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ScanmanDan

Vacuum Chamber or Pressure pot?

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1 hour ago, ScanmanDan said:

Chek - Interesting life you have lead. 

 

 

Only if you're impressed by one litre UK plastic milk cartons and heavy duty rubber kitchen gloves (the only lasting things I worked on the prototypes for), Dan!

Edited by Chek

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21 hours ago, MikeMaben said:

HPH uses only vacuum (apparently)  ...

Watch about 1 minute in https://www.youtube.com/watch?time_continue=86&v=NGeN1DcrSlE

 

 

I'd like to know what kind of resin they are using as it pours almost like water.

I'm sure that helps alot.

 

Hey Dan,  One of the first places I look for just about anything is Amazon.

You guys have it in Australia too : https://www.amazon.com.au/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=vacuum+chamber

 

 

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Mike, I can't recommend highly enough talking to your silicone and resin supplier.

 

I've been out of the loop for 20 years now, so I'm damn sure the technology has advanced in that time.

But talking to your supplier about your specific requirements is the best way toward getting what you need. Most will even sell you a starter pack of a kilo or two of rubber and/or resin to enable you to test out the types they've recommended, so you shouldn't have to invest more than about GBP50 for a trial run.

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I've found the reverse is true from all suppliers be it Axson, Alchemie, Etc although they will sell you the resin and know about cure times and hardness etc they have little knowledge of the actual process of moulding and casting your particular prototypes. I only found the best way was to talk to fellow professional modellers who had experience in producing products.

Graham 

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Greetings all; bear with me as this is a lengthy reply to a timely topic to be sure; I have been contemplating a "Resin Casting" thread in Tips and Techniques, but don't have the time to do a full run-through of the process, plus I don't cast anymore. I have, over the years, developed chemical allergies to many common chemicals, and I am paying the price for it as I age. In consultation with a great Allergist, it's fairly obvious those allergies have developed from my 30-plus year career as a professional model maker. So, word to the wise or unwise: WEAR GLOVES! WEAR A RESPIRATOR! WORK IN A WELL VENTILATED AREA! Please, for the love of Pete, don't be arrogant like I once was, protect yourself, and your loved ones (including pets).

 

Ok, enough nanny-state. I don't want to tread on anything my esteemed professional colleagues have mentioned here already, but I will add my own observations, as well as some photos of my basic set-up. You've taken the correct first step - ask questions!!! I learned most everything I know about casting by asking questions of those who did. I am not a chemist nor an engineer. But I am curious, and a logical thinker (a critical skill). As another response said, you don't need to go overboard to produce good, bubble-free castings. But some expenditure will ensure good results just about every time. I will not address availability of equipment overseas, I am in the States and I, too, have used Harbor Freight, but also industrial suppliers such as McMaster-Carr and Grainger. Surely outfits such as those exist overseas (in fact, I'm almost positive both of those have overseas offices).

 

So, in a nutshell, as others have mentioned, I have found two processes advantageous to achieving consistently good, bubble-free castings: 1) vacuum your rubber before pouring the mold, and 2) pressurize your resin in the mold once poured. There are a great many ways to cast resin parts, but vacuum-casting is a whole other ball of wax and a whole different level of expense in equipment. I have only done vacuumed RTV mold-making and pressure casting. Keep in mind that casting resin parts is a time-consuming process. We have a saying in the industry, about trying to do things too quickly: "if you want it bad, you'll get it bad".

 

1) Mix your RTV and then vacuum de-gas it. Let it set for a few moments to allow the bubbles to settle, then pour into your prepared mold core-box. Set it on a level shelf and walk away for 24 hours. Most good RTVs take that long to cure. Avoid "fast-cure" rubbers, and avoid polyurethane rubbers, they are for special circumstances. I feel there is no need to re-vacuum once the rubber is poured - it would just make a mess anyway. Note that when you vacuum mixed RTV, it "boils" when it achieves full vacuum. Once full vacuum is achieved, that's it. No need to go on for more than a few moments; shut the pump off and the RTV will settle down into a clean mix.

 

Here are my vacuum chamber and pump:

 

dYi3vR.jpg

 

The chamber is called a "vacuum desiccator" and is large enough to hold a quart (905 gm) yogurt cup which I use to mix my rubber (I've used Smooth-On products for 25 years, they are within an hour's drive from my shop). The pump, while not super large or quick, does the job, since the chamber is only about 4 cubic feet. I like the desiccator because the lid is clear and I know just exactly when the RTV has reached full vacuum and is boiling.

 

8q6m6B.jpg

 

Since it's for vacuum only, the lid simply rests on a rubber seal, and the fittings are all friction-fit. Works a charm.

 

2): Mix your casting resin, pour it in your mold cavities, and then pressurize it until fully cured. Try to avoid a "fast-cure" resin - you want as long a pot life as you are comfortable using. As others have mentioned, generally a longer pot life = a longer cure time. I used Smooth-On's Smooth-Cast 310 for years - it has a 20-minute pot life and 4 hour cure time. Be forewarned, though  - pot life and cure time never seem to be as long as manufacturers claim, but that also may be a result of your shop environment more than anything. Working in my basement shop, I found that in summer the ambient temperature was high enough to cause problems with pot life, enough so to curtail the activity until cooler days. Regardless, I will leave filled molds in the pot for 50% longer than the stated cure time, just to be sure. Remember, you can't rush this.

 

Here is one of my 3 pressure pots, all Harbor Freight "5-Gallon Paint Pots". These are designed to be used vertically, holding a full 1-gallon can of paint, which is pressurized and forced through the spray equipment they are designed to work with. One reads all kinds of horror stories of these "blowing their top" - that should NEVER happen. If it did it was likely a result of two things: over-pressurization and a pressure relief valve that was designed for much higher pressures than we need for resin casting. In my experience there is absolutely NO need to pressurize over 30 PSI. You should have valves and gauges on both the compressor and the pressure pot, set to your 30 PSI max working pressure. And your pressure-relief valve should be for 40 PSI. Safety is key.

 

LbNU4W.jpg

 

In the above view you can see three critical items. First, I replace the factory pressure-relief valve with a much higher-quality and lower-pressure-rated device. This is the brass barrel-shaped piece with the pull ring on it. Second, plumb your airlines with ball valves - these allow you to slowly introduce your pressurized air. Third - replace the factory pressure gauge with a higher-quality device. Simply put, I don't trust the fittings on these inexpensive pots, and nearly every fitting that comes with the pots out of the box is binned and replaced with high-grade, trustworthy components. Note how the clamp dogs are starting to create wear points on the lid - this one has just about reached the end of its useful, SAFE life. Also note the quick-release hose fittings - useful for removing the air line from the lid to make it easier to manipulate. One of the things I did not do, regrettably, is to plumb in a "waste-air" evacuation line for when you de-pressurize the pot (done by pulling the ring on the pressure-relief valve slowly). You are letting a toxic chemical-saturated stew of air back in to your working environment when you de-pressurize. Not good for ones' health. Probably not good for the pressure-relief valve, either.

 

Here is a view with the lid removed:

 

8XDuG9.jpg

 

In this view you can see a couple of items of note. First is a styrene sheet (plasticard) baffle that prevents the inrush of pressurized air from blowing liquid resin out of your filled molds. Second are some scrap acrylic (perspex) shelves to place molds on. You can see the simple wood-and-steel-strap cradle I have made that holds the pot securely in it's re-plumbed horizontal position. I have a wire hook hanging from the ceiling above my casting area to hang the lids when loading and unloading molds from the pot.

 

Another point to make, that has been covered already I think: pressurizing liquid resin does NOT "force" resin into the nooks and crannies of a mold. It simply forces air bubbles into "solution" - it makes them disappear. Tricky science stuff, that. Speaking of tricks, the very best way I have found to help resin into a molds' nooks and crannies - talcum powder. Powder your molds and you will be amazed how resin flows into the tightest spots. I have cast HO scale ketchup bottles and salt shakers for a Diner kit I manufactured years back. Talc improves the flow of resin, I assume by relieving the surface tension. And, It is completely absorbed by the liquid resin (unless you leave blobs of it in corners, etc. I keep a dedicated soft-bristle brush handy for brushing the talc in molds).

 

Finally - use mold release. It will extend the life of your molds and make parts much easier to de-mold. There are so-called "paintable" mold releases, which are fine, but I wash resin parts no matter what. As for mold life, you can generally expect to get 20 pulls out of a silicone RTV mold. Parts design and surface texture affects that, but it is a general rule.

 

The one thing no one has really touched on is mold design. Simply put, there are one-piece molds, and there are multi-piece molds. One-piece molds are called "flat casting" and you usually want some sort of thin material that is laid on the top of the filled open-face mold. This is what creates the very thin "matrix" of resin that you see on a "sheet" of parts. Multi-piece molds require planning and incorporate several features. The halves should be "keyed" in some fashion, so they align properly and stay aligned when filled. They also need a pour stub (think the main sprue attachment in injected molded parts). Multi-piece molds also sometimes need air vents. You can't fill a complicated chamber without some way for air to escape. And as with so much of what we do, planning and parts (and mold) design are critical components of successful casting.

 

So, I hope you've enjoyed my presentation of "Vacuum-Mold Making and Pressure Resin Casting 101". I always enjoy sharing the knowledge I have acquired over the years. I do not believe in "trade secrets" per se, although I understand why some do, I have always been more of an"open source" professional. I'd not be where I am without someone at some point sharing their knowledge freely. Thanks all and happy modeling!

 

Jimbo

 

 

 

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 I use a vac chamber for the RTV, I mix up a batch, vac it, making sure the container will be big enough because it raises dramatically.

Then pour it into the masters, letting it puddle from the bottom up, then back into the vac chamber for another suck. 

The chamber I have  is an old vintage pressure cooker,  it has 6 wingnut dogs, and is extremely well made, heavy and solid. A one inch thick Plexiglas lid with fittings for the vac gauge and hose was made for it. 

The pressure pot I use is the cheapo from Harbor Freight,  I have gone through several over the last 20 years, the only issues with them is the consistency of quality. (check the wing nuts before buying one) Sometimes the seals have to be re-set and I have a mark on the tanks for the alignment when I place the lid on the tank. Once I pour the resin mixture into the molds, I tap the sides (and sometime squeeze the molds) to help release any bubbles.

I have almost no issues with trapped bubbles...

 

I also use Platinum cure RTV, I have an old cookie sheet my wife was tossing out that I use to fast cure the molds. I have a space heater that has no elements exposed, the sheet sits on top of it and is the perfect temperature to cure the rubber in about an hour, it also keeps my casting room the correct temp for the resin

 

I know the "big guys" may do things differently, but this works for me.

 

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Thank you Jimbo and Harold for your passing on your experience and advise.  There is much to unpack in your replies.  All of your folks comments have been very helpful in formulating a plan to move forwards.  First , save a bit more cash.  Isn't that always a good starting point?  Next I'm going to work a bit harder to be more tidy and thoughtful in basic mould construction/ design and casting. 

 

Adding talc are already part of my SOP but I haven't used mould releases as of yet.  Venting of the mould is still new to me and seems a bit of a black art.  Sometimes it works well and other times it's a disaster.  I do tend to arrange my two part moulds so the pour sprue feeds to the bottom of the part and that has helped reduce trapped bubbles.  I find that just the way you orientate the part in the mould and where you place the seam line plays a great role in reducing casting problems.

 

Additionally I'm looking farther afield to source my materials and hardware.  I have also looked at dragooning a more competent friend in to help modify a pressure pot.

 

Thanks agin for all the ideas.

 

Edited by ScanmanDan

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G’day Dan,

 

i use a 10L paint pressure pot as a way to pressurise my resin casts as well as using it as a vacuum chamber to degas my silicone moulds. I had a thick piece of clear Perspex cut to fit over the top of the tank and plumbed it to accept a vacuum line attached to an air conditioning mechanics vacuum pump. This is the one that I purchased: 

 

https://www.supercheapauto.com.au/p/blackridge-blackridge-air-spray-gun-siphon-feed---paint-tank-10-litre/156470.html?cgid=SCA01050104#start=11

 

they have gone up a little in price since I got mine, and you dont need most of the stuff hanging off it. Super cheap usually have these in their sales though, so they do come down significantly in price if you are prepared to wait until the right time.

 

hope that helps,

 

Eric.

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Thanks Eric, I hadn't though of Supercheapauto but that makes sense.
If only we had something like Harbour Freight in Oz :)  Last time I was in the States I picked up a adjustable magnetic holder for my dial gauge for $15 on sale.  But I then had to lug it home :( I could of filled my luggage if SWMBO hadn't been with me.
Can I ask what kind of shop cut your thick Perspex for you?

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Personally I would avoid using talc because you will remove the natural release agents present in the silicon a lot earlier and dry out the silicon rubber, if you're pressurising the resin you Don't need to. I have never found release agent that helpful when it comes to casting and now never use it plus it's expensive anyway. A big issue I've found is being careful with what rubber  you choose, I use a clear silicon and a resin that is known for not being aggressive to moulds. Occasionally I make some fuel pitots for a target drone company and I have to use the resin they specify but its so aggressive ( and smelly ) I barely get more than 20 casts out before the mould is shot.

Graham

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Hi Jim, I have 2 questions.

1) If the goal of vacing molds and pressurising resin is to eliminate

bubbles, why couldn't either vacing or pressurising work for both ?

 

2) Is your recommendation for talcum powder and mold release

an either or situation ? Using both would obviously be rather

messy. Or is it powder first , then mold release ?

 

Thanks much for sharing your expertise.  :clap2:

 

 

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1 hour ago, GrahamF said:

Personally I would avoid using talc because you will remove the natural release agents present in the silicon a lot earlier and dry out the silicon rubber, if you're pressurising the resin you Don't need to. I have never found release agent that helpful when it comes to casting and now never use it plus it's expensive anyway. A big issue I've found is being careful with what rubber  you choose, I use a clear silicon and a resin that is known for not being aggressive to moulds. Occasionally I make some fuel pitots for a target drone company and I have to use the resin they specify but its so aggressive ( and smelly ) I barely get more than 20 casts out before the mould is shot.

Graham

Same here, I have never use mold release nor talc. Although I have sprayed some (just a dusting) into a mold that was an open face mold and I only needed one more part from it before I cut it up to recycle. I used a Q-tip to rub it around and let it dry before pouring. I got one more perfect casting out of it...

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Interesting feedback gentlemen.  I have only used talc at room atmosphere pressure and have had better results so maybe the benefits are swamped out by the advantages of casting under pressure.  As for reduced mould life, I only run a mould maybe a dozen times over it's life and use very benign materials.  In my experience I don't see a reducing in mould life.  
It's great to see all the different points of view coming together in one place, thanks.

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OK I'll ask this of any of you experienced guys.

 

If the goal of vacing molds and pressurising resin is to eliminate

bubbles, why couldn't either vacing or pressurising work for both ?

 

:shrug:

 

 

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