As noted before, polyester loses its elasticity over time and the slightest shock, bang or hard vibration during model transporting may cause a crack. To reduce the danger of such an unfortunate situation and as a part of precautionary measures to strengthen the structure of the polyester model, I decided it would be nice to fill the internal model with the material that should be:
- Solid and able to occupy all the available volume inside the model, supporting internally the walls of the polyester model,
- As light as possible to avoid model’s total weight increasing that could strain the wheel polyester arms,
- Hard, but not completely rigid, that can absorb vibrations & shocks and able to transmit vibrations soflty, throughout model’s mass and not on a single point.
This material is called polyurethane foam. It is a unique industrial material which has excellent physical, mechanical and environmental properties and is mainly used as insulation material. It offers high abrasion resistance, tensile stress, impact resistance etc. The fact that the polyurethane foam material can be easily cut, sawed, sanded, drilled etc and is compatible with most types of scale modelling glue & any type of primer or enamel / acrylic paint with no problem at all, makes it a good choice for many scale modelling applications, as for example in diorama building.
I got one polyurethane foam spray canister with the trigger & tube to inject the material, from my local hardware store. Because cleaning the plastic injecting tube would be quite difficult when the polyurethane foam begins to be polymerized after contact with air, I cut the plastic pipe and replaced it with a straw which I threw out after each use.
Before channelling the polyurethane foam inside scale model, a preparation should be applied to achieve best possible results and to avoid any unpleasant surprises. I chose as the point from which will channel the polyurethane foam, the only opened hatch - the crew commander’s hatch. This is why I closed driver’s hatch, not to let polyurethane foam escape from there. Masking tape 50mm width, applied to cover the upper surface of the FDCV and protect it from possible polyurethane foam overflow.
Injecting polyurethane foam needs full attention and should not exaggerate the filling quantity because it takes time that can reach up to 2 hours from the application of the material to swell and solidify. According to the instructions on the spray canister, the best results are achieved when sprayed successive small amounts of foam and wait to swell and partially occupy all the available volume inside the model. Considering that the expanding polyurethane can multiply the original foam volume up to 4 times, it would be very appropriate to inject small quantities, allow time to swell and since we are confident that it has solidified and will not change over its volume, to continue the process with successive small amounts of foam. To conclude the process, I began to inject small amounts of polyurethane foam, first in the more distant spots, such as the front left or front right under the engine’s compartment etc. After the 2-3 small doses of injectable foam placed in appropriate locations, I wait enough time (over 6 hours) to swell smooth polyurethane and cover the internal volume without endangering the vessel to deform or break (worst scenario) from inside out, under high pressure on walls. Then, I added 1-2 more doses of polyurethane foam to the rear part, being careful to inject small amounts and let the material to expand slowly or escape if necessary, from the crew commander’s hatch.
As you can see into following picture, it seems I was lucky, because while material expanding, the polyurethane foam managed to find a way out and despite a little leaking from the driver's hatch. Although, the contact with air was quick and material became solid before escaping to the outside, covering the panel lines and kit details.
After 3-4 small doses of polyurethane foam, with several hours stall time between injections, the material properly inflated and occupied all the available volume of the scale model’s interior. Because of wrong calculations during last injection, polyurethane foam escaped from the nearest (and only) way out - the crew commander’s hatch. Fortunately, this was the last dose and the excess was removed quickly with a spatula, while having established such a case, the protective coating of the outer surface with masking tape, proved thoughtful choice.
After the last dose of polyurethane foam hardened, the excess material overflow from the driver's and crew commander’s hatches were removed carefully, using No 11 blade on modeling knife.
With the method described in the previous paragraphs, I achieved to:
- Fill the interior of the FDCV scale model with foam that can absorb vibrations and distribute throughout the mass of polyester,
- Apply a hard duty adhesive tape across the inner surface, to hold united the shattered parts of polyester if ever broken.
The above protection measures, are not necessary (and possibly not recommended) for use on 99% of commercial kits marketed. The polyester material specificity however and the possible danger to break after few months or years, after losing its elasticity and becoming harder and inflexible (and therefore fragile), enforce these protecting measures to ensure better luck VS possible shock hits & hard vibrations during scale model transport.