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MDF vac mold making technique

 
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jegner
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Joined: 30 May 2003
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Location: Texas, USA

PostPosted: Fri Jul 08, 2005 9:42 am    Post subject: MDF vac mold making technique Reply with quote

I really like it as a vac-mold material.

Here's how I do it:

First, make a line drawing of the part you want to make. Let's assume the bicep as an example. Just sketch it out using you own arm as a measurement. Then just draw the profile of the part. I have made the MDF molds from both cross sections vertical and horizontal, and I like the horizontal better. Think of the drawing as if you were looking straight down from above. Transfer this outline drawing to poster board, or other rigid paper. This becomes a stencil once you cut this out.

Next, transfer the stencil to a sheet of MDF. For the bicep you may need 4 or 5 copies drawn. Number them. Band saw them out. Stack them together. You will see a block of wood that has the same general shape as what you need. If you are crafty, trim the the top layer so that it tapers down. This just cuts down on the amount of sanding you will have to do. My thighs required a lot of sanding.

Now, glue these pieces together, and align them up so that they can't move around while the glue sets. I use a good quality wood glue. Once it's dry it will sand away and won't have sticky seams between the layers of MDF. Let the glue dry over night.

Sand. This is the art part of the project. Basically the block of wood has the outline shape of the part you need but not the contour or profile. Sand and put a good arch to it. Be sure to check as you go. Take your time, and use a belt sander with a 40 grit sand paper on it. That really takes the material off. Once you get close to the finished shape, switch to a finer 100 grit paper. MDF will fuzz a but, but don't worry, using finer sand paper will minimize this.

Once the sanding is done, the next step is to add the strips for detail. I use basswood from the hobby store, but any thin strips of material would work. Look around the house, you might have some scrap material. I've used chip board, counter top laminate, styrene, and filler putty. The basswood works the best for me. Glue these strips down, and let that dry over night.

Next step, prime it. I use Krylon sandable primer and this stuff works pretty well. Spray it on and watch what happens. Some of the MDF will soak it up like a sponge. Thats ok. Let it. Add 2 or 3 coats. Let it dry for a few hours. Then sand it lightly with the 100 grit paper. The paint will bond with the MDF and some places will be fuzzy. Sand this off. You should end up with a smooth mold.

At this point the primer will show you your flaws. Some areas might need some more sanding, or you may need to add some filler. Thats were the bondo comes in. There are 2 types I use. Spot filler putty, thats red and comes in a tube. This is good for light scratches, and minor imperfections but sucks for anything more than a fingernail's thickness cracks. Use the automotive 'red can' bondo, and get an extra tube of hardener. I mix the stuff up according to the directions, but go a little heavier with the hardener. Not too much, but just a bit more.

Bondo is a wonder material but you have to know how to use it. Here are my tips. Mix well but work fast. Immediately after mixing, the bondo is a paste -like consistency. I wear latex gloves cause the stuff does not like to wash off. Smear the bondo on using those spreaders [tan color plastic gizmos found at the same place you got the bondo from] Don't fiddle with it too long. Try to get it as smooth and thin as you can. Properly mixed bondo will be the same color as the spreaders. FYI.

As the bondo sets, it goes through several stages. First is the paste-like stage, as mentioned above. Next it starts to get firm, this takes place within 5 minutes so watch it, touch it, and you will know what I mean. At this point it's like cheddar cheese. You can even grate it using a rasp. This is good to know because you can remove a lot of extra material if you need to with ease. This stage will last only a few minutes. This is the best stage to really shape the bondo using coarse tools, like rasps, and shru-form tools. DO NOT use any power tools on the bondo at this stage. Then the bondo gets real hard. And hot. Keep working with it as it sets, but one it gets hard, you will have to use sand paper or files to work it.

Bondo has about the same hardness as the MDF so the two go together well.

Always were eye protection, gloves, and a dust mask when working with these materials.

The end result will be a good solid sculpture. The last thing to do is add risers and tapers. The riser is just a scrap board, 1/2 to 1 inch thick that has the same outline as the base of the mold. This raises the mold off the forming surface enough so that there are no unwanted forming artifacts. It suspends the wanted mold, so that all the detail, right down to and below the mold. A taper is a term I use for adding extensions to the mold in an effort to make it release from the formed plastic better. Practice forming without the tapers, but if your mold gets stuck, and is hard to remove, then make some but don't fasten them to your mold. They can just sit against the mold during the pull. Then you can pry these tapers out, and the mold will fall out.

I have a video file you might find helpful.

http://www.imperial-armor.com/videos.html

Good luck, and if you have any other questions, just ask.
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dman
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Joined: 09 Oct 2006
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PostPosted: Tue Oct 31, 2006 11:23 pm    Post subject: Reply with quote

Going thru some old posts here and thought that I would elaborate on the bondo part. I have been around that stuff since staring in the auto body shop in about 1976. I'll break it down by the two items mentioned.

Bondo - Bondo is actually a brand name but the product is widely known by that name outside of the industry. In the industry we call it filler, plastic or mud but for this topic, I'll go with bondo. The general tendency for most folks is to add more hardener in the winter to kick it faster and less in the summer to extend working time. The problem with that is how hard the bondo actually gets when set up. It seems backwards but the more hardener that is used, the softer the bondo will be when cured. If you ever have had bubbles come up after pulling some hot plastic, more than likely the filler was mixed with too much hardener and it is breaking down. Less is always better but if you don't get quite enough, it might become brittle and it tends to leave pin holes when sanded. The optimum color when mixed is a nice shade of pink when mixed with a red hardener.

Spot putty - This is snake oil in the body shop industry any more. Way old school. This is a laquer based product that was used for scratch marks but like Jim says, it is only good for very light scratches and very minor imperfections and it must be spread on as thin as you can get it. If it needs another coat, do it but never try to "fill in" anything with it in one coat. It might work better on molds than it does cars with the newer paints, I'm not sure because I haven't used it in so many years. A much better product is "poly putty". It is a two part filler very similar to bondo but much, much thinner. It sands really easy and doesn't have too much of a pinhole problem but you do have to pay attention to the mix, just like bondo. The roughest sandpaper that needs to be used on it is a quick pass by hand with a block if possible with 80 grit but you really need to spread it very thin so you step down in grits pretty quickly. You can't just glob it on and expect good results. That stuff is pretty wicked but is alot more expensive than regular bondo and I'm pretty sure that you can only buy it at a paint supply store or maybe an auto parts store.


Where your really smooth stuff comes from is by sanding as smooth as possible and putting light coats of primer on there. Never try to fill inscratches or flaws with primer. Especially the spray bombs. That stuff is way too thin to try anything like that. The bombs are also a laquer based product (some are enamel, but most are laquer based) and if you put it on too heavy, you will have lots of problems. Paint, primers, etc cures by evaporation of the solvents and that means they "float up" from the bottom and lay on top of the surface. If you put it on too thick, it will skim over on the surface a lot more unevenly and the solvents will have no where to go. This will cause bubbles or solvent pops which are little blister looking defects. That is why you are told not to wax your car for x amount of time when you get it repainted.
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budoongz



Joined: 24 Apr 2009
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PostPosted: Fri Apr 24, 2009 10:30 pm    Post subject: Re: MDF vac mold making technique Reply with quote

jegner wrote:
I really like it as a vac-mold material.

Here's how I do it:

First, make a line drawing of the part you want to make. Let's assume the bicep as an example. Just sketch it out using you own arm as a measurement. Then just draw the profile of the part. I have made the MDF molds from both cross sections vertical and horizontal, and I like the horizontal better. Think of the drawing as if you were looking straight down from above. Transfer this outline drawing to poster board, or other rigid paper. This becomes a stencil once you cut this out.

Next, transfer the stencil to a sheet of MDF. For the bicep you may need 4 or 5 copies drawn. Number them. Band saw them out. Stack them together. You will see a block of wood that has the same general shape as what you need. If you are crafty, trim the the top layer so that it tapers down. This just cuts down on the amount of sanding you will have to do. My thighs required a lot of sanding.

Now, glue these pieces together, and align them up so that they can't move around while the glue sets. I use a good quality wood glue. Once it's dry it will sand away and won't have sticky seams between the layers of MDF. Let the glue dry over night.

Sand. This is the art part of the project. Basically the block of wood has the outline shape of the part you need but not the contour or profile. Sand and put a good arch to it. Be sure to check as you go. Take your time, and use a belt sander with a 40 grit sand paper on it. That really takes the material off. Once you get close to the finished shape, switch to a finer 100 grit paper. MDF will fuzz a but, but don't worry, using finer sand paper will minimize this.

Once the sanding is done, the next step is to add the strips for detail. I use basswood from the hobby store, but any thin strips of material would work. Look around the house, you might have some scrap material. I've used chip board, counter top laminate, styrene, and filler putty. The basswood works the best for me. Glue these strips down, and let that dry over night.

Next step, prime it. I use Krylon sandable primer and this stuff works pretty well. Spray it on and watch what happens. Some of the MDF will soak it up like a sponge. Thats ok. Let it. Add 2 or 3 coats. Let it dry for a few hours. Then sand it lightly with the 100 grit paper. The paint will bond with the MDF and some places will be fuzzy. Sand this off. You should end up with a smooth mold.

At this point the primer will show you your flaws. Some areas might need some more sanding, or you may need to add some filler. Thats were the bondo comes in. There are 2 types I use. Spot filler putty, thats red and comes in a tube. This is good for light scratches, and minor imperfections but sucks for anything more than a fingernail's thickness cracks. Use the automotive 'red can' bondo, and get an extra tube of hardener. I mix the stuff up according to the directions, but go a little heavier with the hardener. Not too much, but just a bit more.

Bondo is a wonder material but you have to know how to use it. Here are my tips. Mix well but work fast. Immediately after mixing, the bondo is a paste -like consistency. I wear latex gloves cause the stuff does not like to wash off. Smear the bondo on using those spreaders [tan color plastic gizmos found at the same place you got the bondo from] Don't fiddle with it too long. Try to get it as smooth and thin as you can. Properly mixed bondo will be the same color as the spreaders. FYI.

As the bondo sets, it goes through several stages. First is the paste-like stage, as mentioned above. Next it starts to get firm, this takes place within 5 minutes so watch it, touch it, and you will know what I mean. At this point it's like cheddar cheese. You can even grate it using a rasp. This is good to know because you can remove a lot of extra material if you need to with ease. This stage will last only a few minutes. This is the best stage to really shape the bondo using coarse tools, like rasps, and shru-form tools. DO NOT use any power tools on the bondo at this stage. Then the bondo gets real hard. And hot. Keep working with it as it sets, but one it gets hard, you will have to use sand paper or files to work it.

Bondo has about the same hardness as the MDF so the two go together well.

Always were eye protection, gloves, and a dust mask when working with these materials.

The end result will be a good solid sculpture. The last thing to do is add risers and tapers. The riser is just a scrap board, 1/2 to 1 inch thick that has the same outline as the base of the mold. This raises the mold off the forming surface enough so that there are no unwanted forming artifacts. It suspends the wanted mold, so that all the detail, right down to and below the mold. A taper is a term I use for adding extensions to the mold in an effort to make it release from the formed plastic better. Practice forming without the tapers, but if your mold gets stuck, and is hard to remove, then make some but don't fasten them to your mold. They can just sit against the mold during the pull. Then you can pry these tapers out, and the mold will fall out.

I have a video file you might find helpful.

http://www.imperial-armor.com/videos.html

Good luck, and if you have any other questions, just ask.


thanks a lot for those techniques you shared here, maybe i can set up a prototype .. Wink



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jakkspar



Joined: 19 Jul 2009
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PostPosted: Sun Jul 19, 2009 2:37 am    Post subject: Reply with quote

nice concept to your prototypes design, i love it



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