You may never look at a milk jug the same way again.
I'd been thinking about using milk jugs as a source of cheap plastic for learning about injection molding and making some plastic tops which might be less likely to break on our hard surface play areas. After harvesting a large piece of aluminum from some scrapped equipment (aerospace engineering has really large experimental equipment), I designed a two part spintop and the molds to inject them based on the size of the aluminum stock. I had planned to do the machining myself but could not pass on the offer of one of the shop guys to make a piece here and there during slow times. About two or so weeks ago, the final components of the molds were completed. Since I haven't finalized the design of the injector and I couldn't stand letting the molds go unused, I tried casting with them in an oven by simply melting the plastic directly in the lower part of the mold and pressing the upper part into the melt and letting the assembly cool. Although there are some problems, it works well enough to make some very playable tops.
Here's a pic of the molds and some of the milk jug tops. The molds are open to show the assembly on the bottom with the additional mold-half stacked above with a formed part installed for demonstration. On the right is one spinning top assembled with tape and its string with milk jug lid button.
One of the problems is the shrinkage of the plastic upon cooling. I knew it would shrink but had no idea about how to design that into the mold so I just designed it to my intended shape. For the cap in the mold on the right you can easily see the gap between the mold and plastic. Another problem is air that gets trapped when the bits of plastic melt. These air pockets can leave blemishes (hard to see in the picture) on the cooled surface and can leave major hidden voids which affect the balance. Because of voids and variable squeeze out the weight is not very consistent between tops but a cap and base are around 60 grams and a milk jug with the paper label cut out is also about 60 grams so 1 jug = 1 top.
The blow molded HDPE (high density polyethylene) of the milk jugs doesn't melt together really well but what works better is injection molded HDPE such as that found in milk crates and 5 gallon pails. These have the added advantage of coming in lots of different colors as demonstrated by some of the tops cast from 2 liter bottle crates, bread crates, pails, and pallets (basically anything with a recycle number 2).
For a size comparison here from left to right are
1. turned HDPE from a melted pail, 2. Spintastics Trompo Grande, 3. Cast HDPE, 4. Duncan Ripcord, 5. Spintastics Gladiator, 6.Mexican 5 Estrellas Atomo
Currently, the tops with the tips in the middle pic are the ones we've been beating around the halls and have had no casualties. Even Aaron the top-destroyer hasn't broken any while learning suns, so you know they've been launched and slammed into concrete walls, pipes, hard floors, and steel door jambs.
