When faced with the task of joining two plastic components, the engineer has a number of options to choose from: snap fits, adhesives, ultrasonic welding, threaded fasteners and others. When a non-permanent assembly is required, threaded fasteners are a low-cost option suitable for high volume production and offer high strength when properly incorporated into the design. With a number of different thread styles available specifically for fastening in plastics, not to mention the possible resin variations, properly incorporating the fastener into the design can be the largest hurdle for a robust fastened joint.
Too often the low cost of fasteners allows them to be the final component considered in product design. By the time fastener selection is considered, numerous constraints exist that limit length of engagement, fastener placement, head size and other factors. Many times the associated tooling is already in production, and design changes carry an extreme cost, if they are even possible. The designer is then forced into the undesirable position of “making something work.” The end result is a less than optimal design, which may be subject to assembly, service and warranty issues. By consulting a knowledgeable supplier early on in the design process, a joint can be designed around the fastener allowing for the lowest cost solution and a robust assembly.
When fastening in metals, an engineer may simply choose between metric or standard, coarse or fine threads. With more than 10 distinct thread forms designed specifically for use in plastics, selecting a fastener early in a design can be a daunting process. However, each of these fasteners has design criteria for the fastening site that must be followed for optimal performance. A hole size as little as .002” above recommendations can make the difference between consistent fastening and consistent issues on the assembly line. And that is just the thread style and hole size! Proper consideration of head style, recess type, length, material and finish are also critical in selecting a fastener to best meet design requirements and assembly line demands.
So how does one design in a fastener with all of these choices in front of them? A knowledgeable supplier can assist in weighing the options and consider commercial availability, cost and performance to select a thread style based on a chosen resin. To go one step further, an engineering forward supplier such as Field will then assist with fastening site and boss design, fastener characteristic design and then provide performance testing to validate these choices. The end result is a strong, consistent, low-cost joint that took minimal design engineering resources and eliminated costly tooling changes.
John Medcalf – Applications Engineer, FIELD
Contact a Field engineer today at engineering@fieldfastener.com or John directly at 815-639-4346
Note: This is the second post in a regular series we’re calling “Smarter Fastening.” Designed to be a quick-reference knowledge hub for all things fastening and joining, this series will be published by various members of the Field team, which has over 200 years of combined technical experience in the fastener industry.
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