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.

Here the file to download in order to 3D print it. I hope you enjoy the hard work that went into it.
The company I used in Switzerland in order to print the battery cap is Sintratec.

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    • 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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