Process Notes - Post Pour to Mold Release

To provide some insight into the painstaking process required to produce a high quality Edison cylinder brown wax recording blank, the following notes describe some of the important considerations in the portion of the manufacturing process after pouring the wax into the mold until mold release:

It has been found that after the casting is poured into the preheated mold, that the best results are obtained from slow cooling in still air. This wax has the peculiar properity of being extremely sensitive to any jarring or any tension or compression stresses put upon it while it is in a plastic state.

Tension, compression or sudden jarring creates internal casting defects, which cannot be seen by looking at the shaved surface, but can be heard as noise when played over with the phonograph reproducer.

Any attempt to speed up the cooling of these castings has been found to be problematic. In fact, cooling in still air for one hour at an ambient temperature in the range of 80 to 90 degrees Farenheit has been shown to work the best. At the end of that initial hour of cooling, the tapered mold core is loosened.

Thirty minutes after it has been loosened, then the exact time for the removal of the mold core is determined from a table of delay times which is the result of experience. If the ambient temperature is 70 degrees F, the core is pulled 1.5 hours after the casting has been poured.

The best time to pull the core out is the latest possible time, right before it starts to bind. If the core is pulled too early, then the casting is still so soft that it contracts and becomes very out-of-round.

Seventy degrees F is the lowest practical ambient temperature in which casting can be successfully accomplished with this process. The delay table is as follows:

<p">70 F is a delay of 0  (core pulled 90 minutes after pouring)

75 F is a delay of 1 minute

80 F is a delay of 5 minutes

85 F is a delay of 10 minutes

90 F is a delay of 15 minutes

95 F is a delay of 20 minutes

Interpolation is used for temperatures falling between these listed points.  Casting in an ambient temperature above 95 F can be done, but there are problems associated with it, such as core release being delayed so long that the casting releases from the mold cylinder and comes right out with the core when the core is removed. Then, once it is out, it suddenly cools being in the open air, and so binds on to the core. That is why the table stops at 95 F.

Below 70 F, rotary noise becomes a problem.

The casting must be free to move about as it cools inside the mold cylinder.  It must not bind to either the mold core, or the mold cylinder, because if it does stick and bind, that causes internal stresses to be set up in the wax as it cools, and that will show up as noise in the recording blank.

This is why a mold-release compound (graphite) must be used to coat all surfaces of the mold before the wax gets poured in. The graphite must be applied very lightly and rubbed thoroughly on to the metal surfaces and burninshed in to the microscopic pores of the metal, leaving a slick surface for the cooling wax to combine with so that it is free to contract without binding. If any amount of free graphite is left on any part of the mold, it gets in to the wax and creates inclusions that can be heard when played over with the phonograph reproducer.

The above paragraphs are meant to illustrate the need for precisely controlled cooling of the mold after the casting is poured. The timing for mold core removal is a variable which depends upon the ambient temperature. The stresses that the wax is subjected to during cooling must be managed and minimized as much as possible.

For a summary of the subsequent finishing steps, see the video on this site