I have a high volume product application in which a plastic tube needs to be press fitted into a plastic socket and must remain retained for ~2 years. Obviously press fits without welding, screws, ribs, locking tabs, welding, etc. in plastic aren't ideal, but press fits on competitor parts are working (though I'd like to end up with something better than theirs) and my first set of prototypes displayed extreme creep / stress falloff within 2 months at just room temperature.
I am working with plastic bottle caps and pickup tubes aka dip tubes, straws, etc. The tube presses into a socket in the cap. The first set of caps were cast urethane with an unrealistically thick (for injection molding) wall. All relaxation happened within the tube, not the cap. The tube has an OD of .305" and an ID of .250. My initial intent was to use polypropylene (cheap, stiff) so I had some PP tubing extruded to my dimensions. That tubing came in with poor quality and simultaneously I discovered that the tubes used in inexpensive ink pens were approximately the correct size. With some adjustment of dimensions (to those above) the first lot of prototypes were made with the tubes from Papermate Eagle pens which had been shortened. These seem to be polypropylene and have what I thought to be an added advantage of having a slight oval section on the exterior (.300-.310) so that some retention could be had from the elasticity of the oval. The dimensional interference of the prototypes I made were not measured (no way to measure the ID which is located deep in the cap). I adjusted the bore in the cap until the two components could just barely be assembled to the full .300" depth by hand. After aging 2 months the retention force was such that an axial tension of approximately 2 oz. would disassemble them. In order for this design to succeed I'll need something closer to 1 lbf at 2 years.
What tube material or strategy should I use in my next attempt given the rules that the tube needs to be extruded, inexpensive, rigid, .250 ID, and the assembly method must be very inexpensive. Would a fiber filled material be likely to give a significant improvement? Would the fibers tend to align parallel with the long axis of the tube and thus be ineffectual? Would the cross linking within an elastomer (it would need to be a very rigid one) reduce creep enough to be helpful? Any suggestions for an unfilled plastic that has better creep resistance than others?
Thanks in advance for any help or suggestions you may give.
Mark
I am working with plastic bottle caps and pickup tubes aka dip tubes, straws, etc. The tube presses into a socket in the cap. The first set of caps were cast urethane with an unrealistically thick (for injection molding) wall. All relaxation happened within the tube, not the cap. The tube has an OD of .305" and an ID of .250. My initial intent was to use polypropylene (cheap, stiff) so I had some PP tubing extruded to my dimensions. That tubing came in with poor quality and simultaneously I discovered that the tubes used in inexpensive ink pens were approximately the correct size. With some adjustment of dimensions (to those above) the first lot of prototypes were made with the tubes from Papermate Eagle pens which had been shortened. These seem to be polypropylene and have what I thought to be an added advantage of having a slight oval section on the exterior (.300-.310) so that some retention could be had from the elasticity of the oval. The dimensional interference of the prototypes I made were not measured (no way to measure the ID which is located deep in the cap). I adjusted the bore in the cap until the two components could just barely be assembled to the full .300" depth by hand. After aging 2 months the retention force was such that an axial tension of approximately 2 oz. would disassemble them. In order for this design to succeed I'll need something closer to 1 lbf at 2 years.
What tube material or strategy should I use in my next attempt given the rules that the tube needs to be extruded, inexpensive, rigid, .250 ID, and the assembly method must be very inexpensive. Would a fiber filled material be likely to give a significant improvement? Would the fibers tend to align parallel with the long axis of the tube and thus be ineffectual? Would the cross linking within an elastomer (it would need to be a very rigid one) reduce creep enough to be helpful? Any suggestions for an unfilled plastic that has better creep resistance than others?
Thanks in advance for any help or suggestions you may give.
Mark