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[mermod]

Hi all, does anyone on here have any experience with vac-forming, I have my heat source sorted but I think I need more suck as I'm not quite getting the crispness of detail I need, should I get a bigger shop-vac or try a compressor driven venturi ? any thoughts or advice appreciated.
Phill
(mermod)

[tigertiger]

It looks like your plastic is cooling before it is fully formed.
Are you heating the mould and doing this in a very warm room?


You could also try using a hot lamp, or a heat blower gun to keep the temperature up in the localized areas that are causing a problem.Keeping the oven door open and working in front of it may also help.
More suck would draw the plastic into the form faster, before it has time to cool.
Remember that if you add more holes you will need more suck.


I have to say that these are pretty extreme edges you are looking for, the plastic is going to get very thin in places. The keel will require a real stretching on the material, that looks like about 3 inches from keel to hull in places. I worked in a plastics factory years ago, I don't ever recall seeing really sharp edges on the inners angles of anything that big. The radius would usually be about 2-3mm in the corners, is that sharp enough for your needs.

[Tug-Kenny RIP]

That does look interesting.

Can you post pictures of the equipment including the bed details please. My first reaction is  'more suck'  and  more holes in the bed, but you've done a great job on that hull so far. They do come out smooth.   

Ken

[mermod]

Thanks for the replies guys, tigertiger the room was quite warm and I had the mold in front of the woodheater before use, 2-3mm rounded corners would be great, I'm happy with the pull on the outer corners its just where the keel meets the hull I currently have a 10mm radius that's not so good, I tried helping it along with a heat gun but I wont put the photos of that one up .
Ken, have a look on you tube for Xrobots UK, I have done exactly the same as that  guy, his example is a C3P0 butt.
Phill

[vnkiwi]

Have you ever seen how it is done by a professional shop?
Something like that would in my humble experience need blowing first to stretch the sheet evenly, then a quick suck to pull it into the corners.
Even heat at all times through blow and suction, then heat off, followed by shut suction.
Let cool for a bit before release from machine. You'd need high volume extraction, and very good suction - no leaks under - to complete.

cheers
vnkiwi

[tigertiger]

I think vnkiwi has is.


As you already have the plug, it is possible that a local plastics factory would do a job for you, on their slack time. You would need to put the plug onto a base to fit the machine, but they could advise. Assuming there is a plastics manufacturer on the island.
They might say no, but they are usually family run business and bosses are approachable, especially if they can get some free media/press out of it.
If they want some money, it may be worth running off a few and trying to sell a few.

[roycv]

Hi first I know nothing about vac forming!  ( How many times have I read "I know nothing" on this forum!!!)

But it seams you are stretching the plastic a lot to form the keel.   Might it not be better to cut the keel away completely and stick a wood or plastic keel on afterwards?
regards Roy

[mermod]

once again thanks for all this great advice guys, I can see where blowing to stretch would be an advantage, might try and build that into my final design, have seen a clip where bathtubs are formed using that method, as for getting them done for me, well were not in the outback but might aswell be when it comes to stuff like that, nearest place is Hobart and even then after a few inquiries most thought I was crazy and should just build my own vacformer and why was I building models anyway.
As for adding the keel afterwards, if it was for myself I would do it but I'm hoping this might be the hull for a kit and would like to make it as easy as possible for a beginner however I may have to resort to supplying the keel separately.
Thanks again, it's so good to have people to bounce ideas off that don't think I'm too mad.(if you all only knew)
Phill

[tigertiger]

@roycv has the best option.
Thinking about it, my Mary J Ward hull was moulded without the keel, but a flat section for location/position of the keel. A wooden keel in three pieces was provided. I will try to dig out pics.



There is another option. But you will need to make new plugs.

Build the hull in two halves. Left/Right. Make the keel pieces full width or more on both sides, but smaller on one plug by the thickness of the moulded (stretched) plastic. So that the two halves interlock, then trim and glue to fit. You can fill the gap with a gloop made of some plastic snippets dissolved in the solvent glue.
More work, but you will then have a very solid keel, rather than one with wafer thin sides. And you should have crisp edges.

[Circlip]

Blow type moldings is for deep drawn items (like Barfs)
 
  With Vac forming you're never going to get a precise angled external corner on a male mold unless you make its female counterpart and compress the material between them, but the base material must be thick enough to allow for thinning on extreme corners.
 
  You could try using a female mold but it would only give a marginally better form.
 
   Never made a fibre-glass molding?
 
   Regards   Ian.

[tigertiger]

Here are some pics. Pics 2-4 are courtesy of Romain Pecks MJW build.


Pic 1 shows the hull form, you can just make out the wooden keel that is glued on with epoxy. The hull has a squared off bottom to attach the keel to.


Pic 2 shows the wood stern post set up and the long square section wood keel.
Pic 3 shows the pre-cut curved stem post clearly.


Pic 4 shows inside the hull and gives an idea of what the plug would have looked like. I think this is a male mould.

[StarLocAdhesives/FiveStar]

When forming something like that on a industrial machine we would blow the plastic to a dome and then raise the former into it then apply the vacuum , the vacuum on the larger machines has a 3 inch pipe with a valve in it from a 200L vacuum tank , the pump runs all the time to raise the vacuum in the tank and then the 3 inch valve is opened to very quickly apply the vacuum to a great big hole in the centre of the table, the table has wire mesh on it to spread the vacuum.
On the smaller machine we have it has no vacuum tank the pump does all the work , we can not form something so deep with sharp corners on that machine, the rapid vacuum from the tank makes the difference
Doing it like large machine you can get a very strong pull so areas like the keel will form, but for the bottom edges were it hits the table we use a overhead plunger to press around it, on that I would use a cut out the same shape as the outline of the shape on the table, sometimes the bottom will web and the former removes it
We use heated tools on the table made from aluminium filled polyester resin and spray water from above to cool the plastic enough to remove the moulding, it is removed by blowing the air through the table and lifting the moulding off the tool, no need for this it just speeds things up as we mainly make seed trays with it
Without the air to blow the dome before forming, you will probably have problems getting it off the tool , if it forms in the corners , air to remove is almost essential and makes it easier to form in the first place