Molds for MUD system

[mbjorkegren]

Hello,
Does anyone here know what is involved in making a minimal mold using the MUD frame system?
What parts do the injection molders generally have, and what do I need to bring?

I'm under the impression that the molders will have MUD frames, and that you can have a small "insert" mold.

I'm assuming I'll have to provide ejector pins, do I also need to provide an ejector plate, or is that part of the frame?

Thanks

[JavaDog]

I feel like an idiot, when everyone was talking about MUD molds - I really thought they were throw away molds made from mud. Liquid dirt.

[cnctoolman]

You will be basically supplying them with a complete mold. It will fit in the frame system( check to see which style/size they have). I routinely use the 4x4.75 size and like them alot for small pieces.

Basic complete mold includes "A" plate and "B" plate( this one generally having the ejector pins and ejector plate). You can purchase the general bases from mud or as I have done in the past make your own.

If I can help explain it a little better just ask, for I don't know exactly how much you know about them and don't want to offend.

[mbjorkegren]

Thanks for the reply.
I'll try to summarize what i believe i know (I've made wax molds for resin, but never seen an injection mold in person)

1. I choose a plate size, based on the frame system the molder has, and the size of the part.
2. I purchase an A and B plate (each half of the cavity)
3. Ejector pins travel through holes in B plate, but are mounted on a 3rd plate (ejector plate) which I must also provide?
4. All plates come with Leader/Alignment/attachment holes? (I dont have to worry about this stuff?)
5. Plates are smooth, and I put them in my mill and cut out the cavities (no need to suface the plates?)
6. I mill a tapered sprue hole through one of the plates (not sure how I interface with the injector nozzle...)
7. air will escape between the plates?
8. I create cooling channels in A and B plate (somehow)?

Any corrections or explanations you have would be a big help. These are all just vague concepts to me right now, except milling the cavity, which I've done a lot of in wax.
Thanks again.

[cnctoolman]

I am going to try to answer without being confusing so bear with me

1.yes
2.,3.4.,5.,6.,7. you can purchase from MUD will be a basic mold base. It will include the "A" and "B" plates as well as the ejector plates. It will already have the leader pins and bushings installed. What you will have to do is add return and ejector pins. The reurn pins are a positive return on the ejector plate. You can put from 2 being min. and up to whatever you like but I usuallly use 4. The ejector pins are what you will use to push your part out/off with. These should be placed around to where they will push on the part. These can vary in size depending on what is needed where( I usually use D-M-E but other suppliers are available.). You can cut directly into the plates without any further working of them. The sprue hole does not always have to be tapered. It depends on how your runner and gate will be going to your part. It will also be determined on where you will shoot the mold from( either the center or the top as an example.) Don't forget you can also subgate the part if it is big enough. Air will not always escape from between the plates. This is where your ejector pins will come into play. Normally you will ream your ejector pin holes onsize and the ejector pins will usually come in from the manufacturer .0005 under. This allows for air and gas to escape. But this is not always enough, sometimes you will need to "vent" the mold. this is done by grinding a tapered relief on the face where needed starting at about .0005 deep out to whatever. Remember a general rule of thumb is plastic will usually not flash up to about a .003 gap, but this is extreme. The cooling channels or water lines will be done by drilling holes either straight through or connnecting them by drilling 3 hole( 2 one way and 1 cross and capping it off) so you can circulate the water through each side. Promenent placement would be directly behind/around your part but you must also consider where your return pins, ejector pins, cavity and cores lie so not to interfere with one another. Proper layout before cutting is sometimes very essential before making your mold.

Hope this helps out some and let me know if I can try to answer any more questions.

[Jim Estes]

Originally Posted by mbjorkegren 6. I mill a tapered sprue hole through one of the plates (not sure how I interface with the injector nozzle...)

Typically, injection molding presses will have a nozzle tip with either a 1/2 or 3/4 Radius (Ball tip). This presses against a matching radius (dimple) in the mold. Some MUD frames have a sprue bushing with this dimple already in it, which means your tapered sprue hole does not need a radius. However, you must make sure that the smallest diameter of your sprue is larger than the largest diameter of their sprue bushing.

You should check with your molder and see what they are using.

[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.