About the "Cracks"

[Dave]

I just wanted to share some experience I have with plastic cooling block "cracks".
I have been reading on this board, about cracks being caused from some liquids, thermo difference between fittings, etc.

It is more likely the cracks where already there.

Please I am not trying to argue with you guys, I just want to explain what I found out myself.

I was making a prototype at AVT when Dan came up behind me and said "don't do that", which happens often. I had machined a piece of acrylic with threaded ports. Dan indicates that you can not cut threads in acrylic without causing cracks.
As is the norm, he does not explain why so I tried to figure it out myself after hours.

I used dye on my first piece and sure enough there were cracks all around the threaded area. Assuming it was only a machining issue, I cut several samples to test with different tools and speeds.

For the first test I used a profile thread mill, and cut samples at 3000,6000,10000, 20000, and 30,000 RPM all at low feed rates. Dye showed cracks in all with little to no difference between the samples.

I them repeated the above test, this time with a single thread very sharp carbide mill. This was sharp enough to scratch glass so I figured it would be the answer. The dye showed I was clearly mistaken, although there was a small improvement.

When I finally had Dan's attention I asked why and all he said was "it is the shape of the tool".

Amazing what I learn at work on a daily bases, that should have at least been discussed at school. I have seen people machine plastic at collage all the time.

Just my experiences please no arguments. I suspect someone in here will explain what is happening in more detail then Dan has.

[lolito_fr]

Some rather pretty pics here

apparently alcohol makes the cracks grow
guess people should know better than to buy wb's with acrylic (let alone polycarb) tops

[BillA]

2 elements:
micro-defect from machining = stress riser = small 'crack'
crack tip propagation energy is far smaller than that required to initiate a crack otherwise,
moderate thermal cycling is sufficient for some materials to fail w/o any applied load;
many plastics are better molded than machined

EDIT: note the widespread use of acrylic to physically model stress, crack tip photos are interesting

[Dave]

Most engineers including myself understand that Bill. It was clear to me when Dan stated "it is the shape of the tool".

I am hoping someone can explain why this type of plastic forms micro cracks under these conditions, so all can understand.

Good pictures

Why don't you guys just mold these parts?
I understand if you mold first then cut, the cracks will not be as much an issue due to stress patterning. Would only be $1-2K for a simple mold like this, we make them all the time.

Even better we use this machine for small runs and REAL cheap tooling.
It has a high price new, in fact you could buy a real good full size used injection machine for same costs. But if you can find a used one it may just be your answer.

http://www.morganindustriesinc.com/index.htm

[jaydee]

I have used acrylic often in my blocks and rarely had an issue. Then moved to Lexan which is much better. There are ways of dealing with the micro-fractures but few take the time to do it. There is also ways of preventing cracks around screws but few seem to listen (one Eddy dude comes to mind).

The largest problem is uneducated end users that think they need 2000ft lbs of torque on each screw so water won't leak. And this is the main reason IMO a manufacture should use copper or brass tops on blocks. Delrin and the likes will work better then acrylic and lexan but I am still not 100% confident in the end users.

All the blocks I design are DIY but I try to think about "what if I sold these" when making them. Thats why all my recent blocks have copper tops with soldered on barbs. I was impressed when Swiftech used the same concept with their MCW6000 series blocks. Small things can come back to bite you in the ass as I am sure any manufacture with experience knows, yet they still take the short cuts that leads to the sore ass.

[BillA]

look at the 'failure' mode
if by plastic deformation (shear, yielding), then no cracks
if by brittle fracture, then all that is possible is to reduce the fracture 'zone' (better coolant, tool design/speed, etc.)

[Dave]

"2000ft lbs of torque on each screw so water won't leak"

[Cathar]

Am happy to say that as perhaps the leading guinea pig producer of Delrin/Acetal topped blocks that I have yet to hear of a single crack occurring, whether through use, misuse or abuse.

Delrin readily deforms to stress pressure, unlike any clear plastic that I'm aware of. I agree with Jaydee116 that it is possible to attempt to largely mitigate the cracking in clear plastics with careful machining, but it is never a guaranteed thing. It's not something that ever can be treated like a metal, where if the metal fails it's because of a major localised flaw, and in that case is extremely rare.

A large number of plastics can be machined quite successfully without much apparant danger of introducing micro-fractures, the problem is that there are NO clear plastics that fall into that category, which I would imagine is due to the highly crystalline nature of these plastics that enables them to be clear in the first place.

Thermal stresses are the primary source of failure though. Yes - the cracks are introduced at machining time, but it is the thermal stresses of warming up and cooling down which will propagate cracks quite quickly.

As for molding plastics, we have to think of the volumes being done, and in some special cases, the need for very, very precise, accurate and repeatable tolerances which is not that easy to achieve with the natural plastic shrinkage that occurs during the mold cooling process.

Still, even with injection molding, depending on how one swings the figures it would basically take a production run of around 2000-3000 blocks to "break even" with production based CNC milling, and really would take well over 5000 blocks worth of production to begin to make the molding process markedly cost-beneficial.

[Jam]

Interesting thread . . .

As a newcomer to this unique corner of the DIY PC hobby, I just built my first WC rig using a DD Maze4-A8N, a TDX and a pair of NV-68-A8N water blocks, each with acrylic tops.

The above setup works well for now and those blocks sure do look sharp . . .

Perhaps in the next months I would do well to put and aluminum catch basin under the case?

,s

Joe

I'm new here and my attempt at humoUr may not have been very successful.

It just seems, after reading this thread, that it is quite surprising the number of folks offering water blocks to the market with acrylic tops - and that one may conclude that doing so may border on the irresponsible?

I am also aware that the folks who post here regularly use brass, Delrin and generally eschew acrylic and have probably learned much of what they know from the "school of hard knocks."

I'd probably have avoided the block choices I made above {at least with the acrylic tops} had I known as when I had my credit card out - as I do now.

[jaydee]

Danger Den seems to do a pretty good job with their Lucite tops. They laser cut most of it which causes no cracks do to tooling and their way of threading the barb holes is pretty decent, at least on the Maze 4 I have. But, there is a reason they now offer metal tops on their blocks.

[Jam]

So far, so good.

The UV dye I added to the coolant hasn't helped to highligt any obvious crazing or cracks, but then again,the system is only a couple of weeks old.

The phrase "time bomb" comes to mind.

Cheers,

Joe

[Dave]

Not sure about your numbers there Cathar.
We have never charged over $2000 dollars for a simple two piece mold before.

I would suggest you would save this much just on your machining costs?

Now if you talking about a multi peice or expandable cavity type mold for barbs, the price increases very quickly.

[Cathar]

Originally Posted by Dave

Not sure about your numbers there Cathar.
We have never charged over $2000 dollars for a simple two piece mold before.

The cost of using a mold is more than just the cost of the mold itself. Still have to pay for the cost of materials, which isn't much different either way (CNC or mold), and then have to pay for the injection mold manufacture process itself, probably with a ~90 second cycle time for a fairly thick chunk of plastic like a wb top is.

Compare that to 2-3 minutes in a CNC mill ($2-3) plus cost of materials.

Still takes many thousands of tops before the mold process breaks even.

[Dave]

We use a MUD system on most of our molds and injection machines, so set up time is very short.

Does a top need to be so thick?
Could you not just rib it along edges and cross over too ports?

Just an idea.

[DryFire]

Wouldn't milling the tops allow you greater flexability for further revision?

I suppose if you must have a clear top molding would be the way to go.

[dacooltech]

Dave cracking of tapped acryclic WB tops is not an unknown issue here... and it happens more often with tapped acrylic tops combined with 1/2"OD metal barbs and cumbersome 1/2"ID x 3/4"OD tubing... we received RMAs for tapped polycarbonated tops in the past, and stopped offering any blocks with tapped poly tops.

I must add, we haven't received a single RMA of tapped lucite Danger Den tops from any customer in the last ~2yrs or so though. So AFAIK the guys over at DD are doing a decent job with lucite tops like Jaydee said. [edit: it's also possible that customers are solving their issues directly with DD, and in that case we wouldn't know.]

If I use a clear topped block in one of own systems I prefer putting poly barbs on instead of metal barbs and never had any bad experiences.

If the top is not going to be clear then like Cathar already mentioned Delrin (Acetal) is the best choice IMHO. Cathar used Delrin for the 1st time as a WB top, and it's most definitely a great material. Definitely better than anodized AL, and also better than Brass or CU too, because it's lite.
But yea it's not clear though...

So I strongly agree with you that the possibility of cracks is there with tapped clear acrylic, metal 1/2" barbs, 1/2"ID x 3/4"OD tubing and plastic injection molding is the way to go for clear acrylic.

But don't you think the same is true for the pumps? What I really don't understand is that -since you're highly recommending plastic injection molding- why didn't you release the CSP pumps with plastic injection molded heads? When the CSP was first announced I spoke with Dan on the phone, and the 1st thing I asked was why anodized AL head instead of plastic injection molded head. and his answer was the required quantities are currently not there to justify the cost of the mold and offer the pump at that low price...
but if we're willing to put up money for the mold, he can make OEM pumps with plastic heads. -guess same is true for the new MAG pumps- he also told me that AVT's been selling large quantities of pumps with anodized heads to non-pc liquid cooling related customers and never had a single issue with anodized AL, so there's really no need for plastic pump heads... I must admit that i was a bit confused at the time if volume OEM orders are there, why not plastic heads for your pumps.
now from your previous posts I understand that you've been selling CSP pumps in large volumes to customers from other industries, yet you never released the pump with a plastic head. I mean if for any reason PIMed housing was not an option, and CNC milling was the way to go, why didn't/don't you use Acetal (Delrin) heads on the pumps?

Sorry this thread wasn't about your pumps, but it's something that i'm curious about, and really appreciated if you can elaborate

[Edit: typed plastic housing for the pump instead of pump head at one point, and got stuck with it :shrug: . corrected ]
[Edit: typo]

[Dave]

Good to hear about the DD tops.

First I was not trying to fault anyone’s product. I do not think the small cracks are an issue. I was just trying to explain these cracks are already there, and likely not an issue. I also do not believe they "grow" to the point they would leak on a straight O-ring type fitting, maybe an NPT fitting in very small amounts. It was an NPT type port I cut when Dan told me not too.

Now about the pumps, most of our 750 customers just purchase the center section of the pump (drive section) and cut or mold the pump housing into there product. In our industrial application, we do the same thing as well. We make a threaded round center section; you just screw the whole piece in like you would a sensor or valve.

For the OEM MAG's, there is a plastic case and a stamped rear section. The pump is very ugly and we at C-Systems believe looks are a big concern. All the insides of a retail MAG are plastic and I guess once reviewers get samples you will see what I mean.

There is also a huge difference in price between a simple two piece mold, and a mold for barb fittings. A retail version will require barb fittings as our early sample tests showed.

For example, at AVT we cut a mold yesterday for a new plastic mounting plate for an OEM MAG, costs to our customer was $850.

The costs of the MAG mold pump section for C-systems parent company product, which incorporates dual piece snap together fittings, for very fast assembly, was a little over $26,000! It was a big job which took several weeks to design, cut and test.

Why did we never cut the pump housing of the 750 out of solid Delrin?
Not sure, I'll ask, but I suspect Dan likely didn't see a reason for it, as we have never seen a pump fail because of pitting. Remember there is a big difference between galvanic corrosion and natural oxidation. I plan on making a Delrin MAG rear case later this week to see how it looks thanks to a suggestion on this board.

As engineers we are still learning what everyone wants. You should know by now engineers look at things differently then most people.

[jaydee]

Let us know how the Delrin turns out.

[dacooltech]

Thanks for the info Dave.

Why did we never cut the pump housing of the 750 out of solid Delrin? Not sure, I'll ask, but I suspect Dan likely didn't see a reason for it, as we have never seen a pump fail because of pitting. Remember there is a big difference between galvanic corrosion and natural oxidation. I plan on making a Delrin MAG rear case later this week to see how it looks thanks to a suggestion on this board.

I'm sure there's a logical reason why AL was used for the pump housing, but am not clear why Delrin wasn't used for the threaded pump head.

yes pls let us know how the Delrin turns out on the new MAG pumps

[JamesAvery22]

Is derlin expensive? And are there different types or grades or whatever?

[jaydee]

Originally Posted by JamesAvery22

Is derlin expensive? And are there different types or grades or whatever?

It is Delrin, search it on google.

[Dave]

Made one today out of black Delrin and looks REAL nice.

Please see other thread.

[JFettig]

I have been making delrin tops for my waterblocks for probably 2+ years now.
From all the consern about this and people asking, I finally made a batch of delrin WW tops that I am selling.

Dave, I made a pump housing for the c-systems pump and used it for some time, worked great. I mainly did it because metal barbs do not screw into your housings at all and I needed them in all the way for good looks


Jon