Screen printing in another dimension
By Tim Wright
The acronym "IMD" is certainly a buzzword around the industry lately. But many people are confused about what "in-mold decoration" means and what opportunities it might offer to screen printers. The confusion is not surprising. There are many ways to decorate a molded product, some of which are classified as IMD, and others that are not. Few of these processes have much to do with screen printing, but where screen printing is used, some exciting opportunities exist.
In the most general terms, in-mold decoration means exactly what it says. Any technology which applies text, patterns, and graphics to a molded part, as part of the molding process, deserves to be labeled IMD.
Imagine an automotive heater/ventilation control panel. Let's say it consists of three controls recessed into a 3-D molding. Around each control is legend text or symbols, usually in several colors on a colored, black, or simulated woodgrain background. The entire graphic may be part of a backlit display. It may also incorporate a polished metal trim or an area matched to the color of the vehicle.
How are these effects achieved? The designs may be applied after the part is molded (post-mold decoration) or during the molding process (IMD). Decorative graphics may be applied directly to the molded part or as transfers--similar to the concept used by garment printers to decorate T-shirts. Because there are so many ways to decorate a molded part, it is worth the detour to summarize these competing technologies. So, before we delve into the most promising IMD screen-printing method, let's define its alternatives. Post-mold decoration
Laser etching In this process, continuous layers of ink are deposited onto the surface of the molding, and then cut back by high power laser to revealed the required colors. This method produces very good results, but is slow and not recommended for high-volume applications.
Appliqué (inlay) This takes place when a printed, self-adhesive label is applied to the molding. This method commonly relies on the screen-printing process and is used in automotive and appliance applications. It is inexpensive and easy to produce, but the label can be peeled and tends not to look like an integral part of the molding.
Aquagraphics In this transfer method, the image is printed onto a water-soluble film. The film is then floated on water until it dissolves, after which the molded part is brought up through the water and into contact with the remaining graphic image. The woodgrain effects produced with this method are impressive, but post-treatment with a protective spray is essential. This method's costs are generally high, and some industry contacts report poor conformance rates resulting from the process.
Pad printing Because of its ability to conform to irregular surfaces, pad printing also is used frequently to decorate molded parts.
Multi-shot, multi-color molding Here, several differently colored polymers are combined into a single molded part, using a highly complex injection tool. The main body of the part is molded, creating the first color. Then the tool is automatically reconfigured to allow a second, colored polymer to be injected into specific recesses to create legends. Third and fourth colors may follow. This method is very suitable for high-volume production and backlit effects. The results of this technique can be impressive, but the downside is the high cost of tooling and the associated costs of responding to design changes.
In-mold foils (transfer process) Polyester-based films (foils) are generally gravure printed by the supplier, simulating woodgrain or other patterns. In the automotive industry, these foils are especially useful for simulating chrome plating or matching a paint color. The image is transferred to the molding during the molding process.
Printed transfers This is a process developed by the Japanese company, Nissha. A release-coated carrier film is printed with a pattern layer; then a transfer adhesive is coated or printed over that, and the image is transferred to the finished part by running the printed film into the injection mold. A hardcoating can also be applied to the carrier film before the pattern is printed. This enhances the durability of the print and can be important, as the printed image will be on the exposed surface of the part and is vulnerable to wear. This method typically does not produce an image that possesses sufficient density for backlighting.
The real opportunity for screen printing
The decoration method for 3-D parts is the one where screen printers can start to really get interested again--insert molding. In its simplest form, this involves a small, flat label which is positioned in an injection mold and bonded in place during the molding process. At its most advanced, insert IMD uses a hardcoated, formable film which is screen printed on the second surface. A selective texture is then applied to the first surface, and the film is deep formed (thermoformed) to fit the mold cavity.
During the molding process, the film forms a complete skin over the surface of the part, complete with the decoration, protective coating, and/or texture. Between these two extremes lie a multitude of possibilities. A flowchart depicting a typical insert-mold decorating process is shown in Figure 1, below.
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Insert IMD is of most interest to industrial screen printers because the number of applications for it are only just beginning to be realized, and it possesses real advantages over other methods:
1. Insert IMD is capable of the deepest formed applications.
2. It can also be used to apply images in close register to the mold profile--within ±0.2-mm positional tolerance.
3. Images can be applied as second-surface prints, where the printing is encapsulated within the finished part, providing complete protection against wear.
4. Selective first-surface printing can also be used to apply gloss or texture variations.
The best news of all is that right now, screen printing is the preferred imaging method for insert IMD. One of the key reasons for this is the adaptability of the screen-printing process, which makes short-run customization possible. The most important advantage that screen printing offers is the opacity of the inks. This allows for the production of stunning, pinhole-free backlit effects, similar to those seen in automotive dial applications.
Applications for insert IMD lie in the automotive industry, as in the example above, but also in appliances, telecommunications, computers, personal electronics, and other products--into areas where screen printing has had limited penetration.
Table 1, below, shows the enormous flexibility of this technique. Not stated in the chart is that the costs of insert IMD are significantly lower than most post-mold operations, due to better process integration and reduced wastage and labor costs.
|Table 1 Mold Decoration Processes and Their Capabilities|
|Process||Form depth||First/second surface decoration||Backlighting possible?||Legend text in register||Continuous pattern capability||In-mold or post-mold|
|Multi-shot molding||Any||Integral||Yes||Simple symbols only||No||In-mold|
|Printed transfers||Low||First||No||Yes (long runs only)||Yes||In-mold|
|In-mold foils (transfer)||Low||First||No||Yes (long runs only)||Yes||In-mold|
|Insert molding||Any||First or second||Yes||Yes||Yes||In-mold|
All right, now we can see the advantages of insert IMD. Let's take a closer look at how the process works and what materials are required.
1. Printing The first step is very much like printing a membrane switch overlay. The image is printed, first- or second-surface, onto a polycarbonate or polycarbonate alloy film (Figure 2). Good quality film is essential, with controlled gauge (thickness), clarity, and minimal internal stress.
2. Forming The printed film needs to be formed to fit the profile of the injection mold. The forming method is completed through a simple vacuum or thermoforming procedure, or through a sophisticated technique such as pressure-assisted vacuum forming or high-pressure forming (Figure 3).
3. Molding Once the printed part is formed, it is trimmed to fit the mold cavity. It is then manually or robotically inserted into the mold (Figure 4). The mold closes to hold the part in place and the melted polymer is injected, coming in contact with the back of the part. The polymer fuses onto the back of the film, and the materials are bonded together. The part is now complete and no post-processing is required (Figure 5). The final product, a cellular phone in these examples, displays the color, dimensionality, and tight registration that has made IMD such a sought-after manufacturing process (Figure 6).
High temperature molding resins are typically required to achieve a good bond. Polycarbonate, ABS, and blends are most commonly used. It can be helpful to source film and resin from the same supplier or supplier alliance, in order to get the best possible technical backup.
Should you explore IMD?
If the insert IMD technique works so well and presents so many opportunities, why are so few screen printers taking advantage of it? The biggest barrier to adoption is the need for access to proficient forming and molding capability as well as screen printing. Injection molders may have forming capabilities, but they rarely have screen-printing lines. Screen printers rarely do forming or molding.
This state of affairs has not come about by accident. Because high-quality screen printing is far from a turnkey process, it is difficult for a molder to get started. Injection-molding equipment comes with a high capital cost; it is unusual for a screen printer to justify the risk of this kind of investment. Some pioneering printers have taken the leap and brought in forming and molding equipment, but generally, the easiest way into the technology is to form alliances between top-notch printers and molders. Suppliers and OEMs are generally happy to broker such deals.
Beyond this, even for skilled technologists, technical barriers still must be overcome before IMD becomes a routine operation. We will take a walk through this minefield and try to pinpoint the areas where things can go wrong. Then we will look for ways to get around them.
Three steps to IMD
If you print dial appliqués or membrane switches, you may consider establishing an in-mold decorating operation in your shop. Though the goal is reachable, there are three primary issues to consider, including choice of inks, image design, and ink drying or curing methods.
Choice of inks For first-surface printing, the choice of inks is straightforward. The main issue is formability of the ink, but you also need to consider whether the print is durable enough in use. For many applications, some form of protection is required over the ink.
On the other hand, if you want to take advantage of the high durability and color saturation of second-surface decoration (and most do!), then the issue of melt damage rears its ugly head. Remember, the ink lies between the film and the molten resin. Very few screen-printing inks can withstand the high temperatures and shear stress during injection molding without washing out.
The biggest problems occur around the "gate" area, or melt injection point of the resin. The number of gates varies with the design of the part. Specific effects of the gate on ink selection and image design will be discussed below.
The simplest way around printed-ink failure is to protect the printed ink with a second layer of film, the so-called two-layered method. Most of the early IMD parts were made this way using a specially formulated polycarbonate with a heat-activated adhesive coating. The method is effective and enables most high-opacity screen-printing inks to be used. The downside is high cost, plus this method carries the risk of delamination should the adhesive fail.
The only real way to eliminate the second film layer is though ink selection. Some conventional inks are better than others for IMD applications, and in areas far removed from the gate, the inks are usually adequate. In areas close to the gate, however, few conventional inks can stand up to the high temperatures of the melt. Here, it is necessary to look at special melt-resistant inks. Several manufacturers are focusing on the development of these inks, which fall into three general categories: thermal-cure inks UV-curable inks high-melt-resistant polymers
In general, the thermal-cure inks resist the melt very well, but require long curing cycles and may not provide the best adhesion to the melted polymer resin. UV-curable inks tend to offer good melt resistance, but any residual monomers must be baked out to prevent problems in the mold. It is also important to ensure that the opacity of these inks is adequate for the job at hand. The melt-resistant polymer inks can be tricky to print, but otherwise give good all-around performance. These use patented chemistry, so don't expect too many clones in the near future.
Some manufacturers are experimenting with printable barrier coats. This appears to be developing into a good solution, and although early prototypes have limited thermal resistance, some do enhance adhesion onto thermal-cure inks.
Image design The design can help reduce the challenges presented by the ink. For example, it is helpful to extend the printed area into a gate so that the ink flows in to replace any ink that is washed out by the hot resin. The placement of graphics is also a critical issue. It is important to position legends and symbols in areas that will remain fairly flat in the finished part. These images should also not be placed too close to sharp, formed edges. This will make forming in register a little easier. We will discuss this further when we look at forming methods.
Ink drying Finally, we all understand the importance of adequately dried or cured ink. In IMD, this is an especially critical factor. Any residual solvent in an ink layer will be trapped in the injection mold and will vaporize. The consequences for ink adhesion are pretty clear--and very dire! For this reason, even when a thorough jet drying cycle is used, it helps to allow the printed parts to stand around for awhile to "air out." This will eliminate all traces of residual solvent. The longer the prints sit, the better the result will be--up to several days where practical.
One of the most critical areas in IMD is the accuracy of print-to-mold-profile registration--one of the primary differences between IMD and traditional thermoforming applications. Because of this, don't make the mistake of thinking any old thermoforming machine will be useful in establishing an IMD system. Instead, seek out IMD-specific thermoformers, such as the one depicted in Figure 7.
Take a look a traditional thermoforming machine. Where are the register pins? There aren't any. It is one thing to form a Darth Vader mask, where the image needs to be within a ±0.5-in. tolerance, but it is quite another thing when the automotive industry demands legend placement within ±0.2 mm. This is an insurmountable challenge for traditional thermoforming, even if the sheet is held precisely in register with the tool.
Uniform heating of the sheet is essential, and because it is necessary to heat the sheet above its softening temperature (typically 140°C for polycarbonate), the film will tend to sag before forming, thus distorting the image. Some manufacturers have addressed these problems with highly engineered equipment. The key points to look for are: register pins precision engineering standards small IR heater units--the smaller the individual heating elements, the more precise the temperature control will be sag control--this can be called the "zero-gravity" method, where air pressure under the sheet prevents sag, or alternatively, a light beam interlock triggers the vacuum when the film sags by as little as 1 mm strong vacuum pull, with some control over the rate of pulldown sequenced vacuum pulldown in different areas of the tool can be useful in maintaining register pressure- or plug-assisted thermoforming can be useful for some types of parts
Another technology used in IMD is hydroforming. This process was originally developed by Hy-Tech (Phoenix, AZ) and involves forming the film over a tool, using high-pressure oil contained behind a polyurethane membrane. Good results are achievable, provided the materials can withstand contact on the first surface during forming. Recent developments of this technique enable the oil to be heated, which provides very good form definition.
Yet another specially developed technology that has gained ground in the last few years is high-pressure forming. This technology is similar to the more conventional "pressure-assisted" thermoforming except for a couple of minor details. The air pressure used in the forming process can be as high as 300 bar, and the air can be heated to a temperature of 150°C. The enormous pressure means that the film can be formed below its softening point, which can improve the print-to-profile registration. This method is also very effective in achieving excellent form definition using the two-layer method described earlier.
The importance of the mold
Another important concern to consider with IMD is the mold design. If you get this right, the printing and forming become almost trivial issues. Get it wrong, and the part can become impossible to manufacture. Here are the main points: Always design the tooling for IMD. Never use an existing tool for this process. Wall thickness is very important, especially near the gates. Thin walls near the gates cause extreme shear stress on the inks, guaranteeing damage to the part. Design the mold with the walls as thick as possible in theses areas. Position the gate, or gates, in areas not containing graphic elements. If this is not possible, do everything possible to minimize the shear. Your film and resin suppliers can be helpful in planning the design with you. Design the mold with gates that assure even flow of the melt across the part. Turbulence can mean death to the inks. Avoid deep vacuum draw close to graphic elements. Use reverse part ejection. Gas venting must be from the back of the tool.
Apart from mold design, trimming the part to fit the mold is the only other real issue. For low-profile parts, simple diecutting is all right, but as the profile increases, the tooling becomes more problematic, as it is essential to support the part during the cutting operation. Hard tooling can be used, and for the most complex jobs, laser trimming is a good solution.
With a good mold tool and a well-trimmed part, the rest of the molding operation is relatively straightforward. Insertion of the film into the mold can be manual, or it can be carried out automatically using the same robot handling equipment that removes the part from the mold.
Since this article is a general summary of the IMD process, a few new developments are also worth a mention before we wind up. We've discussed the developments in ink technology and forming methods that have progressed the commercialization of IMD. In contrast, most of the films and resins have been fairly familiar. Currently, the state of the art in this area is similar to that in the early days of membrane switch manufacture.
Imagine if today we were making membrane switches without the variety of textured and hardcoated films and the selective printable textures and coatings. In the IMD area, products similar to these are well along in their development and will open the doors to a much wider range of design opportunities and applications. Watch for developments in those barrier coatings too.
IMD takes screen printing into the third dimension and creates huge new opportunities for those enterprising enough to form the right alliances and make the investments in time and financial resources. This is one application for screen printing that we can say is just getting started.
Tim Wright is the research and technical manager for the industrial film systems division of Autotype International, Wantage, England. In his 17 years with Autotype, he has managed research and development of masking films, stencil emulsions and films, chemicals, and membrane-switch products. Mr. Wright is currently active in the company's development of systems for IMD, in partnership with GE Structured Products.
In-mold decoration suppliers and manufacturers
Suppliers and manufacturers of IMD supplies and equipment are valuable resources for assistance in all areas of the process, from setting up to sourcing tools and designing projects.
Resins and Substrate Films:
Manufacturers and Specialty Coaters
Autotype Americas, Inc.
2050 Hammond Dr.
Schaumburg, IL 60173
Bayer Corp., Polymers Division
100 Bayer Rd.
Pittsburgh, PA 15205
800-662-2927, ext. 3018
GE Structured Products
1 Plastics Ave.
Pittsfield, MA 01201
16700 Lincoln Ave.
New Berlin, WI 53151
Battenfield of America Inc.
31 James P. Murphy Industrial Hwy.
West Warwick, RI 02893
10901 Dutton Rd.
Philadelphia, PA 19154
215-632-2300, fax: 215-698-8130
HDVF Niebling Jr.
Seeshaupter Str. 58
Penzberg, 82377 Germany
Nissha Offices: Kyoto, Japan (headquarters)
San Francisco, CA
High-Melt-Resistant Polymer Ink
PO Box 429
Weisenburg D-91773, Germany
49-9141-90619, fax: 49-9141-90649
Forming Equipment and Technology
Cannon Thermoformers International Ltd.
Stonehill, Stukeley Meadows
Huntington, Cambs, PE18 6DR
Hy-Tech Forming Systems Inc.
2329 W. Mescal #303
Phoenix, AZ 85029
9705 Commerce Pkwy.
Lenexa, KS 66219
913-541-0066, fax: 913-541-9088
HDVF Niebling Jr.
Seeshaupter Str. 58
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