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In-Molding the Details

Tue, 09/21/2010 - 11:47am
Al Hoeschele and Todd Geisser

Providing easy to follow instructions on a medical device can be a significant challenge; when coupled with ensuring the solution does not create cleaning concerns and will not wear, that obstacle to success becomes even greater. This article examines the benefits of in-mold decorating/labeling for medical devices and how they resolve these, and other, challenges.

 

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Decorated insert can be a flat overlay or formed part.
While in-mold decorating/labeling (IMD/IML) is a relatively new technology to medical products, it has been used extensively in other industries, such as automotive interior, appliance, and hand-held consumer markets. Recent improvements in film and forming have made this technology applicable for new innovations in hospital and consumer medical devices. These innovations include hard-coated and formable hard-coated polyester films, optically clear nylon film and improvements in touchscreen technologies.

IML has given designers greater latitude than ever before by allowing more designs with a wider variety of complex curves and shapes that are more in tune with the needs of physicians and patients.

Chemicals required in the sterilization process of medical devices for super bacteria and viruses are very harsh and are driving new durability requirements for film, surface inks, and coatings. Many times, traditional methods of product decoration, such as hot stamping, heat transfers, pad printing, and painting, are not able to meet the new validation requirements for medical devices. Pressure sensitive appliqués allow foreign materials to contaminate the edges, and eventually areas behind the appliqués, making complete sterilization impossible. IMD/IML has substantially improved the chemical resistance of these devices by creating perfectly sealed surfaces with almost no entry possibility for super bacteria and viruses.

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Insert is placed into the mold cavity and injected with molten resin.

IMD/IML makes it possible to include repeatable registered graphics, optically clear windows, and a wide variety of color and design variations. Conductive entities, such as RFID, EMI shielding, antennas, and capacitive touchscreens can also be added.

 

Economical and Environmental Aspects
By integrating design, graphics, and conductive features into the molding process, secondary operations, such as pad printing and spray painting, are eliminated, which reduces the possibility of scrap from these operations. Additionally, space savings for the total device can be realized. The use of preprinted and color-matched graphics eliminates the need to purchase pre-colored resins, which can cut costs 5 to 40%.

IMD/IML can be considered a green process since the use of similar film and molding resins makes the components recyclable and enables them to meet the requirements of international recycling regulations, such as the European Union’s RoHS and WEEE. Additionally, printing is much cleaner and emits lower levels of volatile organic compounds when compared to other application methods, such as spray coating technologies.

 

The IMD/IML Process

 

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Finished decorated part with perfectly sealed surfaces
The IMD/IML process for medical devices begins by selecting a film that meets the required durability, flexibility, chemical resistance, and hardness properties. These requirements include specifications provided by the customer for their particular application. Without properly defined specifications, incorrect film and ink selection may result in later performance failures. Once the film has been selected, it is decorated through a printing process. Ink selection is based upon the performance specifications. These can include opacity, transmissivity, IR and RF transparency, chemical resistance, elongation, adhesive strength, bright metallic graphics, and conductivity requirements of the ink/film combination.

In most cases, printing is performed on the second surface of the film. The second surface is where the graphical insert image is viewed through the transparent film on which the image is printed. The molding resin bonds directly onto the printed image and the surrounding surfaces of the film. Second surface printing increases abrasion, scratch, chemical, and UV resistance by encapsulating the graphics between the film and the molding resin. With this method, the properties of the film define its resistance to chemicals and abrasion.

Forming is the next process after printing. Three-dimensional forming methods include vacuum, hydro, pressure, or combinations thereof. Proprietary ink and coating systems have been developed to withstand the temperatures and elongation associated with forming and molding.

After printing and forming, the insert is die-cut to fit the mold. The die cutting operation can be automated depending on the size of the part, the application, and the production volume. The insert can be flat or a 3D overlay that is ready to be inserted into the molding tool cavity.

During the in-mold process, the decorated insert is placed into the cavity or onto the core of an injection mold that has been specifically designed for in-mold decoration. The desired molding resin is shot behind or over the insert, bonding its surface to the molding resin and forming an integral finished part.

 

Advantages for Medical Products

The durability, particularly of second-surface insert decoration, is well beyond that of other decorating methods. The graphical insert is permanently embedded in the product and never exposed to its surrounding environment. This contrasts with methods, such as labeling and direct printing, in which the inks and coatings are directly exposed to chemicals, dirt, human or mechanical handling, UV light, and other potential damage.

The in-mold process often produces significantly less scrap than other methods, such as adhesive labels, because the decorated insert is shaped to fit precisely into the mold. The position or registration of the graphic is highly consistent and permanent.

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In-mold decorated meters

During or after secondary operations, labels sometimes shift on a part or collect dust at their edges, making them difficult to clean. In-mold decorating eliminates those concerns because the decoration is fixed on or in the part with no edges. The components can be cleaned with a wide variety of chemicals depending on resistance of the insert film chosen. Resistance to cleaning solutions is particularly important in medical applications where many antibacterial and antiviral cleaning products are available. These include various alcohols, Virex TB, Liquinox, CaviWipes, bleach, Purell sanitizing wipes, and PDI Sani-Cloth HB.

 

Film Selection

The most appropriate sheet or film materials for printed inserts are those with higher surface energy to allow stronger and more consistent ink bonding. These include polycarbonate, PET, acrylic, PBT, PVC, and PS. However, success with some lower surface energy materials, and with those that are self-lubricating, is limited due to inter-film adhesion issues. With proper pretreatment, PP and PE have been successfully used in the in-mold process.

 

Molding Resin Selection

The most common molding resins used are polycarbonate, PET, SAN, PC/ABS, PC/PET, PVC, nylon, ABS, and acrylic. In general, the insert and backing resins do not have to be identical, but they must be compatible. If not, then a special heat-activated adhesive must be used to ensure inter-coat adhesion.

 

Best Applications

 

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In-mold decorated meters with optically clear windows
IMD/IML does have its challenges and limitations. It is less cost-effective if the decorated area is a small portion of the decorated surface. Up-front design considerations are of paramount importance for the process, including printing, forming tool, cutting tool, and injection molding tool design. In particular, graphic locations may need to be printed with distortion before the forming operation to allow the graphics to move into register during the forming process. While not recommended, some existing tools can be modified for IMD/IML. However, new tooling is almost always recommended and generally necessary. This is due to the fact that the existing tool gate configuration may not have been designed with IMD/IML in mind, which can result in gate wash and other molding issues.

With the correct design, film selection, and engineering, IMD/IML has become a process of choice in many applications today. It allows for a wide variety of cost effective, aesthetic, and functional medical designs. In addition, many IML components are recyclable, which reduces the possibility of post-consumer waste and stress on landfills.

Al Hoeschele and Todd Geisser are engineers at Serigraph, a printing, molding, and custom graphics company that specializes in decorative, functional, and brand identity solutions. Hoeschele can be reached at 262-335-7501 or ahh@serigraph.com, and Geisser at 262-335-7536 or tag01@serigraph.com
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