3D printer rings

Fundamentals of digital design for printing and casting

OVERVIEW

In addition to creating beautiful designs, jewelry designers must work within cost and process parameters, paying close attention to wall thickness and weight. Jewelry is generally designed to reduce weight due to material costs (precious metals) and wear resistance, while still giving the illusion of reassuring volume. Maintaining a low weight and low wall thickness helps achieve successful prints in a shorter time and requires less resin to build. A useful general resource on guidelines for technical details can be found in the Formlabs design specifications. nine0007

The casting process has its own success parameters, which will be discussed in this section.

Just because you can print your design doesn't mean you can successfully apply it!

This document provides a broad overview of some of the fundamental principles of digital design for jewelry printing and casting, and therefore will only cover some of the basic principles and explanations. Please note that the following information is provided as one of many approaches to the technical aspects of digital modeling and we welcome feedback if you agree, disagree or use a different way to model your jewelry. nine0007

WEIGHT AND MINIMUM WALL THICKNESS

As described above, significant forces are applied to the print as it is built in layers.

If a part is designed to be very heavy, it will require more support during the printing process than a lighter part. If the part is designed to be very light, other measures must be taken to ensure successful printing and casting.

When designing small, lightweight parts, follow the minimum recommended wall thicknesses for best results. The annular specimen shown below (Figures 4.1 - 4.3) is modeled to demonstrate the results of the same model with three wall thicknesses: 0.3 mm, 0.5 mm and 0.75 mm

The wall thickness of 0.3 mm (fig. 4.1) entails two main problems: the printable walls are not straight, but significantly concave. This is a result of these very thin walls being subjected not only to peel and flow forces during the printing process, but also to any slight shrinkage of the polymer during the print curing process and subsequent post-curing.

Casting not completely filled on one side due to thin walls. If multiple casting attempts are made, the part may or may not fill completely, but there is no way to overcome the warping of the printed part. nine0007

With a wall thickness of 0.5 mm (Fig. 4.2), we see a reliably filled filling, but there are some distortions and concavity in the side walls. We used the file to make some photographic cuts along the surface to show that the surface is indeed concave. It would be possible to continue filling to make this surface perfectly flat, but this would result in dangerously thin walls that would not support the load of a customer wearing the ring.

The 0.75 mm wall thickness (Figure 4.3) works well at this scale with reduced distortion. The same few surface cuts produce very slight surface depressions that can be easily filled flat without significant loss of wall thickness. The result is a structurally strong ring that will withstand everyday wear. nine0007

Fig. 4.1: Hollow center section with a wall thickness of 0.3 mm Pay attention to deformation of the surfaces and incomplete filling.

Fig. 4.2: Hollow center section with 0.5 mm wall thickness Filling the entire flat surface shows contrast with the concave center wall.

Fig. 4.3: Hollow center section with a wall thickness of 0.75 mm Filling the entire flat surface demonstrates the integrity of the flat surfaces.

Large parts must be designed with practical weight for printing and fabrication. If the part is too heavy, the print will need a stronger network of supports so that it doesn't come loose from the assembly supports during the peeling process.

From a practical standpoint, heavy parts increase the material costs of both the final cast metal and the resin used for printing, and possibly cause customer dissatisfaction due to uncomfortable wearing. If a part is designed too light, it faces potential distortion caused by peel and cure forces. Parts that are too light cannot be cast due to walls or components that are too thin to properly fill during the casting process, as noted above for small parts. nine0007

In general, in handcrafted jewelry, large pieces should be designed in hollow shapes with a minimum wall thickness that will maintain structural integrity and allow for everyday wear. The figure shows a 18mm wide hard bracelet that has a wall thickness of 0.9mm.

Three options are shown on the next two pages (Fig. 5.1 - 5.3) The first option (Fig. 5.1) is printed with a solid surface, except for a small hole with a diameter of 3.5 mm, which allows internal cleaning with isopropyl alcohol (IPA) and filling a hollow bracelet with molding sand for casting. nine0007

Figure 5.1: Digital and printed model of a hollow bracelet with one small sand hole. Note the support structure needed to assemble this part (see Orientation and Supports section).

The surface produced on the Form 2 is smooth and consistent, creating a shape and surface ready for small hole plate welding and finishing. Unfortunately, this part cannot be cast. nine0007

The main reasons why any attempt to cast this part will fail are:

Washing and curing: It is very difficult to wash and cure the inside surfaces of this hollow mold, and uncured resin can react with the sand, causing a casting error.

Molding Sand 1 : It would be very difficult to completely fill the inside of the bracelet with sand, and even if it was completely filled, it would also be difficult to remove the sand after casting. nine0007

Sand 2 : When the polymer burns out, a large volume of sand is left in the shape of the inside of the bracelet, floating in a space known as the core, supported only by a 3.5 mm mixture cylinder, which was the hole. This small cylinder of sand cannot support this core, especially when the molten metal is poured at high speed into the flask.

The second option (fig. 5.2) simply adds more and more access holes to the hollow inner bracelet. These holes can be decorative and serve as an integral part of the design. nine0007

Fig. Figure 5.2: Digital model of a hollow bangle with several large holes on the inside to allow for successful addition of sand and casting.

The third option (fig. 5.3) creates a bracelet from several parts. Use this approach if you want to cover all surfaces, or if you want to create a decorative pattern through the inside of a bracelet that has perforations too thin to successfully add molding sand and cast the bracelet as one piece. Building supports and keys in the model greatly simplifies assembly. nine0007

5.3: Digital and printed model of a hollow bangle with a decorative inner surface. This example shows a perforated filigree surface type split into multiple pieces (one of the six interior surfaces shown) that would prevent full and successful sand fillings when printed as a one-piece bracelet.

PROCLE SETTINGS FOR SMALL STONES

Most jewelry can be broken down into its individual components. All of these components must not only be printable, but also usable. If we look at traditional gemstone jewelry, we see separate stone settings (“settings”) and the structure to hold these settings together. Considering the example of a ring consisting of several bartacks, we can break these bartacks into prongs and frames. The ring shown below (Fig. 6) has an 8mm center stone and 2.5mm side stones. nine0007

For these 2.5 mm side stones, the rim thickness was 0.4 mm and the prong diameter was 0.7 mm. Although for such small parts the thickness of the rim can be successfully printed at 0.3 mm, such a thin wall can be problematic for casting.

If the casting is successful, finishing this casting can reduce this thickness to a dangerously thin wall. The 8mm center stone has proportionately larger bezel and prong dimensions of 1mm and this poses no problem for both printing and casting. nine0007

6: Digital, printed and cast model ring for 8mm center stone with 2. 5mm side stones.

When smaller stones are installed inside the walls, similar considerations must be made regarding the thickness of the walls on either side of the stones and the diameter of the claw. For this type of structural and visual application, the wall thickness should be at least 0.5 mm and the prongs should be at least 0.5 mm.

If you create your model with vertical walls and the prongs close together to fill the space with rocks, the prong may merge with the side wall during the printing process (fig. 7.1). Even if it maintains its position, then the sand will almost certainly break at this point, and the prongs will merge with the wall (fig. 7.2 - 7.3)

CASTING AND GROUND NOTES:

Wax casting (or in our case, resin casting) is based on sealing the part with a refractory material, a sand, which forms the shape of the part and allows the molten metal to fill the exact the area of ​​the part, after which this part is completely removed when burning at high temperature. Molding sand is very heavy, but as with very hard materials, it is also very brittle. nine0007

Digital design for metal casting requires an understanding and planning of negative (molding) as well as positive (polymer or metal after casting) spaces.

In the example above, consider how little space is left between the prongs and the walls for sand. After some of the polymer has burned off, this very small piece of the mixture must withstand the force of the molten metal rushing into the mold at high speed. In this case it won't happen. nine0007

This gives two extremely important results:

1. The sand breaks and the metal just runs freely between the prongs and the wall
2. Tiny mixture particles are present in the molten metal mixture. These particles cause voids, and they can also appear in the metal near the surface. There are few things worse than a final polish showing pieces of the mixture.

These images show vertical walls and prongs, and a very thin sand line between prongs and wall. nine0007

Figure 7.1: Digital ring model with 0.5 mm straight walls and 0.5 mm straight prongs.

Fig. 7.2: Injection printed ring, molded to burnout and cast, showing distance between walls and prongs.

Fig. 7.3: Mold sand after burnout of the molded part, before casting, showing negative spaces in the sand between the wall and the prongs.

One way to solve this problem is to add slope to the side walls and prongs (fig. 7.4 - 7.6). The "tilt" turns these walls and prongs so that they form a cone from the thicker base to the thinner top, and usually a 5 degree angle is sufficient to improve castability without being in the way. If the design allows it, then a large angle of inclination will always help better. nine0007

This creates a stronger piece with visually thinner edges and more space between the top of the wall and the prong so that the mix is ​​stronger to withstand casting forces. There may be some space closure where the prong meets the wall at its base due to printed and cast artifacts, but the top of the space will be defined and more easily molded.

This space, which previously only existed in the digital file because it was too small to be cast before the slope was added, allows for safer casting and provides a guide for setting stones. nine0007

Although skew is outside the scope of this article, skew in such details in the model also improves results in the molding process and subsequent wax injection processes. See our document Vulcanized Rubber Injection Molding with 3D Printed Master Molds for more information on the process.

By applying the angle to our prongs, we increase the sand line between the teeth and walls to improve casting success. nine0007

Figure 7.4: Digital model of the previous ring (Figure 7.1) with a 5 degree slope in the side walls and prongs.

Fig. Figure 7.5: As before (Figure 7.2), the revised sand piece shows the enlarged space between the wall and the prong.

Fig. Figure 7.6: As before (Figure 7. 3), the sand after burnout of the revised part, showing the increase in the thickness of the sand between the wall and the prong

HOLES

For all practical purposes, holes should have a minimum diameter of 0.5 mm, and this diameter should only be used for holes in very thin material. You can use smaller holes, but the results may be inconsistent with both printing and casting. In the ring below (Figure 8.1), a 0.5mm hole has been modeled in the center of the bottom bartack bar (most likely this hole is not needed, but it's a good example!). Such a small hole in a relatively thick piece of metal runs the risk of sand breaking during casting and hole filling. nine0007

A filled hole can always be drilled, but as mentioned above, having a filled hole means there are broken sand particles in the casting that can cause a defect. One solution is to simply indicate the ends of the through hole by dividing the sphere at both ends of the hole, providing a guide for the drill (Figure 8. 2).

Fig. 8.1: Digital model of a ring with a 0.5 mm hole that can be filled during casting. Please note that this hole size is for illustration purposes only. If it was a "real" model, the hole would be closer to 1.5mm and cast successfully.

Fig. 8.2: If small holes are needed, drill them after the part is cast. Create a shallow recess for which there will be no problems with casting. This will give an accurate drilling template.

Another solution for printing and casting small through holes (fig. 9.1) is to add an openwork decoration to the back of the hole (fig. 9.2). Not only does this help with printing and molding, but it saves weight and adds a decorative element. nine0007

Fig. 9.1: Casting the ring leaving small round holes behind the stones. Fig. Figure 9.2: Ring casting modeled with rectangular openwork holes around round stones to add visual interest and improve castability.

INTERNAL CORNERS

As we saw earlier, it is important to consider negative spaces in your design, especially internal corners, which can cause the problem of hard sand edges breaking into parts during casting. Consider the flow of metal into the mold and how to make this flow as smooth as possible. Sharp edges, rough finishes, and even sharp or compressed edges from supports that have not been completely removed can cause turbulence in the liquid metal. At best, the casting will be fine. In the worst case, this turbulence can cause porosity (imagine an oncoming wave). If possible, round the inner edges (fig. 10.1 - 10.2), and if the design allows, also the outer edges (fig. 10.3 - 10.4). Spend time finishing your print, smoothing surfaces and removing any artifacts

Fig. 10.1: Sharp corner (negative space) in shank shoulder, positive sand space may break during casting.

Fig. 10.2: By rounding a sharp inside corner, positive sands become much stronger. Overriding this angle by engraving when finishing is a simple solution.

Fig. Figure 10.3: Sharp internal corners in this square hole create turbulence and a potential risk of sand breaking. nine0272