Fixturing Key to Hot Stamped Parts
by Jeff Peterson
Whether choosing an easy-to-stamp plastic material, pretesting the foil, or deciding on the proper type of die to use (metal or silicone rubber,) a hot stamping or heat transfer job can be planned out perfectly with proper preparation. However, if careful planning does not go into the fixturing (tooling) as well, all the other preparations will be for not. Having the plastic part fit into the fixture properly is of utmost importance and short-cuts can cause nothing but problems and create challenges for everyone involved.
Fixtures must be designed to support the area behind the plastic surface being hot stamped. Ribs, knockout detents, and bosses must all be reflected in the fixture. In addition, the design of the tooling must be compatible with the hot stamping press that will be used. It must allow for scuff-free loading and unloading of the part by the operator. Fixtures are most commonly machined from aluminum. However, they can also be made from a liquid urethane or other thermosetting polymer that can be poured directly into a production-approved part. The type of application and length of run can have an impact on the type of material from which the fixture is produced.
In addition to supporting the area to be hot stamped, fixtures also must be used to stabilize the plastic part to avoid any movement during the stamping process. If the part is not secure in the fixture, it may result in registration problems and/or missed areas of hot stamping foil on the printed image. Outboard adjustable arms are sometimes necessary on larger hot stamping presses to secure the part. This is common on applications such as trash containers or wheel barrow bodies. The operator must be able to easily load the part, bring it to a positive, repetitive stop, and release the part in order to activate two safety hand switches. Under no circumstances should a second person be required to hold the part in place while the stamping is done. This constitutes a third hand and immediately negates basic safety guidelines for single station hot stamping presses. It also doubles the labor cost by utilizing two operators. Spending a little more money on the outboard arms will pay for itself in short order.
Type of Applications
Certain applications require more concern with regards to fixturing than others. For blow molded cases or vacuum-formed parts, there is more forgiveness in the surface to be stamped. The precise match of stamping die to part surface and part to the nesting fixture may not be as critical as in other situations, especially if cosmetic quality decoration is not required.
For flat surface decorating of a single cavity injection molded part that has few ribs, bosses, or deep sidewall draws, the mating of the die to the part and the part to the support fixture is usually fairly basic. A few parts from the injection molded cavity and the artwork are probably all that the toolmaker will need to produce a proper fixture.
Projects that offer complex shapes with large decorated areas that are difficult to support require greater attention to the critical interfacing between the surfaces of the die, part, and fixture. The key to producing a properly manufactured fixture, especially in these types of challenging applications, is to utilize production molded parts. And, if there are several cavities in the mold for the same part, it is recommended to get multiple samples from each cavity. When production parts are available, matched tooling can be made using a combination of aluminum framing with cast urethane in the support areas. After final fitting of the various cavities to the fixture, the surface to be decorated is digitized from a part that is properly nested on the fixture. The digitized surface is used to develop the contoured surface of the hot stamping die. An engraved metal die or a silicone rubber die that is cast from an engraved master mold is used for applying the hot stamping foil. In certain heat transfer applications, the silicone rubber die is profiled to stamp slightly beyond the graphics of a heat transfer. This profiling to the outline of the graphics eliminates, or at least minimizes, any possibility of a witness line or halo.
It is not recommended to produce fixtures from CAD drawings or SLA files. If the production parts are not available, computer files can be used to begin the tooling, but finished parts should be tested before production begins. If the final production parts do not fit the fixtures, rework usually involves modification of all surfaces. This is why it is usually best to wait for the production parts if all possible. If the engineering files or drawings or much different than the finished part, the rework may take longer than waiting to do it right.
When utilizing a metal die to hot stamp a plastic part, every effort should be made to soften the support under the stamped area. Whether the part is injection, blow, or roto molded, placing some type of forgiving makeready on the fixture will allow the force of the press to flatten the stamped surface against the flat metal die. This will help compensate for variations in the wall thickness, minor areas of sink, or other unevenness in the part. An example of this is hot stamping large blow molded parts with dies 18 long, when the part thickness variation is up to and exceeding +/- .050. By using a 0.50 thick hard rubber makeready pad, in combination with a low die temperature, and increased stamping pressure, an acceptable hot stamped image can be maintained.
For smaller injection molded parts, it is standard practice to use 1/8 printers blanket built into the fixture. This is usually a better option than makeready (masking) tape and should decrease the makeready time on press. A good example of this is a TPU animal tag project where variations in the thickness of the tags caused intermittent rejects when hot stamping with a sequential numbering head. Occasional stamping of the empty fixture aggravated the situation by leaving heat marks on the thermoplastic fixture surface. The fixtures were redesigned to accept the 1/8 rubber makeready. This served to protect the fixture from the heated die face as well as soften the stamping surface to help eliminate rejects due to part thickness variations.
In sum, it is critical that all parties involved be included early in the fixture and die manufacturing process. This includes the decorator and/or molder, the hot stamping machine manufacturer, and, of course, the die and tool/fixture maker. Final artwork, part design, and the decorated transfer formulations all should be identified before the tooling begins. Getting all of the parties involved early, providing the toolmaker production parts to work from, and utilizing proper makeready techniques with the fixture are all important elements to ensure proper fixturing and a hot stamping operation that runs smoothly with few rejects.
Plastics Decorating would like to thank Bill Morey of Schwerdtle Inc. (800-535-0004) and Vincent Lindgren of Acromark Industries, Inc. (800-227-6675) for their assistance with this article.