Understanding Combustible Dust Hazards Associated with 3D Printing c

Introduction

The 3D printing industry, also known as additive manufacturing, has significantly impacted various sectors, including aerospace, automotive, healthcare, and consumer goods. This technology allows for the creation of intricate designs and rapid prototyping, offering unprecedented levels of customization and efficiency. However, the use of powdered materials in many 3D printing processes introduces combustible dust hazards that can pose severe safety risks. This article delves into the nature of combustible dust hazards in the 3D printing industry, the associated risks, and effective strategies for mitigating these dangers.

Over the past several years, manufacturers in the additive industries have employed 3D printing to increase efficiency on plant floors. Materials used in the additive industries cover a wide variety of plastics and metals. The metals used include alloys such as stainless steels, brass, and copper. The nature of the 3D printing process tends to create fine dusts and typically, dust from metal alloys is not a high-risk concern. However, metals such as aluminum and titanium can pose significant fire risks. The particle size of a dust is a major contributing factor associated with the hazards they may pose. Fine aluminum and titanium dust can be extremely sensitive to ignition and can produce tremendously high temperatures and overpressures.

Typically, an additive can range from 10 to 70 microns, at which size aluminum and titanium dusts are highly dangerous. In these particle ranges, aluminum has an explosion severity (Kst) greater than 400 bar m/sec. Fine metal dusts have also shown to have low ignition sensitivity, such that a static discharge (less than 25 mJ) can ignite them. Such characteristics of a dust indicate potential for a catastrophic event under the right conditions. 3D printing can also create aluminum and titanium dust in particle sizes in the nanometer range. Titanium dust in the nanoparticle sizes has been shown to display pyrophoric tendencies.

3D printing can create hot dust but, even without a credible ignition source, metal dust such as titanium can catch on fire. It is important that industries employing 3D printing with metal additives are aware of the hazards associated with them. Metal dusts such as aluminum and titanium can be extremely dangerous and pose a high risk for a fire or explosion. In addition to having adequate knowledge of the risks, actions should be taken to alleviate these risks. This work seeks to present a detailed analysis of the hazards associated with 3D printing, as well as ways to prevent and mitigate these hazards. The work further provides three fire and explosion case studies resulting from 3D printing metal dust.

Types of 3D Printing Processes and Associated Dust Hazards

1. Powder Bed Fusion (PBF)

Materials: Metals (e.g., aluminum, titanium), polymers (e.g., nylon), and ceramics.
Process: Uses a laser or electron beam to fuse powder particles layer by layer.
Dust Hazard: Fine metal or polymer powders can become airborne during handling and post-processing, posing explosion and inhalation risks.

2. Binder Jetting

Materials: Metals, ceramics, and sand.
Process: A liquid binding agent selectively binds areas of a powder bed.
Dust Hazard: Loose powders and fine particles can become airborne during the printing and depowdering stages.

3. Material Jetting

Materials: Photopolymers and waxes.
Process: Droplets of material are deposited layer by layer and cured by UV light.
Dust Hazard: While less common, dust can be generated during post-processing.

4. Selective Laser Sintering (SLS)

Materials: Polymers such as nylon and thermoplastic elastomers.
Process: A laser sinters powdered material layer by layer.
Dust Hazard: Fine polymer powders can become airborne during material handling and post-processing.

5. Directed Energy Deposition (DED)

Materials: Metals and alloys.
Process: Focused thermal energy is used to fuse materials by melting as they are being deposited.
Dust Hazard: Metal powders and particulates are a concern during material handling and cleaning.

Sources of Combustible Dust in 3D Printing

Combustible dust in the 3D printing industry primarily originates from the materials used in various additive manufacturing processes. Understanding the sources of this dust is crucial for developing effective mitigation strategies and ensuring a safe working environment. Here are the primary sources of combustible dust in 3D printing:

1. Material Handling

Material handling involves the loading, transferring, and storage of powdered materials used in 3D printing processes. This stage is one of the most significant sources of dust generation.

1. Dust Generation: Fine particles are released into the air during the transfer of powders from storage containers to the printing equipment.
2. Dispersion: Manual handling or mechanical conveying can cause powders to become airborne, increasing the risk of inhalation and explosion.

2. Printing Process

The 3D printing process itself can be a source of combustible dust, especially in powder-based additive manufacturing techniques.

1. Dust Escape: Fine particles can escape from the printing chamber during the build process, particularly in non-enclosed or poorly sealed systems.
2. Layering and Fusion: Powder layers can become airborne during the layering and fusing stages of the printing process.

3. Post-Processing

Post-processing involves the removal of excess powder, cleaning, and finishing of printed parts. This stage can generate a significant amount of dust.

1. Depowdering: The process of removing loose powder from printed parts can release large quantities of fine dust into the air.
2. Surface Finishing: Grinding, sanding, and other finishing processes can create dust particles.
3. Handling and Transport: Movement of printed parts for cleaning or finishing can cause residual dust to become airborne.

4. Maintenance

Regular maintenance of 3D printing equipment and facilities can also be a source of combustible dust.

1. Filter Changes: Changing filters in dust collection systems can release accumulated dust.
2. Equipment Cleaning: Cleaning of printing equipment and surrounding areas can disturb settled dust and make it airborne.
3. Inspection and Repair: Routine inspections and repairs can expose hidden dust deposits.

5. Powder Recycling and Disposal

Recycling and disposal of used powders can generate dust, especially when handling fine, dry materials.

1. Recycling Processes: Sieving, blending, and reconditioning of used powders can create dust.
2. Waste Management: Disposal of powder waste, including handling of waste containers, can release dust into the environment.

Risk Mitigation and Management for 3D Printing

It is important for anyone involved in the 3D Printing process be aware of the hazards associated with the system. NFPA 484 provides guidelines on reducing risks associated with aluminum and titanium dust. Operators working in this environment should be made aware on the chances that pyrophoric metal dust can be created and how to address them effectively.

Housekeeping

Cleaning up after the process poses a significant risk to exposed operators. It can be recommended that the area is washed down with mineral oil prior to the removal of pyrophoric metal dust. This will limit any explosion or fire concerns associated with highly reactive metals such as titanium and aluminum. Operators must also be provided with adequate Personal Protective Equipment (PPE) to avoid injury to themselves.

Dust Collection

An alternative (but expensive) method of reducing the risk is to add a dust collection system to prevent the release of fugitive dust. A Local Exhaust Ventilation (LEV) must be placed near the layering of powders to collect any dust produced. The dust collector must be a wet scrubber per NFPA 484, and the media used should be mineral oil. Metal dust in contact with water can release hydrogen gas while titanium dust can easily spark or become pyrophoric. Hence, it is recommended that the media used inside the wet collection system is mineral oil. A well-maintained and documented Preventive Maintenance (PM) program must be in place to avoid upset conditions. The ducts must have periodic inspections to ensure no dust accumulation. The mineral oil reservoir must be properly disposed of. A wet scrubber can placate the hazards associated with housekeeping and ignition sources around the processing area.

The use of a Type D Fire Extinguisher

Any plant that employs metal 3D Printing must ensure Type D fire extinguishers are readily available to operators in case of emergency. Class D fires involve combustible metals, such as magnesium, titanium, potassium and sodium as well as pyrophoric organometallic reagents such as alkyllithiums, Grignards and diethylzinc. These materials burn at high temperatures and will react violently with water, air, carbon dioxide and/or other chemicals.  The specific agents used on each type of fire are chosen for their inertness with respect to the burning material. Class D extinguishers typically require a large amount of agent (several inches) to completely encapsulate and smother the fire which is why they are only available in 30 lb capacity for handheld units. These extinguishers are designed to softly deliver the agent so that it piles up in a layer rather than the typical vigorous discharge of a conventional ABC-type extinguisher [8]. Even if operators are trained to immediately vacate the premises in conditions of a metal fire, Type D fire extinguishers should be readily available and local emergency personnel must be notified of types of dust in the plant.