In order to successfully print your own custom designed parts you must be familiar with fundamental design concepts of 3D printed parts. As a matter of fact, there is no specific and extensive guide on how to 3D design parts for SLA 3D printing. However, some conventional manufacturing techniques already possess some proven methodologies and knowledge. Therefore, in this article we would like to merge currently exploited manufacturing techniques and AmeraLabs experience in resin 3D printing. We believe this will help you get an understanding on how to approach delicate design problems of DLP/SLA 3D printing parts.
This article should not be treated as a step-by-step tutorial. It is more like a reference and can be used as needed.
Resin 3D printing uses photopolymer UV resin that harden under UV light. This so called “hardening” is technically called “cross-linking” of material, when it is transformed from liquid state to solid. This reaction is exothermic. This means that during transition a lot of heat is produced. Sometimes temperatures can reach more than 100 °C (212 °F) and can easily burn your skin. Moreover, the process of “cross-linking” causes unwanted shrinkage and if your part is poorly designed, it can even cause severe damage to your 3D printed object.
It is also worth mentioning that not all resin is cured right after 3D printing process. Although, we have solid object there is still uncured resin within the 3D printed part. This uncured resin continues to cure in the presence of ambient UV light that is all around us. Ongoing curing can still cause cracks and other potential damage, which can be reduced if proper 3D printed part design techniques are used.
So let’s discuss some key design concepts that will help you to overcome these obstacles.
It is well known that it is practically impossible to print large, solid, non-hollowed objects with resin 3D printers. The thicker wall is, the higher stress is put on your 3D printed object. This stress will eventually cause major surface and internal cracks. Moreover, if one uses bottom-up style SLA 3D printer with FEP/PDMS resin tray, large cross-sectional area will also lead to great layer separation forces from FEP or PDMS.
Therefore, hollowed objects with quite thin walls are preferred. Thinner parts also weigh less and use less resin per 3D printed object. This becomes very important for bottom-up SLA 3D printing machines, where object is hanging upside down and is constantly affected by gravitational forces. On average, the wall thickness should range from 1 mm to 2 mm (.080 inches to .160 inches). It is possible to go for a bit thicker walls, but that must be tuned up with overall design. Also you should select proper 3D printing resin with low shrinkage profile. You can also use thinner walls, but when removing supports, increased fragility can cause issues.
It is strongly recommended to keep your walls consistent and uniform. If the walls of DLP 3D printed object are not uniform, the thin sections that consist of less material will shrink less. Those thin segments will also stop shrinking sooner than thicker ones with more material will. As the thick section yields, it leads to warping of the part that eventually causes cracks, because thin segments simply do not yield anymore.
Sometimes it is impossible to have uniform wall thickness. In such cases, change of wall thickness must be as uniform as possible. By following this principle, stress and risk of potential cracks will be reduced. See images below on how very simple formula for wall thickness transition can be exploited.
The most common purpose of bosses in design process is to facilitate parts mating. Typically it is used for attaching fasteners such as screws. However, we strongly recommend to print test parts with your tapping drill sizes separately and see how they work before printing full design. The reason is that resin 3D printing materials end up being quite brittle and tapping becomes even more complicated. Usually any tapping efforts will result in cracks and damaged parts. Therefore, by printing test parts and experimenting, you will be able to avoid problems when tapping your final part. Below we present guidelines for boss design.
Generally ribs are used to increase the bending stiffness of designed part without adding any additional thickness. Ribs increase the moment of inertia that increases the bending stiffness.
Bending Stiffness = E (Young’ s Modulus) x I (Moment of Inertia)
Rib thickness should be lower than wall thickness to minimize risk of potential cracks and additional stresses after 3D printing and during post-curing. We suggest to aim for 60% of the wall thickness. Also it is important that rib would be attached with corner radius as softly as possible. Draft (wider base) for a rib is not mandatory, but in some cases, reducing the angle at which it is 3D printed might be helpful. This can help to reduce the need of additional supports. See guidelines for rib design in the image below.
Moreover, there is one more important facet to consider if you have part that has intersecting ribs. These intersecting areas will have greater thickness and, thus, more material. You can easily remove excessive material by simply hollowing the intersection of ribs to preserve uniform wall thickness and material volume.
So called gussets is another technique to increase stiffness of 3D printed structure. These are support structures that can be designed to minimize warping of the part. In general gussets may be considered a subset of ribs and the guidelines for ribs also apply to gussets.
If gusset is attached to the boss, its height can be 95% of that boss. However, its height should be less than 4 times the nominal wall thickness and the preferred height is 2 times the nominal wall. When it comes to the length of the gusset, it can vary from 30 % to 100 % of the height of the gusset.
Quite sharp corners can vastly increase concentration of stress for your resin 3D printed parts. This stress can lead to severe cracks and, in extreme cases, to total 3D printed part failure. The radius of sharp corners has to be closely taken care off, because the stress concentration varies with radius for a given thickness.
As it is visible in the chart, stress concentration factor is high for R/T values less than 0.5, but lowers for R/T values over 0.5 concentration. The stress concentration factor is a multiplier that greatly increases stress. It is strongly recommended that inside radius would be a minimum of one time the thickness.
At corners the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the materials thickness. If your SLA 3D printed part design allows it, it is strongly recommend to use bigger radius.
We would like to demonstrate, how proper design techniques can affect your resin 3D printing results. For this task we will conduct an experiment of 3D printing a case for a circuit board.
To better illustrate design effects on 3D printing success, we made two 3D designs of the same case: one without design guidelines and second one with design guidelines implemented.
It is important to say that serious warpage in SLA 3D printing actually happens during 3D printing process. With bottom-up style machines like Anycubic Photon, Wanhao D7, Peopoly Moai or most DIY SLA 3D printers that use FEP of PDMS coatings for resin trays, cured layer separation action puts a lot of stress on the part. Final result highly depends on proper support techniques that must be used to reduce severe warpage during 3D printing. After you are successful with 3D printing process, good design implementations come into play. These will help to sustain structure and shape of the object in the long run when exposed to ambient UV light.
After we printed, cleaned and post-cured all cases, we placed them outside on a window sill. We investigated final results after two weeks. Below, we present images for your judgement and evaluation.
After this thorough investigation of cases it was quite clear that design guidelines helped obtain better results. 3D design that followed guidelines warped less and even after two weeks managed to preserve almost flat surfaces. On the other hand, although, we saw no cracks on the incorrect 3D design, this case suffered greater warpage and deformations. Since both cases are quite small, differences might look intricate, but because we all want to be as precise with SLA 3D printing as possible, everything counts here.
We hope, that you have learned some of the key principles needed to design functional prototypes. Now go and put them into practice. If you need some resin for prototyping, we’ve got you covered – check out our IPR-10 SLA and IPR-12 DLP resins:
NO PIGMENT SETTLEMENT for long (12+ hours) prints • HARD. Having 62 MPa tensile strength and 9% elongation at yield printed parts will handle a lot of tension • LOW ODOR, LOW SKIN IRRITATION. Perfect for beginners to start printing at home. Gloves and open window is all you need to comfortably print with this resin • MEDIUM VISCOSITY. No heating required. Easier to clean your parts before post-curing and to maintain all the features of original model • LOW SHRINKAGE. Smooth assembly of prototypes as printed parts keep their intended dimensional properties • COAL BLACK COLOR. Comes carefully tuned with our coal black pigment to take the hassle with pigments off your hands.
Compatible with: Peopoly Moai 3D printer.
NO PIGMENT SETTLEMENT for long (12+ hours) prints • HARD AND TOUGH. Having 62 MPa tensile strength and 13% elongation at yield printed parts will handle a lot of tension • LOW ODOR, LOW SKIN IRRITATION. Perfect for beginners to start printing at home. Gloves and open window is all you need to comfortably print with this resin • MEDIUM VISCOSITY. No heating required. Easier to clean your parts before post-curing and to maintain all the features of original model • LOW SHRINKAGE. Smooth assembly of prototypes as printed parts keep their intended dimensional properties • CRYSTAL BLACK COLOR. Comes carefully tuned with our crystal black dye to take the hassle with pigments off your hands.
Compatible with: 3D Facture Draken, Kudo3D Titan 1, Titan 2, Phrozen One and 1.5, Autodesk Ember, Little RP, MiiCraft, Gizmo3D GiziMate, GiziPro, GiziMax, Atum3D, Futur3D DWARF, Reify Sous, 3D Labs Totem3D, Microlay DentalFab, Morpheus, mUVe Pro, Pro+, 3D Maker, ULTIPro+, ULTIPro MAX and similar 3D printers.