Dumb Question: Why Do We Never See Air Bubbles In Parts?

[Canada Jeff]

There's probably a simple answer to this. We've all seen short-shot parts before, but why isn't it more common to see small air voids in the fine detail areas of injection moulded parts? Anyone who's played around with, or even just researched resin casting knows that air bubbles can easily be trapped and you have to plan out your moulding, venting and pouring to try to minimize the risks of air voids. In fact, filling "pin holes" is such a common part of working with resin parts that we take it as just a normal part of the deal. When you look at some of the fine detail of injection moulded kit parts (like the tiny bumps on a distributor cap where plug wires would attach, or small knobs on dash boards, etc), why is it so very uncommon that we find air bubble voids in our kit parts?

Molten styrene injected into the tools has to displace the air that's already in there in order to fill all the cavities, and I'm sure the kit producers try to speed up the production process as much as possible, so it seems to me that the faster you fill the tools with styrene, the more likely it would be to trap air in a small engraving cavity.

Anybody out there know the facts? This idea crept into my head a few days ago, and it's driving me nuts because I can't think of a simple answer!

[Brendan]

You've kind of answered your own question. The way the manufacturers do it is by high pressure injection, which is less likely to get air bubbles in the plastic. It is also cooled immediately in the die by running cold water through the die.

The reason why you don't see resin casters do this is that it's too expensive to do as well as it puts a lot of wear and tear on the molds. Also they are doing a low pressure injection which is more likely to get air trapped into the material. (They usually pour the resin into the mold.) They can also get air bubbles while mixing the resin up.

A TV Program called How It's Made (Discovery Channel) shows how they do high pressure injections which may explain it better than I can. They have repeats occasionally.

[Harry P.]

You've kind of answered your own question. The way the manufacturers do it is by high pressure injection, which is less likely to get air bubbles in the plastic. It is also cooled immediately in the die by running cold water through the die.

The reason why you don't see resin casters do this is that it's too expensive to do as well as it puts a lot of wear and tear on the molds. Also they are doing a low pressure injection which is more likely to get air trapped into the material.

Actually, resin casters don't use any pressure at all...nor do they inject the resin. They simply pour it into the mold.

[Brendan]

Actually, resin casters don't use any pressure at all...nor do they inject the resin. They simply pour it into the mold.

That was what I thought I had said. That they poured it into the mold. But thank you for clarifying.

[Guest Davkin]

Actually, resin casters don't use any pressure at all...nor do they inject the resin. They simply pour it into the mold.

Not entirely true. The better resin casters out there do use pressure, not for filling the mold but afterwards to compress all the bubbles to the point they are invisible. However, there are resin injection systems out there as well and high volume resin casters, (none in this hobby that I am aware of.) do use these systems. They do cost a fair amount, though not nearly as much as styrene injection, and are used for volumes too small for styrene but to large for single mold pouring and pressurizing but allow cheap RTV molds to be used.

David

[Harry P.]

However, there are resin injection systems out there as well and high volume resin casters, (none in this hobby that I am aware of.) do use these systems. David

Exactly...none in this hobby.

I'm talking about your garden-variety aftermarket resin caster...the typical source of the aftermarket resin parts car modelers buy.

[Guest Davkin]

I'm under the impression, that some casters use vacuum pumps to empty the air from the molds, and aid the distribution of the resin in the mold

Some do it that way, however the most common method is to place the poured mold into a pressure chamber to squish the bubbles immediately after pouring. I don't have any personal experience with vacuum casting, I've only used vacuum to degas RTV, but I do know pressure casting works for any type of resin part you want to cast. The size or type of part to be cast is irrelevant to the method used to eliminate bubbles.

David

[Lyle Willits]

In fact, filling "pin holes" is such a common part of working with resin parts that we take it as just a normal part of the deal.

While this one part of your question may have been true about 20-25 years ago, it SHOULDN'T be part of working with resin products today. There are several top quality aftermarket people who produce resin products for the model car hobby who would not dream of selling items with pin holes or flaws of any kind. Perhaps you need to check out some vendors, other than the ones you buy from now. With the very large aftermarket we enjoy today, there is no need to purchase inferior products.

Edited January 9, 2008 by Lyle Willits

[Harry P.]

But the problem is, sometimes the specific part you need is only available from Joe Pinhole Enterprises and nobody else.

[62rebel]

i do know that metal casters use centrifugal force to get the metal into the entire mold; this also ensures plenty of flash because molten metal will follow every path of least resistance it can find.

i think that the quality of resin materials and the learning curve for finding the right mix is what makes modern resin less prone to pinholes.

and over the literally hundreds of injected kits i've built, i've only encountered two that had incomplete shots in them, and luckily this was back when AMT had a parts-replacement program in place. i once used it to replace a badly warped Impala body that simply would NOT straighten.

the resin casters wouldn't send you a part unless it was either the only thing they had, or couldn't do any better. my .02...

[935k3]

I know Harold Bradford(through MAMA)of Historic Racing Miniatures. His stuff has zero pinholes and is some of the best in the business. He puts the mold in a pressure pot used in spray painting after pouring the resin. He also has figured out the best place to put the resin in and the point where it exits which produces the least molding problems.

[Smart-Resins]

I will try to help out some here. For the big companies, thier injection molding is set up with a big press, heat source and cork screw idea. The styrene is melted, then the press turns down, forcing the molten plastic into a high pressure steal mold, held tightly together in another form of press. The molds will have a escape hole with a door. The press pushes the liquid plastic into the mold and out the other side.Once the plastic has filled the mold pushing the majority of the air out, the door is closed.As the press continues to push the plastic in the mold at literally hundreds of PSI, at around 400-700 degrees depending on material, it litterally crushes the air bubbles that remain.Then the mold is cooled quickly with usually water running through cooling chambers in the mold itself. Now these machines cost any wheres in the neighborhood of a hundred grand. Now for the decent resin casters, they can and do try to imitate this the best that they can. A good caster will try to make thier molds using a pressure pot to try to eliminate air bubbles in the mold itself. Then they will also cast the parts in the pressure pot as well to also try to eliminate the bubbles then as well. And some casters do inject thier molds with a small injector simular to a meat tenderizing injector. key here is to use a resin with a longer pot life and also have a escape hole for the air to escape.The problem is the injection molding system allows time to make the parts.Most resins have a very short pot life.Alumilite average a minute to mix, pour(or inject). Did I mention out of that 60 seconds, you are supposed to stir for 30 seconds.So that leaves you 30 seconds to fill the mold and place it in the pressure pot.Very hard to fit it all in. There are some resins out there with a slightly longer pot life.However for the longer pot life, you tend to trade off other things like durability/strength. Now I am telling this as a bigginner that has talked to several other casters. I just got my pressure pot 2 days ago and will be trying to get more advanced on my casting. Now there is something new out there that a friend has told me about, spin casting that is supposed to be good, working on a centrifugal idea to create pressure.have not heard how that is working yet?Thanks.Jody

[Aaronw]

Most pressure cast resin is put under 20-40psi, the most I've heard anyone use is 100psi, compare that to styrene injection molding where I understand its more like 2000 psi (or was it 20,000 psi, can't remeber how many zeroes).

Also consider that the injection molds are designed by an engineer while most resin molds are designed by some guy who has learned by trial and error where to put pour holes and air vents. I think most of the short shots you see when you do see them in styrene kits is caused by not enough styrene being injected for some reason rather than an air bubble. Between the pressure and better design tolerances air bubbles don't really have a chance with injection molds.

Even without using pressure there are tricks you can use to eliminate most bubbles, pour the resin so it becomes a very thin stream, this pops most of the bubbles in the resin. Also designing the mold so any air has a way out helps a lot. Professional casters should not have issues with air bubbles and from what I've seen they don't. Its the small on the side casters that usually are the ones with lots of holes but as Harrypri said many times your only option is Joe Pinhole, and hey thats why Bondo exists.

[s10chevypu]

I"ve Worked in injection molding for 29 years, Setup, and shooting molds, and taking them apart to repair them. First, some of these molds are shot in a injecton machines between 350-500 ton machines depends on the size of the mold & how many cavity. So it's between 2000-2500 psi. Injected at a high rate of speed and presure. The reason you will not find air bubbles is because the molds all have air vents in them. The bigger the cavity the more air vents. The presure a speed in witch the plastic go's in the mold forces any air out of the vents, witch are about 0.002-0.004 deep. In turn if the vents become dirty the parts will have burn marks on the because the hot air can't get out. I've used scotch tape to add venting to a mold. If you do see a bubble in a model kit its because some of the plastic did not melt right. As far as a kit with a lot of flash on it, three things come to mind. The mold is warn, maybe because of a lot of parts being smashed in the mold from the operator not watching the mold. I get this a lot at work. yes plastic does damage steel 2- The mold is getting over packed with plastic, 3- The mold was in a smaller machine, tonage for mold to small. Not to be a know it all, I'am not. hope this helps Thank you Joseph barbato

[62rebel]

remember; a highly trained and skilled team of engineers built the Titanic, while an old man with no education built the Ark.

anybody got a set of plans for that one they could spare? the Ark, not the Titanic.

i'm working up drawings for a 1:1 replica of the Earth.....suggestions?

[Canada Jeff]

Wow... I had no idea the injection pressures were that high. I guess there's my simple answer; air bubbles don't stand a chance under that kind of pressure.

Thanks to everyone for chipping in their $0.02 here. Now my fevered mind can rest again.

[Luc Janssens]

When I worked for VP we used vacuum pumps to draw the air out of the casting.

The MO was to first put several molds on a tefal coated metal tray, then the tray went into a plexi glass box with a lid and a vacuum pump attatched, then we filled the molds about halfway with a some sort of pistol, on which the plastic nozzle served as the two part resin mixer, the we closed the lid and activated the pump, when doing that the resin seemed to boil, then before the resin boiled over, one slowly had to let the air back into the box, then shoot resin again, 'till the molds were full, close the lid, activate pump, boiling, air back in, open the lid, tray out and up to the next one....

It was a rather big operation compared to other resin casters.

Years later when on my first visit at the Lettermans of VLS, I saw that he used pressure tanks, due to less maintenance (the vacuum air pumps needed frequent oil changes) and less waste of resin

Luc

[Mike Chernecki]

Given these explanations, I have a question. I have 4 '59 Cadillac kits from Revell. Why does each of the 16 bullet tail lights have a bubble in the center of the lens.