Hampton Roads Scale Modelers

Basic Techniques (Con’t)

My basic method of removing parts from the ‘backing sheet’ is outlined below.  However, there are actually some things you want to do prior to removing the parts, if you follow best practices.  These would include:

• Washing of the parts (just as with an injection molded kit) with a degreasing detergent (dishwashing soap) to remove any surface contaminants.
• Taking off needed bulkhead cross-sections pre-removal to capture the “true” part sections while the pieces are still constrained by the backing sheet (see below discussion.)
• Surveying the parts for defects such as sunken surfaces, poorly molded edges and asymmetry (again, while the parts are in their ‘true’ shape.)  I usually will mark any problems areas directly on the part with a pencil.
• Check clear parts – This involves checking the general fit agreement between the clear pieces and their mating parts.  This is especially important with some ‘classic format’ vac kits that may have been mastered with less than perfect precision.  Obviously, the time to find out problems with clear parts is NOT after the fuselage has been assembled, detailed, painted, etc.  Often, such problems can be best identified prior to removing the ‘opaque’ parts from the backing sheet.

In short, do the usual preliminary work applicable to any model project.  But, back to:

Removing the Parts:  Parts removal begins by outlining the part on the sheet with a fine tipped, (my preference) Sharpie brand permanent marker.  Hold the marker at 45 degrees so the ink gets on both the part and the backing sheet.  This ink mark will serve to gauge the ‘as molded’ edge to the part.  Avoid blunting the marker’s point, so the ink gets fully into the corner between the part & the sheet.  It is important that the mark doesn’t have any gaps.

(One alternative I’ve seen others use to establish the parts ‘trim line’ is to give the entire sheet a coat of primer.  This has some advantages for the parts, but as I like to use the waste backing sheet as raw scratch-building material, the primer would interfere with gluing, so I don’t use this method.)

Next, using an appropriate tool (#11 Xacto blade, engraving tool, etc.), score around the edge of the part where it meets the backing sheet.  Score at a 45 degree angle, taking care to score lightly using multiple, short passes so to follow the part edge precisely.  Do not attempt to cut through the backing sheet – that is not the goal.  However, tight corners or small radius curves may need deeper scoring to prevent “tearing”.  Do not leave a border of excess backing sheet – that just makes for more work.  For areas that will eventually be removed such as cut-outs for wings or cockpit openings, leave the molded-in excess to help maintain the part’s rigidity during removal & cleanup.

Although practice will dictate just how much scoring is required, once done, the part is flexed “against the score” (pushed from the side opposite the score which opens the scored groove wider & stresses the plastic so it fails, or ‘snaps’, in line with the groove.)  Depending on the shape, the score may need to be worked back and forth; it shouldn’t take much effort, so if it does, additional scoring is needed. Take special care when dealing with ‘inside corners’ to prevent tearing the break across the part itself.  I usually attempt to cut completely through tight spots just to keep the break along the desired ‘path’.

Once the parts are free of the backing sheet, I usually do a rough ‘dry fit’ of adjoining pieces to get a feel for how things go together.  (It is sometimes true of ‘male molded’ components that the backing sheet thickness is required for a proper fit, unless the masters were properly made with an allowance for the sheet thickness.)  This is where having the clear parts to validate the fit of the matching parts becomes important.

Typically, however, the tiny bit of remaining backing sheet material must be removed for proper fit.  The plastic to be removed is visible because the ink line marks the edge of the cut, giving a trim line.  I start with a #10 Xacto blade (is is more rigid and has a better blade angle for this purpose), or sometimes a single sided razor blade, to shave the excess. Using the blade like a tiny plane, I carve off most of the ‘wedge’, taking care to not allow the blade to wander past the ink mark and into the part itself (or my fingers!)  My goal is quickly remove most of the excess and roughly set a perpendicular edge to the part.  Just take care when trimming to keep your fingers clear & the blade ‘constrained’ (i.e., place your fingers so the blade is braced & cannot move or ‘jump’ very far) to avoid cutting yourself.  Granted, this takes a bit of practice!

Once the ‘shaving’ is done, I finish up with a sanding stick or block.  My sanding tool has to have a flat, rigid backing to achieve the desired flat edge – so no foam sanding sticks or pads (they can deform.)  The sanding direction depends on the part and how much needs to be removed.  Sanding across the edge will remove material faster but with less precision than sanding along the edge’s axis.  Since the part is thin and can flex easily from side to side, cross-cut sanding is used initially with a finish sanding using long strokes along the edge to smooth any unevenness.  (Sighting along the edge helps to keep things straight.)  As always, avoid going past the ink mark.  Be aware that the dry ink may actually ‘flake off’ as sanding progresses, so take care to stop once the marked edge is just about reached.

I will dry-fit the parts during sanding so I can take advantage of any opportunities to improve the fit of the parts.  If there is any undesired asymmetry, ‘over sanding’ the offending part will help the problem but this is usually accomplished once the basic cleanup is done.

With very little practice, removing vac parts is quick and satisfying.

Basic Techniques

Don’s Model Works, home of the late Don Bennett’s vacuform kit outlet (now taken over by his son Chuck) has an excellent writeup of the method similar to the one I use in removing and cleaning up vac kit parts.  I’ve tried several methods over the years and this is the quickest and most foolproof technique I’ve found:

http://www.donsmodelworks.com/ref/Hints_Vacuform_Models.pdf

Also, Brett Green over at Hyperscale compiled a similar vacuform ‘primer’ series you should check out:

http://www.clubhyper.com/reference/vacformbg_1.htm

The primary difference is that I generally have no need to use the old ‘sandpaper on a board’ step – a hand-held block wrapped with sandpaper is usually sufficient.  The key is to minimize the amount of excess plastic that must be removed once the pieces are cut from the ‘sheet’ of parts.  In Don’s writeup, he recommends finishing the file work with a bit of sanding on a sandpaper sheet to smooth the edges.  The trick to this method is when attempting to sand a part flat on sandpaper, it is very difficult to apply even pressure across the entire part – unless the part is ‘smallish’ to begin with – in which case it is then difficult to hold!  Being thin and flexible, vac parts are subject to ‘flexing’ when sanded on a board, giving uneven sanding pressure which then results in a poor fit.  This is the problem with the old ‘standard’ method of scoring around the parts, leaving a ‘border’ or ‘lip’ of plastic around the part then sanding on a sheet of sandpaper until the excess is razor thin and ready to ‘flake off’.   Invariably, the excess would have different thicknesses as a result of the inevitable uneven sanding pressure.   It is just nigh impossible to hold a part evenly, especially when working with larger parts.  Why work so hard!?

One challenge to a vac kit is the fit of the parts (if properly engineered to start) is highly dependent on the modeler’s skill at prepping the parts.  The work is not difficult, as little effort is needed to sand the typically soft styrene used.  In fact, this actually makes it pretty easy to over-sand the pieces.  On less well made kits, constant test-fitting is required to check the fit – which is sometimes better if the edges are not fully ‘dressed’.  Lots of test fitting should be done regardless of a kit’s quality.

Another major point concerns any clear parts provided.  In cases where the clear parts have a critical fit to the fuselage (such as with a B-25, for example, where the entire nose is essentially a clear part), preparation of the clear parts will also dictate fuselage fit issues simply because the clear parts typically cannot be modified to fit better whereas the styrene parts can be shimmed or trimmed as needed.  This is why the clear parts are the first thing to consider before cutting out and sanding the fuselage parts.  A ‘bubble canopy’ (for example) that fits atop the fuselage has less impact on the fuselage dimensions than a canopy that is supposed to have the same profile/ cross section as the fuselage.  In effect, because the clear parts cannot be altered (unless the modeler replaces the parts from scratch), they serve as the primary sizing constraint for any interfacing parts.

Once the parts are fully ‘dressed’ and all critical edges are sorted for the best possible fit, vacuform construction proceeds essentially the same as an injection molded kit, albeit one that will typically may need more scratch-building of details.  Extra support structure such as internal ‘bulkheads’ or ‘spars’ are often needed to properly support thin walled parts or to reinforce joints.   For example, fuselage shells benefit from having short, contoured ‘tabs’ glued to the inside of joints in an alternating fashion so to overlap to the other half, almost like the teeth of a zipper.  This promotes positive parts alignment and increases gluing surface area while providing internal support across the seams.  These tabs, made from the excess sheet stock, should be bent to a matching contour instead of being left ‘flat’ so to provide a better ‘gluing surface’ and prevent distortion across the seam.

Most of the material for the spars, bulkheads and tabs comes from the cut-off (excess) plastic sheet.  This extra plastic is perfect for fabricating needed structure & details including basic internal ‘boxes’ for wheel wells and cockpit bays (if not already provided as molded parts) as it is the same ‘stuff’ as the molded parts.  Again, precision in contouring the mating surfaces will pay off with trouble free and strongly glued joints.

Keep in mind that proper contouring of the mating surfaces are important to get a good bond.  Consider when plastic sheet is cut, the edges will have an angle due to the “V” shape of the blade.  This cut edge usually needs to be smoothed & ‘squared off’ with a sanding stick or block for best fit.  Also, the raised ridge along the cut should be sanded down to prevent it interfering with anything else later – unless it provides some detail!

Precise, exact fit between components is a hallmark of the modern injection molded kit.  For a vac kit, the precision achieved is up to the builder.  This is part of the allure of vac kit construction – the satisfaction that comes from personal craftsmanship.  Plus, there is the side benefit to learning the fine art of “fettling” (adjusting parts fit by hand) which is useful with injection kits as well.  Once a modeler starts looking at parts less as ‘Lego blocks’ and more as unfinished components that *may* need further attention, a whole range of kits become ‘do-able’.  Fully developed, this mentality logically leads into the realms of conversion, modification and even scratch-building of entire models not commercially available.  In short, the modeler has no limits to what he/she can tackle using skills developed mastering the ‘fearsome vacuform’!