There are two impacts of oxygen on 3D printing. In the case of Powder Bed or Laser 3D printing, when using materials like copper oxygen in the material can form bubbles in the part. In the case of Stereolithography (SLA) printing, oxygen can inhibit curing at the surface.
Types of 3D Printers
Before we get into the discussion of trace oxygen, it is helpful to be familiar with the three basic types of 3D printers:
Powder Bed 3D Printers
Powder bed printing uses granules of material (typically metal) that are melted by a laser. As each pooled layer of molten metal is created, the another layer is deposited on top of it. Powder bed printing is typically the most expensive, but can produce objects made of metal at a fraction of the cost of normal machining.
Powder bed laser 3D printing is known by several names including:
- Selective Laser Melting (SLM)
- Selective Laser Sintering (SLS)
- Direct Metal Laser Sintering (DMLS)
- Electron Beam Melting (EBM)
The difference is the type of metal, the addition of nylon or other materials to the metal, and the source of the heat to melt it.
Each type of powder bed 3D printing is accomplished by moving a laser (or electron beam for EDM) across a flat layer of fine metal powder in a sealed chamber. When the laser is turned on it heats the powder to create a small “melt pool” of molten metal. The size and placement of the melt pool are controlled by a computer aided design (CAD) program. After a layer of metal is added to the part from the bottom up, a new layer of metal powder is added. The process is repeated until a metal part is built up inside the chamber.
This 2 minute video shows a small-scale SLM system in action.
Beyond rapid application prototyping, the primary benefit of 3D printing metal parts is that they don't create scrap metal. This is especially important where exotic metals like Titanium are used.
For example, the aircraft industry has been a leader in the use of 3D printing of metal components to prototype and manufacturer parts for their planes.
Stereolithography 3D Printers
Stereolithography (SLA) printers use UV light from lasers or digital light processing (DLP) projectors to harden liquid resin in a process known as curing. In an SLA printer the part is built from top to bottom as it is “pulled” up from a tray of liquid resin. The advantage of SLA printers are their ability to produce parts with extreme detail at relatively high speed. Their disadvantage is higher material costs. While SLA printers typically cost more than FDM printers, they are quickly coming down in price.
Fused Deposition Modeling 3D Printers
Fused Deposition Modeling or FDM printers use filaments of thermoplastic material on a roll that are heated and layered on top of the print bed to build up a desired part. The part is built from bottom to top. FDM printers tend to be the least expensive and easiest to use, but cannot provide the detail created by SLA printers.
Oxygen Inhibition
The majority of light-curing adhesives in 3D printing are acrylic-based. The challenge is oxygen inhibition, i.e. oxygen reacting with the surface layer of the product as it cures. Oxygen inhibition can result in the surface of the adhesive or coating feeling “sticky” after curing.
Because oxygen inhibition is caused by cures conducted in open air, there are several solutions that have been explored:
- Increasing the cure temperature
- Changing the type of material or the wavelength of the light
- Introducing a physical barrier between the part and the air
- Curing the part in a low-oxygen, closed environment
Powder Bed / Laser 3D Printing
For 3D metal printing, a bed of metal powder is swept across by a laser that makes “pools” of material that fuse. Subsequent layers are combined building up until the part is completed. In order to make a quality part, the intensity of the laser, the cooling time, the humidity and the quality of the metal powder must all be maintained. But in addition, any oxygen in the system can result in bubbles or loss of structural integrity in the finished part. This results in not only defective parts, but hours lost to production defects.
For this reason, Powder bed printers may rely on the use of argon or nitrogen as a shielding gas to limit the oxygen present at the intersection of the laser and the powder. Even in a sealed printing chamber, the operator can never insure all the oxygen is removed before printing begins.
One solution to this problem is the TecPen Weld Purge Monitor. This hand-held purge monitor verifies the number of oxygen molecules in the shield gas is effectively zero parts per million before printing begins.
Used in combination with a shield gas in an enclosed printing area the TecPen has the capability of decreasing not only oxygen, but the time and money lost to rejected parts printing.
Case Study: Monitoring Oxygen Levels During Powder Bed Metal Printing
A client needed to monitor oxygen levels for a 3D metal printing project, and we had the opportunity help. The project was to verify low oxygen levels during powder bed 3D printing of metal components.
In order to create high-quality parts, the machine operator had many variables to work with. In addition to verifying the accuracy of the laser, humidity, heat and gas levels inside the chamber must be constantly monitored.
Monitoring the oxygen level is especially important. Since many different metals react with oxygen when heated, in order to create precise parts the chamber is sealed and the air inside replaced with nitrogen or argon.
Large Scale 3D Metal Printing
While controlling the oxygen level inside a sealed chamber for a small part can be done with a single oxygen sensor, how do you control the oxygen level inside a chamber the size of a bus?
Large metal components for cars or aircraft can take over 30 hours to print can now be made using 3D printing technology. To maintain a low oxygen level, nitrogen or argon is constantly pumped into the chamber.
A single air leak or a failing gas pump can result in a part worth thousands of dollars to be ruined, only to be discovered after the long printing process is complete.
Monitoring the Oxygen Level
Our client decided that the solution to monitoring oxygen levels could best be accomplished by spacing oxygen sensors in the top, middle and bottom of the chamber at both the entrance and exit of the chamber. The idea was that the door seals would be the most likely place for a leak to occur.
After discussion, we decided the LuminOX LOX-O2 Oxygen sensor and a 10,000 hour Thomas Micro Pump would be the best solution. The sensor is stable, accurate, and sends a UART digital output that is more accurate than an analog signal. The micro pump is rated to work continuously over 1.5 years. When accounting for tooling downtime, the components will provide years of reliable service.
In order to connect the sensors, the client wired them into an assembly that included the sensor, the pump, inlet and outlet tubes, power and I2C outputs. I2C allowed the sensors to be daisy-chained along an 80 foot path to the control panel. They then added alarms to the system to notify workers if any of the sensors detected < 0.3% oxygen in the system.
While designing the system, the client also used a LuminOX sensor outside the chamber as a control and to monitor the oxygen level in the room as a safety precaution for the workers in the building.
According to the client, “the O2 sensors have been invaluable. I have set up alarm trigger emails and texts to the service guys to run to the office and check the hardware out if the values fall below a set threshold.”
The Future of 3D Printing: Bio-Printing
Controlling the amount of oxygen during 3D printing can have other benefits. For example, a team of engineers from the University of Colorado Boulder found that by regulating the amount of oxygen during a cure in an SLA 3D printer they could control a part’s rigidity. This has potential benefits for bio-medical applications, where body parts need both structural stability and flexibility.
Image by Olivier Cleynen, CC BY-SA 3.0, via Wikimedia Commons
