[SEL] gluing plastic OT

[Jerry Evans]

   At 07:00 PM 10/03/12, you wrote:

     From: Germoamer at aol.com <Germoamer at aol.com>
     Subject: Re: [SEL] gluing plastic OT
     To: sel at lists.stationary-engine.com
     Date: Saturday, March 10, 2012, 7:33 AM
     In a message dated 3/9/2012 9:20:56 P.M. Eastern Standard Time,?
     curt at rustyiron.com writes:
     I'd focus first on identifying the material your hood is made of,?
     then
     seek the appropriate adhesive

     Thanks for all the input fellas!? I think I will contact JD and see?
     if
     they will tell me type of plastic the hood is made out of, then go
     from? there.
     PC-7 sounds mighty interesting and will look into it.? I had? also
     thought
     about drilling a hole at the end of the crack, just like we do on?
     the old
     engines.?
     Right now the entire hood is off the mower so not to damage any?
     further.?
     Thanks,?
     Tom? Schmutz
     Concord, Va.?
     germoamer at aol.com

   Hi Tom,
           I'm sorry to have got into this thread so late but I only get
   the SEL Digest version and that has just arrived. Plastics are my forte
   (been there since the early 70's). Here follows the long version answer
   :-)
           Curt is totally correct - step one is to identify the plastic
   used and then work from there.
           Forget about all the "miracle glues" on offer until you know
   what the plastic is.
           Firstly, you have to understand that there are certain popular
   plastics that just cannot be "glued" successfully. I'm not talking
   about "sticking" some gunk onto a piece of plastic - I mean creating a
   real and lasting bond. Others can be bonded and this is usually
   achieved by incorporating a solvent in the adhesive which will actually
   "melt" the original material to allow the compatible fillers to bond to
   the substrate - your PVC and ABS water pipes use this method.
           Some of those that cannot be "glued" are Polypropylene (PP),
   Polyethylene (PE) and of course Polytetraflouroethylene (PTFE or
   TEFLON). (PTFE can be bonded but only after a rather involved etching
   process). These are often used in applications where their "non-stick"
   properties are required. They are usually "Thermoplastics" (except
   PTFE) meaning (simply) that they can be reheated and reshaped as
   opposed to "Thermosets" which cure by chemical reaction and will not
   soften with the application of heat (but they may burn).
           The next thing to understand is that the manufacturing process
   is often the cause of the failure in the first place. For example, in
   injection moulding and blow moulding the molten plastic resin is forced
   into a mould cavity and then "force cooled" for speed of production.
   This results in what is referred to as "internal stresses" in the
   material. Imagine all those little molecules trying to "hold hands"
   comfortably but they are force cooled before they have had a chance to
   get comfortable so the molecular bonds are under stress. In cases where
   plastics are allowed to cool naturally after moulding there is a lot
   less "internal stress" because, in the natural cooling process the
   molecules actually have time to realign themselves to their neighbours
   (get comfortable). Have you ever experienced a domestic item like a
   bucket or basin which suddenly develops a crack for no apparent reason
   - that is a result of "stress relief" - it's had enough of the
   permanent stress and just lets go.
           In recent years it has been common practise to include
   "moulding marks" somewhere on the inside of the product. This is to
   help recyclers identify and sort the type of plastic used. Typically
   this will be a series of letters etc. actually incorporated in the
   mould and will include date of moulding, part number and other
   information. It will also include the type of plastic used usually in a
   diamond shape eg . PP (Polypropylene) PE (Polyethylene) PVC
   (Polyvinylchloride) ABS (Acrylonitrilebutadinestyrene) and so forth. If
   your moulding has this somewhere then you are already halfway to
   finding a solution by identifying the correct plastic. There should
   also be a "Recycle" logo.
           I would imagine that your bonnet is probably made from
   Polypropelene or Polyethylene - 2 of the cheapest and easiest plastics
   to mould from. If this is the case then forget about finding a suitable
   glue - there is NOT one. (Have you got duct tape :-))
           As Rob mentioned, there is a welding gun available but unless
   you really know what you are doing then do not even try it. I have one
   and it works well if you know what it's all about but one of the main
   problems is finding a suitable plastic "welding rod" - it has to match
   the original material exactly - sometimes I have to cut slivers off the
   original material to use as a rod. The other problem is that in a
   moulded and force cooled product, as soon as you introduce heat you
   release more stress and the hole just gets bigger as the molecules
   "relax" even more. (Do not confuse this "heat welding" with "High
   frequency welding" as used on plasticised PVC - they are totally
   different processes).
           Motor vehicle bumpers today are usually plastic and cracks are
   repaired by welding. Plastic welding is very similar to gas welding
   (acetylene/oxygen) except that the heat is not a flame but rather a jet
   of very hot air which is used to melt the "welding rod" and the
   original surface together. It really is not very effective when welding
   injection moulded articles (like domestic buckets and basins) because
   of the stresses I've already mentioned.
           Apologies for the long answer but certain things cannot be
   explained in one paragraph.
           Feel free to contact me if I can be of more help.

   Keep the revs up (or down)
   Jerry Evans
   Near Johannesburg in Sunny South Africa.
   Etched Brass Engine Plates made to order:
   <[1]www.oldengine.org/members/evans/plates/index.htm>

References

   1. http://www.oldengine.org/members/evans/plates/index.htm