Standard Merriman disclaimer: David Merriman is a talented, yet outspoken modeler. If you are irritated or offended by his comments, you may not want to read this article. Or you may want to skip the introduction. In any event, you have been warned!
ACID ETCHING, THE PROCESS
by David Douglass Merriman lll
This week’s atrocity: a piece on the how-to of in-shop acid-etching.
I take it that most of the visitors to this site are familiar with the ‘Aztec’ patterned acid-etched painting stencils. These after-market painting masks are constructed of thin gauge metal sheet and are used to achieve the off-color demarcation breaks between gore/panel sections on circular and conical structures (ST universe Primary and Secondary hulls). These painting stencils are perhaps the most familiar application of the acid etching process known to the kit-assemblers who specialize in things Star Trek.
Over ten years ago, completing a (don’t get scared) scratch-built twenty-nine inch long, TOS 1701 ENTERPRISE, I employed the acid-etching process there, producing painting masks for all of the markings. With those masks I applied the numbers, letters, bands, and logos with a spray-brush directly onto the vacuformed and cast resin model. However, the use of the acid-etching process to produce painting masks is the exception not the norm. Normally acid-etched parts themselves are produced to enhance the looks of a basic kit or a scratch-build. The acid-etched items themselves are employed as model parts.
So, why are acid-etched parts preferred in some situations over items produced by the more ‘traditional’ part fabrication techniques? Answer: No part fabrication technique, other than acid etching can (and to a less capable extent, metal casting) produce small cross-section items possessing the ability to capture detail, and to have the strength and ductility to survive handling and bending/forming into none linear shapes.
The oblong small, thin, detailed NX-01 model ‘sensor dishes’ I was asked to produce are just such an example of how the acid-etching technique is best employed.
Recently, the better resin and injection kit manufacturers have taken the initiative by producing, or contracting out, and including acid-etched parts to their kits. Such ‘multi-media’ kit manufacturers will package parts produced from two or more of the following techniques: injected styrene; vacuformed styrene; ABS or some other type thermoplastic sheet; fiberglass reinforced plastic (GRP); cast resin; cast metal; or acid-etched.
But, before I get specific with a description of the acid-etching process… the focal point here… lets take a quick look at the other kit part fabrication techniques:
THE INJECTION FORMING PROCESS
This is the staple o ‘model kit’ that has been out there since the fifty’s. (Likely introduced to you when Mom bought one at the five-and-dime to keep you busy and out of her hair while you were home playing sick from school).
Molten polystyrene plastic is forced (injected) into the cavities of a stout tool (mold). The plastic changes state to a solid after cooling sufficiently, it is extracted from the tool, boxed up, and sent off to those lacking the skill to scratch-build a real model. With the advent of highly automated machinery to handle production, kits made using this process are (and cost is a function of numbers produced) the cheapest type of model kit commercially available.
A well engineered injection type kit only requires the snipping off of the parts from the ‘tree’ (the runners used to route the plastic from the sprue to a tool cavity, each rendering the shape of a specific part), and sticking it to the other pieces contained within the kit. Injection kits are the easiest to build – and the least rewarding to those who pride themselves as ‘builder’s’. The majority of the kits seen at hobby shops, K-mart, Wal-Mart, and the like are of the injection type.
The process has the ability to impart to the pieces a depth of detail and complexity that is limited only by the inventiveness of the tool makers and master builders,
Since a styrene plastic kits parts are formed from a relatively stiff plastic with little ability to flex during the de-molding step of production, the machines used are so designed as to assure quick, damage free extraction of the parts from the tool. It’s for this reason that the average injection kit contains so many more parts than would be found in a resin or metal kit representing the same subject. Injection kit parts can not (with some exceptions) posses any undercuts, hence the need to ‘break down’ the subject into bite-size sub-assemblies. So, there are more gluing and seam filling chores for the end-user that builds from a box containing injection formed styrene parts. And the modeler is also tasked with the chore of scrubbing the injection formed model kit parts with soap and water to remove any clinging release agents used during production. Not doing so will likely lead to disastrous results during the attempted priming/painting phase of model kit assembly. But, that’s another story…
THE VACUFORMING PROCESS
As complicated and demanding of a huge capital outlay as an injection kit is to produce, the vacuforming process is as easy. Basically, a sheet of polystyrene sheet (there are other ‘thermoplastic’ materials suitable to the process) is heated, positioned over a plug or cavity, then a differential air pressure is produced between the two faces of the plastic. The higher pressure atop the sheet causes it to conform to the shape of the plug or cavity. Since styrene plastic sheet is relatively cheap, and the plugs and molds are easy to produce with simple shop tools, and the process can be used to form hundreds of forming cycles, vacuforming stands as one of the cheapest and less complex methods of kit manufacture.
This technique is favored when the need is to produce cheap, hollow, medium weight structures possessing minimum surface detailing. In the SF community, such kit parts as Trek type Primary hull halves, display bases, clear parts, and large bodied structures are likely candidates for this type of fabrication.
The major liability is the inability (by most fabricators) to employ equipment that creates enough vacuum or to procure heating devices capable of flashing the plastic sheet quickly to an evenly distributed heat prior to air-evacuation. These problems evidence themselves as kit parts of varied wall thickness, interrupted shape, and loss of surface detailing.
Vacuform kits are perhaps the most difficult type to assemble.
A vacuformed kit often presents the model builder with the problem of cutting the vacuformed structures from the remnants of the plastic sheet from which they were formed. Partially formed parts require build up with a filler and/or insertion of plastic shims or plugs to make up gross gapes between parts. Another problem is that large, thin walled vacuformed structures will likely require design and fabrication of a re-enforcing inner structure to provide the rigidity and strength needed. Vacuform kits are not for the faint of heart!
GRP LAY-UP PROCESS
Glass Reinforced Plastic (GRP) kits are those that feature the major parts fabricated from a laminate(s) of resin saturated fiberglass (or, in recent times, carbon fiber, Kevlar, and other ‘modern’ re-enforcement’s).
Typically this process is employed on large structures that must be light of weight, yet of tough, unyielding form. Practical r/c submarines, flying planes, and static models comprising large sub-assemblies connected together by low cross-sectioned attachment points are obvious candidates for this fabrication method. The larger Star Trek ships are good examples where GRP is the best model fabrication choice. Dennis DeBoer’s recent ST kit comes to mind. Also his SF submarine model, the fifty-seven inch long SEAVIEW – one of which I recently completed as a fully capable r/c model.
The accepted means of fabrication is to lay up laminates of resin-saturated fiberglass within a hard or rubber tool (mold). In some cases (one-shot models for example) a foam core is carved to shape then ‘skinned’ with GRP. However, if the process is to be a repeatable one the GRP is formed within a cavity type tool. The traditional polyester resin has been supplanted by epoxy and urethane resins as the stiffening ‘plastic’ constituent (epoxy is the P in most of today’s GRP items).
GRP possess the best strength to weight ratio of all the fabrication techniques discussed here. But (as always) there’s a catch: GRP lay-up is the most labor intensive of all fabrication techniques. And since time is money, GRP kits are very, very expensive.
One reason GRP is a favored fabrication technique for the motion picture/TV effects miniature houses is that the structures so formed, If a clear gel-coat and clear resin is used to saturate the laminates, is that a semi-transparent structure will result. Such a structure can be back-lit from within! Just mask off the windows, paint the model, removing the masking, and you have lit windows.
(By the way, GRP was Greg Jein’s favorite method of building the effects miniatures for the ST productions he was involved with. That, and other techniques he put me onto, have served me well over the years).
RESIN CASTING PROCESS
Most of modelers know this stuff from the simplistic term, ‘resin’. Well… there are other exothermic curing plastics out there from which a ‘resin kit’ can be produced: Epoxy, polyester, acrylic, and many other chemistries. However, over the years, do to its ease of use and low cost, almost all resin casting outfits have settled on the use of polyurethane casting resins as the ingredient from which to form kit parts.
During manufacture or during mixing by the end-user, polyurethane resin can be tinted to almost any color. However, as is convention, most casters will mix and pour the resin, as is, and dump it into the tools where it changes state from liquid to solid. Most polyurethane resins darken to a light tan color as they change state. Some specialty mixes of polyurethane will cure to a clear and white finish.
Typically, polyurethane resin is catalyzed – a resin and hardening component are mixed together to start the polymerization/hardening, process – and poured into a rubber tool. Once the resin hardens the formed parts are extracted from the tool and boxed up for shipment.
As the tools are rubber, the cavities within can possess both deep draft and undercuts – the rubbers ability to be pulled back on itself to some degree permits extraction of the most complicated of shapes. A clever toolmaker can render the entire model as a single casting. Hollow structures are possible when casting resin – use of a displacing plug/core, slush casting, and roto-casting are a few means to that end.
Small, highly detailed items of complex geometry are the best candidates for the resin casting technique.
The resin casting process is best suited to production runs of low to moderate numbers. Rubber tools, because of the chemical attack the chemistry subjects them too, are good for twenty to eighty pours, then they have to be replaced. The techniques slow production rate/high labor investment – tool prep, pressure/vacuum step, curing time, de-molding and sprue/runner trimming – contributes to the high production coasts and sticker shock suffered by the modeler purchasing a ‘resin’ kit.
Though large hollow items can be created using this technique, un-reinforced resin items of low cross section are prone to warping. Poor packaging design, gravity, and adjacent structure attachment stress contributes to warping.
METAL CASTING PROCESS
White metal (Tin/Antimony) is the alloy of choice here. The metal casting technique shares with resin casting the ability to render parts of complicated shape and deep relief. Metal casting also has the advantage of a very quick production rate with almost zero tool depreciation. I have several sets of metal forming tools – made of the same RTV Silicon rubber I employ for my resin casting – that have over two hundred production shots to their credit and they’re still going strong!
White metals problem is its high weight and cost. This makes the process suitable for production of only small detail parts or very small-scale vehicles and figures. The technique is favored by those producing for the gaming crowd. And, I must point out, that there are the attendant shop risks of working with molten metal poured into quickly rotating ‘spin-casting’ tools – use your imagination here, folks. This model part fabrication process should not be attempted by the faint of heart. Ellie and I have the burn scars to prove it!
On to part 2 and Acid Etching