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issue: November 2003 APPLIANCE Magazine

Plastics Buyers' Guide
The Plastic-Metal Switch


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"We are switching from metal to plastic for our appliance components. How do we assemble them?" This question is increasingly asked by appliance makers.

Demands for lower cost manufacturing, lighter components, and recycleability are forcing manufacturers to switch from metal components to plastic. While the assembly processes are different, some of the same concerns apply. Finding a reliable assembly equipment supplier, defining part requirements, getting them involved early, and choosing the right assembly process are the keys to success.

This chart outlines the different characteristics, capabilities, and requirements of a variety of plastic welding processes. CLICK for the full-size graphic.

Appliance components come in all shapes and sizes, and each one has its own unique characteristics that demand an assembly process to fit. Ultrasonic, hot-plate, spin, thermal, laser, and vibration welding are the most common plastic assembly methods. Choosing the correct method can be difficult. A supplier who has technical knowledge in all of the processes is the best choice. They will have knowledge of all the different process joint designs, can provide assistance in material selection, and can support once the process is in production.

Defining the part requirement before the design of the plastic part is critical. This will save dollars in tooling costs and help assure that the correct assembly process to achieve the requirements is chosen. All too often, the plastic molds are manufactured, the first parts are assembled, and then quality control determines that the parts will not pass a pressure test. This is too late; now significant dollars will need to be spent to correct the problem. Requirements such as a need for pressurization, exposure to extreme cold or heat, cosmetic-part status (requires no blemishes), and parts assembled per minute are all factors in determining the correct process and plastic design.

Each process has unique plastic-joint design requirements to assure proper weld strength. Assembly equipment suppliers can help design the weld area joint design. An example of joint design requirements for ultrasonic assembly is given by Guide to Ultrasonics from Dukane Corporation (St. Charles, IL, U.S.): "Mating services should be in intimate contact around the entire joint. The joint should be in one plane, if possible. A small initial contact area should be established between mating halves. A means of alignment is recommended so that mating halves do not misalign during the weld operation." Obviously, these joint requirements should all be designed into the part prior to machining of the injection molds.

What assembly process is correct for a part? As stated earlier, ultrasonic, hot-plate, spin, thermal, vibration, and laser welding are the most common methods used in production today. Each method has unique advantages.

Ultrasonic assembly is a fast, repeatable, and reliable process that allows for sophisticated process control. High-volume small parts that have very tight assembly tolerances lend themselves well to ultrasonic assembly. Ultrasonic systems have the capability of exporting relevant assembly process data for SPC documentation and FDA validation. Ultrasonic welding can be easily integrated into automated systems.

Hot-plate welding can accommodate a wide range of parts sizes and configurations. These machines offer high-reliability hermetic seals and strong mechanical bonds on complex part geometries. The process is fairly simple; the two parts to be jointed are brought in close proximity to a heated platen until the joint area is in a molten state. The platen is removed and the parts are clamped together until the joint cools off and returns to a solid state.

Spin welding is a very cost-effective method for joining large, medium, or small circular parts such as washing machine tubs to agitator components. Water purification filters, thermal mugs, and irrigation assemblies typically are joined using the spin welding process. Careful attention to joint design is critical for parts that require flash-free appearance.

Assemblies that require inserts at multiple points on multiple planes, like computer or vacuum cleaner housings, typically benefit from thermal insertion/staking. Thermal staking is ideal for attachment of non-plastic components to the plastic housing, such as circuit boards and metal brackets. Dates coding, embossing, and degating are other uses for thermal presses. Thermal welding can be a slower assembly process than ultrasonic, so, depending on the volumes of assemblies required, ultrasonic maybe a better choice.

Vibration welding physically moves one of the two parts horizontally under pressure to create heat through surface friction. Compared to ultrasonic welding, vibration welding operates at much lower frequencies, much higher amplitude, and with greater clamping force. The limitation to vibration welding is simply that the joint must be in a single plane in at least one axis in order to allow the vibration motion. Like hot-plate welding, vibration welding is a highly reliable process that can handle large parts in challenging materials or multiple parts per cycle with ease. Chain saw housings, blower and pump assemblies, and large refrigerator bins are examples of potential vibration welding applications. Cycle times for vibration welds are very short, thus they are ideal for high volume and are easily automated.

Laser welding is the newest technology of the processes available today. One benefit of laser welding is that the weld joints produce no flash or particulate outside of the joint. Assemblies that require absolutely no contamination for particulate, like medical filters, are good candidates. A second benefit is that the assembly is not exposed to heat or vibration. Devices that have very sensitive electronic internal components that may be damaged from vibration can now be assembled effectively. Laser welding requires the parts to be transmissive and absorbive, specifically how transparent the parts appear to the laser beam. One material transmits the coherent laser light and the other material absorbs the light and converts it to heat. Parts that appear black to the human eye can be transparent or opaque at the wavelength of the laser light. Clear-to-clear joints and joints that are optically transparent can be readily achieved by use of special coatings. Depending on the part geometry, laser welding can be a slower process then vibration or ultrasonic welding.

Plastic appliance components are the direction of the future - they can be assembled economically and produce functional products.

This information is provided by Michael Johnston, national sales and marketing manager, Dukane Corporation (St. Charles, IL, U.S.).

 

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