Fastening with Screws and Injection Molded Bosses

 

p/n 31-D106 REV A

 

Fastening with Screws and Injection Molded Bosses

 

Most high-volume plastic parts for the consumer market will ultimately be manufactured with a plastic injection molding process due to its unmatched economy for high volume. The 1-dimensional manufacturing physics of this process often dictate that a structure be built in multiple parts that are snapped or screwed together. Therefore, many printed parts during the design cycle will be fabricated according to plastic injection molding design rules and will often require metal fasteners to assemble. 3D Systems MultiJet Printers (MJP) are ideal for prototyping such designs. The technologies high fidelity, surface quality, and isotropic material properties make it a great tool for all aspects of the design process including visual mockups, engineering prototypes, functional parts, jigs and fixtures, and manufacturing tools, etc... 

To achieve accurate dimensions without geometric defects, design rules for plastic injection molding require parts with relatively thin and constant wall thickness. Therefore, injection molded parts often have many “bosses” which are open-topped cylindrical features that are raised above the surface of the part and are used to assist in assembly using a screw or threaded insert. 

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Drill/Tap, Press, & Screw Diagnostic (Black – Projet 5600, Clear Projet 2500 M2R-CL)

From left to right the following features are shown: 1) Drilled and taped hole with hardware, 2) Press fit precision dowel pins,

3) Screws added to 3D printed injection molded bosses, 4) Press-fit brass inserts, and 5) machined precision holes.

The MJP 3D printed diagnostics above and below show different sizes of screws that were driven into the simulated injection molded boss using an electrical drill. The boss wall thickness was 1 mm.

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3D printed screw bosses with small, medium, and large size screws

 

Self-Threading Screws and Plastic Bosses

Standard machine screws from a hardware store can be used for injection molded bosses. However, special plastic thread forming or thread cutting screws have been developed and are commonly used for high volume injection molding applications. 

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Thread forming screws for injection molded parts

 

Thread cutting screws have features that aid in cutting the threads into the plastic when inserted. They create less hoop stress but require less torque to become lose and less tensile force to be pulled out. Thread forming screws do not have a cutting tip and are designed to displace material in the plastic boss to create a mating thread. Thread forming screws can generates high levels of radial and hoop stress and may result in damage to a rapid prototype printed part. The following 3D printed parts were done on a ProJet 2500 using M2R-CL and M2R-WT and show how sheet metal screws can be used with injection molded screw bosses with production-like wall thicknesses using holes that are standard for the given screw size.

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Sheetmetal or plastic screws can be used with both solid structures and injection molded screw bosses with production-like wall thicknesses using correct hole sizes for the given screw

 

Design Considerations

Injection molded bosses must be strong enough to achieve the desired holding forces and yet not too thick so as to avoid part defects like surface sinks or voids in the plastic. Correct hole size and wall thickness are both very important as there are failures on both ends of the sizing. This is true because threading into plastic with metal screws can result in a few different failures. Anytime the hole is oversize, the joint will fail due to a lack of thread engagement or the thread engagement will be stripped out completely as the screw is driven in. When the hole is undersize, the smallest screw sizes might physically sheer off due to the torsion required for insertion or the screw driver engagement or thread engagement might be stripped out. For large size screws, undersize holes will tend to either strip out the threads or the part will fracture due to radial and/or hoop stresses. 

 The material type selected for these prototyping needs is important to consider along with the hole size and screw size choice. Rigid white, clear and gray materials (M2R-CL/WT/GRY) can be used for all types of metal screw applications. However, they are fairly stiff and might tend to fracture the plastic if the hole size is too small and/or the boss wall is too thin. The engineering materials M2G-CL and M2G-DUR have the highest elongation and toughness and are recommended for many engineering applications. The M2G-DUR (ProFlex) has the most flexibility and will be the toughest material, but threads may tend to strip out easier under torque or tensile stress. The engineering material M2R-CL (Armor) was specially designed with both good stiffness and high elongation to work in injection molded boss applications with both thread cutting and thread forming screws. The M2G-CL is likely the best material for complex engineering applications. 

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Material selection is one of the most important factor for complex engineering applications

 

In some extreme cases, it may be necessary to slightly modify the design for sufficient prototyping robustness. For example, for thin walled injection molded bosses, it may be advantageous to increase the printed hole size by about 1% to reduce the possibility of failure due to hoop stress as the threads are cut into the plastic. Additionally, during prototyping, smaller size holes can be used for ProFlex and Armor for improved screw engagement because these materials are capable of higher elongation before breaking. When rapid prototyping complex injection molded screw bosses that are prone to failure, the designer should always consider making modifications to the geometry such as adding additional wall extrusion in CAD to strengthen the boss. 

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Additional wall extrusion added to injection molded bosses for additional strength

 

These extra features can be easily suppressed when the final tooling geometry is needed. When possible, it is recommended to start with manufacturer’s guidelines for boss and screw hole dimensions. Basic starting point recommendations for some standard size screws can be found in a machinist handbook. It is always recommended to also use best practices in terms of stress reducing fillets and rounds and appropriate wall thickness. Finally, it is also good practice to print out a sample of the geometry in question for fit and holding strength testing before printing the final parts.

 

Self-Tapping Screws

General self-tapping screws are fasteners that are designed to drill their own hole as they are screwed into wood, plastic, metal or 3D printed parts. They typically have a short drill bit on the end with special threads on the shaft of the screw.

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Self-tapping screw with integral hole cutting bit on the end

 

There are both thread-forming and thread-cutting types. Thread-forming screws are typically used for plastics and are designed to stay in place. Thread cutting screws are used for metal and wood and create threads during installation and can be more reliably removed. While these type of screws are designed to cut their own hole, pre-drilling or printing holes is recommended as this is less prone to break a part compared to drilling directly into a solid part. They will work best with M2G-CL or M2G-DUR, which have the highest elongation and toughness. They can be used with the MJP Rigid materials, but will be more susceptible to installation surface cracking and/or failure of a part. If these type screws are used in solid parts, it recommended to install them far apart so their induced stress regions will not overlap which can lead to surface cracking or breaking the part.  

 

Printed Threads

Unlike many 3D printing technologies, the high-fidelity MJP technology allows for a wide range of extremely precise holes to be printed in any orientation and is able to print threaded holes or threaded shafts in sizes needed for most practical engineering purposes (2-56 or 4-40 thread and larger are not a problem as shown below). 

IMG8.pngMJP 3D printed threaded nuts and bolts encompassing sizes for most practical engineering needs

 

Therefore, with the ProJet 2500 it is possible to print the threads with the printer. This can be done in thick solid parts or in an injection molded boss. In this case, it is possible to use traditional machine screws for prototype assemblies instead of thread forming or thread cutting type screws. Reasonable thread engagement will create a very strong joint.

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Left – 3D printed threads, Right – Standard machine screws threaded into 3D printed threads

 

 

Conclusion

For continuity between the prototype and final production part, screw bosses, metal fasteners, and snap fits are frequency used on MJP parts.

  1. Carefully consider material - The engineering materials M2G-CL and M2G-DUR have the highest elongation and toughness and are recommended for most engineering applications. VisiJet Rigid clear, white, gray, black and tan are likely to be more susceptible to damage or fracture if proper design and assembly care is not taken.
  2. Modifications to the design can be advantageous for prototype purposes - Consider making a hole slightly larger to reduce hoop stress. Consider adding material or making walls thicker so they can withstand higher hoop loads without cracking/breaking.
  3. Consider screw type and pilot holes - Different screw types can create more hoop stress. Pilot holes can reduce cracking/breaking of parts, but might have lower pull-out force.
  4. Printed Threads - The unique capability of MJP allows it to easily print most thread sizes which can be used in combination with standard threaded machine screws. Bosses with printed threads will often yield the most robust joint with the lowest insertion force.