vacuum-forming applications for the materials matches several market segments in which screen-printing companies are already active. This fact reinforces the synergy between the thermoforming process and screen-printed decoration of the parts.
Vacuum thermoforming is commonly used to produce objects as varied in design and function as hot tubs, auto components, and signage and promotional displays. For graphics screen shops, the formed promotional items are the category of greatest interest because they have the most in common with the flat-sheet printing applications in which the printers regularly engage. The decorating process employed for thermoformed displays also uses inks and production techniques that are familiar to graphics screen printers.
|Table 2 Common Applications for Thermoformed Plastics|
|Polystyrene||Low-cost P-O-P displays|
Vacuum forming basics
Before we discuss the inks and print procedures used for decorating materials that will be vacuum formed, let’s review the basic elements of the thermoforming process. This review will help clarify how and where the fundamental techniques involved in vacuum thermoforming can create problems for screen printers.
The simplest technique for thermoforming is to heat a sheet of plastic material to a temperature above its Tg and then drape or draw the pliable sheet over a mold, which itself may or may not be heated before or during the forming process. The mold can be made from any material that can withstand the heating/ cooling cycle without experiencing changes in dimensional stability. Typical mold materials are wood, urethanes, metal, and ceramics. The next step uses air blown over the plastic sheet to force the plastic onto or into the mold. This step helps the pliable plastic sheet better fill narrow and detailed areas of the mold.
This simple molding technique is satisfactory for simple shapes, but it is unable to accurately reproduce more demanding and intricate shapes. To overcome this deficiency, a vacuum-thermoforming machine is needed that can apply vacuum force to the preheated plastic sheet and fully pull the material over the mold.
With a vacuum-forming system, plastic is firmly clamped into the machine, heated, and usually prestretched approximately 25% by blowing air into the vacuum plenum below the film, which causes the plastic to bow or balloon. The mold form, contained within the plenum chamber, is then pushed into the pre-stretched sheet and, after a short pause, vacuum is applied to completely draw the sheet into the most intricate parts of the form. Afterwards, the form is moved away from the sheet, which is cooled with air prior to its release from the forming unit. Figures 2A-2G describe this process in more detail.
Because vacuum-formed pieces are three-dimensional, printing must be performed on flat plastic sheets prior to forming. This creates a number of processing and performance issues for the printer and ink manufacturers. The following section will deal with the most important of these issues and discuss the printing techniques and inks available to overcome them.
Figures 2A-2G The Vacuum-Thermoforming Process
Ink and printing concerns
Screen-ink manufacturers are often asked to develop inks and coatings to meet unique and unusual end-user requirements. However, creating an ink that can be heated, stretched, bent, folded, and cooled and maintain its color and cohesion (ability to resist cracking) during and after the forming process is generally a tall order.
Early attempts to create inks for this application used solvent-ink technology. These inks were formulated using resin technologies similar to those employed in the plastic sheets themselves, namely acrylic and vinyl resins. Solvent-based inks dry by evaporation and leave a thin, lightweight deposit that can be formulated to provide a high level of cohesion. This combination of thin deposit and strong cohesion imparts flexibility, which means that the ink film can be stretched and deformed considerably without failure, such as cracking or the appearance of stretch marks. The resins chosen also tend to be inherently elastic, so a typical solvent-based thermoforming ink can be drawn to at least 10 in. without failure. These inks also support sophisticated designs (Figure 3).
While solvent-based inks continue to satisfy many print requirements, legislation surrounding solvent formulations has become more restrictive, forcing ink formulators to look at other technologies. Until recently, the concept of a UV-cured, deep-draw formulation was generally dismissed. The perception was that UV inks produced a thick ink deposit that became too inflexible after curing, which would lead to insufficient cohesion and cracking when even the slightest draw was applied. The alternative was to under-cure the prints in an attempt to promote greater flexibility, but this threatened to make the process uncontrollable in the average shop, where it could lead to problems such as stack blocking of the printed sheets.
The solution to this problem came from fully understanding the conditions that occurred during the vacuum-forming process and developing an ink that used these conditions to its advantage. Under normal conditions, UV inks fully cure when exposed to UV light of the correct wavelength and intensity. The curing process sets up internal stresses in the ink film that affect the flexibility of the film. Coatings that are too hard have high stress levels and crack under the temperatures and pressures of forming. Figure 4 shows typical ink cracks seen on a deep vertical draw. Such defects occur most frequently around intricate bends.0
Overcoming stress in the cured ink film was essential in producing UV inks for thermoforming. Of equal importance was maintaining color integrity in drawn or bent areas of the print by avoiding the washed-out appearance created when the ink-film stretches beyond its elastic limit. The use of high-quality, automotive-grade pigments that resist physical and chemical attack also proved useful in helping these inks maintain color integrity.
The greatest challenge was creating a soft ink film that would follow the extrusion profile of the plastic but also would attain a cure level prior to forming that would allow the printed piece to resist blocking and be handled without print cracking, chipping, or other damage. The printed ink also would have to resist the effects of heat and pressure during forming and avoid excessive softening and smearing.
The final solution involved producing a coating that was touch-dry after UV curing, but was not completely cross-linked at the end of the normal curing process. While the ink is completely dry after curing, total through-cure is not achieved, and the ink film has a lower level of internal stress, allowing it to remain flexible and follow the extrusion profile of the plastic during the forming process. After initial curing, the ink is touch-dry and strong enough to resist normal processing damage; however, the ink film is actually still very weak. To attain a full cure and the strength and durability that comes with it, the ink relies on heat energy from the vacuum-thermoforming process to completely crosslink. The result is an ink film with all the desired resistance properties for the final product.
Shifting production into high gear
As mentioned previously, increasing regulatory pressure has led printers and ink manufacturers to move away from solvent-based inks. However, more than just legislation is at work here. Solvent-based coatings dry by evaporation, a process that is inherently more time consuming than curing of UV inks, which crosslink rapidly in the presence of UV energy. For printers seeking to optimize productivity in forming applications, UV inks provide a clear-cut advantage.
For the vast majority of graphics screen shops, UV inks are already a staple of production, and many companies depend exclusively on UV-printing and curing equipment, including high-speed multicolor UV print/curing systems. Relying on compact UV-curing systems rather than giant convection dryers for solvent inks reduces energy consumption and gives printers room to broaden their production capacity with additional presses. UV-curing technology both increases the speed at which graphics for thermoforming can be produced and substantially expands the production volume that a screen-printing operation can support.
Print, cure, form
Combining UV inks, multicolor presses, and sophisticated modern forming machines has increased the complexity of print designs and molded shapes that can be achieved with thermoforming. Another key advantage that UV technology brings is the ability to fully automate the entire production process, from printing through forming. With the right combination of equipment, forming even can be added as a direct post-printing process, with prints moving directly from the final curing station into the vacuum-thermoforming system.
A growing number of screen shops are adding thermoforming technology to complement their screen-printing capabilities. Rather than working with a separate forming company, which increases the chance of image-to-mold alignment problems and other processing errors, these screen shops maintain complete control over the entire printing and forming process. This gives them the flexibility to quickly produce prototypes so that they can predict how images will distort during molding and adjust the artwork accordingly (a process that involves printing grids and measuring how the patterns change due to forming).
Having the entire process under one roof allows printers to fine tune their procedures and deliver accurate and intricate thermoformed products faster than if thermoforming were outsourced. And when the print shop is also the forming shop, it becomes much easier to accommodate design modifications, seasonal product changes, and other adjustments or alterations requested by customers. Thanks to advancements in UV inks, screen shops are in an ideal position to expand their production capabilities and make vacuum thermoforming a profitable component of their businesses.
About the author
Steve Pocock is senior technical service manager for Sericol North America. A 25-year veteran of the screen-printing industry, Pocock was previously employed with Sericol India. He is a polymer chemist by training and has held a number of research and technical positions with Sericol during his career.
“Mold Making,” C.R. Clarke, C.R. Clarke & Co., Article # 2050, SGIA Technical Guidebook. Plastics Reference Guide, 1999-2000 edition, Regal Plastics, Littleton, CO. Formech Thermoform Guide, Formech Int’l, Harpenden, UK. “Screen Printing for Plastics,” Paul Scheretter, Article # 2051, SGIA Technical Guidebook. “Vacuum Forming,” C.R. Clarke, C.R. Clarke & Co., Article # 2306, SGIA Technical Guidebook.
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