Sub enclosure - two part mold construction, explained. (lots of pix)
Iíve done allot of molding over the last few months (big learning curve), nothing to do with automotive so I didn't bother posting anything. Iím now doing a mold that is more in theme to this forum so I thought I would take lots of pics and tell you what I do along the way along with any useful info I could pass on. This is my first multi part mold.

Here is the plug I prepared earlier, not going to bother posting build pics of this as you all should know how to make one of these by now. Itís a 100% fiberglass enclosure to suit a Mitsubishi Magna 3rd generation boot. The magna has a pretty healthy online community in Australia (kind of like your s-10) so I thought I might try and make some money from my fiberglassing and palm these off on eBay for pretty cheap.


To prepare for mold construction firstly I need a 360 degree working area. Iíve been meaning to weld up a little molding bench on caster wheels but havenít got around to it yet. So I cut out a bit of MDF and clamped it to an old stool. I placed the plug on and chocked up around it so its sits level and stable. I finished the plug with a 600 wetsand, followed by 5 layers of carnauba wax. You should always use PVA (or equivalent) when molding from a plug, this is to block any possible reaction from the plug surface, and for that extra insurance that you plug is going to come out. When laying up a gelcoat surface only wax is needed. Therefore an uber crazy mirror finish on your plug is a waste of time because pva will change the surface back 10 fold.


Ideally this should be done in a sandpit with some casting plaster for a base support and to seal the sand from the clay, but I donít have a sandpit at the moment so I cut up some old hardwood and built up the majority of the wall with this. I sketched out my flange line with a sharpie, I need to spend some time on the line as there are vertical faces and if im sloppy one half might not be able to lift off.



Clay is laid out on pieces of mdf with approximate contours, chocked up a bit past the line with MDF off cuts. I pressed the clay into the plug, supporting the plug at the same time so the clay would sit flush on the curves. I did one side at a time, im using non hardening clay (or plasticine). I then manipulated the clay to give me a clean edge and flat surface using various tools which ill show later.


I repeated this step for every side and then molded the clay together to form one continues parting wall. I used about 3kg of clay to form this wall.


Locater dimples are pressed in using a bounce ball. Locators center the two parts together when bolting up. Masked up the top flange to stop the gelcoat from dripping over.


The tools I used on the clay. Bit of balsa wood, plastic paint scrapers, wooden skewers, bounce balls, two different sized nylon rollers. Donít use anything that will scratch your plug surface because you will have to get close and personal with it. Before I mounted each side, I clamped the clay in between another piece of MDF to flatten it out and give myself something to work with.


PVA mist sprayed, I don't count the coats I just wait till the piece has a nice tinge to it, but it was around 15 very light coats, PVA can be thinned with water on the finishing coats to get the smoothest possible finish, be aware this will effect dry time. I still haven't mastered PVA application, what always happens is the PVA wonít stick to the waxed surface, it just forms bubbles on the surface and dries like that. With this one I sprayed the first coat, then rubbed it on with a cloth in circular motions like I was sanding. This seamed to fix the problem, but I got uneven coat in the end and some runs. I think this method would work but Im getting used to a new spray gun. Nothing to major so I let it slide, I could have washed it off with warm water and restart if I wanted to.


PVA should be left to completely dry before gelcoat, over an hour. The gelcoat im using is tooling gelcoat, but its not needed, standard gelcoat will do fine. Im brushing my gelcoat on, spraying is the ideal way to go when gelcoating because it gives a consistent layer. I brushed on a layer of gelcoat and then went over it with the brush again to level it all out as best as possible. Itís a good idea to buy a quality brush for this part. Because the gelcoat is not consistent you should wait for the gelcoat to completely cure before anything else on top, minimum 3 hours. This is to stop any gelcoat reaction caused by the styrene gases getting caught, which caused gas pocket pathways throughout your gelcoat (alligatoring). Gelcoats do not contain any wax, which means you can leave the gelcoat for weeks and still get prime adhesion, there is no rush here. I brush 2 layers on 4 hours apart followed by an overnight cure before resin. By the second coat my locater dimples has filled up with only gelcoat, another reason why I left it overnight. 200 grams was perfect amount for one coat, catalyzed at 1.5% at 28C temp.


Resin brushed on first, mat is ripped into pieces and placed on then wet out. Im using 225g mat (I think that transfers to 0.75 oz in your system)


I spaced my mat layers out, laid out only 2-3 layers then wait till it cooled and laid the next set. This is to stop to much heat generating that could cause a gelcoat reaction. This shouldn't be needed if your gelcoat was left overnight, but I like to do it anyway just in case. This is a lead up to 10 layers, no ribs are needed here as the mold shape itself is very structural.


Same day as resin lay-up but it has cured hard(but still warm) and I know my gelcoat is fully cured so itís safe to pull the clay out now and prep for the other side. I flipped it over and pulled most of the clay off with help from a plastic scraper. This is where PVA also comes in real handy, I donít know if any of you have worked with non-hardening clay but itís almost impossible to clean off, you basically have to scrape and sand it to remove it. But because I sprayed PVA most of it peals right off, leaving a neat flange behind.


When constructing a two part mold you should never ever remove the first half before you lay up the second half. This is because you can never get the plug to sit perfectly back into the mold again, it also allows for debris to fall in and not sit flush, and also removing the PVA will give a bit of movement that you donít want.

To clean up the seam I used a household sponge and some hot water, the water dissolves the PVA in the seam and also gets rid of any clay still sitting near the seam.


I re-waxed this half again along with the flange twice. Mold release wax (carnauba) has a strong chemical resistant properties and also has a high heat resistance, donít try to use any other wax. Wax-on wax-off is the wrong method, the wax must be left to dry and go white and hard (about 5-10 minutes), then you must use a clean cloth again to wipe it off. When waxing use small circular motions overlapping yourself so as not to miss anything. When the wax comes off you will get a glossy shine and your cloth will slip over it like grease. You only really need a single coat of wax. Re-waxing a surface that has just been waxed doesnt give any more release properties, but you never know if you have missed a spot, so for insurance you should go over a piece multiple times to insure a fully waxed surface all over. PVA is again mist sprayed till a tinge. You will have trouble seeing a tinge on anything but white, The amount of PVA is not really important, as long as its more then about 2-3 coats.


Two gelcoat layers spaced 4 hours apart, masking tape around the edge of the flange to stop any gelcoat dripping over and blocking the wedge seam, tape was remove after gelcoat. For more info on gelcoat refer to the first post.


4 hours after the gelcoat I laid down 2 layers of mat and left it overnight.


In the morning I applied another 3 layers. Because im using waxed resin and I let it cure I must sand the surface of the fiberglass before I lay any more down. During curing the wax gets pushed to the surface and will act as a barrier to more glass which will not give the best adhesion. Im leaving this side light because I may need the flex when pulling this one off, im scared about some overhangs I spotted that could be bad.


I left the mold to cure for 2 days before I try to pop it, mainly because polyester resins will shrink slightly during curing so should be left to fully cure when molding. While I was waiting I trimmed it back to the flange and drilled 8 bolt holes for my bolts, in between the locator's. I then glued some washers on each side.


I also spotted a section of the flange that has a gap in between the gelcoat and mat layers, this was careless of me and shows the importance of the first mat layer on your gelcoat. When I try and pop this now it will cave in and crush parts of the flange. To help support this I put a few beads of resin in there with a syringe.


Time to pop the mold. I ran a plastic scrapper along the flange and thatís all it took for the bottom to come off.


If you canít get the PVA to peel off you can dissolve it with a sponge and hot water like this which will bring out the gloss gelcoat finish.


To pop the other half I soaked the seam with hot water to try and dissolve the PVA around the plug. I then took hold of the flange with both hands, with my foot in the plug I pushed it out with my leg. The first picture is what the hot water did to the PVA to help loosen it. The second is with the PVA removed.


The glecoat was weak where the gap was and crushed. This can be fixed temporary with clay but I want to repair this with a gelcoat body filler down the track.


closed up with a seam shot. Nice and clean seam.


LAYING UP THE MOLD - CREATING A PART.

PVA should not be needed but for the first few parts I want it, just in case. I waxed both sides a few times. I then sprayed the PVA again, I sprayed each piece open and then a few coats while bolted up.


Gelcoat is brush on and just 1 layer. Gelcoat has UV and weather protection and also gives a primed surface to sand or paint. A fan is set up blowing into the mold, this fan will stay here for the entire lay-up and cure. Styrene gasses can build up in closed up areas like this and cause all sorts of weird reactions. Because these gases are heavier then air and go down, they need to be blown out. This is one of the causes for the bubbly gelcoat reactons that can happen (alligatoring).


To keep the parts as one strong seamless piece im going to lay-up these parts with the mold closed, all via the 12" speaker hole. Itís a hassle to join two parts together after and creates more work. This is ensures no weak seams either with a paint/carpet ready box right out of the mold. Usually this is not possible but because in this situation I have a 12" hole I can do this. This is harder then it looks, its quite hard to lay-up through a hole like this specially the flange area which I had to lay-up blind. Thickness is your choice, im doing this piece thin to test the mold. Hereís some various lay-up photos, the fan must be blowing the gases out when not laying up.


Wait at least 24 hours after final mat layer. When I pop this open I donít want the rear part of the plug to be left as there is no prying edge to be able to pop it out without damaging the mold. So I clamp the flange area to ensure the part stays in the top.


Running my wedge around the flange popped it open pretty easily. The clamps did the job too, when I released them the part fell out of the mold by itself.


The gelcoat flange line, I hit this back with 80 grit to leave a seamless surface. clay can be wedged in the mold seam to stop this from happening, I might play around with this on other parts.


Comparing to the plug, a perfect match. And still fits in the car snug.


This part will now be the 'master plug' and will be stored in another location. This is in case anything is to happen to the original plug or mold I have the master plug to re-mold it. These parts will be sold carpeted so gelcoat is not needed on further ones. I can now mass produce these at minimal cost, the whole reason behind the mold. These will be finished with unbacked automotive carpet, speaker terminals drilled and mounted with sub speaker wire soldered and a few 2" double sides velcro strips to hold them to the boot carpet. I can make one of these per day.. in a higher demand situation you could make two molds to double production.

I hope you learned a thing or two in this write-up. Any questions just ask here.