Introduction of Polyurethane Moulding:
There is a noteworthy range of choices when it comes to picking material for producing products. Iron, titanium, wood, a wide variety of plastics such as vinyl, polyester, and sheet molding compound (SMC), not to mention more recent materials. However, one element has come to stand head and shoulders above the others in recent years: polyurethane.
Polyurethane is an organic material in both thermoplastic and thermoset forms that are used. Usually, two elements are used to shape polyurethane: di- or poly-isocyanate and polyol. It was first produced in 1937 in Leverkusen, Germany, by Otto Bayer of IG Farben, where its original application was as a coating for military aircraft of the Second World War.
Today, in a wide variety of industries, polyurethane is used, from vehicles and heavy equipment and roller coasters to shopping carts. Although polyurethane first gained prominence for its lightweight and durable nature, it also has a range of additional advantages that make it one of the manufacturing materials of the highest quality.
Benefits of Polyurethane:
In contrast with concrete, wood, and even other plastics in polymers, polyurethane first made headlines because of its unparalleled lightweight. As opposed to conventional metals such as steel and aluminum, some of the mold polyurethane products are up to 60-80 percent lighter. The light design of polyurethane also makes it shapeable and durable, suitable for use in the manufacturing of foam products such as chairs and insulation sheets, as well as elastomeric wheels and tires. A variety of aesthetic benefits often come from a polyurethane mold. It flows much easier into the mold than any substitute material, allowing parts of complicated geometric forms to be quickly made with great performance properties.
Polyurethane Mold Rubber or Silicone Mold Rubber for Mold Making:
You are in the mold making design planning stages and you need to determine which rubber is more fitting for your mold making process: urethane rubber mold or silicone mold rubber? Both have advantages and drawbacks and there are a lot of different options in each of the product categories themselves (for example, within the polyurethane mold rubber line, there are more than 30 individual alternatives, differing among other factors by stiffness, pour time, cure time, viscosity and color).
We will concentrate on three general mold rubber types for the purposes of this article: urethane rubber mold, tin-cured silicone mold rubber, and platinum-cured silicone mold rubber.
When choosing between polyurethane rubber mold and silicone rubber mold for mold making, there are three main concerns that need to be answered.
- What casting material will I use for the products?
- How many cast products do I need to produce?
- What kind of material would my model make of?
1. What Casting Material will I use for the products?
- Concrete- Polyurethane mold rubbers function very easily and are the "go-to" choice for the making of concrete casting resin products. In certain instances, when casting concrete in these molds, a release agent is not necessary, but it heavily relies on the materials of the concrete mix. Polyurethane rubber options range from A20 to D45 Shore Hardness, allowing a wide range of concrete uses, including concrete statuary, type liners, countertops, stamp pads, and much more.
- Wax or Plaster- Polyurethane mold rubbers are a popular choice for wax or plaster casting.
- Polyurethane and Polyester Resin - These casting resin are often cast with the help of silicone rubbers.
- Polyurethane Foam- Silicone rubber is commonly preferred for use when casting urethane foam since it lasts longer than urethane rubber molds and prior to casting does not require the use of a release agent (e.g. paste wax, PVA).
- Epoxy Resin-Silicone rubbers are often preferred to cast epoxy resins since a release agent is not required; however, silicone rubber is easily worn down by the corrosive curing agent in epoxy resin and results in small castings. Urethane rubbers with an effective release agent is, however, often used instead for the making of epoxy resin.
Most of these materials can be cast in either urethane or silicone molds; general guidelines based on common preference are the choices listed above.
2. How many cast products do I need to produce?
Now when you have an understanding of the most common items for casting those materials, quantity is the next consideration:
- Concrete, Plaster & Wax: Hundreds of these castings can be produced in well-designed urethane rubber molds. In this scenario, if you are able to successfully cast a significant amount of urethane rubber molds (Note: silicone rubbers may induce efflorescence in concrete castings), it might not be worth investing more money in silicone rubber.
- Polyurethane and Polyester Resins: Silicone rubbers are typically the best choice for casting these materials, as stated in the previous section. In these molds, upward of one hundred pieces can be cast. If you only need 10-20 cast pieces, you can prefer to use less costly rubber from urethane mold. You will have a small number of castings and a release agent will need to be used. The release agent can be cleaned off if the cast pieces are to be colored, which may be difficult.
- Epoxy resin: Silicone rubber can be an acceptable choice if a limited number of epoxy castings are required and do not need a release agent (which is beneficial if it is important to paint the cast parts). Polyurethane rubber with an effective release agent is usually the best choice if a greater quantity of epoxy castings is needed.
- Polyurethane Foam: can be made up of one hundred foam castings in silicone rubber molds in certain situations.
3. What kind of material would my mold make of?
Some mold rubber materials, when applied to specific model materials, will struggle from cure inhibition (i.e. failure of a compound to cure against a surface within the prescribed cure time). For example, when poured over sulfur-containing clay, platinum-cured silicones will not fully recover. When added to some plastics (e.g. certain plastics used in 3-D printing), polyester resins, certain epoxy resins, and any model that has previously applied tin-cured silicone rubber or latex rubber to it they can also suffer from cure inhibition.
By very judicious management of a sealing agent to the model (e.g., PVA, shellac, stain, Vaseline, PolyCoat), inhibition problems can also be solved. Polyurethane rubbers and tin-cured silicone rubbers are usually not vulnerable to cure inhibition.
Polyurethane molding Process:
As it can take several shapes and sizes, the method of urethane molding continues to change. There are however a few standard techniques that are used in the sector.
The opencast molding requires pouring a mold or a setting of liquid urethane. This is a low-cost alternative that results in a material that is incredibly robust, but it is not sufficient for those models that need precise detailing.
The process of compression molding requires pouring the liquid material into a structure, which is then pushed through a compressor. To make the finished product, this compressor adds scorching heat and incredibly high pressure to the mold. Manufacturers will find that it is the polyurethane molding process that allows for a lot of detail by using this model. Molding the substance using this technique, however, appears to be more costly.
When the substance is inserted into a rotating mold, centrifugal molding (also known as spin casting) takes place. When it is thrown against the sides of the mold, the substance solidifies. This method gets rid of any air pockets and enhances the strength of the material. Nonetheless, this process can be costly and is not ideal for molding very big assemblies and pieces.
Foaming is a procedure where mixed polyurethane is necessary. It is permissible to set and fill a mold with the material. It is a very long operation, but it provides a substance that can endure several tonnes of force.
The process of injection entails firing the substance at a very high level into a mold. The substance fired into the mold is either in the form of liquid or in the form of polyurethane droplets that are hardened. The injection of liquid into the mold provides a wide range of design capacities, but it is only conducive to smaller parts being made. It can also be costly because creation requires more time. The method of injecting hardened droplets can be relatively inexpensive, as the procedure is very fast and can be automated. It is also possible to recycle any wastage accumulated by this process back into the system. Polyurethane molded use of these hardened droplets, though, appears to be of poor quality; it is usually less resistant to intense temperatures and pressures and can be weakened more readily by its composition.
The process of polyurethane molding can vary, and it is vital to choose a system depending on the individual needs of the business. But a more robust variant of other widely used parts is the end product.
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