Many plastic injection molded parts need to have some form of decoration applied to them after being manufactured. Possible reasons for decorating or marking parts include identification, tracking, branding, notices, warnings, instructions, as a product attribute, or otherwise. There are some standard methods of decorating plastic parts, and each has certain advantages and considerations, depending on the part application, the decorating purpose, and the resin being used.
PAD PRINTING
Historically, one of the more popular methods for decorating plastic parts has been with pad printing. There are three different types of pad printing machines: open inkwell, closed ink cup, and rotary gravure, which is used for printing 360 degrees on a part. Machines have a printing plate or a cliché, which commonly are made out of steel for durability. The artwork to be printed onto the parts is etched into the printing plate.
As the name suggests, the machines have transfer pads that come in a few different shapes and that are made out of silicone rubber with various hardness or shore levels. Pad life ranges from 10,000 to 100,000 cycles with an average of about 50,000.
The process consists of the pad picking up ink in the etched image on the cliché and placing it onto the part. Pad printing uses a type of solvent-based ink specifically designed for this purpose. This type of decoration also requires the fabrication of nesting fixtures to hold the parts in place during the printing process.
HEAT TRANSFER
A newer and usually more effective method of part decoration is thermal or heat transfer. With this process, the image is printed on a polyester film, which is coated with an adhesive so the image will stick to the part. Rolls of this film with the digitally printed images are placed on the printing machine’s reels, which index forward to print successive parts. Fixtures to hold parts are used here, as they are in pad printing. Additionally, a silicone decorating head or pad applies heat and pressure to print the transfer image onto the part.
Some advantages of heat transfer include the ability to create an image with four colors plus white in as high as 1200 dpi. In addition, there is no drying or curing time with decorating plastic parts, as there is with pad printing. Further, heat transfer decorations generally are tougher than pad printing and can be dishwasher safe.
HOT STAMP
Another common technique used to decorate plastic injection molded parts is hot stamping. A hot stamp machine transfers an image with pre-dried ink to a part with a die, again using heat and pressure and a fixture to hold the part, as with the previous methods.
One main advantage of hot stamping is the ability to apply a metallic foil or embossing to a plastic part. If you’re reading this article on a desktop computer, chances are the monitor in front of you has a logo that has been hot stamped on it. It also is a relatively clean process, compared to the wet ink used in pad printing.
LASER MARKING
Another method for decorating injection molded parts is by utilizing a laser marking system. This approach includes laser marking, laser engraving, and laser etching. Laser marking (or foaming) changes the color of the part where the text or graphic is, while the surface stays relatively unchanged. Alternatively, laser engraving removes some of the surface material, whereas laser etching melts the surface resin and creates a raised mark.
Of course, laser marking, engraving and etching all require a laser decorating machine. Additionally, fixtures to hold the parts usually are required, as with other decorating methods, although some fixtureless laser decorating systems have been developed. Some advantages of laser decorating include less wear and tear on the decorating equipment, it’s a precise and repeatable process, and it provides the ability to decorate some hard-to-reach part areas. Due to these benefits, industries like medical, automotive and aerospace often utilize the process.
IN-MOLD DECORATION
The method of in-mold decoration involves placing a pre-printed plastic film or appliqué in an injection mold before the part is molded, and the graphic then adheres to the part after it forms in the mold’s cavity. In-mold decoration requires a uniquely designed and built plastic injection mold and a carrier film feeder. The film can be manually placed into the mold between each cycle, but that greatly increases the manufacturing time and labor costs associated with the parts. However, even with a feeder, parts requiring in-mold decoration generally tend to be more expensive than other decorated parts, due to the specialized design, tooling and manufacturing required.
INKJET DECORATION
Inkjet printing on plastics is a relatively newer process, and it involves digitally printing a graphic onto a part. The system essentially is the same as inkjet printers for paper, with which most of us are familiar, although it uses a different type of ink. These machines typically use either conventional or LED lamps, and the process eliminates labels, films, printing plates, clichés or screens, as required with other decorating methods, although they do require a printhead and ink. Historically, this kind of decorating is somewhat slower and more costly than other forms, although advances are being made.
SCREEN PRINTING
Most people are familiar with screen printing in the context of t-shirts, and this also can be referred to as silk screening. Screen printing can be used for parts with flat or cylindrical surfaces but is not recommended for parts with complex curves and shapes. One advantage of screen printing is that it can print metallic graphics onto parts, which some other methods cannot do. Although providing the option of printing with multiple colors, a separate screen is needed for each separate hue, which increases cost and decorating time. The surface to be decorated needs to be prepared prior to printing, too.
PAINTING
When injection molding a part cannot achieve the desired texture, finish or color, painting the parts sometimes is utilized as a secondary operation. This approach can include spray painting and powder coating. As most injection molders do not perform painting as a finishing operation, the parts will need to go to a secondary provider, which often results in increased costs.
PLATING
When looking for a metal finish on an injection molded part, hard chrome plating and electroless nickel plating are two commonly used options. Plating offers a decorative finish, and it provides a certain level of corrosion protection. One item to note is that any surface imperfections on the part can affect the look of the plating, so consideration needs to be given there. Additionally, few molders also perform plating services, so those two suppliers will need to coordinate their work. There also are some design and material considerations that must be taken into account.
CONCLUSION
There are many different methods of decorating a plastic injection molded part, and each has its own unique advantages and disadvantages, depending on the requirements of the project. When developing a new product with components that may need to be decorated in some way, attention should be given to the part design, the polymer being used, the decorating purpose, the part application, the governing regulations, and the cost considerations. If we can answer any questions, don’t hesitate to reach out to us.