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Thread: Working with Plastics

  1. #1
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    Working with Plastics

    Some points will be obvious to some people.

    Firstly acrylic and polycarbonate are the most relevant materials to cabinet construction so most of the info refers to these two types of plastics.

    plexiglass is a trade name for a material called acrylic. Most sheet form acrylic is cast, this means the thickness is nominal and there is ALWAYS a fractional variation in thickness across the sheet. Acrylic is hard but brittle. It forms well with heat.

    Lexan is a trade name for a material called polycarbonate. Most sheet form polycarbonate is extruded, generally there is less variation in thickness. Ploycarb is softer than acrylic, scratches easier but os very tough. This material retains some water from the manufacturing process and needs to be dried before it can be formed. If it is heated without being dried the water can boil and the sheet will blister.


    Drilling

    I've heard some interesting methods for drilling plastics over the years. Running bits in reverse is a common one but I remember my high school shop teacher taught the "Correct" way to do it was to blunten the drill bit on the concrete floor before drilling plastics. Roll Eyes

    The key to cutting clean holes is to scrape not cut. Any standard twist drill can be sharpened to cut acrylic by taking the leading edge off the cutting surface.





    Spade bits like this can be resharpened to scrape not cut and used

    but NOT spade bits with the points like this


    Forstner Bits and hole saws are suitable also


    When drilling thicker material start with a small pilot hole and drill half the thickness from both sides and never use a lot of force.

    Cutting

    On the cheap a surprisingly accurate and effective way to cut straight lines is to "score and snap". Use a sharp tool with a cross section of about 1mm, like a tile cutter or a scoring blade pictured below to score a deep scratch across the panel using a straight edge. With your thumbs either side of the score and your fingers under the piece you can snap a nice clean line. This is quick and easy but not really suitable for cuts longer than about 300mm.




    Generally speaking Aluminium cutting tools are the way to go. Fine tooth jigsaw and bandsaw blades are the key, I use 10tpi blades in my bandsaws. When cutting with a jigsaw ensure the material is supported as close to the cut as possible. A good trick is to place your acrylic on top of a sacrificial layer of styrene foam or florists foam and cut through both. Make sure the foam is thick enough for the full stroke of the jigsaw Grin. For a circular saw or table saw blades you need a "zero chip" or a zero or negative rake saw blade, this refers to the pitch of the teeth.

    In a pinch you can get away with a hacksaw if you a prepared to clean up the edge with a sander.

    Routing

    Routing is the best way to get good results with limited tools.
    It is as easy as that. If you take one piece of advise from this thread this is the one it should be.

    Using straight cut two flute flush trim bits give a nice clean controllable cut, pic below. Low helix spiral cutters will give a better finish but cost a lot more and are harder to find. 99% of the cutting I do uses straight cutters. I only use the low helix cutters on >20mm acrylic cut on my CNC router. Anything thinner than that I cut on my LASER.



    When flush trimming ensure your acrylic is firmly attached to the guide piece. If you get chipping or chattering you are going too fast, don't have enough clamps or the router bit is too blunt.

    I'm assuming the most common, complex acrylic pieces most reading this will cut will be CP protectors. Along with clamps around the outside consider using screws with big washers in the button holes to clamp the workpiece in the middle.

    Forming and Moulding

    As I'm sure you are aware there are plenty of ways to mould and from plastics, I will deal with only the relevant ones here.

    Line bending is exactly what it sounds like. Bending a flat panel in a single plane.

    Polycarbonate can be bent in a panbreak/breakpress/metal folder.



    In the gap is a heating element and the metal tubes are cooled with a flow of water. The acrylic sits on to until it is soft enough to fold. The closer the metal tubes are the tighter the radius of the bend will be and a reverse is true for a wide gap, to a point. A very sharp bend can be achieved by grooving the underside of the work piece with a V-bit in a router.

    That's not much help if you don't have a line bended, with a hot air gun you can get very good results. In place of the metal tubes use some thick, >12mm, MDF to insulate the acrylic and confine your heat, allow double the thickness of the material on top of the radius you want to bend and don't heat all the way to the MDF. You want a slow EVEN heating of the acrylic.
    Heat like you are painting with a spray gun, don't make changes in direction over the material or you will over heat those spots. The ends require a bit less heat than the middle so vary your speed accordingly ie faster at the edges than in the middle. Anything thicker than 4.5mm will need to be heated on both sides. Use your MDF insulators as clamping bars so the middle is held not just the outer edges.

    Remember SLOW EVEN heating. let the material form under its own weight, don't force the bend.

    Complex curves, drape moulding and blowing domes and bubbles are all possible with household tools. I helped a mate blow mould some shaped windows for his panel van using a BBQ. If you want help just ask.

    Anything else

    Bonding. NEVER USE SUPERGLUE. It will cloud and craze the acrylic and not bond well. Methylene Chloride, the active ingredient in acrylic paint stripper, is a good solvent for light duty bonding. There are plenty of commercial glues available but you can make you own if you have Methylene Chloride. Add small pieces of the acrylic to some Methylene Chloride and allow the solvent to melt the chips to a glue like consistency. You can make coloured glues this way.

    Polycarbonate can be pop riveted. The brittle nature of acrylic makes this an unsuitable solution.

    Polishing plastics can be done in two ways flame polishing and abrasive polishing.
    Flame polishing involves heating the top layer with a very fine very hot flame and melting the outer edge of the acrylic and it will then cool nice and glossy. I have a hydrogen generator that is designed as a jewellers welder. Flame polishing is only suited to cut edges.
    Abrasive polishing is sanding. Using progressively finer grades the surface imperfections are removed. Using finer grits of sandpaper up to 2000 grit then moving to liquid compounds is the way to go. Metal polishes work well, automotive cut and polishes and even toothpaste will do a good job.

    Tapped thicker acrylic can hold a decent amount of strength but only use fine threads. A bit of grease on the tap will give the threads a bit of a polish.

    There is a lot of info there but no doubt someone will want to know something else, or I've forgotten something blindly obvious so Any other queries or questions just ask...

    Cheers
    Damien

  2. #2
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    A brilliant tutorial - thanks

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    Great info selfie.
    Cheers.

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    Excellent post, thanks.
    Dave
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  5. #5
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    Nice trick to make "coloured" glue

    cheers

    DR
    once you go ZAC you cant go back

  6. #6
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    sensational!
    i have some acrylic neon cases that i need to do some work on and was only thinking the other day how best to bend and glue some new pieces in (i was going to get the plastic supplier to cut to size to avoid cutting it as ive had mixed results in the past and never thought to use a router, which i have a couple of).
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    thanks for the guide. this has come in handy

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    Great and informative post.Thanks for guiding...

  9. #9
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    Could I also suggest a step drill bit, they dont bite into plastic and cut great holes.


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