Question on HDPE

[ChrisA Edwards]

I wanted to make a drain board for my RV, so I bought a cutting board, from Amazon, that was large enough to make two. It was 3/4" (18.75mm) thick.

The top I pocketed out 1/4" and left a 1/4" wide rim around 3 sides. The drain slots go all the way through. On the underside, I pocketed out around the edge so it sits in the sink cutout.

This worked great on my MDF prototype, but the HDPE curled/warped like I had released some internal memory stresses. Mostly on the edge where I did not have the upper rim. (Left edge in photo MDF below). The bulk of the board, an area larger than the drain slots is still 1/2" thick.

Of all the plastic cutting boards, we've had for years, run them through the dishwasher, none have warped.

Also looking for suggestions for a better material.

Thanks

[Ted Calver]

This looks like a really nice project. Perhaps a thicker piece of material might not warp? I know a lot of marine applications use a brand of HDPE called King Starboard , which is available in several sizes, colors and thicknesses. No affiliation.

[Mark e Kessler]

Look for “stress relieved” material, stress in plastic can be caused (and somewhat normal) by many things including injection pressure (if injected molded), heat combined with the pressure, the cooling rate….etc…

[Jim Becker]

This issue might also be because of using what appears to be a molded cutting board as your base material. Check for a local plastics supplier to get "raw" HPDE of the type that Mark just mentioned.

BTW, you'll want to be using tooling appropriate for plastics to cut this material. I generally use O-flute, single flute cutters listed as for plastics/aluminum.

[ChrisA Edwards]

I tried an O flute bit initially, but seemed to get a better result with my regular compression (I think) bit.

Looking into stress relieved sheet locally.

[Alex Zeller]

It's not part of my job but we do machine plastics like HDPE. I think you need to anneal it first and possibly use something like water to cool it as you cut it. I've never asked how they anneal it but I think it's with an oven. I'm sure you could find the process on-line.

[ChrisA Edwards]

Yes, as an experiment, I've bought two cheap cookie baking sheets. I'm going to put this piece between them and put it on my outside BBQ and try to control the temp to around 250F and keep it there for 2 hours. I'll put about 10lbs of weight on the top tray to try and hold it flat and then let it cool down to ambient temps.

[Mel Fulks]

They say platinum is good for that kinda’ stuff . Very durable, BUT it only comes in one color , so some women won’t use it.

[Malcolm McLeod]

Yes, as an experiment, I've bought two cheap cookie baking sheets. I'm going to put this piece between them and put it on my outside BBQ and try to control the temp to around 250F and keep it there for 2 hours. I'll put about 10lbs of weight on the top tray to try and hold it flat and then let it cool down to ambient temps.

Mr Kessler is exactly correct about injection molded polymers and influences on stress - only need to add duration of packing* pressure.

*- hold pressure on mold for a few seconds to force just a bit more in as the 'shell' cools and contracts, so keeping the partproperly sized and in tight contact to the cold surfaces of the mold for good heat transfer.

Careful at 250degF with HDPE... I think you are trying to stress relieve this, not liquify it - and that is getting close. I'd probably shoot for 200-210degF, and key will be to heat soak all of the material, not just the outer shell. (Your slots should help a LOT in this regard.)

In a previous life, I processed 350,000-400,000 lbs of HDPE per month thru 2 injection molding machines. It was a tiny facility as such goes, just big machines (700t). It has been a long time, but IIRC we ran the extruder barrels at ~265-275degF? The exact temperature was determined by the melt index of a given batch of raw material (pellets). We WERE trying to liquify it and keep it that way all the way into a 2-3ft wide mold at 40degF.

...just in case you want to go into the molding biz.

ETA - Consider using boiling water - if you have a tray big enough, fill it with water and float the HDPE in it as it boils. Water boils at a constant temperature; you know you'll have 212degF (or close, depending on your altitude?).

[ChrisA Edwards]

Thanks, that might be a good plan of attack.

[ChrisA Edwards]

Found this
https://www.boedeker.com/Technical-R...ing-Guidelines