File: Process.doc

Hasling Audio Systems  2007.

October 10, 2011.  ©


Moulding process for Altec 511 and 811 horn.
Part 1, materials and bow filling.
Part 2 Part 3

Necessary materials:

1: Beach sand.
2: Epoxy resin and hardener.
3: Double sided floor tape.

4: Electronic kitchen scale.
5: Disposable plastic cup 5-600 ccm. 
6: 3-4 mm plywood sides for moulding.

7: Wooden stir stick.
8: Measuring glass (ccm).

Start with measuring the front bow volume, to find out how large a portion you must blend in the disposable plastic cup to fill up the bow minus 2-3 mm from the top.  When you pour the resin blend in the bow, remember that the horn must be adjusted horizontal in both horizontal directions, by adding to the hight at the back of the horn  with pieces of wood  and cardboard, to get the frontbows horizontal.

It is prudent to place a big piece of cardboard under the horn to catch any dripping during the process.

The mixing ratio between the amount of resin and hardener is weight, e.g. 100g resin/22g hardener, see the manufacturers specifications.  The volume of this is typical approx. 122 ccm for 122 grams of blend.

Relationship between amount of resin and hardener is weight, f.eks. 100 resin/22 hardener, see the manufacturers specifications.  The volume of this is typical around 122 ccm for 122 grams of the blend.

Altec 811B horn:  450 ccm/bow, totally 900 ccm for both bows.

Altec 511B horn:  1250 ccm/bow, totally 2500 ccm.

Resin/hardener:  E.g. D-Araldite and hardener, 1 kg can + hardener. Ciba Araldite D AY 103 + hardener HY 956 or equivalent product. Pot life 1 hour.

The blend:

Blend 460 ccm mass in this way. The volume of the plastic cup is the following: mark at 450 ccm, but can hold 500 ccm.

  1. Turn on the kitchen scale and reset the scale to zero grams with the plastic cup on the scale.
  2. Pour 100 grams of resin in the cup, add 22 grams of hardener and mix the ingredients well. 
  3. Pour 400 grams of clean beach sand or other cleaned sand, with particle size between 0,3-1,0 mm max, and mix sand and resin/hardener well. When this is done pour another 200 grams of sand in the blended mass, and mix well again, so there is no dry sand at all in the mixed mass.
  4. The mass is now finished, and shall be mushy, not thin.  (depending on viscosity of resin/hardener, should be thin (low number)).
  5. Pour the mass in one bow, the horn balanced horizontally in both directions.  5mm screws placed like shown on my pics, threaded and glued in the horn with araldite or another 2 component glue, so that no resin/hardener can run through here. The screws just coming out parallel to the aluminium surface on the other side inside the horn. The horn should not be moved earlier than 6 hours after the pouring.  The hardening is finished after 12 hours.

Part 2,  will deal with the damping of the top and bottom bows behind, which I strongly recommend also to carried out. The horns will sound beautiful when finished, very neutral. Any ringing is completely gone. When standing just in front of and close to one of my 511B horns, there are no bad sounds in my usual working  range (500 to 6000 Hz), but clean, clear, well defined sound, and fine horizontal spreading.

Hasling Audio Systems, Denmark.

Copyright 2011

Go to part 2