Temporary Attachments

There are plenty of ways to attach plastic parts to one another. They can be permanently welded or cemented. They can be semi-permanently screwed together, with or without inserts. And, for attachments designed to be easily and/or frequently opened, they


Temporary Attachments

There are plenty of ways to attach plastic parts to one another. They can be permanently welded or cemented. They can be semi-permanently screwed together, with or without inserts. And, for attachments designed to be easily and/or frequently opened, they can be connected using clips or latches.


Clips and latches take many forms. Examples include:


3. A moulded plastic tool box (see fig. 3) held shut by two rectangular latches that hook over protrusions on the body of the box. Flexibility for these latches is provided entirely by bending of the latches and the part, determined by the resin properties and their geometry. The lid of this tool box is retained by a living hinge, a web of resin thin enough to allow repeated bending as the lid is opened and closed.


Depending on the type of closure you use, there are issues of stress and flexibility to consider. Both clips and living hinges have been addressed in previous design tips, but some key considerations are worth repeating.

Fig.3 – Box structure with folding cover



Clips


Because clips move when operated, they must be flexible. Choice of resin helps determine flexibility, but there are several other contributing factors. The first of these is the length of the flexing arm and the second is thickness – clearly if the arm is too thin it will be fragile (see fig. 4). A longer arm pivots through a smaller arc to move a given distance, and the flex can be distributed over a greater length, reducing the stress at any given point along the arm. In other words, longer arm length allows more motion with less stress.
 



 Fig.4 – Good Clip: Long, slender, flexible. Fillets at base to refuce stress concentration. Generous through hole for core. Well drafted. Clip head not too large.





There are a number of sources of information on spring clips and ways to analyze stress on them:


– Some CAD packages include simple Finite Element Analysis (FEA) programs. If you use a lot of spring clips, consider buying a separate, more sophisticated FEA package; it could save you lots of time and money.

– BASF offers a snap-fit calculator.

– Efunda has a page on spring clip design.

– Jordan Rotheiser’s book, Joining of Plastics, published by Hanser Gardner Publications in 2004, has an excellent chapter on snap fits and spring clips.
Living hinges (see fig. 5)









Living Hinges (see fig.5)


For living hinges, you should consider both material and design. Polyethylene and polypropylene, coupled with proper design, are excellent materials for latches requiring living hinges. Thickness of the hinge is a key consideration. Make it too thick and stress created when the hinge is bent may crack the hinge; make it too thin and it will not withstand repeated use and may not fill properly during moulding. The following geometry (from efunda.com) works well for hinges made of either of the resins mentioned above.


Keep in mind that a hinge is a thin area that can be challenging to fill during resin injection. A single gate that forces resin through the hinge area in a mould increases the strength of the hinge but can lead to voids or sink downstream from the hinge. Multiple gates can prevent sink but may leave weak knit lines at the hinge. You can avoid these problems by allowing Protomold to choose gate placement for your design. A well-designed living hinge can be flexed millions of times.


For more information on designing with living hinges, see Penn State University Erie’s Behrend School of Engineering site or online resource efunda.com.


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