Controlling creep in Rotational molded water tanks

[ozchevy]

Hi guys,
I work for a Rotational molding company in Sydney Australia, and we manufacture Polyethylene water storage tanks in a three tier design mold with a capacity of 2000 litres. These are not small molds in physical size. We recently have encounted the age old problem of creep, (pardon the pun) It would seem we have sufficient wall thickness through out, including the kiss off points between the tiers. Rotation ratio seems right with a 5:1 on the plate and arm, oven time is 20 minutes at 280 degrees Celsius, and cooling time is also 20 minutes fan cooled.
We are now finding that many of our sold water tanks are failing (splitting at the base of the kiss offs) where all the load is, when the tank is full of water - approx 1.8 tonne of head pressure. Is there an additive available to mix with the powder to control creep in the molding process, or perhaps cross linking, or should we be looking at tooling modifications in the area of the kiss offs, or adding venturies to gain extra heat in the area of the kiss offs.
Any comments would be appreciated.

Thanks in advance,
Geoff.

 

[ornerynorsk]

Is 20 minutes sufficient for a mold of that size? Seems like a lot of material to heat up.

 

[ozchevy]

Well that is really quite hard to answer, It's a balance of getting a well formed molding and producing as much product as possible in a ten hour shift.

From an engineering point of view, I say no it's not, even with venturies to force the heat into troubled areas that may cause a concern. I have recommended to higher management within our organization to increase the oven time and also to increase the cooling time.
The dollar value is all they are interested in.

From the 700 odd tanks out in the market as sold items, 15 have been returned with fatigued kiss off points, which management see as a tidy percentage on the total made.

I think this problem runs deeper than this has already shown.

 

[btrueblood]

Oz,

is there a sharp corner or step at the "kiss off" points, and is there any way to blend these points to create a smoother/radiused edge? I'm imagining you are describing the join lines where mold sections/segments are clamped/bolted together?

 

[ozchevy]

Hello btrueblood,

Yes you are correct with surmising the parting lines of the kiss offs. The area is reasonably well curved except for the flat sections of the kiss offs or centre bolting sections of the part lines.
We are currently running two of these molds back to back on one arm of a Ferry 4000 Rotary molder. I have just finished today, mounting up six air venturies directed into the base of the kiss off and hoping to run a trial on the molds tomorrow. In saying that though, I now have a concern regarding air requirements to feed the venturies, which will consume huge volumes of air for its time in the oven.
Will post tomorrow regarding the results of this trial.

I do have another question, From the tanks returned with creep rupture, would anyone know if it is feasible to ultrasonic blend (weld) in pieces of plastic to laminate over the area of the ruptures as a fix to our problem. We will purchase a handheld ultrasonics if it would work well.