Fasteners and threaded inserts are small details that have a big impact on product design. How parts are joined affects assembly time, serviceability, strength, appearance, and cost. Many products fail in the field not because of the main design, but because a screw stripped a plastic boss or a joint loosened over time. Our product design team in Los Angeles helps clients choose joining methods early so assemblies hold up.
Start with How the Product Will Be Used
Ask a few questions before choosing fasteners:
- Will the product be opened for service, battery replacement, or repair?
- How many times will screws be removed and reinstalled?
- Is the joint loaded, vibrating, or exposed to heat?
- Should fasteners be hidden for appearance?
- What production volume is expected?
Screws Directly into Plastic
Thread-forming screws designed for plastics can be driven directly into a molded boss, creating their own threads. This is inexpensive and common for parts that are rarely disassembled. The boss must be designed correctly, with the right hole size and wall thickness, to avoid cracking or stripping. Repeated removal will gradually wear the threads.
Threaded Inserts
When screws will be removed many times, or when higher clamping force is needed, metal threaded inserts are the better choice.
Heat-set inserts
Brass inserts are pressed into a plastic hole with heat, melting the surrounding plastic so it flows into knurls. They are popular for 3D-printed parts and low-to-medium volume molded parts.
Ultrasonic inserts
Similar to heat-set, but installed using ultrasonic vibration. Fast and common in production.
Press-fit and expansion inserts
Installed without heat, these rely on interference or expansion to hold. They are quick but usually provide lower pull-out strength.
Molded-in inserts
Placed in the mold before injection for maximum strength (see insert molding). Best for high volumes.
Snap Fits: No Fasteners at All
Snap fits join parts without screws, reducing assembly time and part count. They work well for covers, battery doors, and housings. Good snap-fit design depends on material flexibility, deflection, and retention angle. A combination of snaps for alignment plus one or two screws for security is common.
Other Joining Methods
- Adhesives: good for permanent joints and dissimilar materials; requires surface prep and cure time.
- Ultrasonic welding: fuses plastic parts quickly in production; permanent.
- Rivets and pop rivets: common in sheet metal assemblies.
- Press fits and dowel pins: for precise alignment in machined parts.
- Magnets: useful for removable covers and accessories.
Standardize to Save Money
Using one or two screw sizes across a product simplifies assembly, reduces inventory, and avoids mistakes on the production line. Choose common, readily available fasteners whenever possible.
Design for Assembly and Service
Make screws accessible with standard tools, avoid hidden fasteners that require disassembling half the product, and include alignment features so parts locate before screws are driven. If the product is meant to be serviced, consider using inserts and machine screws in key areas.
Prototype and Test Joints
Printed prototypes with heat-set inserts are an easy way to test assemblies early. We can print variations of boss designs and snap fits to find the best option before tooling.
Design Better Assemblies in Los Angeles
eCadCam has more than 22 years of experience in CAD and prototyping. Visit us at 4212 San Fernando Rd, Glendale, CA 91204, call 213.489.1173, or request a quote to discuss your assembly.