Automakers and suppliers are using plastics, such as nylon, polyphenylene sulfide and polypropylene, in engine compartments to reduce vehicle weight and improve fuel efficiency. Many different grades of material are available to choose from. Ultimately, the application and its requirements will dictate what materials are suited for use under the hood.

Temperature requirements typically determine whether using plastic is a good idea. As automakers use smaller engines, but continue to add new features, such as safety and infotainment systems, engines will run much hotter than in the past. Some plastics are better suited to the task than others.

“The material and who will supply it is always [the best place to start],” notes Bill Heatherwick, automotive market segment manager at Branson Ultrasonics Corp. (Danbury, CT). “Grades and recipes will vary by manufacturer, which in turn will affect your ultimate results. Part size and test requirements play a key role in choosing the appropriate joining process.”

Dan Schewe, regional sales manager at Forward Technology Inc. (Cokato, MN), says he always asks three questions at the quote stage for plastic assembly equipment:

• What is the material? “Some materials do not lend themselves to certain plastic assembly techniques,” warns Schewe.
  
• What are your requirements? Do you want strength, a hermetic seal, cosmetics or a combination?
  
• Production volume and required cycle time. “This will tell you if multiple cavity tooling is needed,” explains Schewe. “Strength may be slightly overlooked for a faster cycle time, if a particular application has a design that lends itself to be vibration welded or hot-plate welded. Ideally, vibration welding would be selected due to its 8 to 12 second processing time vs. 35 to 45 seconds for hot-plate welding.”

Schewe says many under-the-hood challenges he encounters in the field involve applications where end users want to interface metal with plastic. “For instance, if you want to join aluminum and nylon for a radiator application, you must address different expansion rates due to heat, which can create leaks and losses,” he points out.

“The challenge is helping customers at the infancy of a project to retain the best traits found on the metal part, while taking advantage of the properties of the polymer,” adds Schewe. “This step typically takes involvement from everyone from the polymer chemist to the assembly equipment vendor. Each independent automotive component has its own specific need, such as containing fuels, dealing with vibration or durability. Once those needs are defined, how the assembly will be assembled [should be] considered.”

The chemical resistant materials used for under-the-hood applications are semicrystalline in nature, and possess relatively lower elastic modulus values than easier to weld amorphous materials. “While this does not in any way make them unsuitable for ultrasonic welding, it does necessitate a higher degree of diligence in the design and welding processes,” warns Ken Holt, application manager at Herrmann Ultrasonics Inc. (Bartlett, IL).

“Correct weld joint geometry and close attention to good basic plastic part design methods and criteria must be present,” adds Holt. “The old rules of no sharp corners, consistent wall thicknesses, and correct gating locations all still apply. Sinks, warp and molded-in stress, common causes to welding problems, are minimized through the use of good design practices.”

According to Holt, higher amplitude requirements, near-field welding and computer controls on welding equipment are required to produce automotive-quality parts consistently. “With higher temperature resistance comes more resistance to welding and a lower tolerance of poorly executed heat-based joining methods,” he explains. “Matching melt flow characteristics with an optimized force profile will ensure maximized weld strength in these difficult-to-join materials.”

Glass reinforcement in any material requires the correct mold processing of the material to ensure a homogenous mixture of the reinforcement material. Most applications use percentages less than 30 percent. “Correct molding profiles ensure the even mix of the materials and alleviate a ‘glass rich’ area in or near the joining line,” says Holt. “This is imperative to allow the weld to attain the expected strength and weld process repeatability.”

Nylon’s hygroscopic nature is problematic if not recognized and addressed. Nylons of all sorts need to be thoroughly dried prior to molding and kept in a dry-as-molded condition prior to welding. The physical property change that occurs as nylon soaks up moisture can adversely affect welding processes of all types. However, this can be addressed by either welding it directly out of the press (if design allows) or by isolating the molded parts from moisture by bagging them with desiccant material. Problems that Holt commonly sees are weak welds (resulting from the parts not welding in the same manner as before) and loose flash.

“Containment of flash, beyond being just good design practice, is a prerequisite for the application of many joining methods and welding designs,” explains Holt. “But, full containment is particularly necessary in fluid handling, vacuum containment and fuel systems to keep intrusions and particulates from the interiors of parts. The ultrasonic welding of larger parts will require higher power outputs from ultrasonic welding equipment.”

As engine management grows more sophisticated, Holt predicts more plastic-encased sensors will be required. “The degree of precision needed to assemble these various sensors is currently high and will need to continue to be advanced by newer control systems,” he points out. “Small electronic sensors and transducers can be ultrasonically welded in high volumes with minimal investment in assembly equipment,as has been proven in many other industries such as medical and electronics.

“With the advent of different power sources for propulsion, new uses and challenges for integration of plastic components will occur,” claims Holt. “Building upon past experiences and lessons, plastic design and assembly will be able to satisfy new requirements. Already, fuel cell technology is demanding polymer usage and ethanol exposure has been addressed in fuel system components. Battery cases have long been the realm of polypropylene and hot-plate welding.”