- 1 Printer safety (FFF machines): Introduction
- 2 Handling risks
- 3 External safety
- 4 Operation
- 5 Risks
- 6 Safe printer design
- 6.1 Board/firmware design
- 6.2 External control
- 6.3 Electrical part
- 6.4 Fan failures
- 6.5 Fusing
- 6.6 Earthing
- 6.7 Wiring
- 6.8 Heater and thermistor fixation
- 6.9 Heater pad insulation
- 6.10 Limiting power on heating elements
- 6.11 Mains power switching
- 6.12 Relaying the power to stop heat generation
- 6.13 Use thermal cutoff
- 6.14 Electronic enclosure
- 6.15 Electronic cooling
- 6.16 Emergency Shutdown (ESD) button
- 6.17 Using the mains (110/230V) for bed heater or chamber heater
- 6.18 Enclosed machines
- 6.19 Fumes recycling
- 6.20 Smoke detection
- 6.21 Insurances.
- 7 Checking components
- 8 Accidents
- 9 See also
- 10 References
- 11 External links
Printer safety (FFF machines): Introduction
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. In case of faults, hotend heaters can reach temperatures capable to fuse aluminium (> 600°C), which are sufficient to start fires on most flammable materials.
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.
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.
- 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 you shall dump 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. You shall be warned that the triggering of such extinguisher can throw far away burned parts 
- 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 resident current device (RCD) shall be installed on your electrical installation. In north America you can also find arc fault protection interrupters (AFCI) which are a quite efficient protection, unfortunately they are not available for domestic installations in others countries. When installing a printer, it is better to upgrade installation to latest regulation. Printers are machines (and legally considered as such in EU), not appliances.
- 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.
- 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.
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
- Software freeze which left heat running full permanently. In principle there are automatic reset when software locks, but this not always work and in some software can be desactivated !
- Hot parts cooling failure
- Most printers hotends requires permanent cooling while heating. Fan failures are frequent and will drive to temperature raising to the top of the hotend, fusing supports, and hotend will fall on printed parts which may be ignited. Metal supports for hotends are much safer than plastic one.
- Electronic cooling failure
- There could also be electronic board cooling failure, which may drive to component failure, starting thermal runaways or components shorts. 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 flammable parts. A common occurrence 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 components becaming permanently conductive.
- This is typical for MOSFET or SSR and without a general relay on power, may drive to totally unstoppable heating.
- Electronic component exploding or bursting in flames.
- This could be due to components failures, shorts, underspecified or counterfeit components.
- Shorts or sparks on printed circuits.
- This occurs either because of poorly dimensioned tracks or by mechanical part shorting aside the fixation holes, notably for beds.
- Power switch becaming permanently conductive.
- 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.
- User fault when doing maintenance
- Hotends need maintenance and are commonly replaced. A lot of users do that without having basic tool (multimeter)/knowledge to do this operation. Any wiring modification/replacement shall be electrically checked. Many other faults are also possible.
Safe printer design
see Board safety
|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.|
|Checking proper operation of cooling fans||As on computers, fans can be equipped by a third wire which sent a signal for each rotation (the yellow one). This is very easily checked by the board if this wire is connected.||
An external control of overtemperature will help detect these failures, but need another processor and other temperature sensor.
It could be done by a secondary board as a few have done
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. However, A RaspBerry Pi is a full blown computer with complex multitask operating system and cannot be considered as reliable as a simple microcontroller.
On some printers, the LCD panel is handled by own processor. It could be used to do this external control.
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. A lot of fan do have a rpm wire allowing rpm control as this is standard practice in computer industry. This shall be used to check that fans are properly operating. Unfortunately, very few boards incorporate this simple safety.
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.
Printers with IEC plugs often include fuses on the mains input. This is not sufficient. Fuses shall be installed on the low voltage side. Different fuses shall be used for electronic and power. The best way is to have fuses on the board. Resettable fuses (often orange 'polyswitchs') have been prone to faults, unfortunately they are still in use in a lot of boards. They shall be replaced by real fuses. Automotive fuse type are compact and easy to find but not many boards are using them. Heaters driven by separated MOSFET boards or SSR shall have own fuses.
For printers using only very low voltage (12~24V), metallic parts are not always earthed as such voltage is not considered dangerous for humans. However, earthing does not only protect humans, it also protect the machine by helping to trigger the fuses or the residual current device of your electrical installation. There is also the possibility that low voltage circuit enter in contact with the mains in the power supply which is a major source of failures. So earthing metallic structures of a printer shall be done whatever the voltage used for heaters. On DIY kits, the metallic enclosure of the power supply is often not covered by another enclosure. A frequent fault on power supplies is the absence of connection between the earth wire and the metallic case, which is particularly dangerous.
Mechanical shield of mains 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.
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.
Secured wire ends
Whatever the connection type, soldered, plugged or screwed, wires shall not apply any load on the connection because this will after some time drive to problems on the connection, either loose contacts or broken wires creating sparks. Partly broken wires also can overheat to very high temperatures. Problems occurs more quickly on moving wires, but this will also occur on power static wires due to vibrations. So all wires ends of moving wires and static power wires shall be secured just aside the connection, in order to release any load on connexion.
Crimped terminals on stranded wires
Stranded wires are flexible, so they moves and that creates fatigue at the connexion. To avoid breaks at connections on stranded wires, it is imperative to use crimped terminals. This is a regulatory requirement on all CE certified machines. Tin plated wire ends shall never be used as they creates a weak point at the end on the tinned part.
Appropriate wire section
The wire section shall be adapted to the maximum current. That depends of the wire length, ambient temperature, insulation type and core numbers but sufficient section is also required for neat connection to the terminals. For stranded wires, a large number of cores is preferable as it increase flexibility and so, resistance to fatigue, but it reduce the current capabilities. This is why below table have relatively low rating as it is for large core number wires. A large number of cores typically reduce the current rating by half compared to single core.
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 well 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.
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 , 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.
The advantage of such fuse is that it trigger directly at a given temperature and is completely independent from the electronic board operation. However, the replacement is not as easy as for an ordinary fuse because such fuses are supplied with attached wires and not in cartridge form factor. Adding terminals for easy replacement may help.
On heater block
To avoid the risk linked to electric component explosion or bursting, electronic shall be installed in a fireproof enclosure. For self-built printers, one possibility is to use an old PC power supply enclosure. Flammable plastic enclosure is unsafe. For printed parts, difficultly flammable filament does exist but it is quite rare and costly.
see dedicated page Electronic cooling
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 :
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 .
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. However, a fire developing in an enclosed area have great risk that the pressure increase blow off the door as can occur in an oven.
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.
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  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
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.
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 :
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. A cheap analogic multimeter may be more reliable than a cheap digital one. Having larger resistance, 24V bed heaters are easier to check. Calculated data (Resistance = Voltage^2/Power -> Power = Voltage^2/resistance):
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:
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.