by Jordan Rotheiser
This last June, I had the privilege of moderating a seminar entitled “Decorating Hollow Parts” at the Society of Plastics Engineer’s “Hollow Parts Conference” in Chicago. It was sponsored by the Decorating and Assembly Division of the SPE, and presentations were provided by Keith Hillestad (United Silicone), Jodi Laughlin (Pro Mark) and Mark Wille (Mark-It). The “Hollow Parts Conference” concentrated on the rotational molding, twin-sheet thermoforming and blow molding of industrial parts. The blow molding of bottles was not covered in depth because it is largely a high volume system used by the packaging industry with needs and equipment considerably different from those of industrial parts.
Hollow parts can be decorated by hot stamping, heat transfers, screen printing, pad printing and a variety of transfers and labels applied to the surface of the part or in the mold.
Hot Stamping and Heat Transfer
Briefly, hot stamping is a process by which a mark is transferred from a carrier film to the surface of the part (substrate) with heat and pressure. Today a wide range of foil colors and patterns are available. The shape of the foil stamped image is created by the heated die. Lettering is probably the most common mark created, often in the form of product logos. A flat die can be used to mark the top surface of a part as well, such as raised lettering – a variant known as tipping. Hot Stamping is not often used when multiple colors are required because precise registration is difficult. When multiple colors are required, a variation of the process known as heat transfer is used. With registration devices added, hot stamp machines are used for heat transfers as well. In this process, the heat and pressure are used to transfer an entire printed image from the carrier to the substrate. Heat transfers offer the advantage of being able to provide decoration with the full range of color and detail available from a printing process without requiring that process to be present in the molding plant. Unlike hot stamping, heat transfer costs are not sensitive to multiple colors. Both hot stamping and heat transfer can accommodate minor surface blemishes using silicone rubber dies and direct ram equipment.
Hollow parts provide some special decorating problems for hot stamping. For example, hot stamping requires a substrate rigid enough to withstand approximately 300 to 500 psi in the area of contact in order to emplace a quality mark. By the nature of hollow parts, the wall of the part to be decorated cannot be directly supported in most cases – the exceptions being parts with openings large enough to accept a cantilevered support, which can reach into the part and under the area to be marked. For some applications, heat transfers can be a viable alternative because it requires less pressure than hot stamping. Heat transfers normally are applied at 100 to 300 psi, soft goods as low as 30 to 50 psi. Hollow parts made by the injection molding process are usually of small diameter, such as pen barrels, and rigid enough to withstand the forces of hot stamping and heat transfers. In extreme cases, it may be necessary to create the hollow part by injection molding the halves, decorating them and then joining the parts to create the final product.
The bulk of rotational molded products are too large to fit most hot stamp and heat transfer equipment. In addition, the workhorse material of the rotational molding industry is polyethylene, of which only the high-density varieties offer the possibility of providing adequate rigidity for hot stamping. Both blow molding and twin-sheet thermoforming are stretching processes, which result in part walls considerably thinner than the starting thickness. Making the wall strong enough to withstand the force of hot stamping could, depending on the shape, require a change to a stiffer material or a thicker starting wall at an increased material cost. Another alternative is to stiffen the hollow walls by filling them with foam. Hot stamping and heat transfers are used to decorate many containers, particularly for the cosmetics industry where diameters are relatively small and rigid plastics can be used. Both hot stamp marks and heat transfers are limited in size to 12 inches by 24 inches, and at that size, roll-on equipment is usually required to avoid air entrapment.
Screen and Pad Printing
Briefly, the screen printing process is the modern variant of the traditional silk screening technique. In this technique, a screen is placed in a frame .030 to .060 inches over the surface to be printed. Ink is then spread on the screen. There are openings in the screen to permit the ink to pass through when a squeegee presses the screen against the substrate. That deposits the ink on the substrate. Screen printing can print cylindrical parts, but is not tolerant of surface defects and sink marks. The ink is opaque and does not require repeated applications, however it is wet and drying or curing is required between colors and after the last color has been applied.
Pad printing is an indirect gravure printing process. A transfer pad, known as a tampon, picks up the impression from the printing plate called the cliché. The pad lifts and the cliché is withdrawn. The pad then descends onto the substrate and leaves a mark. Pad printing inks are very light and dry quickly so the colors can be applied in sequence in a multi-staged machine and be handled immediately after printing. Pad printing has enjoyed wide acceptance in the plastics industry because of its ability to decorate highly contoured parts. The tampon is soft, so the part to be decorated need not be completely rigid. Most commercial pad printers are limited to decoration of 7 inches by 14 inches, however there are special machines, which can print graphics of 10 inches by 20 inches.
Screen or Pad Printing of hollow parts would probably be more widespread were it not for the predominant use of polymers, which require surface treatment before printing in the rotational and blow molding industries. There key advantage is that both processes are much less demanding of substrate structural integrity than hot stamping or heat transfers. In addition, because many hollow parts are quite large, the printing processes are not as dominantly used because of restrictions on machine size. However, like most decorating processes, special machines have been built to accommodate large hollow part applications.
Labels and Appliqués
Labels and other forms of appliqués are widely used for hollow parts as they are well suited to large semi-rigid parts. As with heat transfers, these methods offer the advantage of being able to provide decoration with the full range of color and detail available from a printing process without requiring that process to be present in the molding plant. Registration is exact, opacity is consistent throughout, and the entire decoration is applied at one time. Little or no capital investment is required.
Of these methods, two or three are unusual and are of particular interest. Mark-It Company, of Batavia, IL, offers a post molding system designed for medium to large parts that applies a multi-color transfer to the surface of the part. The image is screen printed to a paper carrier. The paper is wetted with water or solution to activate the adhesive. The graphic is positioned on the substrate and the paper is then removed leaving the graphic on the part. Adhesion penetrates the surface making it more durable than pressure sensitive labels. The water or solution based systems are selected according to the polyolefin that needs to be accommodated. The transfer can be put around corners, even sharp corners, and can conform to textures as rough as leather because the coatings soften and draw down into the crevices. A wide variety of surface textures can be used without registration problems and, with the proper decal remover, improperly applied decals can be removed to avoid scrapping the part.
There is a system available for polyolefin rotational moldments in which the decoration is permanently molded directly into the part. Mold-In Graphic Systems, of Clarksdale, AZ, offers a process where the graphic is also screen printed onto a transfer paper. In this case, the image is transferred onto the mold surface by burnishing the backside of the paper. For small parts, this can be accomplished in 15 to 25 seconds; a time period which would not delay most rotational molding cycles unless there were multiple cavities. A light coat of adhesive is sprayed on the mold surface to control positioning, however it does not affect the surface finish of the part. The company claims that the decoration is fade, crack, chip, peel and ultraviolet light resistant to enable a rotationally molded graphic to last the life of the part.
The company has another process for adding post-molded decoration to a part called Mold-On Graphics. In this technique, enough adhesive is sprayed onto the graphic to put a tack on it so it will hold position on the substrate. The graphic is then burnished onto the surface and then flame treated into the part. This decoration can be applied any time after molding, thereby providing an opportunity for efficient production by removing that operation from the molding floor. The process can be used with parts manufactured by all the hollow part processes as well as the other major plastics processes.
Industrial hollow parts do create some unique decorating problems due to their relative large size and the predominant use of polyolefins and semi-rigid walls. However, with the various techniques available through the decorating industry, these issues can usually be resolved and excellent decoration accomplished.
Jordan Rotheiser is president of Rotheiser Design Inc. and the author of Joining of Plastics – Handbook for Designers and Engineers, the product design chapter of the Modern Plastics Handbook and the plastics chapter of McGraw Hill’s just published Handbook of Materials for Product Design.