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 A beginners endeavour at mold making. 
Faced with the task of having to make two cowls for my current project, the VeeRod, i contemplated various methods for making them.
I came up with three choices:

  • The first, would be my usual approach of sanding a plug to lay two layers of glass cloth over, BUT! the thought of having to build
    and sand two plugs, the work and mess involved with digging the foam out and sanding both cowls smooth really turned me off of
    wanting to do it this way.
  • The second, would be to build them using ply formers planked with balsa. I think this might have been less work, then again maybe
    not. I almost decided to do it this way because i was already 'into' the planking with the fuse and nacelles. Both these methods might
    only yeild cowls that are a little different to each other.
  • The third way would be to try and make a mold from a plug to make two glass cowls.
    The problem with the last method was that mold making was completely foreign to me. I didn't have a clue of the process and materials needed aside for some resin, fiberglass cloth, and some sort of plug. I knew if i could make them using this method, i would end up with two cowls that are as close as possible to being exactly the same as well as i would have the ability to make more cowls for myself (in case of an incident, cracking or engine swap) as well as being able to offer a set of cowls to other builders that might want to build the VeeRod too.

This brings me to the reason why i wrote this. I didn't find alot of info on mold making processes especially aimed towards the hobbyist who doesn't really care about producing a professional quality mold. Maybe i just didn't look hard enough, but hopefully
this might help give a starting point and some info to others wanting to try mold making.

The Plug
I made the cowl plug by cutting out the top and side shapes of the cowl from balsa sheet. These were glued in a + and provide the top and side profiles of the cowl. These sheet pieces give reference to the center lines of the cowl and act as a guide to get the cowl shape right. Foam blocks were cut and glued into the quarter sections. 1/8" ply formers were cut, one in the shape of the firewall with the sheeting thickness, that is glued to the base and the other being a round disk the diameter of the nose cone that will be used, this is glued to the nose of the cowl plug. The plug was then sanded to shape, quite a bit of time went into trying to get the shape and finish of the plug as perfect as i could.

NOTE: care must be taken to ensure that every surface parallel to the direction of draw needs a draft angle to facilitate demolding, otherwise a multiple piece mold would need to be made.

The plug was then covered with two layers of light glass cloth and epoxy resin. After this the plug needs quite alot of elbow grease sanding it smooth. I worked from using 220 grit up to wet sanding with 1000 grit wet/dry sandpaper. I finally achieved a nice shiny smooth surface.The better the finish on the plug, the better the finish on the mold and the parts pulled from it as well as aiding in their release. It really pays to take time to get the surface as good as you can here.
The plug was given 2 coats of green mold wax Paste #2, buffing the surface after each application. Partall film #10 (PVA) was then brushed on, using a soft fine haired artists brush, in three thin coats allowing each coat to dry completely before the next one was applied. If too much is applied at once it tends to run. The manufacturers instructions recommend spraying the PVA on for the best finish possible, but i found with brushing the stuff on, i could get the results i wanted. PVA is also water soluble so cleanup is easy, when dry it's sort of like a smooth layer of thin plastic Saran wrap. Both these products i bought from Polymer Composites on Ebay.

Plug photos
Rough plug just glued together Plug sanded to shapeFinished plug

The Mold
After the plug was completely finished and preped with the wax and PVA, i placed the plug on a sheet of wax paper on a nice flat surface. As with the plug, i used marine grade epoxy laminating resin, this i also bought from Polymer Composites. I brushed the plug first with epoxy before laying any glass cloth on it, drizzling quite a bit down at the base as heavy glass cloth doesn't conform to sharp corners well and also into the depression that represents the hole that will be cut for the prop driver. I did end up with two small air bubble cavities at the base edge. Maybe there's a better way of doing this part. I used several pieces the length of the plug for the majority of the glassing while using narrow strips to build up the base flange and some smaller squares for the top. In all about 4-5 layers of glass and resin were layed down over the plug. I'd say this is probably the minimum amount to use on a mold this size. The mold needs to be strong and resistant to bending. The thickness of all the layers of resin and glass really depends on the size of the mold. I guess knowing how much will come with experience. I let the resin and cloth cure (or at least get good and hard) before trimming and trying to remove the plug. As i found out, my plug didn't want to release, but i had sort of planned for this because i had a suspicion that there was a slight opposite draft angle in one area on either side of the lower sections of the cowl plug that would hold it from releasing. My fix was to use my dremel with a thin cutoff wheel to cut the cowl into two halves along the horzontal balsa profile plane in the plug to split the cowl. My one piece mold now became a two piece. The plug was now fairly easy to demold from the mold halves using compressed air to blow down the cuts and base edge trying to get the air between the plug and mold surfaces. A stategically placed hole for blowing air into works great too. For a cowl the nose section where the prop driver hole will be is a natural location for one.

NOTE: using compressed air really works well in releasing stubborn plugs and parts from molds.

The two mold halves were then layed on a piece of wax paper and a flange was epoxied around each halve so screws or other fasteners could hold the two mold halves securely together while making a cowl. I've found preping and demolding with the two piece mold much easier than with a one piece. The one downside is a small seam is tranfered to the finished cowls, this is easy to sand off though, so it really isn't much of a downside.

NOTE: be sure to give time for the mold to fully cure before use as well as fixing any problems like air bubble cavities, etc.
Both wax and PVA will need to be used for at least the first five pulls until the mold becomes 'seasoned' then just wax should suffice.

Preping the mold and glassing to make a part is exactly the same as when making the mold, only two layers of glass cloth were needed for my cowls.
This basically ends the procedure i used for making my cowls, i am more than satisfied with the results especially with this being my first crack at it. Now i can make all the cowls i want until the mold wears out. If anyone experienced with mold making would like to add some tips here or even another article on mold making, please send an email to me. tjlewis1@telus.net

Mold & cowl photos
Glass & resin layed over plugMold and plug together after resin had hardened The demolded mold halvesMaking the flange around the base of the mold halvesMold halves after preping with wax and PVA
The mold halves held together with screws ready for useGlass and resin layed into the moldNew demolded untrimmed cowl The new trimmed glass cowlsThe products that i used

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