From a Microwave Oven to a Post-curing Chamber for 3D-printed Parts

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Photopolymer 3D-printing has gained popularity over the years, and different manufacturers have developed resins and process for printing. However, virtually all of them could benefit from a post-curing step to make the printed parts stronger. There are two types of post-curing: heating and extra UV exposition. Both of them allow growing the polymer chains.

What we describe here is quick way to obtain a UV chamber, which can be easily updated with simultaneous heating. The idea is to use an old microwave oven replacing the magnetron by a blacklight.

Once you have the material, the whole project will take no more than one hour to be accomplished. Although the project is quite simple, ask for help to make the electric modification if you don’t know what you are going to do.

Step 1: Disconnect the Magnetron

Unplug the microwave oven, remove the cover, and disconnect the terminals from the microwave power transformer. You don’t need to remove the magnetron or any other part of the oven. By the way, keep the glass dish, because we can use it to allow spinning of the pieces while they are exposed to UV light, which will make more uniform the exposition.

Warning: Even after the oven is turned off, capacitors may remain charged and be dangerous if the terminals are touched. Therefore, don’t touch an exposed electrical contact if you are not sure that it is safe.

Step 2: Put the Lamp Holder

Put the lamp holder on the inner wall. In our case, we chose the perforated region of the wall for sake of simplicity. The holder should be fixed by using nut and bolt, and an additional hole should be provided allowing the passage of the wire.

Step 3: Connect the Lamp to the Magnetron Controller

Connect the wires from the lamp holder to the terminals of the electronic circuit that controls the magnetron. From now on, when the oven tries to turn the magnetron on, the UV lamp will be activated instead. If you wish to include a second UV lamp or a heater, just put them in parallel with the first lamp.

Step 4: Use Aluminum Foil As a Reflector

Use aluminum foil (from your kitchen) to cover as much as possible of the inner walls of the oven. The foil does not need to be form a perfectly flat surface, because the idea is to reflect the light and send it to different sides of the 3D-printed object.

Oops, that is the right time to clean the oven after all these years in the kitchen.

Step 5: Finish the Job by Putting the UV Lamp

Different UV lamps can be used. We used a 25-W blacklight, which emission spectrum is shown in the figure. There is a large band at 365 nm and a small band at 405 nm, which is the same wavelength of the LED from the 3D printer. The absorbance spectrum of the photopolymer resin manufactured by Wanhao shows great absorbance in this entire region.

In addition to the UV emission, the blacklight also heats the oven. For instance, after 10 min working, the inside temperature rises from 25 °C to about 40 °C, which helps enhancing the curing process.

Step 6: Use It!

You can control the exposition time as you would do with a regular microwave oven. Of course, this time should be optimized for each case.

Warnings:

1. If you don’t know about electronics, ask for help.

2. Don’t put the aluminum foil inside the oven until you are sure that the magnetron was fully deactivated.

3. Blacklight is a gas-discharge lamp, which usually doesn’t work in PWM mode. Therefore, always chose the full power mode (100% power) at the oven panel.

4. Pay attention to the emission light spectrum of the UV-lamp. Remember that UV light can cause injuries to the skin and eyes. If needed, cover the oven window with a UV-filter or an opaque plastic, metal, or paper foil.

5. Put the chamber close to a window to help dissipate the released organic vapors.

[C. L. do Lago and E. T. da Costa thank FAPESP and CNPq]

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    4 Discussions

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    ClaudimirLseamster

    Reply 2 months ago

    Thank you.
    By the way, even a broken microwave can be used. Obviously, it depends on what
    part of it is still working. If only the power unit with the magnetron has been
    damaged, the oven may still be useful.

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    gm280

    2 months ago

    The part you have pointed to with an oval and arrow, is NOT the magnetron. It is the power transformer. The actual magnetron is the part attached to the side of the oven at the upper section of the side. Also, you need to inform people that the capacitor can hold a charge and can cause serious issues if it is not discharged properly. So please be very careful.

    1 reply
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    ClaudimirLgm280

    Reply 2 months ago

    The idea
    was not highlight the transformer, but the connectors to be used to power the UV
    lamp. You’re absolutely right: the part from where the connectors were removed
    is the high-voltage transformer. Perhaps our explanation was oversimplified by
    naming the setup with the magnetron, transformer, capacitor, diode, and a few additional
    components as just the magnetron. The idea was to show the unit that should be
    disabled to use the UV lamp instead. Regarding the accumulated charge at the
    high-voltage capacitor, I’ve seen models containing a resistor (like 10 MΩ) in parallel
    with the capacitor or the diode, which let the accumulated charge to dissipate.
    However, the warning is important. We change the text. Thank you.