When it comes to assembling molded plastic parts, selecting the right fastening method is crucial for ensuring the structural integrity and functionality of the final product. Â In this blog, we will explore various fastening options available for joining molded plastic parts and discuss the economic considerations associated with each method.
Snap Fit Assembly:
Snap fits are a cost-effective method of joining plastic parts, especially if side pulls can be avoided in the mold.  Proper design of snap-fit features is essential to prevent overstressing the plastic snap fingers.  Accommodations for self-tapping screws are recommended as a backup in case one of the snaps breaks. Disassemble is possible but limited in frequency depending on how robust the snap feature design is. Several good Snap Fit Design Guidelines are available online. Some manner of locating features (i.e. a boss in a hole) are suggested to resist lateral movement, dynamic loading after assembly.
Snap Fit Feature
Consider a PWBA mounted to a molded plastic chassis. If the electrical connectors are all on one side, they will locate the PWBA in one direction. Depending on board size, three to six snap features will retain the board against the chassis, more if vibration is an issue. Molded stops, ribs or locating features prevent the PWBA from shifting on the chassis. As a PWBA is often a service item, it may require removal from the chassis. In the event of broken snaps, it is prudent to place holes in the board and molded screw bosses adjacent to the snap features to accommodate thread-forming plastic screws as a back up.
Thread Forming Screws:
Thread forming screws, (such as PlastiteTM) into molded holes in the mating part are suited for any production volume when a tight fit is required. Assembly tooling, such as a hand-held electric torque gun, is inexpensive, but the dynamic torque must be carefully controlled to avoid stripping the threads out of the molded boss.  Either the screw thread ODs or molded pins fitting into holes and slots, can be used to locate the parts relative to each other. Thread forming screws are ideal for applications where parts are to be disassembled only a limited number of times, usually 2 to 4. Avoid Thread cutting screws in plastic as they produce a chip.
Hand Held Electric Torque Gun
Thread Rolling / Forming Screw
Access Panel with Locating Slots
Consider locating an access panel or machine cover. Use snug fitting tabs in slots (one narrow locator slot and the rest widder) to position the panel in one direction and secure it against the mating part so that it won’t rattle. Use Thread Forming Screws A/R to secure the edges of the panel or cover. One of the screws should fit thru a narrow hole to complete the panel or cover alignment (in conjunction with the tab in a low clearance slot). The remaining screws fit thru clearance holes in the panel. Be cautious not to strip the threaded bosses when the screws are removed and reinserted. This is usually limited to 2 to 4 removals.
Machine Screws and Threaded Inserts:
Machine screws into threaded metal inserts are appropriate for applications that require frequent disassembly. Part to part location protocol is similar to that used for thread forming screws. This method is not suited for recycling due the metal inserts not being regrind friendly. If the inserted part is designed so that the inserted feature can be easily snapped off, recycling is enabled.Â
Threaded Insert in Plastic
All of the same locating and fastening protocols apply as discussed with Thread Forming Screws, except, the molded bosses contain metal threaded inserts. These are required when multiple screw removal and replacements are required! If the inserted plastic part design can be adapted to allow for the plastic containing the insert to be snapped off of the part, then it will facilitate regrind for recycling.
Ultrasonic Welding:
Ultrasonic welding of plastic parts provides a permanent assembly.  While this method may involve higher initial equipment costs, it becomes less significant as the production volume increases.  Ultrasonic welding is an ideal choice for applications where a permanent and robust assembly is required. Self-locating features are required to position the parts correctly, possibly incorporated into the ultrasonic welding features. Flash may be visible from the melted plastic. Most commonly used thermoplastics can be ultrasonically welded. Avoid welding of dis-similar materials.
Selected Ultrasonic Welding Options
Consider an inseparable assembly of a box with a lid. Because they are separate parts, various features can be molded into the box and lid interior. Many options are available for a flush lid design. A flanged lid, similar to a manhole cover geometry, is quite common and very practical.
And the appropriate placement of the energy directors, triangular sections that concentrate the US Energy and melt to form the welded joint, would direct flash, if any, toward the inside of the box. The result is an inseparable, sturdy torque tube box. There are several good Ultra Sonic Welding Guide Lines available on line.
Economic Considerations:
When selecting a fastening method for molded plastic parts, it is essential to consider both assembly and part costs. Â Snap fits are the most cost-effective method, provided that side pulls in the mold can be avoided. Â Thread forming screws are suitable for any production volume, while machine screws and threaded inserts are ideal for applications that require frequent disassembly. Â Ultrasonic welding, although involving higher initial equipment costs, provides a permanent assembly and becomes more economical as the production volume increases, provided dis-assembly is not required.
In conclusion, the selection of a fastening method for molded plastic parts should be based on the specific requirements of the application, including the frequency of disassembly, production volume, and the need for a permanent assembly. Â By carefully evaluating the economical and functional aspects of each fastening method, manufacturers can ensure the successful assembly of molded plastic parts while optimizing costs and performance.
AUTHOR
Henry T. Bober
Subject Matter Expert, Srushty Global Solutions
Henry is a seasoned Mechanical Design Engineer with 40 years at Xerox Corporation, specializing in Product Development, Cost-Effective Design, and Technology Development. He holds degrees from West Virginia University and the University of Rochester and has 35 patents to his name. After retiring, he founded Fast Forward Engineering, consulting for clients like Xerox, Diebold, NCR, and Siemens Medical Products. Now a Subject Matter Expert at Srushty Global Solutions, Henry lives in Fairport, NY, with his wife Leslie and their pets. He enjoys Western-style horse riding, Japanese garden landscaping, woodworking, naval warfare history, and animal welfare advocacy.