by Jeff Peterson
Molders and decorators often are looking for a decorating process that offers a permanent, mar-resistant image with easy set-up and few rejects. Hot stamping and heat transfer decorating can fit this need, however, the key to success is to analyze the application carefully and choose the best type of hot stamping equipment to fit a particular need.
The customer’s application and production needs determine the type and complexity of the equipment. Quality hot stamping and heat transfer equipment manufacturers will review the part samples or drawings and work with the customer to recommend appropriate equipment designs.
The very first step in determining the right hot stamping press is to consider one of three basic hot stamping/heat transfer designs.
Vertical Presses. The most common method of hot stamping and heat transfer decorating. They are designed for decorating flat or slightly contoured surfaces from very small intricate parts to massive molded parts. Silicone rubber, magnesium, copper, and other metal dies can be used. Vertical presses range from simple hand-operated presses to custom automated systems.
Roll-on Presses. Roll-on presses are ideal for applying hot stamping foil or heat transfers to large surface areas using a silicone rubber roller. Roll-on presses can be utilized for decorating flat, round, and irregular-shaped plastic objects.
Peripheral Presses. This type of press is used most commonly for foil banding on plastic caps or round cosmetic containers. It can also be used for applying heat transfer labels to certain surfaces. Automated peripheral equipment can band parts 360 degrees in one machine cycle with typical yields in the 2400 to 3600 parts per hour range.
Once the type of press is determined, there are many other factors that must be considered before choosing the right equipment. Hot stamping equipment is rated in pounds or tons of pressing force. It is very important that the hot stamping press has adequate tonnage for the particular application. This is determined by the size of the hot stamping die or plate, what is the actual plastic substrate, and whether a silicone rubber or metal die will be used.
For an application using a metal die (magnesium, copper, brass, etc.), the general rule is to utilize 2000 pounds of pressing force for each square inch of stamping area. This would apply to high durometer plastic materials such as polycarbonate, nylon, and even styrene. Working with softer durometer plastics such as polypropylene, polyethylene, urethane, and vinyl, 1000 pounds of pressing force is usually adequate. Silicone rubber dies with their inherent ability to conform to uneven surfaces only require 500 pounds for the same square inch of contact area. This pertains to both molded dies with the text or image raised from the surface or flat rubber dies used for tipping raised characters or images.
Hot stamping machine manufacturers will point out that the customer may want to consider purchasing a press with a few more tons of pressure to allow for more flexibility with potential jobs in the future.
The next item to consider is the size of the part to be stamped. How high is the part, and will the press have enough “daylight” or space under the stamping head? Where is the image being stamped on the part? These questions are relevant as to how the foil or heat transfers will be pulled and whether or not there may be interference from an adjacent raised area in the molded part. Sometimes it may be necessary to reorient the part 90 degrees from what was originally planned or change the foil feed from a front to back or back to front set-up to avoid the foil stripper bars. If these adjustments still don’t accommodate the part, it may be necessary to move up to a larger press to gain additional daylight opening or throat depth.
An additional area of consideration includes the size of the heated head or plate. Sometimes the image area is small, but may be spread out in three or four areas. This means the tonnage requirement is low, but a large head will be necessary to accommodate the die size. Foil feed requirements (will more than one color of foil be stamped at a time) and productions rates (will the parts be decorated one at a time or will multiple parts be decorated), must be considered as well. Even the environment that the hot stamping machine will work under is important. For instance, clean rooms have a different requirement for a press than a standard press used in a general molding area. Special considerations need to be addressed for contamination for clean room applications.
Accessories and Options
Once the exact type and size of hot stamping machine has been chosen, it is time to consider other options and accessories. As stated earlier, it is important to analyze whether the machine will be used for other applications in the future. Do I need a machine with both hot stamping and heat transfer registration capabilities? Ordering the machine with a heat transfer system now verses having to potentially add it in the future can save a great deal of cost and time.
Probably the biggest question regarding additional options is whether or not the hot stamping machine will be converted with a multiple station rotary index table or automatic slide table verses positioning and stamping parts one at a time. Rotary index tables, in most instances, are capable of producing parts at a rate two to three times greater than a single operator loading, stamping, and unloading one part at a time.
Since most rotary table systems offered today are powered by variable speed D.C. motor drives, cycle speed may be gradually increased to attain an optimum output as the operator becomes more proficient at loading and unloading the parts. Rotary table systems can even become more automated with an air ejection system or through the use of a pick and place device.
Another potential option is a slide table to bring the fixture out of the press and permit loading of high profile parts. This is sometimes necessary to position the part over and down onto the fixture while in the press without contacting the foil or placing the operator at risk of being burned from the heated platen. When combined with an automatic cycling device, the slide table can also provide a means of pacing an operator by cycling the part in and out at a controlled rate. Again, as with the rotary index table, this cycle may be shortened as an operator becomes more adept at loading and unloading.
In addition to these automated options, many hot stamping or heat transfer presses are utilized at the molding machine, either by an operator or as part of a totally automatic system. This may require a riser or modified base to elevate the equipment to an optimum load height, plus an umbilical cord to link the P.L.C. in the hot stamping press with the robotic controls.
As with most decorating equipment applications, getting the machine manufacturer involved early in the design of the plastic part is always recommended. In many cases, a small adjustment in the part design can end up saving valuable time and money. The amount and accuracy of information provided to the hot stamping press manufacturer is the real key to choosing the right hot stamping/heat transfer equipment. Preparation can make the process smooth for both the customer and the press manufacturer.
Plastics Decorating would like to thank Dennis Cook of Kensol-Franklin (508-528-2021), Jon Rocca of Trekk Equipment Group (636-271-1391), and Keith Hillestad of United Silicone (716-681-8222) for their assistance with this article.