Posted on: July 27, 2018, | By Candy, WayKen Marketing Manager
Product developers will inevitably reach a moment where it is time to make a small batch of prototypes for testing. Or perhaps the whole size of the batch is small. In any case, making a steel injection mold is just not a good option when your whole batch is just 50-500 parts. What other options are there? CNC machining may be viable, but it just isn't really suited for plastics, you'd have to lower the feed and the time for cutting would be too great. 3-D printing? Takes too long and the surface finish is not so great, so you'll have to post-process the thing a lot. Now, there is only casting left.
Urethane casting or vacuum-urethane casting a small batch may become a good solution. But first, let's look at what the process itself looks like.
How It's Done
1.Making a Master Form
Urethane casting is based on copying an initial part. It is called a master-model. The quality of the cast solely depends on the quality of the master model, so you should aim to manufacture the master model with the highest surface finish you can. The master model can be of any material that won't come into a chemical reaction with the molded silicone. Your best options are other neutral plastic, metal, wood. The manufacturing method depends on the material but generally, the best way is to get a wood or aluminum stock and CNC machine it. Another good way that has become popular in the last decade is 3D printing. You should note, however, that some printing techniques won't suit urethane casting because they won't yield a good surface finish. The best options are SLA and jet printing. After the form is done, it's time to make the mold.
2.Setting Up The Mold
With a small exception, all molds consist of two halves. You have to somehow get the part out, right? Well, that's how it works: you put two silicone molds with cavities that mimic the manufactured product. In order to create the mold, you have to fill the part in silicone and the part has to be in the middle of the tank so that you can make two halves.
Usually, the master model is hung over the tank on a thin shaft. When the tank is filled and hardened, the shaft trace will serve as an inlet hole. As it stands, the model and the whole are not enough to complete the mold. Some more holes must be made to let the air out during the pouring process and to indicate that the whole volume has been filled. Some other thin shafts are added to the system to provide the holes. Now, the mold tank is ready to be filled with liquid silicone.
3.Manufacturing the mold
First, you have to mix the two parts of the silicone so that it will harden in time. Then, the liquid is poured in the tank and the whole thing is put into a vacuum chamber. Vacuum gets all the air bubbles out of the tank while the silicone is still liquid. Otherwise, the mold will have small defects that will ruin the mold and the part.
After the mold is done, you cut it into two parts and get the master form out. There, it's done. Time to cast.
4.Casting and Curing
Mold hardening takes a few hours and then you can cast the parts. The process begins with fixing two halves of the mold firmly together and getting the mold set up in a vacuum chamber. After that, the air is pumped out and an automatic device (you can't pour the plastic yourself in the vacuum, right?) pours the liquid material into the mold until all the small channels you've done in stage 3 start leaking. That's when you know that the mold is full.
Right when the last of the plastic is in, the vacuum is turned off and as the air rushes in, the pressure gets higher and the air pushes on the plastic so that it fills the mold completely. Now, some more time for the plastic to solidify and the product is done. Just get it out of the mold, cut off all the inlet and outlet additions and cast it again.
Why Use Urethane Casting?
Now that you know how the thing works, let's look at the reasons why it's a good way to manufacture prototypes or small plastic batches.
Urethane casting is like injection molding on a budget. It's true. The silicone mold costs at least ten times less than a press-form for the injection molding process. Sure, it will last 10 or maybe a 100 times less too. Average silicone mold is good for 20-500 uses. It can't compare to the steel one but you just don't need that many copies yet. So, why pay more?
The strength of the part is the same as that of injection molding or machining and each part cost is much lower.
Abundance of materials including clear and biocompatible. There are a lot of different polymers that can be liquified to be cured later at low temperatures. Among them: hard and strong plastics, flexible, transparent clear ones, and even some biocompatible polymers approved by the FDA.
Why VACUUM Urethane Casting, Then?
Now, you may ask, why use the vacuum version when it is literally possible to just buy the plastics and do all of that at home. Here is why.
Vacuum ensures that there are no air bubbles. The main issue with urethane casting is that when the plastic is poured into the mold, there is air in it. It will get trapped inside and your part will have cavities. However, the vacuum chamber lowers the possibility of the bubbles forming and their size up to a microscopic level.
When the vacuum is released, the plastic fills the mold better. So, when you release the vacuum, the atmospheric pressure becomes normal in the chamber or, in other words, increases. So, the polymer is pushed further into the mold after having been in the vacuum.
What to be careful about
As always, there are some cons and considerations in every manufacturing process and that involves vacuum casting as well.
You need an initial master model manufactured separately. That is a con to be considered as you will need extra time and expenses to manufacture a first example. After all, the process is just copying the original.
The mold has a limited number of uses. Depending on the part size and material, the mold may last for 20 parts. In other cases, it may last a hundred. You will have to make another one to finish the batch. It is best to choose the silicone for the mold in a way that ensures either one mold for the whole batch or an even distribution of the parts between the molds.
Surface finish is only as good as the master. With urethane casting, the end quality of the part spends solely on the quality of the master model, so, you will have to put even more effort in creating it.
Some plastics can't be used. The process depends on the material of the part not reacting with the mold. A lot of plastics will glue to the mold and that's the end of your casting process.
Well, that's about everything you have to know to decide whether urethane vacuum casting is a choice for you.