Vacuform Aircraft Modeling – Part 3

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:

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

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’!

<Go to Part 2
Go to Part 4>
This entry was posted in Modeling and tagged , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *