Thinking Inside Out
The best way I have found to create a mold is to first design the part you intend to cast. Then you build the mold around it. To make the explanation as simple as can be, I made a simple part: a washer. The mold for this is pretty straight forward, but it does cover everything necessary for mold design. It should be helpful to get anyone, at any skill level, started in making molds. I should point out that this isn't exactly a chronological process. Depending on the original part, you could apply some features before, or after others. All have to be taken inconsideration as you design though.
First, Sizing Up Your Mold.
To get started, start with a block with a cavity inside of it. The block has to be big enough to give yourself some material to work with. When the time comes to add clamping and aligning features, the space will be helpful.
I do this by extruding a box over the washer, then deleting the washer to make the cavity.
Implementing Mold Features.
The key features for making a mold work are:
- Split Line
- Feeding Line/Gate
- Air Vent/s
Secondary features are bolt holes and alignment pins.
If a mold is pretty apparent in its design, you can implement a lot of the features before you have to split the mold. It's more convenient to deal with one body, rather than multiple. I added bolt holes before I split the mold as an example (and out of habit).
1. Split Line
The split line, no matter how simple the part, is the most difficult of all to figure out. When incorporating the split, you have to consider a few constraints;
- How will I remove the cast out of the mold without damage?
- How will I inject the material and push the air out?
- And how can hide that ugly split line!?
For the washer I went right down the middle. There aren't any overhangs that deter the mold from breaking apart easily. The design isn't eccentric, so the urethane can be injected from wherever.
2. Inlet and Air Vent Position.
These two features are solved in tandem and have to be in your mind when making your split. It is helpful to have the two run along the split for easy removal.
For the inlet, I like to have the urethane enter the cavity from the lowest point possible, pushing air up and out. This helps reduce bubbles in your part, making it more robust and appealing. When creating the air vent I like to put that at the highest point that ensure I can get air out.
3. Alignment and Clamping.
The requirements for bolts are quantity and placement. There should be enough so that there is an even distribution of load and place bolts close to the part, to ensure optimal sealing. Having the nuts sitting in recessed pockets makes assembly and disassembly a smooth process.
With the room that's left, add alignment pegs to make sure the segments don't move around. You only need two, and they should not have any interference fit.
Printing and Prepping.
For printing on a Form, there are specific procedures that have to be followed for successful printing. Our constraint is; orient the parts so that the mating surfaces have as minimal support as possible.
Keeping those surfaces as clear provides; better sealing, ease of alignment, and less sanding. One other factor we do have control over, is print resolution. The better the mold looks so will the cast.
I hope this quick walkthrough makes mold design a little easier for you. It can be time consuming to make one when first starting out. Even now, if I am making something with complexity, CADing up a mold can take a few hours. This whole process was refined through trial and error, so there could methods for doing this in a better way. For any questions/suggestions please reach out.