Posted on: July 23, 2018, | By Candy, WayKen Marketing Manager
Modern developers and designers have high demands when it comes to prototyping. In order to impress potential investors, they need to show them a high-quality impressive product. In some cases, the recent giant in rapid prototyping, additive manufacturing may not be enough to support the required demands concerning surfaces, coloring etc. Injection molding, another efficient way to produce plastic parts of high-quality, is just not affordable to people who are just testing and presenting their design and need only 1or two copies of their product. This is where urethane casting becomes useful.
What is urethane casting?
As the name suggests, urethane casting is a technological method of creating plastic parts by pouring molten polymer into a mold made of silicone. The result of urethane casting is a functional plastic part that can be presented as a model or used in a fully functional product. The process of casting is much easier than injection molding, however, it still has some nuances, one has to take into account. A number of equipment and supplies is required in order to create a high-quality urethane cast.
The process of casting is roughly divided into a number of steps. The first thing you need to know is that in order to cast a part, you need to have its physical representation already. It means, that if you are making a urethane cast, you are making copies of your initial prototype. This initial part is called a master pattern and is manufactured by some other method. Afterward, the master pattern is submerged in a molten silicone bath and left to solidify. The silicone is cut into two parts and so the mold is created and can be used to create cast parts.
Making a master pattern
As it was mentioned earlier, a master pattern is basically the original part or a representation of it. It has to be made by some other method and can be used to create molds for future casting. There are quite a few different techniques for creating such parts. The easiest way would be to use a high-definition SLA 3D-printer and create a rapid prototype. However, this method yields the worst surface finish compared to others because layer-by-layer curing creates steps on the surfaces of the part.
A way to counter such an occurrence would be to process the part further. Some ABS plastics can be processed by using gaseous acetone. Machining can be a viable option in cases where the part does not consist of complex surfaces. Other viable methods for creating master patterns are machining of metal, wood or even plastic. In some cases, using an already existing modified part can be effective.
Molds: materials and manufacturing
Urethane casting molds are considered to be soft tooling. Unlike their injection molding counterparts which need to have outstanding precision, strength, and rigidity because they are always used in dire conditions (high pressure and temperature), soft urethane casting molds do not need to be regulated so strictly. They will be used for not much more than 6-10 parts and then utilized. This fact lowers their price and manufacturing time considerably (the costs may be tens of times lower than those for creating injection molds).
The materials for urethane molds are usually special silicones or casting resins that can be liquefied under normal temperature using some chemical reagents and after that, they solidify, so the process doesn’t require any heating. This fact increases the precision considerably. It should also be noted that not every type of silicone or resin can be used in urethane casting. A typical mold usually consists of two halves with a pattern that forms the final part when connected together. Mold halves need some fixing elements in order to stay in the same
A casting system is basically an arrangement of elements that are required to execute the process of casting correctly. It usually consists of funnel and casting channel, a urethane mold release agent, fixtures to hold the mold halves together and air channels.
So, a funnel and a casting channel are usually simple plastic tubes which are connected to the mold through a hole in it. This is where the material is poured. The higher the funnel is (or the longer the channel) the higher the casting pressure will be and so it will be easier for the polymer to fill all the features of the pattern.
Now, the air vents are situated on the opposite side of the mold pattern (but on the upper half as well) and they are necessary for the air to have somewhere to go when the pattern starts being filled with plastic. They are also useful for understanding when the pattern has been filled completely as the plastic starts to flow upward from them.
A urethane release agent is an oil screen that prevents the cast part from sticking to the mold for any reason (a thin-walled protruding element that is hard to delete because of the friction between the mold and the part is a general example of why the release agent is used. It also makes the part surface a little bit smoother because the screen covers some microscopic holes.
Urethane casting is an important rapid prototyping and low-volume manufacturing method. It allows the developer to present his product to the investors and allows the manufacturer to accept orders for a low amount of plastic parts and still get profits by saving money for the tooling. He doesn’t need to create hard metal injection molds that would be used for 10 parts or so but still obtain high-quality, fine surface parts.
One of the advantages of using urethane for creating such parts is that coloring can be used. Even transparent parts can be colored to give them the desired tint. An abundance of different urethane types opens wide opportunities for manufacturers. They must only have the supplies necessary for casting urethane that was described before. They can create flexible or hard casts, transparent or matted parts, some of urethane plastics even imitate the texture of other polymers, so spare parts are a viable option.