Stereolithography (SLA) 3D printing or resin 3D printing technology is utilised by scale modellers to produce finely detailed scale parts with high accuracy and excellent precision compared to other additive manufacturing techniques. Whilst this technology brings great benefits and rewards it is important to remain mindful of good working practice when handling and working with chemicals and polymers.

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SLA 3D printing uses an ultraviolet (UV) light source to cure a light reactive thermoset material, resin, into solidified rigid geometries. During the SLA printing process liquified resin is added in thin layers and cured before the next layer is applied. Due to the nature of the process it is not uncommon for resin prints to print with some defects such as miss-aligned layer lines or gaps. Every effort is made to ensure parts print well and all parts are inspected. Any minor issues deemed ‘reasonable’ are passed so on some occasions it may be necessary to sand or fill these defects. Any such rework should be minimal and considered within reason when finishing parts to a good surface finish. We could scrap all parts that are not considered perfect but with such an increase in scrap cost parts would become uneconomical.

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SLA 3D printing utilises support structures to hold parts in place during the build process. Parts are printed in such an orientation as to minimise any support material on the cosmetic face of the part. Support material is removed during post processing but the evidence of touch points (small dimples) are not removed. Often these touch points are on the inside or back face of the part so do not need to be removed. Touch points can be easily removed by gentle sanding, usually at the same time as the part is being prepared for painting.

 

We recommend that SLA 3D prints are primed, painted with acrylic paint, due to the level of UV resistance that they provide and lacquered. We have had excellent results using standard ‘rattle can’ primers and acrylic based modelling paints.

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Parts are supplied washed and post cured. There should be no uncured resin remaining on the parts and parts should be safe to handle. If you are concerned about coming into contact with uncured resin then suitable precautions should be taken such as wearing protective gloves when touching new unpainted resin parts. When working with any polymers, not just 3D printed parts suitable precautions should be taken to avoid breathing in fine dust when sanding and/or machining.​​

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Will my resin prints melt in the sun?​

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The answer to this is, it depends!

It is true that resins (used in SLA printing) and filaments (used in FDM printing) have a much lower glass transition temperature compared to their injection moulded counterparts. ABS-like resin used often in engineering parts can have a glass transition temperature as low as 40degreesC so any temperature above this will start to soften the parts. This is both a blessing and a curse! as we often warm parts up using a hair dryer or submerging the part in to warm water to correct the shape of any deformed parts.

 

SLA resins which have a higher glass transition temperature once cured tend to also be more brittle. These resins are better for producing highly detailed parts compared to softer materials which do not reproduce detail well so a balancing act needs to be found to find the most suitable and practical resin for the application. SLA printing at a hobby level is very much an emerging technology with new resins come to the market frequently. Also blending resins is very common and makers find their own ‘secret’ formula producing a resin that faithfully reproduces detail and is resilient enough for the intended application.

 

So what does this mean in practice?

Ambient temperatures, especially here in the UK, are not a problem as low 30`s degreesC is considered a heat wave! However, the trouble can come from leaving models in car boots or trailers where temperatures can easily reach our glass transition temperature. But this does not mean the part will collapse in to a pool of molten plastic, our parts are designed to have sufficient structure to be self-supporting however if a load/heavy object was leaning against the resin print part at these elevated temperatures then some deformation could occur.

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What’s our experience?

Our Class 37 has been out and about for 2 years and has been in operation during our ‘scorching’ summer temperatures which at one point hit ~30degreesC. With the model sitting out in the sun in steaming bays we did not experience any issues or deformation and once on the rails with a draft the surface temperatures dropped to levels far below anything that would cause an issue to the resin prints.

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We have tried to be a little bit scientific and measured the temperature at different points on the model, unfortunately for us locomotive roofs tend to be dark grey or even black. Dark colours are better at absorbing heat radiation compared to shiny colours and therefore the part heats up more quickly. We put the model out in direct sunlight during one of our 30 degreesC heatwaves and measured surface temperatures as high as 66 degreesC on the cab roof (executive dark grey colour) and much lower temperatures on the yellow parts. Although these hotter parts were softer to the touch we did not experience any permanent deformation and no corrective work was required. The solution to this potential problem is to move the model in to shade on these hot (for the UK) days or cover the model up. Inspired by the protective covers used on composite aircraft we have designed a silver cover for our Class 37 and have it at the ready but so far not needed it – it may be used more as a rain cover!

 

In summary

We have to be mindful of the material properties and adjust our care and maintenance accordingly. The quality of appearance of the model which can be achieved is well beyond anything previously offered using more familiar methods such as fibre glass mouldings and this benefit far out ways any limitations the material properties may bring – in our opinion!