Post by thread:[OT]:Casting plastics?

Check hobby and craft stores, there is casting resin you can buy by the pint or gallon. Mix with a little bit of catalyst, pour it into the mold, hardens into clear plastic - commonly used for paperweights and such. There are colors available too, and I also found Web sources. Search for casting resin, I think. Dale -- "Curiosity is the very basis of education and if you tell me that curiosity killed the cat, I say only the cat died nobly." - Arnold Edinborough On Wed, 27 Mar 2002, Jay Hanson wrote: > I would like to encase a small circuit board in some sort of clear plastic. > Should I melt lexan, plexiglass, or ....? Is there is good liquid for this? > Fiberglass resin? > > What are the rest of you doing? -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

There is a product used in setting flowers and the like. You mix two portions and pour it on your project. It will harden to a clear plastic. Check your local flower shop/gift shop for the product. Rick Jay Hanson wrote: {Quote hidden}> I would like to encase a small circuit board in some sort of clear plastic. > Should I melt lexan, plexiglass, or ....? Is there is good liquid for this? > Fiberglass resin? > > What are the rest of you doing? > > Jay > > -- > http://www.piclist.com hint: To leave the PICList > piclist-unsubscribe-requestKILLspammitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

At 01:43 PM 3/27/02 -0500, you wrote: >There is a product used in setting flowers and the like. You mix two portions >and pour it on your project. It will harden to a clear plastic. Check your >local >flower shop/gift shop for the product. >Rick > >Jay Hanson wrote: > > > I would like to encase a small circuit board in some sort of clear plastic. > > Should I melt lexan, plexiglass, or ....? Is there is good liquid for > this? > > Fiberglass resin? > > > > What are the rest of you doing? The stuff they use for flowers is usually a polyester (activated resin + MEK as catalyst). Epoxies will handle a bit more heat but generally the tg points are in the 150-250°F range (TG = transistion to Goo ;-]) I'd think the commercial conformal coating resins would be modified for a higher temp. Dave -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

> > I would like to encase a small circuit board in some sort of > > clear plastic.Should I melt lexan, plexiglass, or ....? Is there > > is good liquid for this? > > Fiberglass resin? If you use fibreglass resin (polyester) you can improve the look of final casting by pre-coating the board and components. The reason is that the reaction between the polyester and the peroxide catalyst is exothermic, which causes the resin to shrink away from the potted object as it sets rigid. What you'll see then is silvering as a gap is created between the resin and the object The higher the level of catalyst, the more exothermic the reaction, sometimes to the point of cracking the resin. In the lab we used to see test curings often melt nylon potting cups and on one occassion actually catch alight, so don't go mad with the peroxide in a big casting. Depending on the shape and volume of the resin, anywhere from 0.25% to 1% is about normal, but check out the particular resin's promoter content (often metal salts like cobalt naphthenate) and manufacturer's recommendations The trick to getting a good finish is to pre-coat the object, particularly metal as it doesn't bond well to p/e. As the volume / surface area is low and heat can escape better you can use a higher level of catalyst, 2% or more but YMMV. The pre-coat surface after curing will not be hard, as oxygen inhibits the cure, (which is why a higher level of catalyst helps) but it bonds extremely well to the next layer of resin. To get a good hard flat final surface you can use a glass sheet (with release wax - furniture polish is OK but no excess !!) or the plastic used for laminates, I think a polyethylene derivative. This doesn't bond to polyester at all, but finish isn't as good as glass. To buff up the casting, use brass or silver polish You don't need to let the pre-coat set absolutely rigid. To do so causes a visible line, at some viewing angles, between it and the final potting layer. It's a little bit of an art to find the time at which you pot after the pre-coat to create the appearance of a single volume of resin with no join lines, but a bit of experimentation will put you straight. Perhaps the best advice is not to be in a hurry. Speeding up the process by over-doing the catalyst or heating the resin is "more haste, less speed". To get really slow stable cures for decorative objects we'd put the resin in the fridge for a while. This lengthens the working time, one benefit of which is that there's longer for bubbles to leave the casting -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

Hi Everyone, Another trick is that with the "casting" resins, you can do it in layers and as long as you're in a clean environment to stop dust settling on the layers, you won't see the line between the layers as it will bond together. Doing it in layers means that there is less resin to cure at a time and the heat generated is much more easily decapitated so you can use a faster curing rate by using more catalyst. This is how most of the embedded objects are done IIRC, the first layer provides the base, then the object is placed in it and the mould filled. Cheers, Ash. --- Ashley Roll Digital Nemesis Pty Ltd http://www.digitalnemesis.com Mobile: +61 (0)417 705 718 > {Original Message removed}

> The stuff they use for flowers is usually a polyester (activated resin + MEK > as catalyst). Epoxies will handle a bit more heat but generally the tg points > are in the 150-250°F range (TG = transistion to Goo ;-]) I'd think the > commercial > conformal coating resins would be modified for a higher temp. You might be able to get it cheaper in bulk if you buy it at the hardware store (polyester resin for fiberglass). Also, in the original post the poster was talking about melting plastics. The plastics being discussed by everyone else are thermosets, which means they don't melt. The Tg is more a measure of where they degrade, not a melting temperature as with thermoplastics such as polyethylene. Sounds like other people here are more expert at this stuff than me, so correct me if that's wrong. I got a B in my polymers class :( -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 07:48 AM 3/27/02 -1000, you wrote: >I would like to encase a small circuit board in some sort of clear plastic. >Should I melt lexan, plexiglass, or ....? Is there is good liquid for this? >Fiberglass resin? There are several types of castable plastics- epoxies, urethanes and acrylics. The first two types are binary system, you mix two substances and it sets in a while. A vacuum chamber is useful to get the bubbles out, some of them are very low viscosity (almost like water) so they fill nicely. Others are thicker. Sometimes you want a thixotropic type so you can just dip the item. Melting thermoplastics like polycarbonate (Lexan) or acrylic (Plexiglass) won't work very well, they are very gooey. They are normally injection molded or extruded with thousands of PSI of pressure. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" @spam@speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

We do a lot of potting on surface mount circuits and perhaps these recommendations may be helpful: i) If you are working with surface mount, it a good idea to apply a few coats of silicon conformal coating to your circuit before potting. This prevents the potting from flowing underneath chips. Some potting materials expand on drying, causing enough pressure to literally 'pop' off the chip. ii) Appication of heat speeds drying but can be very counterproductive. Excessive heat may damage components or make the potting so viscous that it simply flows off the board. A 100 watt light bulb is often enough to speed the process considerably. When in a rush, we cure pottiing in a 80C oven, but I like to plan ahead and allow the potting to set overnight at room temperature. iii) If you are using an encapsulating material that is not specifically designed for electronic components be certain you understand its electrical properties. The encapsulating material may affect circuit performance. Also, you may find that your circuit doesn't work properly until the potting has fully set, and this may take days. Myke Predko mentions such a problem in his PIC book. The following link is for a potting system from M.G. Chemicals. If I remember correctly, this product sells for under $15. It is designed specifically for electronic circuits. http://www.mgchemicals.com/products/832.html Shawn ----- Original Message ----- From: "Ashley Roll" <KILLspamashKILLspamDIGITALNEMESIS.COM> To: <RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU> Sent: Wednesday, March 27, 2002 5:22 PM Subject: Re: [OT]:Casting plastics? > Hi Everyone, > > Another trick is that with the "casting" resins, you can do it in layers and > as long as you're in a clean environment to stop dust settling on the > layers, you won't see the line between the layers as it will bond together. > > Doing it in layers means that there is less resin to cure at a time and the {Quote hidden}> heat generated is much more easily decapitated so you can use a faster > curing rate by using more catalyst. > > This is how most of the embedded objects are done IIRC, the first layer > provides the base, then the object is placed in it and the mould filled. > > Cheers, > Ash. > > --- > Ashley Roll > Digital Nemesis Pty Ltd > http://www.digitalnemesis.com > Mobile: +61 (0)417 705 718 > > > > > > {Original Message removed}

Just be sure you never want to access the circuit again. One of my clients found out the hard way on this...and now brings out the ICD pins outside the cast and then clips them after testing. If you want to protect for moisture, there is some sprays I think that will coat it pretty well. Another possible solution is to actually coat it in parafin (wax) and that way you can scrape it away. _________________________________________________________________ MSN Photos is the easiest way to share and print your photos: http://photos.msn.com/support/worldwide.aspx -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads