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Old 19-05-2015, 16:28   #1
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Thermoforming A Curved Acrylic Hatch

Finally getting around to making a replacement for my curved acrylic companionway hatch. Figured I would share some pix.

Pretty simple really. Always helps to have the right tools of the job but I have done some junkyard forming in the past with a variety of heat sources and forming techniques with both acrylic and Corian.

In the case of my hatch, I had access to a CNC machine to cut the curved plywood frames for my form. I also used a vacuum bag to laminate the face of my form, which is made from a couple layers of MDF with a final layer of masonite on top.

(Masonite - Wikipedia, the free encyclopedia)

What you see in the photos is a hot platen press with a vacuum bag on top. You start by cooking your sheet to about 340 F before laying in the form and pulling a vacuum on the bag. Let it cool. Pretty simple.

You can see in the one photo of the part coming out of the platen press where the heated acrylic is quite supple. That's 1/2" material.

It's nice to have access to the right tools but it ain't rocket surgery, it's just plastic. I have done some junk yard stuff so if you are ever considering a similar project and have any questions, ask away.

I'll post some photos of the finished piece when I pick it up.
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Old 19-05-2015, 17:19   #2
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Re: Thermoforming A Curved Acrylic Hatch

That's pretty cool... looking forward to seeing the finished product!

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Old 19-05-2015, 17:50   #3
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Re: Thermoforming A Curved Acrylic Hatch

Thanks for posting
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Old 19-05-2015, 17:55   #4
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Re: Thermoforming A Curved Acrylic Hatch

I've seen DIY curved motorcycle/motorscooter acrylic windscreens formed in a household oven using just gravity over a form.

Requires careful monitoring and sure to upset the wife, but it can be done.

Not sure what the thickness limitations are, though. The ones I saw were about 3/16".
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Old 20-05-2015, 08:26   #5
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Re: Thermoforming A Curved Acrylic Hatch

What would you do if the curve was slight, maybe 2 total inches in a 36" x 14" sheet of 3/8" plexi with only a propane torch or heat gun for tools. Is it possible?
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Old 20-05-2015, 09:09   #6
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Re: Thermoforming A Curved Acrylic Hatch

2" bend in 36"? You don't need heat for that. Just bend it cold and fasten it in.
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Old 20-05-2015, 10:19   #7
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Re: Thermoforming A Curved Acrylic Hatch

Quote:
Originally Posted by captainwireman View Post
What would you do if the curve was slight, maybe 2 total inches in a 36" x 14" sheet of 3/8" plexi with only a propane torch or heat gun for tools. Is it possible?
I've often bent thin (1/8") plexi in sharp bends using a heat gun, but I suspect bending thicker stuff -- especially evenly over large radii -- requires a more general, gradual application of heat, as with an oven.
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Old 20-05-2015, 11:04   #8
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Re: Thermoforming A Curved Acrylic Hatch

Thanks for the post. This is a topic of great interest. Please describe your machinery in the images. Also, can you recommend some basic references for gaining a comprehension. I have been looking at some Polyethylene applications and am interested in creating some sandwich composite structures. Good sources to learn? Thanks.
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Old 20-05-2015, 11:15   #9
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Re: Thermoforming A Curved Acrylic Hatch

I used to make some plastic windshields for small planes. Built an oven from plywood, lined the inside with foil to keep the fire danger down. An oven heating element and a good thermometer. The temp range is narrow and you need a way to support the sheet while it's heating also.
Using heat guns, hot wires etc. work only some kinds of parts. You need an even temp over the whole part to get good optics. It's not to hard to do simple curves but compound curves are more difficult and you need a mold, which is a whole other project.
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Old 20-05-2015, 18:50   #10
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Re: Thermoforming A Curved Acrylic Hatch

Quote:
Originally Posted by Guy View Post
I used to make some plastic windshields for small planes. Built an oven from plywood, lined the inside with foil to keep the fire danger down. An oven heating element and a good thermometer. The temp range is narrow and you need a way to support the sheet while it's heating also.
Using heat guns, hot wires etc. work only some kinds of parts. You need an even temp over the whole part to get good optics. It's not to hard to do simple curves but compound curves are more difficult and you need a mold, which is a whole other project.
^^What he said^^

Heat gun and hot wires for line-bending, build oven for larger pieces, compound curvature requires a compound curved form. The hatch I made is a section of a cylinder so much easier. Also, compound curvature requires either stretching or compressing the material so is also sort of a whole different game.

See video.



For reference I think my curve is about 7/8" deep over about 32" width, it's actually a radius that was like about 10' and my material is 1/2". Thicker material takes longer to heat up and cool down. The key to precision is the uniform cooling. Sorta like with welding or casting metal.

The only way to make something a "perfect size" is to form the piece oversize and trim after. In the case of my companionway hatch, the edges will be concealed so I don't care if the are a little wavy. We'll see.

Yes, you can cold form acrylic but it has it's limits. Polycarbonate is much better for cold forming. Either material will turn slightly opaque at the bend, acrylic will break much quicker at a larger radius bend.

The only way to get optical qualities on a bend is using heat but even then you will see striations. You see line-bending all the time on like bakery display cases or other POS.
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Old 20-05-2015, 18:53   #11
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Re: Thermoforming A Curved Acrylic Hatch

I plan on a no fastener method. Sorry I do not have the 3M specific product recommendations handy but I will use 2 side tape (vhb?) with adhesive sealant. The plexiglass manufacturer recommended no holes drilled or use of screws if possible. Sounds like you feel there will be more than enough flex. I have a sample of the tape and it is very strong. Do you agree?
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Old 20-05-2015, 19:00   #12
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Re: Thermoforming A Curved Acrylic Hatch

What does your hatch look like, what are the dimensions?

Edit: I see above 36" x 14" but having a hard time imagining it. Are you replacing something? 3/8" thick to cold form acrylic. Doubt VHB will hold it til the adhesive caulk cures. Must clamp. Would recommend polycarbonate but scratches easier and doesn't last as long.
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Old 20-05-2015, 19:08   #13
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Re: Thermoforming A Curved Acrylic Hatch

These are windows. My friends at the local pub insist I use safety glass! Weight to strength ratio plus the fact there is a curve to almost all got me researching and decided to not do what these "pros" say. Max 2 "radius in the 36 length of a 36143/8 section.
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Old 20-05-2015, 19:26   #14
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Re: Thermoforming A Curved Acrylic Hatch

Fixed port lights are a little different in that the depth of curvature is usually relatively slight and the areas are usually long and skinny and the materials are commonly much thinner. On monos at least.

If you can cold bend it to the shape you need and get it to bond you'll probably be fine. Don't know about the folks down at the pub. Pictures are worth a thousand words so could maybe be more helpful if you could post some.
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Old 20-05-2015, 19:42   #15
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Re: Thermoforming A Curved Acrylic Hatch

Sorry, no pics but you are correct in assuming the curve is slight. Two inch is overkill but I wanted to be sure. I will check with the plexi manufacturer and see what his opinion is. You can easily google Dean Oceancomber 10M and see specifically what my upcoming project is. The present windows had undersize drilled holes that all spider web cracked at the screws probably from thermal expansion and contraction and/or overtightining.
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