HDPE and sag?

[rennjidk]

I've found a place that mills and finishes custom hdpe. I'd like to make ridged bases for my rimless nano tanks to rest on. The issue I'm running into is that while most furniture can easily handle a 5-10g tank, the actual resting surface deflects under the load. That's fine for say a rimmed tank, but having a rimless not being fully supported in the middle keeps me up at night.

Anyways, how thick would you go for a ridged base that won't deflect under the 50-70lb load across roughly 16 inches? I can get up to 1", but price goes up exponentially with thickness so I'd prefer not to over engineer too much.

 

[Timfish]

I'm curious what surface you'd place a tank on that would deflect, a desk top, small table? FWIW I'd use some oak or 3/4" plywood to support the HDPE. Assuming your tank is glass keep in mind any glass tank with a frame has large areas under it unsupported. If acrylic, acrylic is stiffer than HDPE. WIth either deflection will be less if the tank is resting directly on a surface than if it was supported with trim.

 

[rennjidk]

I'm curious what surface you'd place a tank on that would deflect, a desk top, small table? FWIW I'd use some oak or 3/4" plywood to support the HDPE. Assuming your tank is glass keep in mind any glass tank with a frame has large areas under it unsupported. If acrylic, acrylic is stiffer than HDPE. WIth either deflection will be less if the tank is resting directly on a surface than if it was supported with trim.

Right now I have a 7g rimless resting on top of a drawer chest, but pretty much any "flat pack" furniture will deflect over 4g in my experience, unless it's a tv stand or something more robust.

I can certainly use plywood and have before, but I'd like something more visually appealing like the pedestal stands IM makes for their desktop tanks, hence machining a piece of thick colored plastic.

 

[theatrus]

Can you describe the deflection you are seeing? Flat pack furniture is usually chip board or MDF, and more modernly a very engineered hollow MDF beam. Do you care the tank is going to be unlevel if you just support directly under the tank, or do you want a topper for the entire table surface?

 

[rennjidk]

Can you describe the deflection you are seeing? Flat pack furniture is usually chip board or MDF, and more modernly a very engineered hollow MDF beam. Do you care the tank is going to be unlevel if you just support directly under the tank, or do you want a topper for the entire table surface?

It's a solid sheet of mdf as the top of the chest. I'm just looking to add some rigidity under the tank itself so that all the glass on the bottom makes contact. Here's an exaggerated sketch I did on my phone. The actual sag (in red) is only maybe 1/16". It's just more than I'd like a leveling mat to make up for. There are numerous sag calculators for wood, I just can't find one for plastic.

 

[kyleinpdx]

I may be misunderstanding but.... why not properly brace/secure the current dresser MDF top? Say a 1x2 running back to front? Almost any material that goes over an unsupported span will have some sag/deflection.

 

[rennjidk]

I may be misunderstanding but.... why not properly brace/secure the current dresser MDF top? Say a 1x2 running back to front? Almost any material that goes over an unsupported span will have some sag/deflection.

There's not room underneath. The top drawer almost contacts the bottom of the mdf. In fact, when I tried to set my 13g Evo on it a while ago, it caused the drawer to stick as it was resting on it under the sag. I just threw a 1" piece of plywood on top of the dresser and it was fine for over a year. I could do that again, but I want something that looks nice.

 

[chonsworth]

I guarantee it you're overthinking. Unless the item in question is made out of cardboard, I wouldn't stress about that kind of deflection. 1/16 is definitely something a mat should take care of if you're worried about - but tbh the tank might not be heavy enough to dig in to the mat to reap the benefits.

edit: if it's binding the drawer with a 13 gallon tank I'd be really curious to know what the surface is for that dresser

 

[rennjidk]

I guarantee it you're overthinking. Unless the item in question is made out of cardboard, I wouldn't stress about that kind of deflection. 1/16 is definitely something a mat should take care of if you're worried about - but tbh the tank might not be heavy enough to dig in to the mat to reap the benefits.

Possibly, but it's only $30 for the machined piece of plastic. I was really hoping some engineer would have a formula of "X thickness over X span with X weight = X sag" the same as wood workers have for shelving, only with platics.

edit: if it's binding the drawer with a 13 gallon tank I'd be really curious to know what the surface is for that dresser

It's 1/2" mdf over a 27" empty span. ~170lbs in the middle will definitely make it sag, lol.

 

[kyleinpdx]

There's not room underneath. The top drawer almost contacts the bottom of the mdf. In fact, when I tried to set my 13g Evo on it a while ago, it caused the drawer to stick as it was resting on it under the sag. I just threw a 1" piece of plywood on top of the dresser and it was fine for over a year. I could do that again, but I want something that looks nice.

Ok, I follow now. My understanding of why you cant find any deflection calculators for HDPE is that there isn't a single formulation or production process. HDPE is polymerized ethylene whose molecular arrangement is due to the addition of a catalyst, as there are many different catalysts (and combos) that can be used, so there are many different potential end products.

why not go the plywood route, either adding another layer atop or replacing the current top. furniture grade plywood with some stain/varnish and maybe even some routed edges.

 

[chonsworth]

You could double the MDF, lazy math says another 1/2 would maybe just be 1/32" sag (or less).
Paintable, easy to sand. Cover it in a boat load of poly.

 

[rennjidk]

Ok, I follow now. My understanding of why you cant find any deflection calculators for HDPE is that there isn't a single formulation or production process. HDPE is polymerized ethylene whose molecular arrangement is due to the addition of a catalyst, as there are many different catalysts (and combos) that can be used, so there are many different potential end products.

why not go the plywood route, either adding another layer atop or replacing the current top. furniture grade plywood with some stain/varnish and maybe even some routed edges.

Thanks for the info. Honestly, its just more time and money than I want to put into a cheap flat pack dresser. Id just buy a solid pine one if I were to go that route, and probably will in the near future. It's $38 for the 3/4" finished piece of HDPE, so I'm just going to try it.

 

[Timfish]

Right now I have a 7g rimless resting on top of a drawer chest, but pretty much any "flat pack" furniture will deflect over 4g in my experience, unless it's a tv stand or something more robust.

I can certainly use plywood and have before, but I'd like something more visually appealing like the pedestal stands IM makes for their desktop tanks, hence machining a piece of thick colored plastic.

I would think Acrylic would be a better material structurally. You might be able to find thick "shorts" or left over pieces at local plastics fabrication shops. Could your design be easily modified to have metal or wood supports hidden under it?

 

[rennjidk]

I would think Acrylic would be a better material structurally. You might be able to find thick "shorts" or left over pieces at local plastics fabrication shops. Could your design be easily modified to have metal or wood supports hidden under it?

Yeah, after looking into it further or some boating forums (its a popular material), it's not as ridged as I thought. Acrylic would probably be better, but it's much more expensive. I think at this point I'm just going to get a solid wood dresser if it continues to bother me.

 

[theatrus]

From a stress-loading on corners of a rimless tank, your 3/4" would probably be fine (disclaimer, I am not a materials engineer and as others have said, the calculations require some constants which vary a lot).

Thats a lot of material thickness for the weight and, sitting on top of the bending MDF, would probably do the job just fine. Plus having a black or other nice color actually would look aesthetically pleasing. And it doesn't absorb water or otherwise have issues getting wet like even plywood does, so no visible degradation.

Dead tree carcass materials are pretty cheap though Personally I use a sheet of 1/8" plywood on small ADA rimless tanks on MDF IKEA tables as a buffer and its no issue whatsoever.

 

[FUNGI]

You mean something like this? BUT, as others have said, plywood

 

[rennjidk]

So, I went to open the top drawer today and noticed it was rubbing. I took a piece of paper and can freely slide it in the red area, from front to back of the tank. The blue areas have solid contact. Would you trust this in a 7g, 4mm rimless or should I drain the tank and replace the dresser now?

 

[All_talk]

Looks like a great application for a foam pad.