Safety

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Printer safety (FFF machines): Introduction

This page is about printer safety and how to handle it. Health and Safety is another page. There is also a page about Gaseous_pollutant_filtration.

A 3D printer using filament deposition is a complex machine involving electricity, parts in movement, hot temperature elements, flamable parts and high energy content consumables. It does present significant risks.

A lot of machines are open and if not, they could be opened in service, so there is some risk to have fingers pinched. However, motor strength is not large and the risk of serious injury is low, even for children. Being powered electrically, there is some risks associated with the power supply and associated wiring.

The power supply part is something quite standard, and it is dangerous only if cheap components are used, which is frequently the case.

With temperature which may rise to 300°C in service and high energy content consumables (the filament), the biggest risk is fire and there has already been accidents.

The 3D printer world is not technically mature and there are no standards. In addition, a lot of people involved in machine development, notably in reprap world have very limited experience in safety handling, so the design often does not take into account basic safety rules. In addition, there are a lot of low cost equipment, particularly in electronic area, with chosen components known to fail in very dangerous manner.

300°C is often the maximum design temperature, but could be largely exceeded in case of failing component, driving to temperature above the ignition point of a lot of parts or consumables in the printer. Users have experienced incidents with temperature capable to melt aluminium.

Handling risks

There are three ways to handle risks, which shall be used simultaneously:

  • By machine design.
  • By installation of external safety components
  • By doing operation in a manner taking into account the risks

As for now, without safety standards and incitation to proper design, with low cost components and absence of real certification (even when compulsory as in EC), consumers 3D printer are dangerous equipment and shall be handled as such.

External safety

  • Fire alarm
A fire alarm is an imperative requirement, and it may be compulsory in some countries (EC). While the triggering of a fire alarm may save your life, it may be too late to stop a fire involving a significant amount of high energy content products (the filament). Locating a sensor or an alarm near the printer will help early alarm, which is critical to limit or stop fire expansion.
  • Manual extinguisher
A manual extinguisher of sufficient size (4 to 6 kg) -dry powder ABC- shall always be readily available and functional. A printer fire is more difficult to stop than other kind of fire which may occur in domestic life and small extinguishers are not sufficient. Everyone in the home shall know where is the extinguisher and how to use it. Participate to training sessions. Check the manometer regularly and send to maintenance when required. If there is no manometer (for a permanent pressure extinguisher), buy a new extinguisher. You cannot check yourself a cartridge based extinguisher, so it is better to avoid them.
  • Fire blanket
A fire blanket may help stop a small fire without the mess created by a powder extinguisher, which is aggressive chemicals.
  • Automatic extinguisher
An active safety, say automatic extinguisher -dry powder ABC- is not very costly and could really improve safety. Such equipment designed for boilers is easy to find. Being designed for fuel, it shall be capable to stop an hydrocarbon fire. You shall choose an extinguisher of sufficient size (4 to 6 kg). The research key words are 'boiler automatic extinguisher' and they could be found for 50~100 euros. Here also an incorporated manometer is imperative.
  • External electrical power shutdown
Easily available electrical power shutdown. You shall be able to easily shut down the electricity, while the printer and equipment aside is burning, so relative location of the breakers and printers shall be carefully chosen.
  • Proper electrical earthing
Earth shall be connected to a valid earth, never on plumbing. Use a socket tester.
  • Electrical safety breakers
If not compulsory in your country, a 20mA safety breaker shall be installed
  • Easy evacuation
How could you escape if your printer and filament is burning ?
  • Fire containment box
This is a possibility, but is complex and make printer operation less easy. However, that may be the only solution yet to run a printer unattended, provided an automatic extinguisher is installed inside or outside the box. If inside, the extinguisher flow shall access all compartments, including electronic and filament.

Operation

  • No printer shall remain unattended
As printing is often a quite long process, people are really tempted to leave their printer unattended. With the state of the art, it is unreasonnable to leave printer unattended, especially while these printers are built with cheap electronic, known for frequent failures.
  • Safe location
Printer shall be located in a place where an eventual fire will have difficulties to propagate and where access is easy to combat a fire
  • No filament storage aside the printer
Filament are hydrocarbon and they burn quite similarly to liquid fuel, so they shall be installed in a place which will be the last to be reached by a fire. As soon as filament storage space is burning, fire became uncontrollable with simple extinguishers.
  • No flamable part aside the printer
Frequently printers are installed in DIY areas, with a lot of flamable stuff (wood, paint, solvents, etc.). That shall be avoided. Remember that drywall, brick or concrete are the best way to limit fire propagation.

Risks

What are the problems ?

  • Thermal runaway
There is in a printer at least one heating element with a control loop. If for any reason, which could be related to software or hardware, the heating cease to be controlled, the temperature could rise, sometimes relatively quickly to a value capable to start a fire
  • Hot parts cooling failure
Most printers requires permanent cooling while heating. Fan failures are frequent and may drive to excess temperature, which does not always start a fire, but may drive to destruction of supports, hot parts falling down to easily ignited area
  • Electronic cooling failure
There could also be electronic board cooling failure, which may drive to component failure, starting thermal runaways. see electronic cooling.
  • Mechanical failure
In case of mechanical failure, due to uncontrolled movements or more frequently, hot part support failure, the hot parts may come in contact with flamable parts. A common occurence is the hotend heater falling down on print or bed.
  • Wiring failures
Movements of the printer creates a lot of stress on the wiring and connections and failures are frequents. That may drive to shorts or contact between wires and hot parts, causing harm to the control board, which may end badly. There is an example of someone who started a fire on its control board because of a short in stepper wires.
  • Electronic component exploding or bursting in flames.
This could be due to components failures, shorts, underspecified or counterfeit components.
  • Power supply failure
PC power supplies are standardized and relatively safe. However, notably for voltage over 12V, power supplies often used in 3D printers are the one designed for LED supply and they are frequently very poorly designed and manufactured. Also, earthing is not always properly done.

Safe printer design

Board/firmware design

see Board safety

Thermal runaway
Causes Defense
Control loop software being freezed in heating A control of coherency between the measured temperature and the target shall be done
Whole firmware freezed, letting the heaters on Shall be controlled by a 'Watchdog' being it incorporated in the processor or being a physical watchdog on a processor pin
Control loop software perturbated by temperature measurement sensor failure Thermistor failure/disconnection shall be detected by the software, as they give off the range values
FET failure, locked conductive. Known to occur frequently on cheap RAMPS boards With designed as done on nearly all existing printer, such a failure could only be controlled by stopping the heating power (12/24V). If the power supply is an ATX PC power supply, the firmware could stop the 12V via an input on the power supply. For other kind of power supply, a relay is needed, but rarely present. This is one of the most common hardware design fault.
Heater failure Intermittent contact may trouble the control loop. Cut circuit simply stop heat. More dangerous is internal short, as it will increase the power. In case of bed heater, that may lead to local overheating, not detected by the temperature sensor.
Coherency between power input and temperature change rate A frequent incident is the heating cartridge dislodged from the heat block. That will significantly modify the control loop answer and that shall be checked continuously, as a cartridge can ignite printed parts quite quickly.

External control

An external control of overtemperature will help detect these failures, but need another processor and other temperature sensor. One user reported doing this external control with an external computer RaspBerry Pi, which run this task in addition to the printer and camera control. The computer being already existing on a lot of setups, the extra-cost is limited to temperature sensor and programming work.
On some printers, the LCD panel is handled by own processor. It could be used to do this external control.

Electrical part

  • Do no forget earthing
  • Power supply '-' shall be connected to earth
  • see power supply

Fan failures

Due to the very low quality of most fans installed on printers, fan failures are frequent.

If they are in charge of electronic cooling, that can drive to electronic failure.

If they are for hotend cooling, there will be a slow increase of temperature of the hotend which may end in complete destruction of the support and the fall of the hotend causing wire shorts or ignition of materials. Insulating hotend supports may help reduce this risk.

Wiring

Moving wires

When cables are connected to moving parts, it is important to use very flexible wires, and this is critical for heaters cables. Stranded cables are flexible but they are not all equal, some have a few strands (less than ten) and shall NOT be used for moving cables, as they will break after a certain amount of movement cycles, which can be dangerous if the break is close to the terminal as it induce heat in the near broken area. You shall use stranded cables with a lot of thin strands.

It does exist special cables for robots, capable to handle safely millions moves. For cost reasons, this may not be used in 3D printer, while printer movements are more demanding than on a robot, as it is faster.

You could find flexible wires with silicon insulation used in RC models, which generally have more strands than ordinary wires. A sheath around the wires help maintain them and limit local stress.

Fixed wires

For fixed cables, while there is no movement, it shall be taken into account that most printer vibrate, so the use of flexible stranded wires, as used in car or industry, is preferable.
For stranded wires, it is imperative to use crimped terminals.

Heater and thermistor fixation

Incidents frequently involve the hotend heater cartridge or the thermistor falling down from the hotend. This is generally related to the fact that these equipment were maintained by polyimide tape (Kapton). This is very bad practice and these components shall be mechanically secured, well taking into account the thermal cycling. A set screw is the most frequent mean used to maintain the heater cartridge but there are other solutions. The thermistor could be maintained by cement or often by locking its wires. This locking shall be effective. Thermistors with wires on both side (like resistors) are better maintained as they are installed through the heating block with wires on each side. Also, it is absolutely imperative that the software detect thermistor wire cut or short and also loss of control, when the thermistor signal is no longer linked to the power applied to the hotend heater.

Heater pad insulation

To preserve heat leakage, there is most of the time an insulation between heater element and nearby panels. The minimal insulation is sometimes only an air gap.
If something go wrong in heater pads, temperature could became very high, destroying the heater pad and sometimes ejecting high temperatures parts. Silicon pads inflate with significant pressure if overheated, then could make contact with adjacent flamable part and start a fire.
Some are using simple cardboard as insulation which could easily catch fire and this shall be avoided. If there is only an air gap, there could also be problems, particularly with silicon pads.
So a real insulation with fireproof material is required, with the side advantage of increasing heating rise rate. One solution is to install elements used by plumbers to protect stuff while using flames. The typical research keyword could be 'plumber pad' or 'soldering blanket'. Stuff like fiberglass blankets may be better installed with multiple layers. Ceramic fiber pads are fragile and costly, but they are one of the best solution.

Limiting power on heating elements

It is possible to design equipment in a manner that while running at maximum power continuously, heated elements (hotend or bed) may not be capable to physically attain temperature causing ignition. That mean a smaller power, so less reactivity of the printer components. That may be the price of safety, but fairly easy to implement for manufacturers.


In case of incident, if possible, do not forget to do a 'post-mortem- checking of the component, to help track history.

Mains power switching

You always shall be capable to unpower totally the printer, so the printer power switch shall be on the mains. If there is no power switch, you can add a switchable plug which shall be near the printer and labelled. Power switches are NOT safety equipment and they can fail permanently conductive [1], so if you don't have an emergency shutdown button (ESD), the plug shall be easily accessible.

Relaying the power to stop heat generation

As FET often fails in a conductive position driving to unstoppable heating, using relay to shut down power is recommended.

DC/DC SSR generate a lot of heat and need heat-sink. Mechanical relay cost lest and don't create heat but they are actuated by a coil needing more current than could be supplied by a processor output, hence it is required to use a transistor as first stage to command the coil. Board relays with opto-insulator for command are easy to find but the quality of their design and safety have been questioned.

It is good practice and could be required by some standards to use 2RT (2 contacts) relays and use both contacts in serie for a safer shutdown.

Use thermal cutoff

A few users have installed on their machine thermal cutoff (TCO), which open electrical circuit while a given temperature is reached. This kind of device have a trigger temperature and a lower temperature where you shall normally stay for safe operation. Operating within these two temperature may rise to random triggering.

If there is no general power relay, this may be the only way to cut a power circuit in case of FET (or SSR) failure shorted.

See this thread and cutoff datasheet example or another datasheet.

On bed

You can find thermal fuses for the temperature range of bed, which rarely exceed 130°C in service. However, bed are rarely a critical safety as the power applied to the bed is generally the temperature limiting factor.

On heater block

A lot of risk comes from overheating hotend heat block, but the available temperature range of existing thermal fuse, with a maximum usable temperature of 230°C (for a fusing temperature of 280°C), makes it only usable for PLA, which is an important limiting factor.

On bottom of hotend cooling block

The temperature range of the cooling block is appropriate for a thermal fuse. That will protect against a fan failure, but to get a protection against hotend heat block overheating is more delicate and need some tests. This is used notably in delta printer SeeMeCNC Rostock MAX V3.

Electronic enclosure

To avoid the risk linked to electric component explosion or bursting, electronic could be installed in a fireproof enclosure. One possibility is to use an old power supply enclosure.

Electronic cooling

see dedicated page Electronic cooling

Emergency Shutdown (ESD) button

There is a lot of confusion and bad practice in terminology used in 3D printing. An emergency shutdown is just that, a total stop to answer to a real emergency. The hierarchy of stops is :

  1. Print stop - with possibility to resume
  2. Print abort - no resume
  3. Reset. - board/firmware reset: can be hardware/software or both
  4. Emergency shutdown - Kill ALL POWER - Restart need physical action.

An emergency shutdown button labelled as such shall kill all power and shall have effect on the main supply. Wiring an ESD button on something else than the mains is misleading and non compliant with most safety rules. In case of emergency, you may want to intervene with fire extinguisher, fire blanket, etc. If a button (physical/software) drive to a board/machine reset, it SHALL BE labelled 'RESET' and nothing else.

An emergency shutdown button is recommended and imperative if your printer does have heating on the mains (for bed heater or chamber heater). If there is only a general on/off button it shall be clearly visible, easily accessible and its position shall be unambiguous from remote view. Mechanically self-locked shutdown buttons installed directly on power line are the most reliable solution, but the switch nominal current shall be adapted to the printer current with large margin. Safety relays are too costly for home 3D printer, so a relayed ESD shall use a 2RT relay with both contact in series.

Any external power block shall be installed on an accessible switchable plug.

Using the mains (110/230V) for bed heater or chamber heater

For large printers, the power needed by a bed can be significant and it does not look very clever to transform current just to make heat. So some people supply their heat bed and chamber heater in 110/230V. Earthing shall be properly done and due to risk of cable wear this is only acceptable for fixed bed (e.g. for deltas)

If the control loop is done by an independent controller, this could be an electrically safe solution. But that add another source of risk, the controller [2].
However, it does have some advantages to have the bed and chamber controlled by main board, which needs a thermal sensor connected to the board. The weak point is the insulation of this sensor. For cost reasons, galvanic insulator are never installed on sensors, so an electrical insulating problem on the sensor may end having the mains voltage connected to the control board, which could be dangerous. A power supply act as an electrical insulator, a safety you loose with direct mains supply.

Enclosed machines

Machines could be enclosed to reduce the noise, add heat chamber or recycle fumes. A fire which developed in a confined space will stop while oxygen is depleted, so the design shall be as such as there is no input and exhaust of air. Indeed, for recycling fumes, machine shall be somewhat tight. Melaminated wood act as a fire retardant because melamine release Nitrogen while burning, however while burning melamine products toxic fumes. The electronic protection shall be handled separately.

Fumes recycling

See Gaseous pollutant filtration.

Plastic may generate toxic fumes while heated, because of presence of toxic chemicals or degradation of the plastic. A recycling system with activated carbon filter may stop most of the volatile organic compound()VOC and notably the aromatics, but cannot remove heavy metals which could be found in opaque cheap filament.
Recycling is desirable to limit heat extraction from the chamber.
Mattress of fabric containing activated carbon could be found easily as they are used in recycling kitchen aspiration units. Use at least two layers of fabric.

Smoke detection

A well located smoke detector close to a printer raising an alarm could make early detection and saves a lot if you are at home. It shall be able to detect both electronic fire and hotend/bed fire.

Most smoke detectors are independent products of low cost. It is better to have a smoke detector coupled to a general home alarm, as it could warn neighbours and drive to a faster firemen intervention.

It was proposed to install a smoke detector inside the printer to shutdown printer electrical power supply. There was schematics on forums and a Kickstarter campaign [3] which have not became an official product. Shutting down power may, in some cases, stop the fire ignition and prevent the accident. However, it is difficult to evaluate the percentage of cases where this effectively prevent the accident. And it shall start a fire alarm, which was not the case of the product proposed in the Kickstarter campaign.

Remember that the role of a smoke alarm is to save your life, but it may not save your home.

In some cases, tiny smoke detectors like the Atom smoke detector have been installed directly on the print head, see this example

Insurances.

There was one report of an insurance refusing to insure home if a 3D printer is used inside. With increase of use of 3D printers, this kind of insurance disclaimer may expand and you may check with your own insurance if you are covered for this use.

Checking components

For cost reasons, cheap components are used in 3D printers. Also, mistake occurs in delivering components are when assembling a printer, so basic safety shall be carried out after printer assembly or any modification. Check shall be done in two phases :

  • First check of component (where possible)
  • Check of components after assembly, in case of wrong wiring/connection

Checking power of heating elements

It occurs regularly that the wrong voltage components are sent (12V instead of 24V). It is also frequent that there is no marking at all on heating components (bed or hotend heaters). And also, builders sometimes mix components. In any case, you shall not trust marking. For every new assembly, you shall check the resistance of all heating components. While this resistance change with temperature, cold resistance is a relatively reliable way to check power. However, low resistance measuring is not very accurate and you need a good quality multimeter to check 12V bed heaters. Having larger resistance, 24V bed heaters are easier to check. Calculated data (Resistance = Voltage^2/Power -> Power = Voltage^2/resistance):

- 25W 30W 40W 50W 80W 100W 150W 200W 300W 400W 500W
12V 5.76Ω 4.8Ω 3.6Ω 1.44Ω 1.8Ω 0.96Ω 1.39Ω 0.72Ω 0.48Ω 0.36Ω 0.288Ω
24V 23.04Ω 19.2Ω 14.4Ω 11.52Ω 7.2Ω 5.76Ω 3.84Ω 2.88Ω 1.92Ω 1.44Ω 1.152Ω

Checking wiring connections

Wrong connection occurs, often on the crimped part, so you shall check the component where doable after their assembly. There is no simple way to check board connections, but if there is intermediate connection on a wiring, like is frequently done on hotend, you shall check component through the wire harness:

  • Unplug you hotend/heater, etc. from the board
  • Measure the heaters, thermistors, fans through the complete wires at the board connector.

Accidents

Fire started by a dislodged heating cartridge.
IMG 4973 Lars Kuur.JPG

See [Forum thread]. Owner had made an analysis and tests to reproduce the incident. Probable cause: Non-secured wire harness dislodged the heating cartridge which fall down on the print. Firmware had only a timeout on setpoint temperature but did not check the temperature change rate against the power and did not detected the fault. See board safety.

Fire started by an electrical short during an overnight printing, which hopefully self-extinguished
Testing chamber heater, overheated silicon pad inflated, closing the 10mm air gap and started fire on the adjacent wood panel
Wood panel burned by silicon pad

See also

References

External links

References

Accidents

Tests

Technical papers

Safety equipment

Insurances


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