help with picking material for bumper application 4

[yoshi900]

Hi all,

Looking for some advise on picking a material for making some plastic bumper guard. These bumpers are long thin curved shaped. Since there are many different variation, I'm looking at injection molding the material, then machined to fit each application.

The materials that I'm thinking about are HDPE, ABS, Delrin, and Nylon. These bumpers are more for preventing scraping than hard hitting impact. The concern I have is the machinability and cost of each material. Also, I might be considering painting these bumpers (rather than molding a bunch of different color) and I don't know which one is possible for paint to stick to.

If someone can give me some pointer on any of these material or if there is another material that I should consider, I would really appreciate the help.

JC

 

[Pud]

What size are they? Do you mean by "bumper" what the US call a "fender"?

Rgds

Harry

http://www.tynevalleyplastics.co.uk

 

[MacGyverS2000]

...trying to imagine a car's bumper made out of Delrin. Now THAT'S an expensive bumper!

Dan - Owner

http://www.Hi-TecDesigns.com

 

[yoshi900]

No, it's not like a car bumper. It's just a thin curve piece. It's more like about 20" long. cross section is about 1"x1".

 

[KENAT]

You may get some idea on cost just by looking up somewhere you can buy plain sheet material.

For instance I believe Delrin is fairly pricey, not sure if you can vac form it either, though others here can say for sure.

I believe some car bumpers are made of ABS and they can be painted.

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[MacGyverS2000]

Delrin is very machinable, plenty slick (scratch-resistant), but getting paint to stick will require some extra work.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Nylon and ABS seem to meet all your stated requirements. I am very concerned that there might be a few undeclared requirements.

Do you have any idea what an injection mould costs.

When you say 30" by 1" by 1" do you mean a hollow or open section like box or U section or do you men 1" wide and 1" thick.

Regards
Pat
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[yoshi900]

Pat,

It's approx 1"x1" solid section. Length wise is about 20" long but curved. To cut it out of a 1" sheet would be wasting a lot of material. That's why I'm looking at injection molding. The quantity will be there to justify the cost of the mold. >4k pcs.

Looking at buying these material in sheet form, the cost difference seem to be (from high to low) Delrin>Nylon>ABS>HDPE
Would this cost difference hold true when injection molded?

So both nylon and abs can be painted?

JC

 

[Compositepro]

This sounds like an application for an elastomer rather than plastic.

 

[btrueblood]

How sharply is the profile curved? Why not start with an extrusion (straight), and then form it to the curve as needed? A shop I almost went to work for was creating a thermal "blanket" to use for in-situ hot bending of polyethylene planks for decorative/archy type fancy decks for houses. I think the same idea would work for all of the noted materials, as long as the curve is not too sharp.

 

[yoshi900]

compositepro: I did think of using some type of rubber at first, in the end decide the mounting needed to be bolted on. Plus, it might need to be paint for decorative purpose.

btrueblood: interesting suggestion. but i think curving it would add to the cost/pc. since the quantity will be enough to justify for the price of a mold made oversea.

 

[patprimmer]

At 1" thick a thermoset material might be more effective. Glass reinforced polyester seems a reasonable choice. It can be moulded in cheap low pressure moulds but may require hand finishing.

1" section thickness involves cycle times running into quite a few minutes for thermoplastics.

If you must use thermoplastic, cycle time might impact on cost to a greater degree than material cost.

Nylon with a suitable foaming agent might be worth a look.

Regards
Pat
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[Pud]

A 1" thick section (imho) is much too thick for injection moulding for a an impact application). Internal voids (notch producers = impact reduction) will be a problem. It will also look crap as the faces will not be flat (sinkage)

I would guess cast nylon may be a possibility, although quantities have not been mentioned as this is also a slow process, but cheap tooling.

cost difference seem to be (from high to low) Delrin>Nylon>ABS>HDPE
Would this cost difference hold true when injection molded?

Yes, with Acetal and nylon close, with acetal almost impossible to paint. (Actually, it's easy to paint - the paint just won't stay on the stuff!)

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

That is why I suggested foam.

With a 1/2' by 1/2" gate he can pack out the voids and sinks but pack time and therefore cycle time will be quite a few minutes.

Bottom line, I agree it is impractical to injection mould and should be made from a thermoset, either by compression moulding or casting.

Regards
Pat
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[yoshi900]

Pat,

What thermoset (machinable) would you suggest I look into?

I know very little about compression molding (so they put the plastic pellets into a mold, squeeze and heat, right?) How's the cycle time (cost) compare to injection molding?

Regards,

JC

 

[patprimmer]

Compression moulding is more expensive than injection on thin parts, but cheaper on thick parts.

I think glass fibre reinforced polyester will be the best. It can be supplied in several forms including bulk moulding compound which can be compression moulded. The glass will create a lot of wear on the cutters.

I think Harry has made the best recommendation of cast nylon which can be poured into a mould.

When casting or compression moulding you can economically add colour to the resin before moulding to give through colour.


Regards
Pat
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