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Thread: Question: Bowfront Mold Materials

  1. #1
    DIYFK member DailyLunatic's Avatar


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    Question Question: Bowfront Mold Materials

    I've made or repaired a couple of tanks in my day. Nothing larger than 30 gal, and all glass.

    I'm wanting to start a new build, but out of acrylic this time because I want this to be a long, thin, short, bowfront along my kitchen bar top

    I've been reading up here, but don't see much in the way of the mold creation for a bowfront. (search terms of bowfront and mold did not reveal much)

    I will be heating the acrylic and allowing it to relax into the mold shaped for my bar.

    My first concern is that if the mold is not smooth, the acrylic will pick up 'texturing' or seam lines. I'm thinking that lining with plastic or a hardboard type material might work, or fill/paint to smooth.

    The second concern is that the acrylic might adhere to mold. I would hate for flecks of paint or plastic to bond with the face of the acrylic.

    Are these viable concerns, or are there any suggestions on the materials that come in contact with the acrylic while heated? New to acrylic, so I hope I am concerned about nothing.

    Thanking you in advance,
    -sterling

  2. #2
    DIYFK member Meep's Avatar


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    If heated properly with a decently smooth buck the acrylic won't pick up any noticeable texture unless forced or pushed down on the buck...

    My plastic forming experience is limited to thin plastics, as I don't have a huge properly calibrated oven (or specialized heating blankets) to properly heat thick acrylics...

    That said when I have seen thick 'clear' acrylic formed done, they use some type of 'fabric' like coating stretched over the buck, I don't exactly know what it is it just appears to be some type of fabric...

    Also note, that acrylic is hygroscopic that means is absorbs moisture from the air, this trapped water is what caused acrylic to 'bubble' when heated, to avoid this you need to very slowly over a long period of time heat the acrylic (below forming temps) until you drive out all the water, then once the water is out you bring it up to forming temp... If you don't drive out the water first, when you bring it to forming temp the water will turn to steam, expand and create a ton of micro-bubbles...

    You can see the 'fabric' over the bucks in this video...



    And briefly in this video at about the 1:50 mark...


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    DIYFK member DailyLunatic's Avatar


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    Thank you for the post.

    The fact that acrylic is hygroscopic was new to me. I will work this into my plans and slow down the heating process. Thanks.

    My initial plan was to use a modified toaster over and move slowly along the 96" length. Couple of inches an hour for instance. Haven't worked out the details yet. Still gathering info.

    The acrylic calculator is giving me just under 1/4" as the thickness for the planned tank. I'm planning to bump up to about 3/8".

    A recent internet search found a page referencing flannel as a material upon which to drape form acrylic. This site would not yet allow me to post the Plaskolite URL. No idea if flannel is what is being used in the videos. While the forming process that I envision 'at this point' is not technically anything mentioned on this referenced page, drape forming comes closest.

    To those that have formed acrylic in the 3/8" range: What heating options would be available to a DIY'r wanting to bend a 96" x 12", 3/8" strip strip of acrylic? Too large for a home oven, and bending machines create too sharp of a bend. Would the a fore mentioned modified toaster oven crawling along its length be an option? ...or should I be concentrating my research on creating something akin to a heating blanket?

    Thanking you for your help,
    -sterling

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    DIYFK member Warfie's Avatar


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    Well look at what I found! It's older, and is only for smaller parts, but you get the idea...




    Meep, I think it's some micro fibre of some kind. Helping to keep scratches to a minimum.


    DailyLunatic, I love your project and that you want to try this! My approach for something this size would be this way...

    Need to make a full size form, and put the time into it to get it square and right.

    Build a wooden frame work and use 1/4" drywall for the "skin".

    Cut the first side of the form from 3/4" plywood, OSB would be fine. Sand it to ALMOST finished shape. Clamp (screw) the first side to the sheet of 3/4" plywood. Use a router with a templating bit to cut the second side. Leave them clamped together and finish them both to the FINAL shape. Separate the two sides. Brace across the two frames, 3/4" plywood on the back and 2x4 blocks along the curve, keeping them below the curve of the outer frames.

    This is like a practice run for the acrylic... Cut a length of 1/4" drywall larger than your form, width is more important and DO NOT use the tapered edge. Wet both sides and let it drape over the form, it will probably take more water than you'd think. Let it ALMOST dry out and attach the drywall to the form with screws along the edge of the form only. Start in the middle and work the drywall out to the ends. I wouldn't think you'd need to fill the screw heads, like in a finished wall. Use a flush cutting router bit to clean up the edge of the drywall even with the form. USE A DUST MASK AND GLASSES!

    Cut a length of acrylic larger than your form, just like the drywall. Make sure the acrylic sheet is LINED UP with the form, the closer it is, the easier life will be... Clamp the acrylic to the form in the middle and use framing squares to align the ends to the form.

    The forming range for acrylic sheet is 290-320F (142-160C). REMEMBER to remove the protective covering from the acrylic sheet!

    Use of 2 heat guns (2 people working the guns) to heat up the sheet should get it there, and allow plenty of time and heat to work the sheet. Work the acrylic out to the ends, letting gravity get the "rough" shape of the form. Once cooled, clamp the acrylic to the form and flush trim the acrylic with the router.

    If this is "good enough" for the shape, ALL GOOD.

    If not, make a smaller, say 12" upper "slip form", to hold the shape while it sets up. Weight the upper form to help keep the form in place and "force" the sheet into the desired shape. Clamping the slip form may be a better option...

    Any scratches should be easily polished out.


    It's a LOT of work and a little bit of skill is needed. Might be easier and "cheaper" ($75 to $100 for the form, not including any tools) to have some company form the piece for you...

    Good luck!

  5. #5
    DIYFK member DailyLunatic's Avatar


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    Not familiar with 1/4" DryWall. Thanks for the suggestion. I'll look into it.

    Question:
    As the Acrylic strips are narrow, do you feel that a one piece mold would be sufficient? ...or does the Acrylic have a memory while cooling?

    -sterling

  6. #6
    DIYFK member DailyLunatic's Avatar


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    Click here to enlarge Originally Posted by Warfie Click here to enlarge
    It's a LOT of work and a little bit of skill is needed. Might be easier and "cheaper" ($75 to $100 for the form, not including any tools) to have some company form the piece for you...

    Good luck!
    What sort of company would do this sort of work? A couple of hundred to avoid the labor would be worth it.

    -sterling

  7. #7
    DIYFK member Meep's Avatar


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    Click here to enlarge Originally Posted by DailyLunatic Click here to enlarge
    As the Acrylic strips are narrow, do you feel that a one piece mold would be sufficient? ...or does the Acrylic have a memory while cooling?
    In my experience with forming plastics they don't necessarily have much 'memory' aka residual stress if heated EVENLY to the proper forming temps and held it place until it cools a bit and sets (unless it's a high rubber content plastic) but the edges do tend to 'curl' and 'lift' if not held down at all times...

    The biggest problem with most DIY heating is that it's not properly controlled heat... It will not heat evenly and this will cause as you said memory issues, also the temp is not controlled so it's very easy to overheat an area and burn the plastic... Plus as I said due to the hygroscopic nature of clear acrylic that you want to remain optically clear it's real hard with DIY heaters to slowly drive the moisture out of the acrylic unless you have a way to more properly regulate it... I have formed some 1/8" clear acrylic on my vacuum table with mixed results, but what I have found especially true is that the longer and slower I heat up the acrylic (within reason) allowing the heat to slowly drive out the moisture the better the results... It's probably been about 10 years since I formed clear acrylic but I believe I heated the 1/8" sheet for about 1 hour at 150F before I formed it... This isn't necessary for many other forming plastics like styrene, ABS, PVC or HDPE or even opaque acrylic sheets where the micro bubble probably won't be noticed... You could probably drive the moisture out of a sheet fairly well using a crude plywood box and laying the sheet on a common bed sized heating blanket at the bottom of the box... A bowl or pan with decadent (crystal cat litter) in the DIY oven will probably even improve drying results as it will trap any moisture in the DIY oven...

    This site suggest 'drying' time can be up to 24 hours at 176F, also note this site actually gives a clue on to what the bucks are covered with, they say it's billiard table felt... I do argue that their claim that "Predrying acrylic sheet is rarely necessary" I have found it absolutely necessary for clear acrylics if you want to maintain optical quality and avoid micro bubbles, but I suspect that how much drying will change by your local climate and storage conditions and obviously the thickness of the material...

    http://www.plasticsmag.com/ta.asp?aid=2260

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