Project 7: Molding and Casting

This week was all about molding and casting. I was hoping to use an CNC mill to make my molds, but the only mill we had access to was the Roland SRM20 for this week. I despise that thing, and couldnt imagine how long it'd take to machine something with the tiny mills. I relied on old faithful instead: the Prusa Mk4. I decided to see if I could make a solf-plastic fishing lure with a swimming tail.

Let's Start with the Group Project

I made little square cups in Solidworks with drafted walls to test the material properties of all the molding and casting materials. The hole in the bottom can be taped over, so that the mold can be pushed out.

I then molded little cubes of the following materials: Ecoflex 30, Ecoflex 50, Dragonskin, Sorta-Clear 37, Oomoo 25, and Oomoo 30. Each are mixed in 1:1 ratios of two materials, and have different working and curing times.

Each material had different stiffnesses. The qualitative ordering was ecoflex 30, ecoflex 50, Dragonskin, Oomoo 25, Oomoo30, softa-clear 37. Mold-Making Material Comparison

Mold-Making Material Comparison

Material	Process	Safety Concerns
Ecoflex 30	Mix Ratio: 1A:1B by volume or weight Pot Life: 45 minutes Cure Time: 4 hours Used for soft prosthetics and flexible molds	Use gloves, avoid skin contact Ventilate to avoid fumes Uncured Component B can irritate skin
Ecoflex 50	Mix Ratio: 1A:1B by volume or weight Pot Life: Shorter than Ecoflex 30 Cure Time: 5 hours	Similar safety as Ecoflex 30 Use protective equipment
DragonSkin	Mix Ratio: 1A:1B by volume or weight Pot Life: 20 minutes Cure Time: 5 hours Used for durable molds, prosthetics, special FX	Requires gloves and ventilation Durable but toxic when uncured
Oomoo 25	Mix Ratio: 1A:1B by volume Pot Life: 15 minutes Cure Time: 75 minutes Quick and simple mold making	Minimal ventilation required Low-hazard, but avoid prolonged contact
Oomoo 30	Mix Ratio: 1A:1B by volume Pot Life: 30 minutes Cure Time: 6 hours	Same safety as Oomoo 25 Good for beginners
Sorta-clear 37	Mix Ratio: 1A:1B by weight or volume Pot Life: 25 minutes Cure Time: 4 hours Clear silicone for detailed visual casting	Use gloves to avoid irritation Ventilation required for fumes

Making the Lure

I started by designing my lure in solidworks by lofting a protrusion between two faces, guided by splines. I kinda just went by what looked right. I also cadded molds, with the hope that I could mill them out.

So we didn't have cnc mill capabilities at Harvard. I sold out to the 3D printers.

I thought it would be interesting to print my lure, smooth out print striations, cast around it, then split that cast to make both sides of the mold. Those sides would then be used for making duplicate lures.

After 3D printing, I sanded the lure.

I wanted to try UV epoxy to smooth out the print. The reef had none, and the people in charge suggested superglue. I made a jank cardboard stand (ensuring to use a screwdrive as intended), then proceeded to trash a paintbrush.

I used my sacrificial paintbrush for two layers, but it still didn't look great.

I then rummaged through materials in the casting room, and found the perfect resin: XTC-3D, which is literally used for filling 3D print striations. It worked pretty well.

Layer 1

Layer 2

I used Sorta-Clear 37 to make my mold, knowing that I'd have to cut it in half. The clear material would best facilitate that process. I started by suspending my smoothed 3D print in the silicone.

I then hacked the cured sorta-clear 27 in two without removing any of my digits.

I elected to use ecoflex 50, and put a tiny bit of yellow die on the end of my popcicle stick before mixing. I casted the material in the mold. I put the mold in a cup to provide circumpherencial pressure

The cure time for this material is 2 hours. I gave it 3, and tried to remove the mold. The silicone stuck to the sorta-clear 37, and the silicone had not fully cured. I was left with goop.