Eight options for improving your 3D printed part’s look and performance.
By Michael Jermann • Assistant Editor
3D printing is taking the manufacturing industry by storm these days. But no 3D printer, no matter what size, style or resolution, can produce objects with the same level of surface quality as traditional manufacturing methods. No matter how well designed an object is or how advanced the printer is (at present), the basic nature of 3D printing will not allow for the same uniform surface that injection molding provides.
Fortunately, there are steps that can be taken to give that object the look and even feel of an object that has been produced through other means. These steps can be as simple as sanding an object down by hand or as complicated as electroplating. Regardless of the method used, the goal is to make the 3D printed part look and feel as close to the finished product as possible.
The first step in finishing an object, for just about any printing technology, is to remove the supportive material. This material is there to keep the object from succumbing to gravity or, in the case of powdered-metal-based printing, keeps the object anchored to the print bed so it doesn’t curl up on itself.
The method and ease of removing these supports varies by printing method and build material. In the case of polyjet technology, such as the kind used by Solid Concepts, supportive material is different from the material used to build the part. As a result, this material can be removed easily with a water jet.
Stereolithography (SLA) based 3D printing technologies use the same material for building as they do for support, so removing them comes with a little more risk. These supports must be removed by hand, but in doing so there is a chance that they could leave behind residual material or even leave a divot on the surface of the object. Fortunately, both of these issues can be addressed with later finishing methods.
The supports created by fused deposition modeling systems have the advantage of being water-soluble. However, the process is not as simple as dunking your new print into a bathtub or running it under a tap. The material requires a hot lye solution to be dissolved. The object can then be left to soak until the supports melt away or the process may be expedited by using an ultrasonic cleaner. Higher temperature materials used in FDM, such as PPSF and ULTEM, are not soluble and must be removed by hand.
Removing the support structure from powder-bed systems is as simple as digging the object out of the powdered build material and dusting it off. The very nature of the SLS technology means that the unused build material acts as a support for the object.
Once the supportive material is removed, numerous options open up for what further steps a designer can take to finalize a printed piece.
The most likely finishing step to follow support removal would be sanding. This can be accomplished by hand or by machine, but the goal is to remove any traces of support structures and reduce or eliminate the “stair step” surface texture found on every additively produced object. Removing these steps is important, not only for enhancing an object’s appearance, but its function and durability as well.
If you used 3D printing to create a mold that would be used to make an object with a flow requirement, stair steps could cause turbulence where you want laminar flow. In 3D printed metal parts, stair steps can be a stress riser in the object and could lead to the part failing prematurely.
Sanding by hand is time consuming and expensive in terms of labor and does not work well for pieces with fine surface details or surfaces in hard-to-reach places. But it does entirely remove stair stepping and can be used on parts made by any additive manufacturing process.
“If you’re looking for a show part that’s going to be at a trade show and it has to look like the final manufactured part, the only way to get there is with hand sanding,” said Patrick Gannon, engineering manager for rp+m.
Originally developed for metal work, but adapted for use with FDM materials by Stratasys, mass finishing acts as both a sander and polisher for large quantities of parts simultaneously. The process works by jostling parts around in a tub of sanding/polishing media. As the media comes in contact with the part, impact forces and friction work to sand and polish away imperfections on the parts surface. As a result, parts with delicate features may not hold up well to vibratory finishing processes.
The media can be ceramic, synthetic, plastic or corncob and each is suited to a different material. Each of these media, save for corncob, come in a variety of shapes to accommodate different part geometries.
“You really have to dial in the vibratory finishing to the specific part geometry that you’re finishing,” said Chuck Alexander, product manager for Solid Concepts. “So it doesn’t necessarily provide a low cost benefit for doing one of prototypes or one to five, it’s really something you want to dial in for a low volume production run of parts.”
The process of vapor smoothing is a fairly uncommon method of finishing used by rp+m and a few other companies. The technique, mainly suited for ABS materials used in FDM processes, exposes a part to vaporized solvents for a few seconds that melt its outer layer to give it a smooth, glossy finish. Vapor smoothing cannot completely eliminate stair stepping, especially on an angled surface, but for vertical surfaces it can virtually eliminate stepping.
“If you were going to be doing any other kind of finishing like filling and painting, it reduces the amount of work required,” said Gannon.
Vapor smoothing works for high-volume applications and can smooth surfaces that cannot be reached by hand sanding. However, vapor smoothing may not be as effective with complex internal structures. Thin, flat pieces are prone to warping during this process. Since the part spends only a short amount of time exposed to the solvents, finer surface details are preserved, however the glossy finish can expose imperfections on the surface of a part. A small amount of strength is gained by the part when finished with vapor smoothing as the tiny peaks and valleys on its surface that could be stress risers are reduced.
As with hand sanding, nearly any additively produced object can be painted. Parts can be painted in numerous ways such as simply spraying them down with spray paint or treating them with different dyeing techniques. Designers should keep in mind that filler or primer might need to be applied before a part can be painted properly. Beyond the aesthetic benefits of painting, some specialty paints can enhance textures to a part. Solid Concepts, for example, offers a rubberized paint that will give a part a rubberized feel.
The process of electroplating is used commonly in the airline industry to give lightweight plastic parts the look of a more high quality material. The benefits being that you get all the aesthetic appeal without the added weight. Alloys such as chromium and metals like nickel, copper, silver and gold are the most commonly used plating material.
Electroplating also carries the advantage of giving the part added strength by forming an exoskeleton around it, thus eliminating the cost and weight of a solid part. This added strength varies with the thickness of the plating, which typically ranges from 0.0001 in. to 0.020 in. (0.0025 mm 0.508 mm). In applications where sensitive electronic equipment is housed within a part, the natural conductive properties of the plating can help keep electrical signals from getting in or out.
One of the biggest limitations of 3D printing systems is their limited build volume. A way around this is to cut your object up into smaller parts and arrange them so they fit within the build space. Doing so can increase the speed of the print and cut down on the amount of support material needed and reduce finishing times. Once they are printed, those pieces must be bonded together. There are multiple ways to accomplish this including: solvent bonding, hot air welding, super gluing and epoxies. Bonding is most suited for objects made using FDM processes and materials.
Super gluing is as straightforward as one would think. An adhesive is applied to the matching surfaces of an object and joined together. Epoxy based bonding uses two components that are combined to form an adhesive, which is applied to the part through dispensers, brushes or infiltration. The parts are then joined and the epoxy is allowed to cure.
Solvent bonding uses chemical solvents to melt the plastic on the matching surfaces to bond them together. These water-thin solvents can even be injected into cracks to repair an object. Unlike epoxy and super glue bonding, once the solvent has evaporated, only build material will remain. This method is not suitable for chemically resistant FDM materials such as PPSF or ULTEM.
Hot air welding has a lot in common with metal welding, but instead of a flame, a hot air gun is used and instead of metal filament, a filament of plastic is used. The filament of build material is drawn along the seam of the matching surfaces and melted by the hot air gun. The melted plastic then fills the seam and bonds the two pieces. The advantage of this method is that it’s extremely low cost and the part is ready to go as soon as it is cool to the touch.
Ultrasonic spot welding uses high frequency sound waves to melt localized areas of a part, where it can then be joined with its match. Since this process does not introduce new material to the part, the overall accuracy of the print remains relatively unchanged. To further strengthen the bond, ultrasonic welding techniques may be combined with other forms of bonding.
If you are looking to strengthen an otherwise fragile 3D printed object, infiltration is a possible solution. Infiltration uses an epoxy resin to strengthen and seal an object. It works best with materials with a porous or semi-porous surface. The epoxy is brushed onto the surface of the part and then allowed to sink into the tiny spaces within the material, though the process can be helped along by the use of a vacuum chamber.
The epoxy is then cured in an oven to complete the process. Infiltration creates an air and watertight seal and makes the part chemically resistant. Parts sealed through infiltration can withstand pressures up to 65 psi.
With so many different finishing methods available, some may find it daunting to figure out which method is suitable for their part and their needs, but according to those in the industry, figuring out the purpose of the part is the first big step.
If your part is intended for marketing purposes, more aesthetic finishing may be appropriate, but if the part is intended to demonstrate a form and fit application, it may be better to skip the aesthetics and focus on ensuring that the part is dimensionally accurate.
Production size should also be taken into consideration when choosing a finishing process. One or two prototypes could be treated with more detailed finishing processes and not break the bank, but for low volume production runs, costs could be reduced by using simple or mass finishing processes.
“Most finishing does cost money and it can double the price of your part,” said Alexander. “So you want to make sure you’re only buying what you really need for the application that you’re going to use the part for.”
“Work with your team and 3D printing experts to truly understand what type of finishing you need,” said Gannon. “Finishing is an art and you want finishing experts to meet your expectations up front. While finishing can become expensive, your 3D printing partners can also help alleviate costs if the opportunity is there.” MPF