What is Resin 3D printing

Resin 3D printing is a group of additive manufacturing technologies which cures liquid photopolymers to build an object layer-by- layer. Resin printing is not an official title for the whole group but since these machines work with resin materials, it’s an easier way to describe them all.

So, to start with, we must admit that nowadays there are various 3D printing technologies that work with liquid resins. What they all have in common is that:

  • they use a principle of photopolymerization to create objects
  • they operate according to a code (or a machine creates a code straight from an uploaded digital model)
  • as a material, they use resins (photopolymers)
  • the material is stored in a vat (bath/resin tank)

Photopolymerization refers to the chemical reaction of a special liquid and changes in properties when subjected to light (UV, laser etc.). The scientific terms for such liquids are photopolymers or light-activated resins, and in 3D printing, they are just referred to as resins.

There are different types and effects of photopolymerization reactions, but in 3D printing, it’s the solidifying effect of a liquid resin under a light source that is required. Light sources can be different – lasers, projectors, LCD screens, all which can influence the technology type.

Typically, resin 3D printers consist of a light source, build platform (or build plate) where the object is formed, and a resin tank. Other tools and features vary depending on the technology.

Resin 3D printers can form an object normally or upside down, depending on how the components are positioned and whether the build platform drops down or lifts up. Upside-down orientation is more common for desktop machines because it’s easier to operate and fabricate without losing out on quality. The critical factor with upside-down resin printers is the print adhesion and supporting structures, preventing a part from falling down due to gravitational pull.

How does Resin 3D printing work?

  • To start printing, a resin tank should be filled with material either manually or automatically. If a specialist fills the resin manually, they should be careful to avoid skin contact because some resins may be toxic.
  • Then the machine descends the build platform down (or lifts up) to create a thin layer of resin touching the build plate. Afterwards, a light source draws the pattern of a single layer in the resin, causing it to solidify.
  • The platform moves to allow uncured (non-solid) resin to come through and form a new thin layer above the plate. If a unit is equipped with a roller, it would go across the building area to make sure the resin lies perfectly. (CLIP printers skip this step)

These steps are repeated until the entire object is formed. Once the process is complete, the object is removed from the machine for post-processing.

What types of post-processing do resin prints require?

Compared to other 3D printing methods, resin printing is a technology that requires at least the minimum post-processing. The primary steps for an object to be ready are:

Washing – it’s needed to remove uncured resin from a print. Since material is a liquid, it flows over the build plate and the object, so prints are put into alcohol, water, or at least left to dry on some paper.

UV-curing – this step is required when objects are exposed to sunlight or a UV-lamp to strengthen solidify better. While printing, each layer is exposed to light for just several seconds, so post curing is always recommended.

Supports removal – this involves removing all supporting structures from an object. Resin printing requires supports to hold an object, make it stick to a build plate, and prevent it from falling into the resin tank while printing. Usually, there are many supports holding a part, and despite the fact that they are quite easy to remove, it can still be a time-consuming and careful process so as to avoid cutting too much from the object’s volume.

In addition to the aforementioned post-processing techniques, resin prints can go through other post-processing methods such as:

Sanding - to remove taps from areas where supports touch an object and smooth the surface or make it matte. It can be performed with dry sanding tools or wet sanding. The latter involves using special liquids, polishers or just adding water to common sanding papers. Wet sanding is capable of giving a smoother surface to a print.

Patching escape holes – to reduce the cost of photopolymers, models can be designed with vent/escape holes. These holes help the resin to run out when an enclosed hollow object is printing. However, what you receive is a part with several holes that are not aesthetically pleasing to say the least. So patching is a post-process step when these holes are manually closed with resin or other materials to improve the structure and appearance.

Finishing - with special compositions or applying coatings is done to strengthen a part and hide imperfections. It may be replaced by priming, which pretty much does the same thing, as well as priming the surface to receive a lick of paint. For some resins, this step is important to prevent material from curing and becoming brittle over time.

Painting - is available with a range of colors and compositions for special requirements.

Polishing - using various compounds is done to make the surface even more clear and smooth, especially to bring back the transparency after sanding.

All the processes mentioned above can be done manually or with special tools and devices. There are washing, curing, and sanding machines available ranging from desktop semi-automated units to turnkey industrial complexes.

What SLA materials are there?

As stated earlier, resin 3D printers cure liquid photopolymers (resins). These resins can be manufactured differently to achieve numerous properties of a final cured product for different manufacturing and prototyping needs. So, let’s take a look at the types of resins 3D printers can work with.

Standard resin – usually just “resin”. It’s an average photopolymer suitable for most applications. Objects printed with standard resin come out solid and tough with some rubbery texture. The most easy to work with is translucent resin – it has an orange color as orange is the most sensitive to UV light.

Resin part by Harp Concepts

Resin part by Harp Concepts

Glass-reinforced resin – polymers of this kind have some glass additives for improved strength. Prints come out more rigid and tough and resistant to deformation and wear.

Durable resin – a type of material which is ideal for parts exposed to mechanical stress and wear and also requires some flexibility. Compared to non-photopolymers, it’s quite similar to Polypropylene (material used in tough jerrycans).

Flexible resin – can be characterized as “rubber” due to great elasticity and flexibility. It’s widely used in parts requiring bending properties, available to deliver parts which can keep their form through deformations.

Tough resin – also known as “ABS-like” resin for its increased stiffness properties. It’s mostly used for tough parts and prototypes which will be subject to stress without deformity. Prints aren’t that elastic like the ones printed with ordinary resin, but they are able to keep their structure well.

Tough Resin

Tough Resin by Harp Concepts

Dental resin – some photopolymers are biocompatible and can be used in the medical industry for end-use products and devices like retainers. They are also resistant to wear and deliver precision while forming an object.

High-temperature resin – used for prototypes and parts that face high temperatures or even direct flames from a fire. Depending on the manufacturer, prints can withstand temperatures up to 536 degrees Fahrenheit (280 degrees Celsius).

Castable resin – ideal for mold-makers and jewelers. Aside from producing accurate structures, these range of resins can be used as master models for investment casting, delivering no ash and clear burnouts.

Shell casting resin – type of resin which can be used in shell casting to create soft parts. The print is a mold itself, which reduces time and costs in the process.

Ceramic resin – is a photopolymer with ceramic additives created for prints to mimic objects made from ceramics. If made with such resin, an object would feel, look and act like a true ceramic part, keeping all geometrical possibilities of resin printing.

Clear resin - maybe not a unique type of photopolymer but deserves a special place in the list because this one is really…clear. After sufficient polishing, clear resin prints can reach nearly optic transparency, which is harder to achieve with so-called “transparent” colors or other resins.

High-detail resin – these resins have different names depending on the manufacturer. However, the key factor is that improved formula and color helps to achieve better resolution thanks to photopolymer reacting better when exposed to a light source. Usually, these resins are dark – like deep black, but since more precision is possible with them, the trade-off is worth it.

What colors are possible? Some Resins are compatible with pigments, which makes it possible to achieve almost every shade needed. Manufacturers also offer custom resin colors or sell pigments and photopolymers, which can be mixed together into a resin color required.

What are the advantages and limitations of resin 3D printing?

Resin 3D printing methods vary, so each technology may have more or less advantages and limitations. It also depends on the power of a light source inside a machine, properties of photopolymers used, and other components of the 3D printer. Basically, resin 3D printing is great to use because:

  • High quality and details. Even if a resolution (minimum layer height) of a resin machine is the equivalent or worse than another printer (such as FDM), the result would still be more precise. Since photopolymers are loaded into a printer as liquid, they are easier to shape in complicated geometries than melted plastics.
  • Smoothness of a surface. With resin printing, there still would be some layer lines however; they are extremely small and nearly invisible on some colors of resin. Polymerization creates a solid structure resulting in a more attractive print.
  • Wide variety of materials. Although the range of resins isn’t limitless, it still manages to covers all common applications and needs, and the list is growing rapidly. Each photopolymer resin consists of working components required for reaction and additives like dyes, visual and functional supplements. Varying different combinations of those resins can be manufactured for a certain function of a final part.
  • Some geometries require less supports. Resin printing requires supporting structures all the time, which is devastating if a print is simple. However, some complicated geometries can be produced with less supports compared to binder jetting or FDM technologies. That’s because a resin tank is filled with resin all across the perimeter, so if there are some lightweight overhangs, they just can be attached to the main body of a part while curing.
  • High speed. Compared to other prototyping methods, resin printing is much quicker, and modern machines can be extremely fast. Without quality loss, resin printers can produce high-detailed parts within hours or a day depending on the size.
  • Lower costs for some applications. If printing a batch of end-use figurines with a resin printer, it may be an expensive way to do so. However, for some applications like prototypes, custom dental devices, and master models for molds, resin printing saves the budget.
  • Prints are waterproof. Compares to plastics or powder materials, photopolymers come out solid and waterproof, they don’t absorb moisture from the air, and most of them can be used for water-tight applications.
  • For those who are looking to buy and own a resin printer, the extra advantages are:
  • Possibility to nest many objects onto a build plate. Supports created by software let parts “fly”, so combining several models to print in one go is easy and effective without negative consequences.
  • Growing interest in resin printing. Manufacturers, hobbyists, scientists and many other clever people tend to push the limits of technology, offering new materials, methods, and improvements of vat polymerization methods. This results in more perspectives and a variety of products on the market as well as cutting down the costs (affordable desktop resin printers appear a lot). The technology becomes more investigated, so today, it’s quite easy to find knowledge bases, software, resins and other stuff required to become a resin printing professional.
  • Safe process. Desktop machines, as well as some modern industrial units, don’t maintain heat or produce any smoke. Thais means that resin printing is safer than others, so someone can run a printer in-house and even leave it alone without any dangers.
  • Easier 3D designing. Stating that resin printing doesn’t have special requirements to digital models would be a lie. Still, the list of recommendations and geometrical limitations is less compared to some other printing methods, so more creativity and details are possible to produce.

3D print with Resin

As for limitations and downsides, the main ones are:

  • Some large size objects are impossible to print as a single piece, or it would be too expensive. Since resin printing is used more for smaller precise models, average printers aren’t so big. It ‘s also caused by the need to fill the whole tank for tall prints with expensive photopolymer – and for industrial machines that can be counted in tons.
  • Some design limits and requirements. Resin printing still has a list of things which need to be included into a project while preparing it for production. For example, engraved details should be at least 0.4 mm wide and thick. Part orientations and mesh quality also influences the printing, so some knowledge and experience is required for a model to be made perfectly.
  • Many supporting structures. For the best quality, experts advise on placing an object a certain way that it won’t touch the plate and “fly” with a 45 angle. That means many supporting structures. They are usually fine to remove, but still, it’s an extra step to do after printing. Plus, after being cured, supports are solid and useless – they can’t be recycled into fresh resin.
  • Necessary post-processing. We have already mentioned at least 3 things to do with resin prints before they can are ready and neglecting those would end up in weaker and possibly ruined parts.
  • Some reactions keep running after the print is done. Polymerization is a tricky thing – after resin is formed into an object, some reactions continue inside a part (that’s why post-processing is important). Depending on a part and material, this can lead to shrinkage, cracks and deformity.
  • Price tags. We weren’t kidding when we said resin printing is more affordable today. However, it’s worth noting that for resin printing, “affordable” is around $3,500-4,000 for a desktop machine, and $100-200 per liter of resin. As for industrial grade machines, priced may range between $80,000-250,000 per unit.
  • Tricky resolution parameters. An interesting fact about resin printing is that resolution parameter (precision/quality) depends on several factors, which means for high-detailed prints, everything should be kept in mind. Also, this plays a role for a machine owner – even if a 3D printer is powerful enough to provide accuracy down to 50 microns, the type of resin used can make it impossible to produce parts better than 100 microns.

Applications of resin 3D printing

Vat polymerization in general is quite a flexible fabricating method with a range of materials and technologies suitable for many industries and tasks. Typically, resin printing is best recommended for:

Creating master models for jewelry and molds. Precise, smooth and good-looking models are produced in short time periods. Using castable resin, these models can be sent straight to casting services to create the final piece in metal. In the jewelry industry, resin printing is one of the main tools now, check it out

Custom dental devices like bridges, crowns, aligners, night guards, impression trays, surgical drill guides, splints, and more. In dentistry, resin printing helps to produce training models and end-use devices. In medicine, every correcting dental device must be custom-made for a certain person. With CAD compatible software and dental resins, specialists can create those easily and quickly straight from a CT scan (with minor improvements to the 3D model). More information on resin printing in dentistry

Model-making, props, animation. Digital format is now commonly used in visual graphics, films and cartoons. However, props and real action models are still needed. Designers, animators, movie-makers, and toy makers use them for a better quality, for improving their digital design or straight in making films. Despite prints being monochrome, figurines come out lively.

Prototyping and product development. For large size objects with simple geometries, resin printing may not be the best choice. However, products with complex forms and textures would look amazing with this method. Prototypes and demonstration models would be precise, impressive and functional.

Industrial vs Desktop Resin Printers

Resin 3D printing is complicated when it comes to separating “good” and “bad” machines, because the technology itself is highly subjective. As far as being used for producing small parts, desktop 3D printers can’t simply be called unprofessional. Still, some types of resin 3D printers are slightly worse than others, so let’s try to analyze them all.

The first major difference among resin 3D printers seems to be in their price tag – low-cost units for personal use can be purchased now at around $300-400 (or $100 in some extreme cases) to $3,500-4,000, while professional machines are still a huge investment.

But is there something more than just thousands of dollars? Of course there is.

Build volume – in 3D printing, this is a parameter which limits the dimensions of an object to be produced on a machine. Simply put, this is the working area which can’t be hacked or overcome easily. The build volume of the Wanhao Duplicator 7 is only about 120x68x200 mm, while machines involved in big manufacturing are about 500x500x600 mm or more.

Resolution – the quality of a part. Resolution stands for the lowest possible detail size, or to be accurate, a possible lowest printing platform movement. The highest resolution of a machine matches the lowest layer height. For resin printing, it can go down to 10-25 microns for some units.

Power of a light source – a light, involved in polymerization should be precise and strong to work faster and smoother. The quality of this light source (projector, laser, LEDs, LCD) is one of the main factors of a print’s accuracy. Also, it has an influence on the printing speed, materials range and durability of a machine.

Type of a light source – there are various resin printing technologies using different sources of light. Some of the methods tend to produce better parts. For example, a laser vs projector usually wins because a laser beam is smoother and smaller than a single pixel produced by projector.

Compatibility with different resins – some machines are manufactured to support only one type of resin or just some types of photopolymers. That’s one of the consequences of a certain light source – depending on its power, it can be used only for certain types of liquids.

The last and perhaps one of the most critical aspects that separates a resin printer from the others is the specialist who operates it. Advanced 3D printers can perform automated processes but the human input such as preparing a model, choosing the settings and the angle, advising on the type of material and post-processing is still critical to the success of 3D prints.

Cover image by Creative Tools