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Nick
Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
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quote:
what kind of glue could i use to "weld" my plastic frame up? I can't find shoo goo anywhere so i'm assuming it's only available in america.
There is a special glue for polycarb - unlike any other type, it melts the surfaces of the parts and fuses them together when it dries. It may be available from Mulford plastics and its sure to be expensive. It is also possible to heat weld polycarb but it takes skill & machinery.
All glue joints in ploycarb will be brittle and likely to snap on the first impact. The two methods I would recommend is heat forming and screws. To make a rectangular frame, you can cut a strip longer than the four walls, mark the bend points, then heat the polycarb and bend it around a former like a piece of waterpipe. You can use a paint striping heat gun or a propane torch (very carefully) and you might get better results by heating the former rather than the plastic directly. It is important to use a gentle radius at the corners for strength. After the fourth bend, the ends of the strip can be cut off so they overlap a little and screw them together.
The other method, which might be easier, is to cut the sides and make corner brackets from small aluminium angle. You can drill & screw the polycarb sides to the angle with M4 screws for a rigid side frame although it will weigh more than the first method.
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ALSO superglue makes weird white powder .
Super glues make very brittle joints; if they flex they will break quickly.
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And also, is it ok to spray paint polycarb?
You can buy special paint in R/C hobby stores. Regular paint is said to weaken polycarb, although I suspect its partly urban myth. The R/C stuff sticks better too.
_________________
Australian 2015 Featherweight champion
UK 2016 Gladiator champion
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Thu Nov 17, 2005 12:47 pm |
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kkeerroo
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane
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And nobody wants to ask me?
quote:
Originally posted by TDT:
I hear chloroform is one of the chemicals you can use to chemically weld polycarb together, just don't breath any of the stuff in, you might be out cold for a few hours 
The glue is called Weldon. There are different types and I believe the difference is down to the drying time. I remember it being Weldon #4 and Weldon #16, but I could be wrong. The active ingredient is Methelyne Chloride.
I tried to buy it from Mulfords but the bloke on the phone was very rude and refused to sell it to me in the quantities I needed.
quote:
All glue joints in ploycarb will be brittle and likely to snap on the first impact. The two methods I would recommend is heat forming and screws. To make a rectangular frame, you can cut a strip longer than the four walls, mark the bend points, then heat the polycarb and bend it around a former like a piece of waterpipe. You can use a paint striping heat gun or a propane torch (very carefully) and you might get better results by heating the former rather than the plastic directly. It is important to use a gentle radius at the corners for strength. After the fourth bend, the ends of the strip can be cut off so they overlap a little and screw them together.
The other method, which might be easier, is to cut the sides and make corner brackets from small aluminium angle. You can drill & screw the polycarb sides to the angle with M4 screws for a rigid side frame although it will weigh more than the first method.
1) Welded joints using the Weldon glue have proven to be stronger then the rest of the polycarb.
However it is true the Weldon is the only glue that will work.
And you will not be able to use the Weldon Glue on 3mm polycarb due to the insufficiant surface area at the joins. You will need to use lap joins which will require bending for 90 edges, its just not worth it. You might as well bend the on sheet in the first place.
2) Heat bending of polycarb DOES NOT WORK. Trust me I have tried and researched more methods then anyone else has. The polycarb will begin to break down before it reached the correct heat. Actually you end up having water vapour forming with in the polycarb as it breaks down which then bubble to the surface. I have tried taking polycarb to just below this heat and then bending it but it did not become flexible enough to be worth the effort.
3) You second method won't work either. M4 bolts in 3mm polycarb?
Even if you tried tapping into the surface of the sheet 3mm is not thick enough to be really useful and I garentee thats where you cracks will start.
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Super glues make very brittle joints; if they flex they will break quickly.
Super glue need a porus surface which is not in polycarb. It will only coat the surface with out bonding. Methelyne Chloride based glues are the only ones that will work
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You can buy special paint in R/C hobby stores. Regular paint is said to weaken polycarb, although I suspect its partly urban myth. The R/C stuff sticks better too
This is not a myth. You must used a special paint for polycarb because other paints will degrade the polycarb. In fact even duct tape will degrade polycarb. One of the front armour panels in Scarifier has no cracks forming from the hits from robots such as Vertical Limits, Prancing Queen and Reboot, but there are two cracks which started from the section where I just used duct tape to hold the broken panel onto the robot. These cracks appeared from light hits from rammers.
Paints and addheasives will degrade polycarb unless they are designed for the use with polycarb.Make sure it says it can be used with polycarb before you start using it.
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if u cant find that my guess would be some of that uber noice 5 min epoxy u can get for 2 bucks each tube at warehouse.
WIth spraypainting polycarb i reckon itll be ok but there has been rumours to painting polycarb has weakened it caus of the chemicals or something.
Sorry Andrew, but this is wrong.
Ok, and now for what I know does work for polycard.
1) Sharp internal corners are bad. When cutting out your shape drill the internal corners first and then cut to the drill holes. This will give you a rounded corner and no cracks will start there.
2) Remove all the burrs from cuts. The burrs will have small cracks at their bases and this is where your polycarb will break. Either sand back all the edges or do what I do and run a steel rule along the edge to remove the burrs.
3) Clean out all the drill holes. Drill will leave burrs on the edges. Use a large drill to lightly countersink the holes.
4) If you are bolting a top panel to a steel frame alway drill the holes over size. This will allow the polycard to flex under impact. If you don't the polycarb will crack at the bolt holes. Isn't that right Glen.
I am also going to be small peices of silicon tubing between the holes and the bolts to remove shock loading.
5) Try to make the whole frame able to flex. That is the biggest problem with polycarb robots. People think they can treat it like steel.
6) Cold bending of polycarb does work. I stick it in the vice so the bend line is along the top of the vice and then put all my weight on it. It takes a bit of force with some of the longer bends and you my have trouble bending past 75 degrees, but it works beautifully and I found no cracks travelling the bend.
_________________
Get Some!!!
Secretary of the Queensland Robotics Sports Club inc.
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Thu Nov 17, 2005 5:50 pm |
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Daniel
Experienced Roboteer
Joined: 30 Aug 2005
Posts: 2729
Location: Gold Coast
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quote:
Originally posted by Nick:
If you couldn't heat bend Polycarb,then you were doing something wrong! There are literally thousands of plastics businesses on the web offering this service and Polycarb is, after all, a thermo plastic. It seems the plastic needs to be "dried" for several hours before bending at about half the bending temperature to drive out any water.
Ok, but how thick is the polycarb that these guys use. The paint that the hobby shops sell is for use on the polycarb R/C car bodies, which are made from polycarb. These bodies are less then 1mm thick and it seems it is the same with all other processes I have seen.
The problem is you need to get a constant heating through out the polycarb. If you just hit the polycarb with a heat gun, oven or strip heater you are unable to control the heat distrabution in thick peices of polycarb. And I'm refering thick as being greater then 1mm. Basically the outer surface of the polycarb will become hotter then the center of the sheet. And since polycarb is a poor conductor of heat it makes it very hard to transmit the heat through the full depth of the polycarb.
Yes I have tried the exact method you have suggested and it does not work. I have also set my oven to the correct temperture for getting floppy 1mm polycarb and left the 3mm polycarb to roast in the oven for over 2 hours and the heat simply could not penatrate deep enough.
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I can't see what's wrong with using M4 screws through 3mm polycarb into a aluminium angle. M4 washers have a diameter of around 8mm, plenty of clamping force and pretty hard to pull through the plastic. Anything larger in a beetleweight will add too much weight. I used 6g screws with smaller heads than that in a test frame and it had absolutely no problems. I havn't had an cracking problems yet due to all the edges, corners and holes being well rounded. I don't bother tapping ANY size threads in the edge of Polycarb sheets; its going to break no matter what you do!
Cracks! Tapping will cause lots of small fractures in the surface of the hole which can not be cleaned out. It is alright when end tapping polycarb because it is harder for the crack to start from that point. Also tapping in that direction may mean the polycarb would be locked down and not given eough room to flex. Espicially if you are screwing it into a aliminium or wood or even a UHMW base. You have to let it flex.
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The guidelines for cold bending polycarb mention a minium radius of 12 times the thickness; For 3mm sheet, that gives a 36mm radius, which is pretty big for a beetleweight sized frame.
Thats if you must keep a constant cross sectional area though out the bend. The way I do it I end stretching the polycarb which reduces the cross sectional area. This does reduce the strength but not enough that I want to worry about it yet.
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Fri Nov 18, 2005 8:01 am |
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RoboWars Australia Forum Index
-> Technical Chat
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any places you can get it in the city? (central CBD) 
