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Molding plastic
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  gundoctor

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Posted: November 16 2008 at 11:51am | IP Logged Quote gundoctor

Here is what I am thinking.  Going to need pressure to inject something with reinforcing fibers.  Still trying to determine what to use for fibers.

I intend to try make a disposable injection system.  It will be based on pvc pipe (disposable) with a reusable aluminum piston.  It will use cheap tubing to connect to mold.  Probably cost less than $2 for an injection and nothing to clean up.

Use the shop press to power my injector. The mold will set on the press by the injector so there is a minimum of dead space and wasted materials.

The mold will be prepared and connected to the disposable injector.    I might use a vacumn in the system to evacuate air from the mold / system prior to injecting.

Compliance parts (stocks, grips) and some stocks for some of my custom one-off's really interest me.   I also think a composite receiver with some aluminum inserts in the casting is a real possibility.

Thoughts on mold materials?  Silicon?   Say for example if you were making a pistol grip.
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  dcorb

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Posted: November 16 2008 at 12:01pm | IP Logged Quote dcorb

This is getting interesting.

I am interested in starting with grips and stocks.

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  midmichigun

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Posted: November 16 2008 at 1:41pm | IP Logged Quote midmichigun

Ladies and Gentlemen

Yes, it is getting interesting. Keep the thoughts flowing.

Point to remember that is depending on pressure, you may have problems with your hoses/ tubing. I have seen alot of tubing fail due to being used beyond a pressure rating or temp range. Reminds me of the day that a PHD decided to overfill a 3neck flask without thinking about thermal expansion. He tried to siphon out the hot overflowing material with a plastic hose. It was pretty funny except for the burning hot liquid spilling everywhere....

When I go to Lowes or Home Despot, I will look at their reinforced tubing with I believe nylon fibers. It should be able to handle what we are looking at.

Several statement to make on bore vs. stroke of a piston injection system. If you use a 2 or 4 inch PVC pipe, and neck it down to say 1/4 or 3/8 inch, outlet you will have alot of pressure build up. Your AL piston may do the trick with "thick fiber" fluids. Obviously a shop press is going to be an awesome thing to use. I would try a solid rod piston first. I have seen shop presses be pretty wobbly. The syringes we use in labs have a "driving seal". The piston column is x-shaped for strength (good polar moment of inertia). If you can get decent stroke, I would be interested in going with a 1in PVC barrel (outlet 1/4in) and a 5 or 6 inch stroke.  

PVC is wonderful stuff, I think that it is the ticket! It would be a great mold also if you could carve it.

The mold will be the problem. I think that with the pressure being moderate a flexible mold is not a good option. The silicone will deflect under pressure. For example, with my two piece mold I have to be very careful applying pressure otherwise it will squat. If I have time, I will post more mold pics (not gun related, but interesting from a defect/ problem standpoint, with the same soft mold technology that the receiver was made from). If you squeeze a whole bunch of "tooth paste" into your mold it may either give (deform) or in turn squeeze out un- needed material. If you have a multi- piece mold, the material will more than likely get into that seam.

One thing to try may be a semi- permanent type mold of plaster. This would be workable but stiff. Not as enduring as steel, but something that could be done on the kitchen table. More importantly, it can be used many times. Teflon sprays would work, but you could also vaseline this.

Avoid latex molds. These have alot of problems that we don't need. They are flexible, don't last long, and react to many solvents. Hence my initial silicone route.

If possible, a metal mold would be ideal. However, I think that it would only be great for high production rates and for those of us with a talent for metal working.

Pistol grips by far are the easiest. Followed by handguards. Stocks would be next. I have several extra ak pistold grips. May create a mold to demo the technology. Since I don't need extra AR or AK grips, I have been not going in that direction. However, it is a good experiment and others may need them.

For a custom part, you could coat your original in clay (pistol grip for example) make your changes. Then you could cast plaster around it. I would avoid trying to cut a plaster mold apart (you remove the thickness of your blade from the mold and resulting parts). Cast one half, let it harden, slather it with Vaseline (only need a light coat) and finish pouring. When hardened, it should pop apart with ease. Obviously work out teeth or alignment rods for your production run.

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  gundoctor

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Posted: November 16 2008 at 2:59pm | IP Logged Quote gundoctor

Thoughts:   can eliminate tubing by attaching cylinder directly to mold rigidly.

Second.  What about making a ceramic mold.  Use ceramic clay and fire it? 

Lastly, what about going thermoplastic?   For most of the parts we are talking about making would fit in an over (molds, injection cylinder and all).   Could you take an old oven and make an injection system basically inside of it?   What temps are needed for injection molding.   Just brainstorming.
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  Mad Machinist

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Posted: November 16 2008 at 4:15pm | IP Logged Quote Mad Machinist

Another possibility is to make a wax model from your mold.......invest it in plaster....burn the wax out in a furnace.....and then make a simple spin casting machine and do it in aluminum......or maybe brass...

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  gundoctor

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Posted: November 16 2008 at 5:30pm | IP Logged Quote gundoctor

Just thinking some more:

I kind of like the grips and stocks on the vz58, and my understanding is that they are woodchips / sawdust in some sort of epoxy?   Looks good to me and this might be fine for stocks.
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  midmichigun

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Posted: November 16 2008 at 10:17pm | IP Logged Quote midmichigun

Mr. Mad Machinist,

Bad! Bad!  Now don't go stealing my thunder! I happen to have several fried lawnmower engine blocks, along with some AL from a few appliances. Low and behold... huh... I have 1.5 lbs of paraffin wax! Now what to do with my lowly AR mold? Look for more in the Metal Casting thread (when i get time).

I have around 20lbs of brass/ steel from spent ammo laying around. Some is military that seems like nobody wants to reload.... I have never thought about a brass AR lower.

Gundoctor, Never held a Vz58 so I can't comment on wood dust. However, it is found VERY often in other circles. The dust makes a good cheap fill, that when combined with a good epoxy makes a pretty mean part! This is a good idea. However, i don't know how "ground up" the wood has to be (particle size). My tablesaw has pretty fine dust... perhaps the stuff you pull out of wood shop air cleaners? Marine grade plywood is just wood.... and a good dang epoxy.

Thermoforming is a great way to go. Hand and hand... with injection molding. I figure that suddenly there will be a lack of pop/ milk cartons in peoples houses!

Just got back from a date with the wife.... the whole time I was looking at a decorative pipe at the restaurant thinking that it would be a cool barrel for an injection machine!!! Pretty bullet proof!

You can get alot of different plastics with various melt temps. I believe that milk cartons melt at a low temp. I will have to lookup PET to see its characteristics. I have a few heat bands from other projects that would do the trick. However, I want to concentrate on ways to do it... that are compatible with things that people have in their house/ shop.

Keep thinking... and building!

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  bikergunnut

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Posted: November 16 2008 at 10:21pm | IP Logged Quote bikergunnut

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  bikergunnut

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Posted: November 16 2008 at 10:22pm | IP Logged Quote bikergunnut

Photo stolen from a gunbroker auction .
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  Inabadhood

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Posted: November 17 2008 at 11:49am | IP Logged Quote Inabadhood

I know this is a thread about casting plastics, but has anybody here ever done any work with forming/casting of PolyIsoButylates (PIB's)???

It's basically a form of artificial rubber - rather than a 'plastic' per se...  Anybody have a source where I could find some PIB material for 'casting' (which involves a chemical solvent to make it maleable/castable)???

 

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  midmichigun

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Posted: November 17 2008 at 6:58pm | IP Logged Quote midmichigun

Guys and Gals,

Looking good so far. Nice pic for the vz58! I see what you mean. Looks like they just used wood as a filler. I am curious to see what the West System would do with that! Guys use it on wooden boats all the time (pretty water resistant). Now I will have to go out and buy a gallon of it... to play with! Haven't been able to afford it in awhile.

I have been staring at my ak74 pistol grip. You can see the fiber swimming in it.

I would LOVE to cast PIB's. I don't think it would be hard... just need a supplier. If you wanted to coat a handgrip or something... it would be a great asset! I was looking at "The Blue Press" and they had "synthetic rubber" for rails and mounts... so you could make them into grips, and protect the surface.

We need a full time person to chase down resins/ plastics/ rubber on the internet! :)

Anyone ever deal with Bakolite (SP)? I remember working with it. The experiments we did on it, involved a powder (bakolite), heat and pressure. Pressure to keep thinks square (or round in the mold) and heat to melt it. There should be a source for it on the Internet. It was pretty durable.... I think it cracks over time... but nothing immediate (they still have jewelry from the early 1900's that hasn't cracked). I think it took a little abuse from us, when we researched it. Any opinions?

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  weaponeer

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Posted: November 17 2008 at 7:33pm | IP Logged Quote weaponeer

a few more VZ58 pics...

and yes, it's mine...  lol

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  weaponeer

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Posted: November 17 2008 at 7:36pm | IP Logged Quote weaponeer

Zytel is a trademark owned by DuPont and used for a number of different high strength, abrasion and impact resistant thermoplastic polyamide formulations of the family more commonly known as nylon, often with varying degrees of fiberglass, from 13% to 60%, added in for additional stiffness[1]. Some of the grades are homopolymer nylon66, other copolymers with nylon6 repeat units, and further grades involving rubber toughening are available.

 

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  gundoctor

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Posted: November 17 2008 at 10:12pm | IP Logged Quote gundoctor

The vz-58 appears to be an early version of "oriented strand board (osb)", except it is not oriented and used chips about like would come from a chainsaw instead of bigger ones.  It also appears to use a high quality polymer or epoxy.

I have been in an OSB plant, and a lot of pressure is involved.  I think the chips are piled several inches deep (with adhesive) and then pressed into the boards about 1/2 inch thick with a tremendous press.
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  midmichigun

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Posted: November 17 2008 at 10:43pm | IP Logged Quote midmichigun

Mr. gundoctor,

I believe you are absolutely correct on the pressure.

I need to spend more time on this "Right" (notice I am NOT using the word "Hobby" anymore). However, with so much work... I will post some pics on other things I am working on... so you guys don't think I am a slacker!! :)

 

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  dcorb

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Posted: November 23 2008 at 12:20am | IP Logged Quote dcorb

I have been thinking about this and what I would like to start with is a G# type gun. I plan to scratch build the receiver, a small metal tube one top of a bigger metal tube. The metal receiver would provide the strength for the things that make the gun go bang. I want to mold plastic (mostly for looks) to cover the receiver and to provider an uber cool looking magwell cover. 

Not sure if it can be a two piece molding or have separate pieces for the magwell.

I am not sure on how to approach this, maybe make a clay model? My wife would like the idea of firing clay. Heck I would be given the OK to build a kiln.

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  gundoctor

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Posted: November 23 2008 at 3:04am | IP Logged Quote gundoctor

I am moving along on this, learning about the different resins, their viscosities, demold time, tensile strength, heat resistance, etc.

Two choices on molds:   Silicone and Polyurethane.  Polyurethane is cheaper, but doesn't have the heat resistance of silicone (if you are heat curing or using a fast curing exothermic casting material).  I am going to make some molds out of both.

Two choices on part material:  Polyurethane and Epoxy

I like the Epoxies:  longer working times--but also longer demold times.  They are also pretty thick in the stronger resins.  You can also get filled resins, high temperature, etc.

I managed to find a great company through a friend that specializes in polyurethance polymers.   I am not going to put a number here because they usually deal only with commercial interests.   They have some good stuff, especially if you are willing to build a vacuum chamber to degas the pour and the mold.   If you are willing to go all the way---vacuum, heat cure.........some of these water thin pourable polyurethanes will approach nylon in strength and heat resistance.  NASA grade stuff.   They are tinatable, paintable, and tough.    I am working up a custom formulation now---it will be Kevlar reinforced--and once I have done some molds and tested it I will make the information and possibly small quantities of whatever I get working well available.   I am going for two products, one just for open air pouring (like Alumalite) and one for working with a vacuum chamber (high tech / supper high quality).     I worked on the vacuum chamber some today--actually it will be both a vacuum and pressure chamber so that I can eliminate any bubbles in the end product.   I lucked into a drying oven--complete control of temperature and humidity up to 200 degrees--for free--and I didn't even know I needed it when I got it.

I am planning to offer some hard to get compliance parts out of this project as well.  Might as well use the molds once they are made.  Another thought I had was to make the molds available to others once/if I am done with them.

There is one more neat thing about this set up.    It is perfect for making molds and casting wax for investment casting metal parts.   I had started on this a while ago and set it aside, but since I am going to be molding anyway I have been picking out some parts to cast in wax and take over to a nearby investment casting facility where they will weld them together (with wax) on a tree, dip them in ceramic slurry, burn the wax out, and the cast them in metal.   They have the ability to cast almost any metal---including high carbon and tool steels.   I really lucked out in finding someone willing to work with me on this project as well.

With the economy slowing down, it is getting easier to find people to work with you on small to medium sized projects.  

I can see the way to make nearly any part I could ever need now, and that is exciting.



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  weaponeer

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Posted: November 23 2008 at 3:15am | IP Logged Quote weaponeer

for potential projects....  these are the two I like (based off of what internals? or donor weapon??  I don't know  Maybe a hybrid AK SCAR or FNC2000)

FN2000 (real)

I say real, because thay make very good looking airsoft rifles, which could be used in the mold making process.

http://www.airsplat.com/Items/ERM-JLS-E03-FN.htm

You can even find Youtube reviews..  http://www.youtube.com/watch?v=EI-uS9NS6hc&feature=relat ed

SCAR (real)

and yes...  they make airsoft  http://www.airsplat.com/Items/ERM-JLS-E02-B.htm

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  dcorb

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Posted: November 23 2008 at 10:15am | IP Logged Quote dcorb

Along time ago I purchased one of the with the intention of copying the stock in wood and inserting an AK parts kit into it. Maybe plastic will be the way to go.

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  whiteing

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Posted: November 23 2008 at 11:53am | IP Logged Quote whiteing

http://lindsaybks.com/

http://lindsaybks.com/prod/sub/machine.html

Great source for the old machining books, technigues and so on. I purchased a book from them on how to build your own plastic injection molding machine. The catalog comes out every 3 months.

Look thru the whole website, there is alot to be discovered.

As a Tool and Diemaker and Moldmaker, I'm following this with interest.



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