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Help needed for selectiing a plastic resin; PP, Acetal, or ?
3

Help needed for selectiing a plastic resin; PP, Acetal, or ?

Help needed for selectiing a plastic resin; PP, Acetal, or ?

(OP)
I am currently designing a snap together two piece hinged plastic tube/disk part for an outdoor application to be used from AZ to Alaska.  Each disk/tube is nestled inside one another and are supposed to rotate freely on each other and spin.

The characterics of the plastic are:  excellent UV, excellent weatherability (ten years would be great), fire retardent, low friction, high impact, electrically insulating, low temperature stabile and max service continuous service temperature of 104 deg. F, abrasion resistant and must be inexpensive and come in black.

I chose PP because it tends to be less expensive than some other engineering plastics but I need help in determing whether it should be glass, talc, calcium carbonate filled and how much, Can the Friction be low and can the material withstand long term outdoor exposure.  

I also like Acetal because of its lubricity features, it tend to be a more rigid material which gives it a good "Snap" when locking the tube and disk pieces  together but I have some concern that an outdoor application in 0 degree weather would cause the part to be brittle and break.

I am a real fan of Delrin and Nylons but they are costly.

Please reccommend a varity of resins that would work and different grades and fills (glass, talc or ??) with an idea of per pound costs.    

RE: Help needed for selectiing a plastic resin; PP, Acetal, or ?

Weathering concerns should be addressed by contacting the supplier for which resin to use.

For UV resistance, your selection of black is excellent! Neat PP or PE will be your least costly resin. Fillers or reinforcements invariably lead to molding problems, in my experience.

Acetal offers greater rigidity and stength at about four times the material cost. Use it only if you absolutely must.

Design is critical. Have it done by a person that really knows what he is doing, like me.

RE: Help needed for selectiing a plastic resin; PP, Acetal, or ?

You write about a plastic hinge. Live plastic hinges will only work if the plastic is above it's Glass transition temperature. To use PP you may have to consider an impact modified grade to maintain toughness at low temperatures.

With your disks nested inside each other and freely rotating make sure you can have plenty of tolerance for heat expansion and contraction.

Your requirements of Fire resistance, electrical insulation, and UV resitance could be obtained from PVC. PVC is quite rigid but also ductile so you can also use it for your live hinge. The cost is also low.

Hope this helps.

RE: Help needed for selectiing a plastic resin; PP, Acetal, or ?

The biggest drawback will be in finding a cost effective flame retardant product. Acetal cannot be made FR. PP can but the cost will be too high. PVC is inherently FR but you will have choose flexible grade. Unfortunately I am not that familiar with PVC. Contact several PVC producer's websites for more info.

Unfilled PP is not an engineering resin. It depends upon how much you will purchase but PP is pretty cheap 30 to 40 cents per pound. Flame retardent PP will be well over $1.00/LB. Acetal will be around $1.00/LB unless bought in some volume.

RE: Help needed for selectiing a plastic resin; PP, Acetal, or ?

You may want to consider a 70D ether based castable polyurethane. Parts made in this material will exhibit the properties your after, but material costs are high.  Tooling costs on the other hand should be far less.. depending on shape and processing method... quantities etc.

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