Key Points in the Design and Construction of Injection Molds for In-Mold Decorating

By John Berg, director of marketing, and Alan Kastelic, tooling manager
Sussex IM

Every experienced mold designer and toolmaker understands the basic best practices of their profession. Our readers are all familiar with the credos of appropriate draft, uniform and graduating wall thickness, a generous radius at corners / avoiding sharp edges, ribs to support walls…you can likely recite all of them in your sleep. When a customer’s part design includes special functionality, multiple materials, nontraditional surface geometry or atypical aesthetics, additional elements of consideration come into play.

The application of placing a preprinted film or pre-form into the injection mold to create a decorated surface adds several critical and unique aspects to the design and build of the mold. In-mold decorating (IMD) – used here as the general term for all things related to adding a visual element to the part (along with the inherent color of the selected resin) – is a value-added technology absolutely requiring its own subset of best practices. The finished product, when done successfully, gives little indication of the additional steps involved. The result often is a colorful and engaging brand design that integrates seamlessly with the part’s surface.

You can certainly refer to the decorative element as a label, but the overall effect, both aesthetically and functionally, is generally superior to a post-mold applied self-adhesive fi lm. Art applied “in-mold” becomes part of the surface and is likely to never delaminate the way nearly every conventional adhesive-backed decal or label eventually will.

In-mold decorative art can be applied to the final surface of the part when placed on cavity steel during the open mold cycle, and it can be applied on core steel, looking through a clear resin as a second surface. The art can be applied on a surface parallel to the parting line (a screen or appliance bezel, for example) or it can be applied to the sides of a part (cylindrical geometry, like drinkware). Key areas of consideration and focus for the mold designer and builder are as follows:

• •Where and how to gate the part to ensure plastic flows properly over the fi lm – and balancing that with where and how to gate the part to ensure its structural stability
• How to guide and hold the fi lm in place prior to and during the injection phase
• Cooling circuitry dedicated to both part geometry features AND label placement and label material characteristics

Gating location

Gating location is a critical factor because resin flow, direction and force all play a significant role in successfully bonding and merging the fi lm to the part surface. However, the gate also can be a challenging area because the temperature of the resin at this point can degrade the in-mold film, and its forceful flow can move or dislodge it. We want to direct material flow to the back side of the label, missing the leading edge of the film. A pocket in the steel needs to be created, typically no less than twice the thickness of the label, to act as a location guide – not necessarily a retaining mechanism.

The ultimate goal of gating an IMD part is to create a flow that continuously moves outward. Material fl ow moving straight across a label often will produce ripples. A slight hesitation in flow front may cause a crease in the label.

The flow of resin should be designed to pin the label against the steel. We can use a rib, which fills first, then feeds material to the back of the film, pinning the label to the steel to assure placement accuracy. Using this technique, we reduce heat concentration to the injection point, helping to alleviate degradation to the label. While it does add considerable cost to the system, valve gating can effectively regulate material volume, speed and flow.

The mold designer and toolmaker also must integrate part geometry to develop a detailed level of understanding of the part design’s impact on how material will flow around coring and create knit lines – which can cause labels to crease or pucker. If part design is such that gating directly at the back side of the label is not possible, creating a pocketed area to seat the label below the flow surface is necessary to reduce the chance of flow getting under the film.

Decorative film placement

In the production environment, the flat or pre-formed film often is robotically picked from a stack of films (magazine) and delivered to the injection mold via an appropriate end-of-arm tooling (EOAT) fixture. One of the most common and reliable methods for holding the label to a specific and repeatable location in the injection mold is to use a controlled static charge. This process charges an insulator – in this case, the in-mold fi lm or pre-form – causing it to attach itself to a properly grounded injection mold. The EOAT will have guide pin(s) to match the guiding holes machined into the injection mold.

Cooling strategy

Temperature control is especially critical when gating to the film. A water circuit, baffle or bubbler directly opposite the gating point behind the label is critical for label integrity. This focused water requires its own circuit, as most likely it will be run colder than the surrounding steel. Directing water near the gate on both halves of the injection mold to control heat is an important factor in establishing a process where the fi lm or pre-form is not subject to damage.

Business as usual

The IMD process does not normally present any unusual challenges or requirements for part ejection or venting. These mold features are entirely dependent on part geometry and resin characteristics – as if the part was not being decorated / there is no inserted film involved. There also are no specific rules or concerns as far as mold surface finishes / polish are concerned. The in-mold film will take on the surface characteristics of the steel. Texture depths of 0.0005″ to 0.001″ commonly have little to no effect on label performance or aesthetics.

Beyond the decorative aspect, in-mold technologies also can add a level of functionality to the finished part. Conductive and luminescent inks can carry current or illuminate. Digital printing can add limitless art design changes in a fraction of traditional lead-times and facilitate the inclusion of digital watermarks. Scanning these watermarks with the appropriate smartphone app can open a specific web page or online content to provide unique user experiences or act as a conduit to aftermarket sales and service. The ability to serialize the decorated product is possible with digital watermarking technologies, enabling product traceability or evidence of counterfeiting.

Utilizing the attributes of IMD to facilitate branding, user instructions and product security are common to many brand owners for good reason. This particular method of value-added injection molding provides the ability to change art without halting the production process. The films can be given performance characteristics such as abrasion resistance or a soft-touch feel. The fact that this decorating technology is far more environmentally friendly than plating or painting is another inducement for expanding its applications.

Sussex IM is a proven, long-standing leader in custom injection molding, with a history of more than three decades. In 1977, Sussex Plastics, Inc., was formed in Sussex, Wisconsin, by an ambitious immigrant from Hungary, Lorand Spyers-Duran. His small but industrious group of employees grew to be the modern, integrated manufacturer that is now Sussex IM. For more information, visit www.sussexim.com.