Feature first published in the September/October 2022 issue, then online in February 2023. PostProcess Technologies describes its solutions at that time. Technical clarifications added during review refer to manufacturer instructions.
Post-processing helps resin-printed parts achieve the required properties and finish, whether for prototyping or end use. In stereolithography (SLA), DLP and LCD printing, the resin, printing process and post-print workflow must suit the application.
The article below aims to discuss the various methods/tools that can be leveraged to give a resin 3D printed part a great finish. It is part of two series of insights: a series that ambitions to provide a thorough understanding of each post-processing task, and another one that ambitions to understand the secrets of resin 3D printing. This latter series already made us delve into the various considerations that need to be taken into account to choose a resin 3D printer, and the different forms of toxicity and solutions explored to reduce it in such processes. These are all crucial information you should be aware of before utilizing a resin 3D printing technology.
These technologies can produce fine details, but achievable feature size depends on the system, material and geometry. Orientation and supports can leave marks requiring finishing. Washing and post-curing also serve distinct purposes relating to quality and material properties.
Three operations recur: cleaning, support removal and post-curing. Their sequence and settings vary with the resin and part; this overview is not a universal processing recipe.
“The process can be daunting for any organization simply because of the number of steps required to finish a part. It includes multiple steps that are traditionally done manually by a technician and require both time and skill. This can become even more of an undertaking when companies scale their process into full production. Performing all these steps manually may be fine for a few desktop printers, but with multiple large format printers or a farm of dozens of smaller format machines, manual labor, and management of the excess resin can become problematic quickly. Many post-processing steps need to be performed in a resin workflow, including removing parts from the platform, removing uncured resin, traditionally with caustic solvents in tanks, and manually removing support structures. Additional steps can include UV and/or thermal curing”, an expert from PostProcess technologies told 3D ADEPT Media.
Before handling materials, consult the resin and washing-agent safety data sheets and equipment instructions. These specify compatible gloves, eye protection and ventilation. Avoid contact with uncured resin and control light exposure while handling it. The quotation’s term “caustic” does not describe every solvent: IPA is notably volatile and flammable and must be kept away from ignition sources.
1. Cleaning the part
Remove the part from the build plate using the tool and method recommended for the printer. Use a washing agent compatible with the resin and equipment: IPA, TPM, DPM and ethanol are not automatically interchangeable. Some resins have a water-based washing protocol; follow their instructions rather than a blanket prohibition on water.
Cleaning must remove residual resin, including from accessible channels, without exceeding the permitted immersion conditions. Geometry may require a specific procedure. The timing of support removal depends on material and part. After washing, let the part dry completely before post-curing.
A washing station can make batch processing more repeatable. For ultrasound, explicitly check liquid compatibility with the equipment and its instructions. Do not pour IPA or another flammable solvent into an ordinary ultrasonic tank: this can create a fire or explosion hazard. The Elmasonic Easy manual, for example, prohibits flammable liquids.
In such cases, complete automation to the integral resin 3D print operation would be worth exploring. “Our resin removal solution automates the excess resin removal step in the resin workflow. Once the build plate is removed from the printer, the entire plate (for most printers) can be placed directly into a PostProcess DEMI series solution, where both the parts and the plate are cleaned of uncured resin. Our solutions are automated thanks to a combination of proprietary software, advanced hardware, and sustainable chemistry”, PostProcess Technologies says, speaking of their solution. The company attributes reduced waste to its detergent’s resin-loading capacity compared with some conventional solvents. “It is also a safer option than traditional solvents, as it has a significantly higher flashpoint than IPA. The PostProcess comprehensive system of software, hardware, and chemistry is proven to reduce post-processing steps by 50% or greater. The proprietary process reliably removes resin materials from even intricate or delicate 3D printed parts while increasing productivity and reducing cycle time, consistently cleaning full trays in under 10 minutes”, PostProcess Technologies says.
The 50% gains and sub-ten-minute cycles are supplier-reported results, without a detailed comparison protocol in this interview. A higher flash point does not remove other chemical hazards; assess the safety data sheet and complete process.
2. Removing supports
Supports may be removed before or after post-curing, according to the validated workflow. Material, thickness, delicate details and dimensional stability guide the choice. Removal either too early or too late can damage some geometries.
Suitable flush cutters can help remove supports while limiting stress on the part. Maintain the required protection. A surface that is no longer tacky does not by itself prove complete cure or suitability for use.
3. Post-curing the part
Washing removes residual liquid resin; post-curing then treats the clean, dry part. Curing a layer of resin left on the surface does not replace proper cleaning and can obscure fine details.
Polymerization is not necessarily complete when printing ends. Light and, depending on the material, heat help achieve specified properties. Medical and dental uses require the validated workflow; arbitrary UV exposure does not establish biocompatibility.
Wavelength, light dose, temperature and duration must suit the resin and equipment. No single setting applies to all SLA, DLP or LCD parts. Formlabs describes resin-specific post-curing settings.
This step can also be a great fit for parts with a wide range of thicknesses and unsupported overhangs that suffer from warpage due to irreversible deformation during thermal cure. As seen in Carbon Digital Light Synthesis™ (Carbon DLS™) 3D printing process, this “baked-induced inaccuracy typically originates from differential shrinkage between thinner and thicker walls of parts, and parts softening at elevated temperatures. UV post-cure can potentially address these issues by reducing mass loss during baking and increasing green strength.”
This possibility does not apply to every resin and does not automatically replace thermal curing. Carbon workflows distinguish treatments by material; dimensional stability requires checking for the particular part.
“There are different workflows for different resins. This can require a very specific workflow to create the desired end product. For example, most prints will require a UV curing step, but the UV exposure can vary between each resin to achieve the mechanical properties advertised by the resin manufacturer. Removing excess resin from parts is done using solvents, but we cannot use one solvent for every resin. Differing resin properties may call for different solvents”, PostProcess Technologies warns us.
Some manufacturers offer a complete workflow; others partner with specialists, such as Carbon and PostProcess Technologies for resin removal. A nail lamp, improvised chamber or sunlight guarantees neither a controlled dose nor the required properties. Functional parts need a documented, validated workflow.
Other finishing options
Depending on surface requirements and end use, blasting, sanding, painting, coating or metallization may complete the workflow. Check compatibility with the resin and required dimensions, together with protection appropriate to the dust and products involved.
This article has first been published in the 2022 September/October edition of 3D ADEPT Mag.
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