by Dave Schoofs, product development, Central Decal Company
A common subjective test to evaluate a bond is to cut an “X” through the IMD/IML insert and slightly into the molded resin. Then use a knife to attempt to peel the insert from the resin.
Over the past 30 years, in-mold decorating and labeling has become a common method of providing labeling, signage and user interface on many durable and consumable products. There are a few common benefits to IMD/IML and in-mold foils, including permanency of the graphic, reduced labor content, simplified supply chain and an overall impact on product design.
Considering why IMD/IML is used, the assumption of permanency and/or improved performance as compared to other decorating methods may not always be accurate. In addition, the IMD/IML performance requirements may not always be defined and/or verified for end use, specifically the colorfastness, durability and the overall adhesion of the IMD/IML to the molded resin.
Yes, many companies have developed engineering specifications and include measurable requirements and test methods. Other companies may reference specifications for traditional decorating methods or reference industry/market standards. These may include accelerated weathering, thermal cycle and chemical tests that require no appreciable loss of adhesion, fading, cracking or blistering.
When developing a performance specification for IMD/IML, what should one consider?
On a macro level, there are several attributes to evaluate before and after molding. As with any printed graphic, the printing technology, type of pigment and film deposit all affect color retention. In addition, an overprint clear or the face film may provide additional protection to UV degradation, oxidation, chemicals and abrasion.
Next consider the adhesion, not only of the IMD/IML to the molding resin but also consider the adhesion of the various layers within the IMD/IML. For IMD/IML, it is recommended to test the inks and coatings before and after molding, as the molding process can affect the performance of the inks and coatings on the IMD/IML.
Evaluating permanency or adhesion
First, it’s always a good idea to review the common IMD/IML constructions.
- First-surface IMD/IML – the image is printed on the face or “first surface” of the film. There are two variations to this version, with a printed clear or an over laminate.
- Second-surface IMD/IML the image is printed on the back or second surface of the film. This type of IMD/IML is considered very durable, as the inks and coatings are sandwiched between the base film and molding resin.
Why does an IMD/IML adhere to the molding resin?
For first-surface IMD/IML, the heat of the molding resin melts the rear surface of the IMD/IML base film and the two materials bond to one another. For second-surface IMD/IML and in-mold foils, a very similar process occurs, and the final printed layer or coating will melt and fuse with the molding resin. In either case, when the resin cools, the two materials are basically fused together. This bond is often described as a chemical/thermal bond, as the two materials have melted together and become one.
Testing and evaluating bonds
When it comes to testing and evaluating adhesion to determine if a permanent bond has been achieved, there are quantitative and subjective test methods. Whichever method is selected, molders should consider testing the adhesion shortly after molding and again after environmental tests.
A common subjective test to evaluate the bond is to cut an “X” or a rectangular strip through the IMD/IML insert and slightly into the molded resin. Next, use the end of the knife to attempt to peel the insert from the resin.
In the case of a quantitative test, typically a rectangular strip would be cut and the peel value measured. Some companies benchmark and require peel values in the 2.5 to 3.5lbs/inch range, similar to the required peel values for pressure-sensitive adhesives.
Consider performing the adhesion test near the gate as well as away from it. Why? Knowing that heat is required to achieve the bond, keep in mind that gate design, part geometry and wall thickness can affect how the resin flows and cools within the tool.
In addition, test the bond or adhesion of the printed coating to the IMD/IML with either a tape test or traditional cross-hatch test. It is recommended that the test be conducted, at minimum, one to two hours after the molded part is cool to touch and again after accelerated weathering, thermal cycle, chemical or immersion tests are complete. The molding process can affect the IMD/IML.
Common test results for first-surface printed IMD/IML:
- If the IMD/IML does not delaminate with effort or “chips” from the molded resin, then a permanent bond has been achieved.
- If the IMD/IML delaminates from the molded resin with little effort, the resin and IMD/IML may not be compatible, or there is an issue related to the resin temperature.
- If the IMD/IML does not delaminate near the gate but does delaminate away from the gate, there may be a potential temperature-related issue.
- If IMD/IML bubbles after time or thermal cycling, then there are issues related to the IMD/IML compatibility and/or the molding process.
Common test results for second-surface printed IMD/IML are similar to the first-surface test results; however, there are a few added considerations to those listed previously.
- If the printed image adheres to the molding resin but peels entirely from the base film, this indicates ink and base film issues.
- If the printed image adheres to the base film but peels easily from the molded resin, this indicates a potential compatibility issue.
- If IMD/IML bubbles after time or thermal cycling, several issues may be related to the IMD/IML as well as the molding process. The root cause of this failure can be different to the same type of failure for a first-surface IMD/IML.
- For metallic coatings and other large pigments, the base film may delaminate from the resin. This condition can be considered acceptable if metallic coating or pigment is present on the IMD/IML and the resin.
Are the test methods and anticipated results the same for a formed as a flat insert?
The short answer is yes. Additionally, it is possible to encounter what has been described as a mechanical bond or friction bond whereby the insert is held in place by the geometry of the IMD/IML and the molded part. There also may be partial adhesion. If a weak bond is present, it may be possible to observe “oil canning” of the insert, where the insert is flexing or moving independent of the molded resin. It is recommended to verify the adhesion in numerous areas of a formed part.
Furthermore, it is important to focus evaluations on the areas where the parts are stretched during forming, as the inks and coatings may crack and create a potential failure.
Regardless of the application, identify the performance requirements and verify that the IMD/IML meets those performance requirements. Don’t assume; always verify.
Dave Schoofs has more than 35 years of experience providing durable printed solutions to a wide variety of end markets including automotive, appliance, lawn and garden, marine, sporting goods and handheld electronics applications. His current responsibilities include supporting product development at Central Decal. For more information, visit www.centraldecal.com.