3D printing uv sensitive resin

Photopolymer is a complex liquid mixture based on acrylates or methacrylates that cures under the action of light. MSLA uses two types of photopolymers - "UV" under 405 nm illumination and "daylight". The word "UV" (UV) in this case is a marketing ploy, 405 nm is not ultraviolet, but the border of visible light. Hereinafter, the conversation will be exclusively about "UV" photopolymers and 405nm printing. nine0003

The photopolymer consists at least of a mixture of oligomers, active diluents and a photoinitiator (mixture of photoinitiators). The base is acrylates or methacrylates, often modified with epoxides or urethanes. Photopolymers of the "first wave" have a peak of reactivity in the UV spectrum, at 405 nm there is a "tail" of reactivity, many modern photopolymers have a reactivity peak shifted to 405 nm.

Illumination consists of 405nm LEDs. The choice of wave is associated with a compromise between the reactivity of photopolymers and the bandwidth of LCD matrices - matrices poorly transmit radiation shorter than 400 nm. nine0003

Matrix for printers

Illumination is carried out through the LCD matrix. In the first generations of printers, standard RGB matrices from smartphones / tablets with remote illumination were used, rarely projection matrices, now specialized mono matrices.

The main characteristics of matrices are diagonal size, resolution and pixel size. Now matrices are mainly used 2k (2560x1440 or 1600, 1620 pixels) and 4k (3840 x 2160 or 2400), 4k + (4920 x 2880) sizes 5.5, 6, 8.9, 10.1, 12, 13, 15.6 inches with a pixel size from 31 to 90 micrometers.

The matrix can be covered with protective glass from above.

Protective glass protects the image sensor from damage, but reduces print accuracy.

Illumination

Illuminated with 405nm LEDs underneath. Illumination can be carried out by different light schemes, the light must fall perpendicular to the surface of the matrix and evenly. If these two conditions are not met, the print quality deteriorates and various distortions occur. To date, the most advanced schemes are "paraled" and "creality halot". nine0003

Illumination accuracy

The matrix can provide illumination accuracy of 1 pixel only if the following conditions are met:

- light falls perpendicular to the matrix.

- there is a minimum distance between the matrix and the photopolymer, ideally there should be neither a protective glass nor an air gap.

In fact, illumination accuracy is always worse, and often very much worse.

Print Accuracy

In addition to illumination accuracy, print accuracy is affected by the characteristics of the photopolymer, in particular its accuracy and color. White and light photopolymers in most cases do not give an accuracy of 1 pixel. nine0003

Due to layering and pixel illumination, a characteristic pattern appears on the surface of the printed part - pixel moiré. Pixel moiré can be reduced by anti-aliasing (sub-pixel smoothing) and/or by using light inaccurate photopolymers and imperfect lighting schemes.

It should be remembered that the complete absence of layering and pixel moiré indicates low print accuracy, and if someone writes that he printed with amazingly accurate photopolymer, and as a result got smooth, accurate models without layering and pixel moiré, he is either an idiot or marketer. nine0003

Photopolymers shrink, which also worsens the accuracy of geometric dimensions.

Spurious flare

Increasing the XY dimension from the "ideal" case is called "spurious flare". It can be associated both with the imperfection of the light scheme or thick protective glass, and with the properties of the photopolymer. As a rule, with increasing exposure time, spurious illumination increases. Light resins always have increased XY flare due to light scattering. nine0003

Penetration of light to a depth greater than the thickness of the print layer is called parasitic illumination along the Z axis. Parasitic illumination along the Z axis is related to the optical properties of the photopolymer and the lower it is, the stronger it is colored and the more precisely the exposure time of the layer is set.

Bathtub

A bathtub with a transparent flexible bottom is placed on top of the LCD matrix. The bottom is made of a material that has low adhesion to the photopolymer - FEP film, fluoroplastic, etc. A photopolymer is poured into the bath. nine0003

Platform (table)

The printed part is formed on a platform (table) which is lowered into the bath.

Before printing, the stage must be calibrated - installed plane-parallel to the LCD-matrix with a minimum gap.

The table is made of a material with high adhesion to the photopolymer - steel, aluminum, glass can be used. Sometimes a coating is applied to the table that does not have good adhesion to photopolymers - it is better to clean it off with fine sandpaper. nine0003

The table must have a flat bottom plane, all bulges and deflections impair the adhesion of the first layer.

Moreover, the LCD panel must also be flat, without downward deflection.

Print layer formation.

The formation of the layer is as follows - the table is raised to the Z height, then lowered to the Z-thickness of the layer.

When the stage is lifted, forces associated with the separation of the part from the film are generated - the stronger, the larger the separation area and the higher the lifting speed. When lifting, the flexible bottom of the film bath bends and stretches upwards, and the lifting height Z must be greater than the stretching of the film upwards - otherwise the separation will not be complete and the part may break in height. When detached, you can hear a characteristic click or pop. nine0003

When lowering, there are forces associated with the need to extrude the photopolymer from under the model - the stronger, the greater the thickness of the model, the greater the viscosity of the polymer, the higher the lowering speed and the smaller the layer thickness.

Exposure time depending on the layer thickness

The exposure time of the photopolymer depends on its reactivity, illumination power, transparency of the LCD panel and layer thickness. Mono-panels are more transparent than RGB, the greater the layer thickness, the longer the layer exposure time (the dependence is non-linear, the difference in thin layers is almost imperceptible, and layers thicker than 150 ... 250 microns often simply cannot be illuminated on colored photopolymers). nine0003

Features of the first layer(s)

The first layer requires a longer exposure time than the others. Usually increase the illumination of not one, but the first 5-6 layers. The more carefully the platform is calibrated, the less the first layers are needed and the shorter the exposure time of the first layer.

If the exposure time of the first layer is insufficient or the number of first layers is insufficient, the part may not stick and print. If the time of the first layers is too long, the layers turn out to be too “smooth” and the main layers do not stick to them - the part delaminates during printing, in addition, when printing with highly colored photopolymers, they cannot be illuminated to a great depth - because of this, careless calibration or curvature of the table is often impossible fix the exposure time of the first layers. nine0003

Due to the longer exposure of the first layer, as well as due to the reflection from the metal table, the first layers are always wider than the main ones - something like an "elephant's foot". This is an unremovable feature of photopolymer printing and for printing "from the platform" the model should be made with a chamfer or with a ledge.

Duration of photopolymer storage

Photopolymer can be stored for a very long time (several years) in hermetically sealed opaque containers at room temperature; properties up to complete unsuitability for printing. Therefore, buying an expired photopolymer is playing the lottery, and you should not pay attention to comments like “I have had a photopolymer for 3 years and it prints like new” - it all depends on storage conditions. nine0003

Necessity of photopolymer mixing

During long-term storage, the photopolymer can delaminate; before use, the photopolymer that has been lying idle for a long time should be thoroughly mixed for 10 ... 20 minutes, and recently used - for at least a minute. If you store the photopolymer directly in the bath - before printing, the photopolymer must be placed with a finger (gloved) or a soft spatula for at least 1 minute.

Effect of ambient temperature on printing

Photopolymer reactivity and viscosity are highly dependent on temperature. When printing at an ambient temperature below 20 ° C, the photopolymer may simply not stick to the table if your printer is in a cold room or on a balcony - you need to warm up the photopolymer before printing (it is not necessary to insulate the printer itself - it will heat up the photopolymer in the bath during printing by itself).

High air temperatures during printing can also lead to unpredictable results as the photopolymer gets very hot during printing and needs to be cooled down. nine0003

During printing, the photopolymer heats up both due to the heat from the LCD panel, which is heated by the absorbed backlight radiation, and due to internal heat release - the polymerization process is exothermic.

Photopolymer level in the bath when printing

How much photopolymer to pour into the bath - there is no consensus, a layer of 1 ... be higher than the lifting height of the part so that the part does not come out of the photopolymer - this allows you to avoid air bubbles and water hammer in the layers when the table is lowered. nine0003

Slicers and model formats

3D model and slicer required for printing. Most slicers work with STL and OBJ model formats.

Slicer - a program for "slicing" models, which produces a print file as output. Different printers require different print files, they have only one thing in common - this is not g-code, but a compact file with slicing frames. The main extensions are zip, cws, cbddlp, cbt, pzh, wow, pws, photon, potons.

Some printers are able to "slice" stl|obj files directly "onboard" the motherboard, but it doesn't make much sense - slicing "onboard" takes much more time than on a normal computer. nine0003

The most versatile slicers are cheatbox (free) and lychee (paid). The remaining slicers are designed for a limited range of "their" printers. Which slicer to use - cheatbox or "your own" depends on many factors.

Speeds, lift height, exposure time, number of first layers, anti-aliasing in most printers are set by the slicer. Jerks and accelerations are either hard-coded in the firmware or configured by M-codes by sending a text file with the gcode extension for printing. nine0003

The cut print file can be viewed and edited in various programs, the coolest of which is Uvtools.

Printing Hollow Models

To save on photopolymer, it makes sense to print some models hollow. A hollow model can be made in special programs (for example, in a mesh mixer), but most modern slicers (including chitoobox) support the ability to convert a solid model into a hollow one. The wall thickness is usually sufficient 0.8 ... 2 mm. Be sure to make two holes for washing the model inside and draining the photopolymer. nine0003

Some slicers support printing a model with internal filling, but this has no practical value - just a tribute to FDM traditions.

Model and support placement

When slicing, it is important to correctly position the model and place the supports correctly.

There are a lot of ways and ideas on how to arrange models, the main thing to remember is that your own experience and common sense are more important here than other people's opinions.

When placing supports, it is important to remember:

The first - in the photopolymer printing of supports - not supports, but thrusts that work in tension and experience heavy loads. Therefore, there are never too many supports…

Secondly, the surface from the side of supports is always of poor quality, so important surfaces should be “on top”, and from below, more supports should be placed around the perimeter and on the lower edge.

And the third thing is to avoid suckers.

Everything else comes with experience.

The cheatbox slicer allows you to make support contact with the model in the form of a ball ("sphere") - a very useful feature that facilitates the process of removing supports. nine0003

Principles for choosing the layer exposure time

When printing, it is important to choose the right exposure time. Most of the tables on the web and on the websites of manufacturers - give only an approximate time, because. different LEDs (or even with different operating time) even within the same series of printers can differ in luminosity, the same applies to the transparency of the matrices.

The shorter the exposure time, the less durable the exposed part is, including supports, which can lead to its tearing during printing. nine0003

The longer the exposure time, the stronger the parasitic illumination, the stronger the gaps swim and the thin ribs increase in thickness, but the detail and supports are stronger.

If the exposure time is increased beyond a certain limit, the layers will turn out too smooth, adhesion between layers will deteriorate and the part may break when printed.

Dry air and static electricity

Dry air is very important when printing. Dry air builds up static electricity that can cause print failures, flash drive read failures, and even damage to the flash drive. During the heating season, it makes sense to put a wet rag next to the printer or humidify the air with a humidifier. nine0003

About rinsing

After printing is complete, allow the resin to drip off the platform and part, separate the part from the platform with a sharp knife or spatula, and wash the part.

Parts are washed in isopropyl alcohol (water washable photopolymers in water). It is possible to use other mixtures - vodka, ethyl alcohol, a mixture of water with acetone. Almost any photopolymer can be washed with water with surfactants - both with special products, such as Hardlight Cleaner, and with various effective household compounds - Fairy, Doctor Peper, etc. nine0003

It is advisable to use only pure alcohol and several changes of alcohol. After washing, alcohol can be partially restored by exposing it to the sun and waiting for the illuminated photopolymer to fall out.

Rinsing can be done manually, in special baths with agitators or in ultrasonic baths. The accuracy and sharpness of printing depend on the degree of washing intensity - an ultrasonic bath washes everything and everywhere, and it is very difficult to manually wash the photopolymer from long holes or narrow slits. nine0003

Some photopolymers (usually high strength, tof or flex) retain a residual tack after washing. It can be removed after exposure by repeated washing in acetone or 646 thinner for a few seconds. It is also possible to remove by filling (coating) with coconut oil for 2-3 days, followed by washing with water with an energetic surfactant.

About re-illumination

After cleaning, the part must be re-illuminated.

The duration of additional illumination depends on the goals and objectives - 5 ... 10 minutes of 365-405 nm lamps are enough to fix the surface, at least 2 hours of the spectrum of 400 ... 450 nm are required for polymerization to the maximum depth. nine0003

For additional illumination, you can use ordinary nail service lamps or various devices, both home-made and purchased.

The best way to illuminate is sunlight, but it is not available to everyone and is not always available.

Illumination is preferably carried out at a temperature of 50°C to improve the mechanical properties. If backlighting is performed at a lower temperature, then the printed parts after backlighting can be heated to a temperature of 50 ° C with an exposure of at least 30 minutes to improve mechanical properties. nine0003

A little about removing supports

Supports need to be removed. When to remove support - there is no consensus. They can be removed before washing, so that they do not interfere with washing (dirty!!!), after washing before backlighting, after backlighting. Supports are removed with sharp side cutters, nail files, a sharp knife or scalpel. With some training, supports can be removed by breaking off, especially if the place of contact between the support and the model is a ball (such supports are formed only by a chitoobox, if the place of contact is not a ball, then when breaking off, it is quite possible to get a hole on the model). nine0003

Photopolymer shrinkage

After photopolymerization, the photopolymer shrinks in volume.

Photopolymer shrinkage occurs in three stages:

layer-by-layer formation.

2) with additional illumination - unevenly - maximum from the side with more intense illumination.

3) after pre-exposure - within about a week - evenly along all axes

Shrinkage during pre-lighting can cause distortion of parts, so it is important to pre-light evenly from all sides, or fix the part so that it does not “behave”. nine0003

After three stages of shrinkage, shrinkage in XY is greater than in Z, shrinkage unevenness can lead to distortion of the geometric shape of the models.

Photopolymer types.

There is no generally accepted typology of photopolymers, and what may be called “nylon” for some “hardflex”, nevertheless, the following groups of photopolymers can be distinguished … can be of any quality and mechanical properties

- eco-friendly - made with a claim to the environment, but just as harmful as the usual ones.

- water washable - water washable photopolymer, does not require alcohol

- ABS, xABS, zABS, ABS-alike, industrial ABS with ABS does not have