Metal / plastic interface cracking

Can anyone tell me how I can avoid this?
I want to design a part using the same
kind of metal - to - plastic interface as on
the Krups 203-70 coffee bean grinders but I
want to avoid the cracks in the corners.
If it gets posted it will be:
formatting link

Will this require say, a compliant adhesive
gasket between my metal surface and the
inside of the plastic reservoir or is this flaw
primarily caused by a different problem, say
a mixing issue that causes the plastic to be
really brittle?
Thanks!
--Winston
Reply to
Winston
Loading thread data ...
Those are stress cracks that result from the molding process, plasticizer migration and shrinkage over time. Thin walled parts are especially prone ti these over the lifecycle of the part. You also have to be careful when selecting a cleaning solvent (soap).
Reply to
John R. Carroll
(...)
Thanks! This plastic stuff is complicated. Clearly I will have to turn that design task over to a specialist.
Apparently not too much time is required for the cracks to develop.
I shot the photo about 10 minutes after taking the grinder out of the box for the first time. I never even turned it on.
--Winston
Reply to
Winston
I didn't realize it was new. Sheesh. The part has probably cracked at the knit lines. Poor knitting has a number of causes but chief among them is improper resin processing or low mold temperatures. IOW, somebody screwed up either when they designed the gates/runner system or when running the mold.
Reply to
John R. Carroll
'Knit Lines'. That is a new one on me.
Ah Ha! So it passed final inspection while hot but grew the cracks in the Amazon warehouse? Wow!
Thanks again.
--Winston
Reply to
Winston
Use a coffee grinder instead of a coffee basher? ;-)
Reply to
Steve Ackman
When I mounted splash shields on the table of my R2E4 (cast acrylic, and VERY prone to cracking and "spidering" from holes), I drilled the holes in the plastic to accept a grommet. IIRC, the holes were 7/16". I don't even want to have special bits around for plastics, so I drilled them way undersized, and used a tapered reamer to enlarge them without cracking the plastic.
The grommets were made for whatever hole size I drilled, and themselves had a bore of 1/4". The 1/4"-20 screws with a washer under the head fit just loosely in the grommets. When you squish 'em a little, the hole swells shut, and it holds firmly to both bolt and plastic, without any tendency for the plastic to be stressed at the union. Unless you use a shoulder bolt, you'll have to put a bit of threadlocker in the screw hole, since you cannot tighten the bolt too far without undoing the whole purpose of the exercise.
LLoyd
Reply to
Lloyd E. Sponenburgh
"Lloyd E. Sponenburgh" wrote in news:Xns9E599A26A11FBlloydspmindspringcom@216.168.3.70:
Just make sure you keep the threadlocker AWAY from the plastic. I'm not sure about acrylic, but I know from sad experience what happens to Lexan when you get Loctite on it. We had a rush job that was just about finished when an overzealous technician decided to applied threadlocker to some 3/8" Lexan parts it took several days to machine. They swelled up & cracked like mad. Had to start all over again, with even less time.
Doug White
Reply to
Doug White
Help, Sir Coffee!
Is this one all right?
Can you recommend a better unit?
--Winston
Reply to
Winston
So my intuition about an elastomer gasket wasn't too far off then, aye?
Excellent! Thanks Lloyd.
--Winston
Reply to
Winston
Reminds me of a misadventure I had with acrylic and an acetone spill once.
Thanks for the reminder.
--Winston
Reply to
Winston
It could be a material problem or maybe the part that broke was designed to use the minimum amount of plastic necessary in order to cut manufacturing costs. You could try your hand at plastic welding, maybe with a small soldering iron and proper tip. Another possibility might be solvent welding if it=92s made of PVC. I=92ll bet that the part wouldn=92t have broken if the sections were thick enough.
Reply to
Denis G.
formatting link
formatting link
(....)
I'm not going to repair the existing grinder, especially since I need a different type anyway. (Who knew?)
My application will use softer plastic, so perhaps cracking won't be an issue.
--Winston
Reply to
Winston
Winston fired this volley in news: snipped-for-privacy@news2.newsguy.com:
Winston, you have mentioned the "other alternative".
Instead of holes, grommets, and bolts, some applications have a channel frame all-round the work with a u-channel-shaped gasket capturing the edge of the plastic sheet.
The only problem with this method for the 'ordinary' shop guy is getting gasket and frame material that will match up with one-another and the thickness of the plastic.
LLoyd
Reply to
Lloyd E. Sponenburgh
The only issue with a softer plastic might be a greater likihood of staining, but maybe it won't matter for your application.
Reply to
Denis G.
(...)
You are right. This will be a 'liner' of a larger insulated container that will very rarely see the light of day.
--Winston
Reply to
Winston
I like my B&D model CBM210 . I'm the only coffee drinker in my house . An 8 second grind makes just the right amount of coffee for my little 5 cup coffeemaker . I'm set like 4 or 5 clicks from the finest grind . And it's around 30 bucks at Walmart .
Reply to
Snag
I had a very similar challenge sealing the end of a 30 gallon HDPE drum to the inside of a cylindrical plastic flange. The application did not require a lot of strength.
After I cleaned and degreased both pieces, I assembled them and flipped them 'opening down'. I poured some castable elastomer into the gap between the inside of the flange and the outside of the drum.
It worked a treat!
The elastomer was just 'runny' enough to find and seal the gap between the two cylinders, without leaking out of the gap. It was this stuff IIRC:
I expect that one could pour this stuff into a machined mold to create a gasket of just about any shape, too.
--Winston
Reply to
Winston
(...)
Thanks, Snag!
--Winston
Reply to
Winston
According to a Consumer Reports article some years ago, burr grinders work very well, especially for making a fine grind for expresso, but need a lot of cleaning to keep rancid coffee oil flavors.
We use a Krups Fast Touch Coffee Grinder, which is oval and of the impact type, and have not had problems with cracking. I recall that the broken grinder was not oval, and the cracks formed at the corners.
They retail for $13 to $18, so one does not repair them. Not that it's been needed, and the current one is at least ten years old.
Joe Gwinn
Reply to
Joseph Gwinn

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.