Effect of plastic elements thickness on colour transmission intensity with light behind

. This article looks into the possibility of transmitting different colours in 3D-printed lamps using different thicknesses of plastic. During this work, an experimental stand was created to review the quality, structure, colour and translucency of printed elements. The analysis of decorative properties of plastic depending on the environment and colour was carried out. The colour was determined and such characteristics of the coating as colour, saturation, purity were calculated.


Introduction
3D printing is one of the most promising technologies in the production of various products. With the development of this technology, there are more and more applications for 3D printing in design [1,9].
The idea for 3D printing originated in 1986 with Charles Hull, who co-founded the 3D Systems Corporation. He invented a technology called stereolithography to create threedimensional objects by modelling them on a computer.
Since then, 3D printing technology has continued to evolve. In 1999, Z Corporation developed the first 3D printer with the ability to create full-colour objects [2].
Today, 3D printing is used in various fields such as medicine, the aviation industry, architecture, and design [10][11][12].

Methods and materials
When creating luminaires of different shapes by 3D printing, it was noticed that when different thicknesses of plastic are shined through, different saturation and brightness of colour can be obtained, which characterizes the intensity of colour [13]. The first step was to research the history of development of 3D printing, then made a stand [6][7][8] to conduct experiments with different thicknesses and different colours of the plastic, based on the data obtained, the information was structured and made certain conclusions about the dependence of colour on the thickness of the plastic.
PLA plastic and Wanhao Duplicator i3 3D printer [4] were used in this work. PLA (polylactic acid) is a biodegradable thermoplastic polymer derived from natural resources such as corn, potatoes, sugarcane, etc. PLA plastic can be used in 3D printing and is widely used as a replacement for traditional plastics such as ABS (acrylonitrile butadiene styrene) because of its environmental and health benefits [3,15].
PLA plastic is lighter and harder than ABS, making it an excellent choice for model building and prototyping [14]. It is also more UV-resistant, making it the most rational material to use in creating objects that will be outdoors.
PLA plastic also has lower toxicity [3] than other materials used in 3D printing such as ABS and PETG. This makes it safe for indoor use, especially when creating toys and other items that will be used by children.
Despite all its advantages, PLA plastic has its limitations [3]. It has a lower melting point than ABS, which limits its use in some projects. In addition, PLA plastic is less strong than ABS, making it less suitable for creating objects that will be subjected to mechanical stresses.

Creation of an experiment stand to conduct experiments with elements of different colours and thicknesses
To test the hypothesis about the influence of the thickness of plastic inserts on their colour intensity during transmission, a stand ( Fig. 1) with a removable top panel was designed to allow a 9W Uniel LED bulb (G9 socket) to be installed inside. The LED bulb was selected to have a minimum size and high brightness. The panel was provided with 6 triangular holes, each of which can be fitted with a prefabricated element.
To achieve different colours in the intended luminaire and to find the most rational thickness of the insert, several yellow elements with a thickness of 0.6, 1.0, 2.0, 3.0, 4.0 and 5.0 mm were printed.
These elements are smooth to the touch at the bottom (the side that was in contact with the base of the printer) and slightly rough at the top, with sharp edges. The colour is pronounced on each element, but changes when viewed through the light.
Creation of an experiment stand to conduct experiments with elements of different colours and thicknesses.  Next, the colouring was done in Adobe Photoshop ( fig. 3), using the photos obtained and the "dropper" tool.  Figure 3 shows the colour transition from a pale yellow to a rich ochre colour. It is obvious that by varying the thickness of the printed parts, different colours can be obtained, and this should be used in products.

Creating a colour palette
After getting the results, it was decided to print a panel with 24 holes to explore more colours. The colours green, blue, purple, and pink were explored (Fig. 4). Their thickness was also 0.6, 1.0, 2.0, 3.0, 4.0, 5.0 mm. The differences in colour are most noticeable in daylight, while in darkness (Fig. 5) the difference between 4 and 5 mm is practically invisible.   Figure 6 shows the resulting colour palette. Not all colours are darkened evenly, most probably depending on the density of the pigment in the plastic. This confirms that at the smallest thickness the violet colour gives the greatest saturation, and at the largest thickness it becomes almost black.
A colour palette with a dark (Fig. 7) and a light (Fig. 8) background without an internal light source was also created from the photographs to assess how the product would look without backlighting.

Results
Based on the findings, the unlined colours with dark backgrounds have inverse tonal gradation, i.e., the smallest thickness becomes the darkest, as the darker background shines through from underneath it. With a lighter background, the palette is similar to the original highlighted colours.
Also without backlighting, all colours become duller and take on a pastel hue [5].
With the first palette, the saturation and brightness of each colour was calculated. The data was recorded in Table 1.

Discussion
Thus, the use of such tables will help you decide which colour to use to achieve the desired result. For example, if you want to draw attention to a certain colour, you should choose the thickness that allows you to achieve the greatest saturation. It is also possible to specify different combinations of brightness and saturation and select the most appropriate colour using the RGB model.

Conclusion
In this paper an experimental bench was developed to systematise the colour data of PLA plastic for 3D printing. Based on this research, palettes were compiled, considering different environmental conditions, as well as a comparison table with the calculation of saturation and brightness.
With such palettes and colour tables, it is possible at the stage of product design to predict the features of its aesthetic characteristics in different lighting. The results of these studies make it clear that a wide range of shades can be achieved with different thicknesses and basic colours are available.