by Paul Hurley and Tony Ward, Turnkey Technologies, Inc.
Thermal Processing of plastic parts has been utilized for many years. It is a reliable and cost effective secondary added value process. Many advantages of thermal processing versus other conventional methods can be recognized, including the following:
- Capital expenditures can be considerably less, making projects more amiable.
- Cycle times can be reduced resulting in increased productivity and savings.
- Similar operations can be combined and processed simultaneously utilizing the same tooling, resulting in increased productivity and additional savings.
- Tooling designs can be incorporated allowing for unlimited and simultaneous operations.
- Thermal processing can provide larger processing windows compared to conventional methods, reducing strain on the primary part and sub-components.
Secondary applications related to thermal processing include insertion, staking, de-gating, stamping and embossing. Within each of these categories are several approaches dictated by each unique application. These processes and/or approaches are briefly explained below.
Insertion is the process of installing metal component parts (inserts, studs, etc.) into plastic parts. This process is considered an added-value assembly operation and is often desired as opposed to the cost of incorporating features into an injection mold. The strength and integrity of the secondary installed components can easily exceed requirements otherwise provided by the molded-in features.
The installation of components can be accomplished with direct or indirect heat sources. An example of a direct heat source is the coiled heater around a stake, attached to a heater platen or pneumatic thruster. A protruding beryllium copper or A2 steel tip is threaded into the stake to make contact with the insert(s). This also can be accomplished using cartridge heaters installed in a common block with the same protruding tips. Indirect heat sources are hot air or infrared. These are used as a secondary heat source and/or in conjunction with a direct heat source. The advantage is to preheat the metal components prior to insertion resulting in a reduced cycle.
In all cases, tips make contact with a component and physically press the component in place. Tip geometry varies widely and is dependant on application. The benefit of this type of machine is it will consistently control process and stroke, allowing heat to work and not the machine tonnage. A pressed or forced in component will always result in reduced jack-out/torque-out physical properties.
The machine should incorporate multiple platen extend speeds to minimize process cycles. Individual temperature controls with full PID capabilities are ideal. In cases where cartridge heaters are utilized, the temperature controls should allow for a ramp-and-soak capability to prevent premature heater burnout. Shuttles are recommended to prevent inadvertent contact with heaters.
Staking is a secondary process utilized to assemble metal/plastic components to parent plastic parts. This is required to facilitate product assembly where part design deems it impractical to incorporate in the injection mold. The mating component/s assembled will have molded-in features/holes that fit over the mating molded-in stakes/features incorporated into the parent part. Heat is applied to the stakes making them pre-molten and pliable. Custom designed tips traverse over the stakes and form a desired geometry thus encapsulating the component or mating part. Depending on the process selected and the polymer processed, the tips will be cooled with compressed or vortec air. This allows for proper re-solidification of the polymer prior to tip retraction. The pre-formed stake and mating part geometry is critical to produce acceptable results.
Machines made for the installation of components can be incorporated with direct or indirect heat sources. Direct heat sources are identical to processes described for insertion, but beryllium alloy tips are typically incorporated due to its hardness and thermal recovery attributes.
Hot air is another indirect heat source used as a primary heat source for staking. It is applied to the stake at a specific volume and time to reach a desired molten state. A cold (ambient) metal stake traverses into the pre-molten plastic stake and reforms the material to a desired geometry. HACS (hot air/cold stake) is an excellent process for polymers that tend to string but the process typically requires more compressed air. HACS works well with larger staking requirements.
Infrared is an indirect heat source that utilizes a light or energy source to preheat the plastic stake. The cold metal stake reforms the material to a desired geometry. Infrared is a technology that should be considered as it continues to be developed and can compete with long-term existing processes.
Again, each machine should incorporate multiple platen extend speeds to minimize the process cycle. Individual temperature controls with full PID capabilities are ideal. Cartridge heaters should only be utilized when space limitation is a factor. Shuttles are recommended to prevent inadvertent contact with heaters.
De-gating thermally is the process of removing runner/s from a molded part. Although larger gates can be thermally cut, the process is typically selected for smaller runner systems that can be easily be placed in a fixture. Hardened steel tips are best for maintaining an edge but thermal characteristics could be compromised. Beryllium material is well suited for smaller applications. Design of secondary tooling and fixtures are critical to part placement in the de-gating process. Upper tooling should have hold-downs or stripper plates incorporated. The heater source is always a direct contact type – coiled heaters on stakes or cartridge heaters in a block.
Stamping and Embossing
Stamping and embossing is where a part surface is thermally marked/identified with a date code, serial number, symbol, etc. The heater source is typically a cartridge heater inserted into an indexing or stationary numbering head. Machines incorporating multiple platens extend speeds and minimize the process cycle. The temperature controls should allow for a ramp-and-soak capability to prevent premature heater burnout.
The base polymer processed should be considered prior to selection of a secondary thermal process. Knowledge of both physical and mechanical properties of the polymer selected, and its reaction to the proposed thermal process, is critical in selection of the optimum secondary thermal process. Additives in resins also will affect the decision of the final thermal process. Use of plastic regrind materials must be considered.
While thermoplastics are excellent candidates for secondary thermal processes, thermoset plastics are typically not chosen due to high temperature processing requirements.
Once a process is selected the joint designs must be considered. This can be accomplished in reverse order but ideally the joint will be designed around a selected process. Further considerations include the amount of automation desired, overall process cycle, desired process flow, and the environment the machinery will operate. Each application is unique and should be reviewed for applicability to the process and the heater sources.
The above processes, as well as hot plate welding and decorating, are thermal processes well suited for most thermoplastics. The selection of the optimum process for a particular application is paramount for the success of the project. Thermal processing can provide a cost effective savings over conventional methods when properly designed and integrated, as well as provide increased productivity at lower operating costs.
TurnKey Technologies, Inc. is a custom machine builder and consulting firm specializing in plastics assembly, painting and decorating processes, located in Concord, N.C. with more than 64 years of combined experience in secondary processes related to the plastics industry. TurnKey can be reached by calling (704) 782-7281 or by emailing at [email protected]