HP-28S Battery Compartment Door

HP-28S Battery Door
HP-28S Battery Door

HP-28S Battery Compartment Door

The redesign of the whole HP-28S scientific calculator case is quite difficult and very time consuming. In most cases the only thing broken is the HP-28S battery compartment door, so I decided my first step was to get that re-designed.

Print testing phase of battery door

The 3D-print of the HP-28S battery compartment door is being tested right now (05/2020) from a few volunteers. I have reached out to a few 3D designers asking what printing method would be best in making this part. See below for PLA printing picture (not). The answer is Selective Laser Sintering or SLS printing using polyamid (PA). The characteristics are robust and the pegs can be melted down to keep the metal plate or battery contact, in place.
Having no experience with 3D printing, I’ve now learned that the following printing methods have certain advantages and disadvantages.

  • Fused Deposition Modeling or FDM printing
  • Stereolithography or SLA printing
  • Filament PLA printing (see below)

With FDM printing it is not possible to have the part mechanically resilient. The problem with printing via Stereolithography is with melting the material afterwards. Please find some printing companies here.

Small Assembly of Battery Compartment Door needed

You should keep your original metal plate from the battery door. After printing the part, you just re-insert the metal plate and melt down the 3 pegs, and voila you have a new battery door.


  • 3D Viewable PDF (Adobe Reader needed)
  • After testing the STL file print, I will make it available for download

PLA printing results

Feedback from one Tester: PLA filament does not print very well. Adjusted were temperature and speed of print after 3 tries, below pictures show the best it can do. That is the limitation of an entry level 3D printer.
Best to use laser type 3D printer, hence SLS printing.

SLS printing results

Today June 12th 2020, the first parts arrived and I must say the quality is quite good. I compared the original with the new printed version and I have to make a few adjustments. But now I sourced a printing company close by and progress should be much quicker now.

Yesterday July 9th 2020, I finally had some time of from work to pick up the 2nd set of printed parts. Very nice quality and the color is just right. The metal plate fits right into the grooves and the 3 pegs are ready to be melted down. Take a look for yourself. I added 2 drops of glue just to be save below the metal plate.

Here the file to download in order to 3D print it. I hope you enjoy the hard work that went into it.

Ordering and pricing

I was asked if I had printed a few battery compartment doors and if some could be ordered. So I did some checking on pricing and I ordered 40 of them, which makes sense for that size machine as a minimum order.
So if you do NOT want to print them yourself or do not have a SLS printer, you can contact me for payment information.
Cost €5 a piece + shipping & packaging. (I would recommend to order 2, so you have a spare)
Packaging: €1 (padded envelope)
Shipping: €2 (for 2 parts) within Europe and all other countries
Payment fee: €0 using IBAN, €1 using PayPal
As of Oct 2020, I have them available.
Once received, do not forget that you need to melt down the 3 pegs to secure the metal plate (you can also use a drop of glue). I used a small soldering iron.

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Skip to comment form

    • Guillermo Ramos on 02/02/2021 at 10:50
    • Reply

    Hi ! I want to order some 28S Battery Compartment Door. Could you please tell me how to proceed?

    1. Hello Guillermo,
      I will send you an email outlining it a bit more then in the article above.

    • Aurele on 27/01/2021 at 21:32
    • Reply

    Hey Marcel, I just received the two HP28 calculator covers. Really well made!

    Aurele from Ontario, Canada

    • Ruben on 12/01/2021 at 21:29
    • Reply

    Hi Marcel,
    I received my parts today and just wanted to let you know. The color is spot on.

    Thank you
    Ruben – Texas

    • Matt on 15/12/2020 at 21:24
    • Reply

    Hello Marcel,
    today I received my parts and I must say a perfect fit.
    Matt from Seattle

    • Laurent Chavanis on 23/09/2020 at 23:38
    • Reply


    Congrats for the great job !
    Is there still a possibility to order a battery cap for my HP-28S ?

    Thanks and best regards,

    1. Hello Laurent,
      yes you can order them, contact me via the contacts page for payment options. I will mail them in the first week of October 2020, that is if you don’t want to print them yourself.

    • Wyatt Ward on 27/08/2020 at 13:05
    • Reply

    I’d still be interested in a full case; my shell has breaking heat stakes and also has splintering around the battery door, but the cover itself is still intact.

    That said, this is a step in the right direction as well.

    1. Hello Wyatt,
      the spacing between the keys is small. In order to use the inside board again, you can not change a lot. So basically we would need to reverse engineer the case and then print it and use the same method to secure the case together. But if it lasts another 20 years that should be ok. So now all it takes is a 3D-designer to create the case and some funding. 🙂
      You can sign up for the ML if you like. If I continue to do more work, I would notify that way.

    • Barth on 14/08/2020 at 03:33
    • Reply

    Hallo Zusammen,
    das ist ja wirklich klasse! Bestünde vielleicht die Möglichkeit, eine solche Batterieklappe von Euch zu beziehen?
    Viele Grüße Kersten Barth

    1. Hallo Kersten,
      eigentlich schon, Problem ist um den Preis einigermassen günstig zu machen müsste ich 40 bestellen.

    • John Gonder on 22/06/2020 at 10:51
    • Reply

    I’m interested in the STL files for the 28S Battery Compartment Door, when your testing is complete.
    Thank you

    1. Hello John,
      thanks for asking, the modified files are in printing the 2nd time. We’ve made an additional 6 corrections.
      I expect them this week.
      Best would be to sign up for the mailing list HP-Calculators, so that you will be notified of a new posting.

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