Looking for plastic suggestion

[Cliff]

I have been trying to make a part out of abs, but it is thick, so it sinks a lot. (I'm replacing a machined part.) What are some other materials that are not as likely to sink (I.e. do not have much thermal expansion), and can take adhesive, as abs can?

Thanks.

[Airwinghma]

What melt flow of abs are you using? A high melt ABS will flow and fill better, 15-20, with less heat. If you are running a good melt I would suggest you try a blowing agent, I run lots of ABS and DO use Cel-Span 840 ( at .5%) on some to eliminate/reduce sinks and specifically on a plateable ABS.

Scott

http://www.phoenixplastics.com/products/chemical-foaming-agents/cel-span-cfa-840/

[rickbatey]

What's the thickness? Too thick and the sinks will never go away. I think your design needs tweaking and then you won't need a CFA (your only other option). The gate may be freezing off too soon as well plus poor tool cooling.
Rick.

[Cliff]

Thickness .4 od x .31 long.

Material is sp-100:
http://www.southlandpolymers.com/products_abs.shtml
It's says melt flow is 18, but there may different test conditions. I can't go softer, or it will wear prematurely.

Running the mold coolant from my hydraulic chiller, at 60F, so the mold stays pretty cool. On a hot humid day it builds up condensation if I'm not running it non-stop.

Due to the long residence time (that's a small part), I ran it down around 375f in the barrel.

Can't redesign the part, that would defeat the purpose of the mold. Ends must be solid, and the point behind molding it is to reduce cost vs machining.

The mold ran sample parts on a BaByplast, a hydraulic bench top. I think the problem lies with my machine, it's got an oversized screw, so pressure is about 50% lower. If I can swap out the material, I can avoid investing in a $6k injection unit with a smaller screw (too give higher pressure) in a machine worth only $5k.

[Cliff]

...I would suggest you try a blowing agent, I run lots of ABS and DO use Cel-Span 840 ( at .5%) on some to eliminate/reduce sinks and specifically on a plateable ABS.
Scott,
I've never used Cel-Span--what does it do to the properties of the material? I need the outside to maintain hardness, so it won't wear out.

Thanks.

[rickbatey]

Blowing agents create bubbles internally in the molded part. The cosmetics will suffer with silver streaks (splay) but exterior will be solid. I think a Chemical Foaming Agent will give better bubble/cell structure as well as part cosmetics. But you'll need a weight blender to meter agent (.5-1.5%) and a shut off nozzle. You can get a mechanical shut off nozzle.
To me the root cause is residence time too long and too low injection pressure. Find a better press for the mold and you should be able to warm mold up and make a MUCH improved part.
Rick.

[3NGLENN1889]

Heed everything rick said in previous posts. My question is you mention adhesives, can you be more specific? In my years of dealing with ABS I've never used chilled water, always in the 80-120* range. Have you considered glass filled nylon? (Warning: could have warp/bowing issues with glass filled nylon depending on gate size/location along with dimensional changes) Whole different animal. Good luck! G.P.

[JayDub]

Given that your available pressure is low, can you open up the gate?

[Cliff]

Found a problem with machine, fixed it, and now I don't get sink. I got bulging until I reduced pressure.

But the reduced pressure brought what I think is nasty jetting, because there are seams (creases?) all around the part.

[JayDub]

You could start by running the tool at mfr's recommended temps - 120 - 160 instead of 60 F, & see if that improves the surface.
If the problem really is jetting, then your solutions are either to increase gate size or slow injection speed. If you slow the injection speed, make sure you have 95-98% fill at transfer.