by Eric Larson, owner, Art of Mass Production
In-mold decorating is used to mold high-precision plastic parts with exceptional color and appearance. The process involves placing an appliqué – a pre-made form made from a printed sheet of plastic, which then is formed and cut to size – into an injection mold and molding behind and around the appliqué. It provides all the benefits of injection molding with the added advantages of modern digital printing.
In a simple sense, in-mold decorating has been around forever. People have been making things with decorative patterns in molds for centuries, and the technology has been used in the injection molding industry for several decades. Many have pointed to the cellphone boom of the 1990s as a turning point for IMD technology. Housings with detailed graphics, vibrant colors and countless variations became commonplace thanks to IMD.
The promise of IMD
IMD allows a manufacturer to make plastic parts with a virtually unlimited set of appearance options. Parts can be made with unusual colors, patterns and textures. Parts can be imbued with realistic images and even holographic images. Parts can be made that are opaque in one area, translucent in one area and clear in another. There are parts that glow in the dark or in the sunlight or only in certain conditions. There are parts with a hard coating in one area and soft-touch in another or any combination of the above.
Basically, if something can be printed, it can be used in the IMD process. All of this is achieved not with a label or a decal but through having the decorative element as an integral component of the molded plastic part.
The challenge with IMD
At first glance, IMD seems like a great idea. A pre-made form is placed in a mold and molten plastic is injected around it. The result is a fully decorated, high precision part that needs no secondary operations.
However, a peek behind the curtain (or appliqué) reveals that, compared to other plastic decorating methods, IMD is a complicated, highly specialized process. There are challenges at every phase of the development process from concept to production release. There often are issues with design, tooling, vendor sourcing and supply chain logistics. The development will take longer than expected, and the final part cost probably will be higher than initially quoted. While the quality of the production parts should be exceptional, more than likely there will be some noticeable part-to-part variation and a high scrap rate, especially in the early phases of production.
There is a reason why the plastics industry represents one of the world’s largest manufacturing segments. Injection molding is an amazing process. Molten plastic is forced into a mold cavity under high pressure to make highly complicated parts at a near net shape with low part costs. The process doesn’t stop there, however. When it comes to applying something like an appliqué – this weird thing – and putting it in the mold prior to molding, there are some key things to consider.
What does this thing look like? How big is it? How thick is it? How stiff is it? How will it be placed in the mold? How will it be oriented? How will it be kept in place? How will it react when hit by a wave front of molten plastic at 400°F moving at close to 300 mph at a pressure of 10,000 psi (think of what happens during a volcanic eruption)? How will it be designed? How will the rest of the part be designed so that it functions as it should – and still work with the appliqué?
IMD requires more tooling than standard injection molding – a lot more. First up is the IMD appliqué itself. Some tooling might be required for printing, depending on the graphics. Next will be a forming tool to fabricate the IMD form. If using a flat appliqué, skip the forming tool. Next, a trim tool is needed. The trim tool might be as simple as a cookie cutter, or it might be a highly complex form used on a programmable CNC machine. Finally, there will be tooling to create a tray or nest for storage and transport of the trimmed appliqué. As for the actual molding, there will be an injection mold, tooling to load and position the appliqué into the mold, and possibly some additional tooling for measurement and quality control.
Most of these tools should be standard. The challenge lies in project management and in coordinating the fabrication and production schedules. Assume there are two tools, each being made by a different vendor, and they both committed to having their tool ready on the same exact day. Also assume all the purchase orders and down payments are in place, and there are no internal delays of any kind. What are the odds that both tools will be ready on the agreed-upon date? At first glance, it could be said to be a 50/50 chance, but there are a lot of variables, and much will depend on the vendors, their quality systems, their expertise in project management, etc.
Now, assume there are 12 tools spread out over five different vendors involving a mix of materials, technologies and precision, and all the vendors have committed to having their tools ready on the same exact day. Again, assume all the purchase orders and down payments are in place, and there are no internal delays of any kind. Now, what are the odds that all these tools will be ready on the agreed-upon date? The answer is zero – the proof is left to the reader.
The point is that IMD tooling is complicated. Manufacturers can create all kinds of project schedules, using GANTT charts, PERT charts, PRITTY tables, etc., but it is impossible to account for all the unknown variables and delays. Those schedules likely will be out of date before they are even distributed.
Injection molding is a proven manufacturing technology used by companies all over the world. There is a comprehensive network of resin suppliers, tool makers and molding companies. For whatever part that needs to be molded, there are usually dozens – if not hundreds – of molders capable of producing that part. Some of those molders also might have capabilities in decorative processes, such as pad printing, painting, laser etching, etc. If not, finding a supplier to provide these secondary operations is usually straightforward.
IMD, on the other hand, is a specialized process. The vendor base is small, and some vendors may further specialize in a specific kind of IMD process, or they may only mold parts of a certain size or type or production volume. Finding a vendor that can produce a given part can be an arduous task. Another thing to consider is whether that vendor is a suitable business partner. Do they have bandwidth available for tooling and development? Do they have expertise in new product development and project management? Do they understand the performance requirements of a given application? Are they capable of providing design support? Is this a vendor the company will be comfortable working with for the next three to four years?
Supply chain logistics
One of the often-claimed benefits of IMD technology is in supply chain management. By making the injection molder responsible for the final, fully decorated part, a manufacturer can reduce work-in-process inventory. This can be a real benefit. However, the logistics of work-in-process inventory hasn’t gone away. The responsibility simply has been shifted to the molder.
In some ways, the supply chain has gotten more complex. All the normal logistics of injection molding (the IM part) still apply: materials need to be sourced (including color matching and color approvals), resin needs to be ordered, molds need to be fabricated, production schedules need to be managed and parts need to be molded.
On top of that, there is the decorative aspect (the D part): Film needs to be sourced, graphics need to be developed (including images and text, color matching and color approvals), printed proofs need to be made, thermoforming tools and trim dies need to be fabricated, production schedules need to be managed and the formed parts need to be sent to the molder.
Development takes forever
The biggest issue with IMD is the time required for development. As mentioned previously, there are a number of design variables. There are all the typical variables of designing and manufacturing an injection-molded part, plus the variables of graphic design and printing, plus the variables of thermoforming and trimming, plus a whole slew of interrelated variables. There also are going to be questions about tolerances. Each phase of the process has associated tolerances, as does the final molded part. In many ways, the final molded part is an assembly, and there will be assembly tolerances and tolerance stack-ups. Unlike a traditional assembly, from which components can be removed and replaced, an IMD part is an inseparable assembly. Once it is fabricated, it is together forever. If there is an issue with any component, the assembly is scrap.
There also are lots of tools involved. While most of these tools can be fabricated independently, many of them are interdependent, e.g. parts are needed from the forming tool to debug the trim tool, and parts are needed from the trim tool before the IM tool can be debugged. Then, while the IM tool is being debugged, the forming tool may need to be modified, and the iterative loop of tooling changes and mold trials starts all over again. This adds weeks of development time and complicates the development schedule.
Finally, there is data management. There is going to be a lot of data exchanged in the development of an IMD part. There is design data – color chips, color recipes, graphics files, 2D drawings, 3D CAD files – and production data – schedules, measurement reports, quality control documents, etc. This data will consist of different file formats created in different places using different programs and systems. How is the flow of data managed? How is proper version control of every component and every file ensured?
Remember, multiple tools and inseparable assemblies are involved, so an IMD part with two different IMD appliqués (different colors or patterns) will require different part numbers for the final molded part. Ditto if different colors are used in the resin.
Add all these factors together and it is easy to understand why development takes so long. Art of Mass Production recently worked on an IMD project involving seven molded plastic parts, including three parts that were decorated using IMD. More than a dozen tools were involved, and it took more than 16 months from project kick-off to production approval. As a comparison, a typical non-IMD plastic project can go from concept to full scale production in four to six months, depending on the industry.
So why should manufacturers consider IMD? Without a doubt, IMD offers some advantages, and high-precision parts can be made with unique appearances that could not be made any other way.
Eyes wide open
To evaluate the process, manufacturers need to be clear about their needs and goals. Don’t select IMD just because it’s cool, or it’s going to improve work flow. Use IMD because it offers a cost-effective way to create parts with unique appearance requirements. Be prepared to spend more time in the development process and set realistic schedules for design, development and production launch.
Establish clear design targets
Consider appearance as an aspect of performance and create specifications and tolerances for that appearance. Most in the design business are familiar with color matching and color tolerances. In many ways, printing is substantially more precise in terms of color than injection molding. Printing also is very precise in detail. However, the printed film is going to be formed and cut and then put in an injection mold and subjected to high heat and pressure. There is going to be some distortion of the printed film and perhaps a small color shift, both of which will affect the appearance of the film in the final molded part. There also may be variations in position due to tolerances of forming and trimming and the placement and movement of the appliqué in the injection mold.
Also, remember that appearance is more than just color. Appearance is highly dependent on lighting conditions, as well as gloss and texture.
The gloss level of a printed appliqué is easily controlled, but it could be affected by the injection molding process. Similarly, the texture of a printed appliqué is easily controlled as is the surface texture of an injection mold – but they may interact in unexpected ways.
Appearance specifications should address all these issues.
Use the technology wisely
The KISS principle – Keep It Simple, Stupid – refers to the concept that most systems work better if they are kept simple. It is a useful concept, but how can it be applied to complex systems? It is recommended to use simple designs for complex processes and use simple processes for complex designs.
As an example, printing is a simple process. It can be argued that injection molding is a simple process. Take advantage of these processes and add complexity to the design of these components.
Thermoforming is also a simple process, as is trimming. But the fabrication of a 3D IMD appliqué involves multiple processes (printing, forming, trimming, putting it in an injection mold) and, thus, becomes a complex process. So, make the design of the appliqué as simple as possible.
Manufacturers who decide that IMD is right for them can look forward to producing parts with a look and feel that is absolutely exquisite. As Billy Crystal might say, “That looks marvelous!”
Eric R. Larson is a mechanical engineer with more than 30 years of experience in plastics engineering. He is the owner and chief engineer of Art of Mass Production (AMP), an engineering consulting company based in San Diego, California. AMP provides services to manufacturing companies in the consumer electronics, wireless and medical device industries. Larson is also moderator of the blog plasticsguy.com, where he writes about plastics and their effect on people and the planet. For more information, visit www.artofmassproduction.com.