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Mold half trimming

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Old 01-31-2018, 02:33 PM
  #1  
Mms_citrus
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Default Mold half trimming

i am looking for advice. When I lay up the part I wax the mold and spray pva. I trim pieces after I lay them in to the edge with some angled scissors. When the part sets up or at least gets a good gel I trim with a razor. When I trim it peels the pva from the mold part. I did a fuse already. I mix a little paste up and bond the mold parts with that. It squeezes a thin layer out onto the mold partings, and because the pva pulled up a tiny bit of it stuck in some areas when I split everything, I had to scrape it off. So far, is this making sense?
So do I need the wax it a bunch more? And not worry about the pva peeling, or should I try to brush more pva on the parting surface after trimming? Or is that just normal.
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Old 01-31-2018, 06:19 PM
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GREG DOE
 
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Mms citrus There are as many answers to your question as there are experts! You stated that you" mix a little paste up and bond the mold parts with that". This leads me to believe that you are joining the parts in the mold? This is standard practice, but often reserved for production situations so that the molds can be cycled faster. Also standard practice is to use joint tape, which is much stronger then just "paste". With joint tape there is usually less "migration" of the resin into the flange areas, so less of a problem releasing. Keep in mind that most of us recommend PVA as well as wax, but that the wax on the flanges will usually release OK. My suggestion is a little more time consuming, but worth considering. If you are only making one, or a few parts I would suggest that you join the parts outside the mold. I start by taping them together with small pieces of masking tape every inch or so. Then I use CA glue to join the halves. Finally I push joint tape over the seam, and brush on resin. I have brushes on long sticks for just this job. In the case of a fuselage, the tape is often applied through a wing saddle hole, but sometimes you have to be creative on how you accomplish the job. If you really want to join in the mold, you might consider putting masking tape on the mold flange. That way if you pull up some of the PVA on the flange when you trim, the the halves won't get stuck together. I hope this is clear. Good luck, Greg
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Old 02-01-2018, 05:13 PM
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Mms_citrus
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THanks for the reply. So I think I will try taping this time.tell me if you think this will work. I make my parts. Cut out my access hatches, gear doors, etc. use your recommended method to seam the fuse thru my access points. Use masking tape to hold in place and seal my panels, the gear doors, access hatch, etc. reinsert into mold, pour foam core.

i have no experience using polyurethane expanding foam. I was going to experiment on my overweight fuse before doing the good one. Iíll link the build again for you Greg. You saw it before but just to freshen your memory so you can see if it would work the way Iíd like it to.
Scratch build McDonnell XP-67
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Old 02-01-2018, 08:22 PM
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I use a two component liquid, that when mixed together turns to foam. It is used to fill voids in a 7.5 lb. commercial project that I make. The foam is available from Wicks Aircraft, and Aircraft Spruce & Specialty. If you decide to do it this way, I would suggest that you experiment with the foam to see how it performs. For one thing it can exert a lot of pressure, which might force your halves apart, even if you put them back in the molds?. The venting holes that you mentioned should help relieve the pressure. Now that I have refreshed my memory with your project It's possible that joining the halves with "paste" might be the only practical way to assemble your model.
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Old 02-07-2018, 08:11 PM
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in my experience,once You used pva on the plug to make the mold,wash it very thoroughly to dissolve the pva,off the the mold.After that ,just apply 3 coats of wax,and buff to a mirror shine.I always seemed to get a better casting ,without the PVA on the epoxy mold,as long as there was no fillers
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Old 02-08-2018, 02:52 PM
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I had bonded the two halfís after I trimmed them without pulling them from the molds. This time I pulled the parts. Cleaned and sprayed new pva on the mold and the outer surface of the plug. So....this time although it will still spooge out it shouldnt want to stick to anything. Iím going to foam it while itís still bolted up after the bond is cured. I foamed my practice fuse and I gained some precious knowledge. I built a hot box too, I really want to get maximum thermal,expansion out of this foam. I was really surprised at the pressure the foam can generate. I have 3 holes in the mold to pour into and I blocked 2 with my thumb when they started to overflow and apparently the top hole expanded and sealed. I moved my finger and got just sprayed with foam lol. Was fun. If this flies well I will be remaking my molds. First mold Iíve ever made and I could have done better
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Old 02-08-2018, 04:53 PM
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if you have no bondo(styrene),you dont need pva...just wax lightly ,and polish slick,before starting your laminating.Pva will leave ridges.you dont need to use foam,3 layers is more than enough
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Old 02-08-2018, 07:04 PM
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3 layers? Of cloth? Itís only 2.5 oz. if I donít want to make a bunch of spars and bulkheads I think foam core is my only way. Iím doing 2layer laminate with some focused reinforcement layers, but itís mostly a 2 layer build up. Itís thin...no composite, as in 2.5oz, some foam sheet, another 2.5. That would give me enough strength probably but getting something like divinycell would move me straight onto vacuum bagging and Iím neither experienced enough or rich enough to buy the equipment right off. Unless of course you meant something else, which in that case ignore all

if There is something Iím doing wrong and 2layers of 2.5 should be that stiff Iíd like to know. Iím using minimal resin just enough to wet it out, and Iím keeping the mold and everything super hot so it flows really easily.
Im using a slow cure epoxy mix. I need all the time I can get doing a wet layup and keeping everything as warm as I am.
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Old 02-08-2018, 07:37 PM
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make one,nothing else.it costs so little in effort..to see what you need from there.
don't make it hot,as it will bake your surface coat

Last edited by alex5; 02-08-2018 at 07:38 PM. Reason: add
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Old 02-08-2018, 07:40 PM
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keep it to the recommendations from the manufacturer. see what You get,then adapt it
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Old 02-08-2018, 07:43 PM
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too hot and it will wrinkle the surface,before you lam the rest of it
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Old 02-08-2018, 07:45 PM
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don't go heavy on laminations..3 should be enough
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Old 02-09-2018, 09:28 AM
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Iíll video the next layup. Iím going to redo the bottom half.
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Old 02-24-2018, 04:58 PM
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Here is a two piece mold I made for a Goldberg Chipmunk cowl. I lay up each half just up to the edge, but not quite touching it with two layers of .50 cloth, then join the halves just as the second layer set so it doesn't sag, then run my center strips of cloth to join the parts together using 1 ounce cloth over .50 ounce cloth. Only reason for the /5 cloth is for a smooth matching finish in case the cloth gets too close to the surface.

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Old 04-12-2018, 11:35 AM
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nothing but trouble with foam !
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