The Following Comments are by David W. Braun, with The Back Shop

 

In many cases, your plastic original can be converted to a brass casting. What follows will be a brief run-down on the process, known as “investment casting” or in slang terms, “lost wax.”

 

WHY CONVERT PLASTIC PARTS?

Obtaining a brass part from a plastic part can be done for reasons of strength. As for example:

 

Model Railroad: ladders, stirrup steps, brake wheels and the like.

Model Aircraft: landing gear, antennas, steps

Armor: small, breakable detailing items like tow cables, shovels, axes, jerry cans.

Model Automobile: suspension parts, driveline, windshield wipers, door handles

 

A conversion can also be done as part of pattern work, i.e., fabricating or converting plastics to brass for metal masters for casting. Also, plastics can be converted to brass for subsequent spin-casting masters (i.e., “soft metal” or “white metal”). And, some folks just prefer working in brass, as opposed to plastic!

 

UNDERLYING BASIS OF THE PROCESS

The investment casting cycle relies on the combustion process as part of the mold preparation. Any master, whether wax or plastic, is literally vaporized out of the investment mold, and in doing so provides a cavity or “mold” for subsequent metal casting. Another words, your plastic original is destroyed.

 

NOT COPY CASTING!

There is a misunderstanding on the part of some that this process is a form of copy casting. Not so! If for example, you want 5 brake wheels, you must provide 5 brake wheel plastics because of the combustion stage cited above.

 

WHAT KIND OF PLASTICS CAN BE USED?

Low- or medium-impact styrene, Delrin or its competitor, Celcon.  Evergreen styrene is very much usable.

 

WHAT KIND OF PLASTICS CANNOT BE USED?

High-impact styrene, ABS, 1940’s-1950’s style acetate styrene, plastics that are plated, painted or have a lacquer coating (of which, more in a moment).  Some Plastruct “plastics” are actually ABS; be wary.

 

WHAT IS THE DIFFERENCE?

The combustion cycle cited above will cleanly burn out the styrenes and Delrin. The others leave an ash in the cavity—even lacquer will not burn out at 1300 degrees F. Any casting made from these sources will be extremely pitted (reflecting the ash deposits).

 

HOW DO I TELL THE DIFFERENCES IN THE MATERIALS?

Very generally speaking, the difference between low and high impact styrene can be measured by how a sprue cuts with sidecutters. A hard “snap” indicates high-impact. The Delrins and Celcons are very slippery to the touch and will not react to solvents. ABS is extremely hard to detect by eye, except some types actually shatter or spaul when cut.

 

If there is doubt about the nature of the material, a test cast can be made on a sprue or a runner, or other scrap parts, to determine the burn-out capability.

 

WHAT ABOUT PLATED OR PAINTED PLASTICS?

Plated or painted plastics cannot be burned out as is because of the ash problem cited above. Plated or “chrome” plastics are plated by an electrostatic process, involving an underlying lacquer coat onto which the plating is applied. The plating must be removed with oven cleaners or the like. The lacquer coat in turn must also be removed with oven cleaner or similar.

 

Warning: the oven cleaners and similar solvents are extremely unforgiving of the human skin. Use gloves and do the work outside, never indoors. Further, some of the “paint removers” sold in model railroad shops (to strip paint from plastic freight cars), as well as the “Adios” nail polish remover (cited in some of the model car magazines) have to be used with a great deal of caution.  Left unattended not only will they remove paint, plating and lacquer, they will go on to etch the plastic itself. Left unattended for extended periods, and the plastic itself will finally dissolve. Use these (and the old stand-by, automotive brake fluid) with extreme caution if you value your plastic. Soak the materials for no more than a few minutes at a time, and frequently flush with water and scrub with an old toothbrush. You are forewarned!

 

We have also found various painted and/or plated plastics react differently to different chemicals. Recently, we have encountered plastics made in China reacting violently to the “Adios” nail polish remover. The plastic literally melts within seconds of being immersed. Further, we have also encountered Chinese-made plated plastics impervious to oven cleaner, yet responding very well to the model railroad “Polly-S” paint remover.

 

The point to all of this is to proceed cautiously. Always use a sprue or other scrap piece, to test the chemical, prior to proceeding with the paint, plating or lacquer removal.  Take it from one who has been through all of this before, who has destroyed plastic and has tried every chemical cited above!

 

 

CAN RESINS BE BURNED OUT?

We have run resins through our shop for almost four years and it is our considered opinion that casts of resins are good only for pattern work. The underlying problem is not only the ash consideration cited above, but another, more subtle situation. Resin, despite what you might read or be told is not a dense material. Part of the investment casting process involves a vacuum stage. If a pattern is the slightest bit permeable, the vacuum will draw the investment medium into the pattern. In part, this could be circumvented by spraying the pattern with a plastic coating, to form a barrier of sorts, but we have found the end results to be mixed. We will do resin burn-outs but with the understanding that the resulting casting may not be of the best quality.

 

And yes, to anticipate another question: since by now the reader has seen that anything combustible is in theory castable; yes, we have cast bugs, spiders, flowers, paper, cloth, you name it. Work of this nature is generally just to create conversation pieces rather than anything practical.

                       

WHAT NAME BRAND PLASTICS HAVE YOU CAST?

Renwal, Hasegawa, Revell, Monogram, Frog, Academy, Red Caboose, InterMountain, GrandtLine, Cox, Athearn, MPC, Strombecker, San Juan Car, PSC, AMT, DML, MDC, Lindbergh, Kemtron, Atlas, Aurora, Ertl, Williams Brothers, Life-Like, Pyro, Accurate Miniatures, Cannon & Co., Westwood, Detail Associates, Details West, A-Line, Simpson, Jordan, Bachmann, Roco and some we have probably forgotten about!

 

HAVE YOU TRIED TO BURN OUT SOMETHING WHICH YOU THOUGHT WOULD AND IT DID NOT?

Yes, most definitely. Although seldom seen today there is a casting material called “Cerrobend.” This material can literally be melted in a double-pan situation on your kitchen stove and poured into open face or two-part RTV molds. It is an extremely-soft material, but many outfits in the 1950’s and 60’s used it extensively.

 

Years ago, we were told on various occasions, by several “old timers” this material could be burned out. We have run tests over the years with some of the most spectacular “mis”-results ever seen. Either we are doing something wrong, or the stuff won’t burn out. Needless to say, unless we can get better information, we won’t go there again.

 

If you have a “rare, original” Cerrobend casting from which you want duplicates, it will have to be molded in RTV and then run through the normal waxing process. Contact us if you need further information about this point. And, if you have “rare, original” plastic you not want to have destroyed, ABS, acetate plastic, etc., again contact us and we will discuss what’s to be done.

 

 

WHAT IS THE OVERALL SHRINK BETWEEN MY PLASTIC ORIGINAL AND THE RESULTING CASTING?

Exactly one percent (by exact measurements, somewhere between .99 and 1.0 percent). If pattern work is contemplated, there are other shrink considerations, and we ask you contact us prior to starting any work of that nature. But if you merely want a brass from your plastic, you will be handed back something that is one percent smaller in all three axis.

 

OK, I HAVE PLASTICS FOR BURNOUT, WHAT DO I DO NOW?

Contact us by mail or Email, and we will arrange matters with you. Naturally, there might be some give-and-take, and we absolutely do not mind spending time with a new customer, to get all of her or his questions taken care of.

 

We have no minimum casting or dollar amounts. If you want one piece of plastic converted, that’s your business. Costs for this service are in relation to how much plastic in amounts and volume you want to run. A single GrandtLine model railroad brake wheel in O scale will cost you a dollar. The GrandtLine 6-ton Porter locomotive kit is $125.00. A 1/25^th scale mag wheel would be about $3.75. A 1/35 scale 50-cal. machine gun on a tripod would be $15.00 or so. The Hasegawa 88mm Flak 16 in 1/72 scale, complete with its transporter would be around $45.00. And so on.

 

Yes, we give a firm quotation prior to starting the work.

 

ONE LAST QUESTION, DO YOU WORK IN OTHER THAN BRASS?

Yes, we do. We have a metal called “Nickel Silver.” This is a rather interesting metal, and in some forms it is a variant on the yellow brass alloys. Old-time foundry men called it “white brass.” Today it is known by several names including white manganese bronze and “Bronwite” (A trade name).

 

Oddly enough, it has no silver in its alloy, and in some cases no nickel, as well. Think of it as a yellow brass alloy with a high manganese content. It is also a tricky metal to cast, and some foundries want nothing to do with it.

 

For model work, its chief appeal is that it takes a very high polish, akin to plating. Any of the traditional metal polishing methods can be used with this metal.

 

To decide between brass and nickel silver, use this rule: if it is to be painted or plated, use brass. If it is to be used for pattern-making, choose brass. If it is to be polished, and if no plating is contemplated, choose nickel silver.

 

Thank you for taking the time to read this “tome.” If you have any questions or comments, please feel free to contact us as below---David W. Braun

 

THE BACK SHOP

PO Box 15285,

Sacramento, CA 95851-0285 USA

 

Email: dbraun@jps.net

Homepage: http://home.onemain.com/~thebackshop

 

 (This page may be freely copied and distributed by the reader, if done so in its original form, with no changes or deletions. Otherwise, simply refer an interested party to this URL. Thanks!) 

 

 

UPDATE: 8 June 2001:  CORING CONSIDERATIONS

 

It should be kept in mind that in most cases, cored (i.e., holes) plastics can be cast with the holes in place.  These would be holes which run clear through the piece, not blind cores. Casting blind cores is a very tricky process, involving some special foundry techniques.  We would prefer to discuss blind cores to your individual requirements, as to further explain it here would require some very esoteric foundry practice discussion.

 

However for holes in general, we can cast them in place. Holes can even be tapped before hand! We take pride in casting intricately cored pieces, so be sure to look over your job with the idea of using cores, and thereby saving time having to drill out a subsequent casting.

 

--dwb.

 

 

UPDATE:  2 August 2001:  FURTHER COMMENTS ON PLATED PLASTICS

 

We have been asked if we could strip plated or painted plastics on behalf of the customer. Prior to the disaster with the Chinese plastic (discussed above), we would have been accommodating. However, now that we are aware of newer plastics reacting differently to some of the old “stand-bys,” we must respectfully decline to do such work in the future.  Again, try out your chemicals on a sprue or a scrap or surplus piece, First!

 

--dwb.

UPDATE, 05 March 2004: THE COLOR OF STYRENE

Styrene, in its natural state is usually neutral in color ( and some are completely clear). The color seen in a styrene piece is generally from colorants, dyes or powders, depending on both the maker of the styrene and the injection-molder. Depending on a great deal of variable factors, the color itself can present burn-out problems. There is generally no way of predicting in advance how a piece of styrene will 'run' in the casting process, meaning once again, samples will have to be run from scrap or sprues.

dwb.

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