[Jim Estes]

Around my neck of the woods, and every shop I have worked in, the most popular size od MUD insert, is the 08/09 size. This system is extremely versatile. I have built multiple cavity molds to produce many very small parts, or a single cavity mold to produce a very large part. I even have a little trick that I use, to have multiple cavities of different parts in the same mold, but only process one part at a time.

When you buy the insert it comes as a set, the "A" "B" inserts, and usually two plates with no holes (ejector plate and back-up plate). There are many variations, but the basic mold is just that.

The A and B sides will have the pins and bushings, and the two halves will be blanchard ground on some sides and will be surface ground on at least one side. THe two halves will be marked on one corner and will only match up with that corner aligned. You will have to drill bolt holes to bolt the ejector plate to it's back-up plate. and you will have to drill and counterbore the ejector plate for the ejector pins. just a tip for you, always work off of the center of the plates when drilling the holes, the edges of the plates may not be perfectly square or parallel. Be sure that your holes align in the ejector plate and "B" half of the mold. I always use four return pins and I place then at the four corners of the ejector plate. I put springs between my ejctor plates and the "B" half, to push the ejectors back, but that is optional.

Some molders will want you to attach the ejector plate to the mold inserts, you can use shoulder bolts to hold then together and allow then to move forward and back. This helps keep the ejector pins from coming out of their holes, if they come out, you have to unbolt the backup plate to get them back in their holes (a real pain when you have a lot of pins).

With all of the MUD base molds that I build, I have to add a sprue pin, which is just an ejector pin just oposite the sprue. I provide a "puller" which is basically an undercut in the ejector pin hole. This undercut is what breaks the sprue away from the injection nozzle, and holds it onto the "B" half of the mold. The sprue pin pushes the sprue out along with the part and runner.

If you want to have a runner and gate, rather than just injecting the part via the sprue and then cutting off the sprue, you will need to machine the runner and gate accordingly. You can use a trapezoid runner (a tapered channel cut in only one side of the mold), or a round runner (cut with a ball endmill in both halves of the mold) or just a half round runner (cut with a ball endmill in only one side of the mold.
If you have a long runner, you may need ejector pins under this to get it out of the mold. If you put ejector pins under a runner, leave the ejector pins depressed some under the runner, this will keep them from interfering with the flow of material.

There are many different types of gates, everything from just a slot cut to allow material to go straight into the edge of a cavity, to a curved "cashew" gate to send the material underneath the cavity and then up into it hiding the gate. You can use sub terranian gates that trim themselves when the part is ejected. If you use a "sub-gate" then you should put an ejector pins under both the runner and the part close by the subgate, to push the part and runner together so that it trims evenly. Also the ejector pin under the runner side of the gate should be cut so that the plastic flows below the depth of the sub-gate.

One you have the ejector pins, runners and gates, cavity and sprue done. You can then add the water lines. The cooling water is usually the last thing I do on a mold because I would not want to drill an ejector pin hole thru a water line and have water leaking everywhere. I usually plan the water lines along with the cavity, but I don't drill then until after all the details are done. A couple of good rules for water lines is to try and make then the same on both halves of the mold if possible, this makes setup easier for your molder. Make your water lines simple to understand, such as drilling straight thru the mold and tap both sides with a pipe thread. Try and keep your water lines as close to the cavity as possible, without risking damaging the cavity while drilling them. Also, you must make sure that your water lines are accessible while the insert is mounted in the MUD Frame. On the 08/09 Frames, you can't have waterlines in teh lower 3/4" of the insert thicknesses, unless they are on the top of the insert.

Other things you should consider before machining anything:

Do you have enough room in the ejector plate to add pins where you need then under the part?

Do you know which side of the mold the part will stick to when you open the mold?

Where should the part be gated, and how will the material flow thru the cavity and where will be the last place to fill? (this will help you determine where to put venting)

For anyone unfamiliar with plastics, the thickest section the the part takes the longest to cool, so that is what needs the most cooling, consider thinning certain portions of the part that don't have to be thick. These "thinned" areas are made by adding material savers. One example, imagine a wheel, it could be molded as one solid round disc, but if you only molded a round tread and then several spokes to keep it round, it would cool much faster and therefore you save time and money.

Then once you have your mold design and know the specifics of the molders requirements, then you just figure out the fastest and least expensive way to build it.

Good luck with moldmaking.