rereading this thread all i can think about is that i wonder how slowing down everything having to do with heat would effect the tops being created... i wonder if stepping down the heat would effect how much the plastic shrinks...
also i wonder if when you start to melt the plastic, if you did it at a slower rate over a longer period of time, i wonder if it'd have fewer air pockets/voids...
and on a somewhat different note: who cares if it makes a thorough proper mix or not: i think you should try mixing higher densities of materials with lower densities of materials... a sponge form built around/within/through a sponge form... if i were you i'd be finding out what happens when i mix different hotglues with the milk bottles and other things you've been melting...
For melting, the most effective way to reduce air trapping is by adding small amounts of material at a time to minimize the trapping events and minimize the material through which any trapped air must travel to escape. Degassing the melt with vacuum before adding more material also helps. Of course injecting a continuous flow of melted material would be better.
For cooling as ta0 commented, the thermal contraction is a function of temperature so the high temperature at which the form is assembled is what controls the amount of contraction experienced. Because my mold is square and not round, the cooling can be nonuniform with the metal corners holding more heat than the narrow sides. Thus stepping the temperature down slowly reduces the variations around the top and helps promote uniform shrinking but does not reduce the amount of shrinkage.
The HDPE plastics I’m using have close but varying densities. A thorough or proper mix of multiple colors helps with a uniform distribution of mass which promotes better balance for the top. If PE plastics of significantly different densities are mixed (like some HDPE with some LDPE), the different thermal contractions result in inconveniently textured surfaces. Mixing different types of plastics with significantly different melting temperatures may result in decomposition of the one with the lower melting temperature when subjected to the high melting temperature of the other one. Additionally the various plastics may not bond to each other.
There are lots of possible improvements to test and think about, however, most of them take more time to implement and increase the time and effort required to make a top. One thing I hope to eventually try -- based on some descriptions from Spinningray about reinforcing his large tops on the inside with (I think) fiberglass -- is to melt the plastic with some type of cloth liner becoming embedded in the plastic.
In general though, my process already requires an absurd amount of time to produce a flawed top which still requires machining to clean it up and make the cap fit the body not to mention the tip. As long as there are great playing tops for $20 (or even $100) available, this technique is a ridiculous way to get a top to play with. I do it because I have more time than brains and get some satisfaction from the process.
Then I found out about the requirement list, and my hopes of ever getting one fell...... I wonder if it would be possible to have an international requirements list... something one can fulfill without having to live in the US? 
I recognize the geographic/economic discrimination created by my requirements to ‘earn’ one of these tops, however, my willingness and motivation to invest the time and effort to fabricate these tops is to provide a token somewhat unique 'thank you' to those spintop enthusiasts willing to commit their time and travel expenses to visiting and supporting top spinning in Ohio, particularly my favorite event in Miamisburg.
