What is urethane casting? Casting urethane methods 2020 Guide

Very Basic concepts about vacuum casting & its benefits

What is urethane casting?

Urethane casting uses a vacuum chamber to solidify liquid plastic by pushing it into a pre-made mold. The mold is usually made of silicone rubber and is based on the master pattern model.

Table of Contents

1. What is urethane casting?
2. Why Urethane Casting?
3. Vacuum casting and silicone parts molding
4. 3-step process
5. The many advantages of urethane casting
6. urethane casting: how does the urethane method work?
7. Plastic urethane casting
8. Products and uses of urethane casting
9. Rapid prototyping with urethane casting
10. Small series with urethane casting
11. Conclusion on urethane cast

Why Urethane Casting?

Urethane casting is the perfect solution for rapid prototyping and short-run manufacturing at a low cost. The molds can be made inexpensively and are made in great detail, requiring very little post-production for prototypes. Each mold can print about 25-30 sheets and can be produced quickly. 

For short running batches, it is more economical than plastic molding. However, high-quality urethane casting is less suitable for long-term batching.

A little-known technique yet so useful

Unlike stereolithography and sintering, more used in prototyping, vacuum cast allows to create small series with over-molding: inserts, shafts, cables ... In addition, this technique offers the possibility of low volume manufacturing in many materials: food resins, flame retardant, suitable for electrical uses, filled with fiberglass, transparent.

 These different materials used in urethane/vacuum casting make it possible to withstand the various mechanical stresses which, depending on the object, can prove to be significant. The hardness of the parts can also be adapted as needed, which allows the creation of soft or elastic parts. Advantage also on aesthetics: it is possible to color the parts in the mass.

“Vacuum cast is rarely put forward when talking about 3D printing, even though it effectively meets certain needs that are partially covered today by additive processes. 

Vacuum casting and silicone parts molding

The series of parts resulting from vacuum cast, also called silicone molding, result from the creation of this type of mold made from master pattern parts produced in stereolithography.

Tool life depends on the complexity of the part and the resin used for the castings. This can range from 10 to 30 pieces.

It is a process commonly used for the manufacture of small series or pre-series.

Polyurethane resins

There is a wide range of resins that can be used in vacuum casting:

• Transparent
• Soft (elastomers)
• Similar to PP, PA, ABS, PEHD ...

Some of these resins can be colored in the mass.

There are standardized resins for the food industry (solid contact only), medical, electronics, etc.

3 Step casting process

Step 1: the master pattern model, also called a master.

In most cases, it is produced using stereolithography and therefore follows the various stages of this manufacturing process.

The part is cleaned, sanded and / or sanded according to the desired finish on the parts of the series.

Step 2: The tools

The master pattern parts is placed in a tool often made of wood and then silicone is poured around it.

A pause time is then necessary to allow the silicone to harden, then the wooden tools which surround the silicone are removed.

The mold is then cut into several parts following the pre-established parting lines and then the master is gently removed.

Step 3: parts duplication

The mold is closed and is placed in a vacuum chamber. The resin cast is then carried out in this chamber.

Then, the mold filled with still liquid silicone resin is placed in an oven and will follow a baking cycle determined by the nature of the resin and the type of cast.

The operator can then remove the polyurethane resin part from the tool. The part is then deburred and passes the quality control.

In most cases, plastic components are manufactured using the injection molding process. However, due to the relatively large effort and the associated high costs, the process is only worthwhile for a certain number of pieces.

 A suitable alternative to this is the urethane casting process. It comes between 3d printing and injection molding. Moreover, vacuum cast provides a large production rate.

 If a newly developed product is to be tested for its function, a small series is to be produced or components are to be manufactured in small quantities, vacuum cast is in many cases the better choice. The most important prerequisite for production is a master pattern model or a prototype that can be duplicated using urethane casting.

The many advantages of urethane casting

The use of vacuum cast parts in the industry has numerous advantages, especially compared to injection molding that requires hard tooling. Manufacturing companies can choose between a limited variety of materials, ranging from elastic to rigid and from transparent to colored.

 Since the procedure is basically carried out under vacuum, air inclusions are excluded. It means that the resulting plastic part is guaranteed to be free of bubbles with very smooth surface. The vacuum process can also be used to produce molds and components in a wide variety of sizes. Small parts, which are characterized by high precision, are just as possible as very large components.

Compared to the conventional molding process, the urethane casted process enables fast, efficient, and inexpensive low volume production of the desired components and molded parts. With so-called 2K components, two different materials such as soft and hard materials can also be combined during production. 

The integration of inserts (thread inserts, magnets, etc.) into the component is also possible with the urethane casting process.

Below is a brief overview of the main advantages of urethane casting:

• Small series of 25-30 pieces possible by silicon mold
• Inexpensive production of components and silicone mold
• Short-run production possible (approx. 5-8 days)
• High production accuracy process
• Complex geometry possible
• Integrated functions can be carried out
• Smooth surfaces with an erosion structure can be achieved
• 2K parts and inserts can be produced
• UV-resistant depending on the material
• No  hard tooling

urethane casting: how does the urethane method work?

In the urethane casting process, an original form or a prototype is first required, which is to be used as a template for manufacturing the parts to be produced. If the original form is not yet available, it can be produced using various methods such as 3D printed or CNC manufacturing.

 All stable materials that can withstand a temperature of 40 ° C during hardening are suitable as material for the original form. In most cases, these prototypes are made of plastic.

After the prototype has been produced, it is hung in a cast box, which is completely filled with liquid silicone rubber. The silicone is then heated to 40 ° C. This hardens the impression of the prototype. The sample remains in the casting box until it has completely cured.

Fine holes are placed around the prototype, which are used to suck out the air during the later manufacturing method of the molded parts. As soon as the silicone has hardened, a scalpel knife is used to cut a wavy parting line in fine "up and down" movements that penetrate the component. Then the master model is removed from the silicone mold and the silicone tooling is ready for use.

Actual production of urethane cast

The next step is the actual production of the cast part. If the master pattern is removed from the mold, the cavity reproduces the negative shape of the original.

The surface quality or every tiny scratch that is on the cast model will be reflected in the cast! For the short-run production of parts, the cavity is filled with a specific polymer that meets the customer's exact requirements.

 Transparent and opaque parts are just as possible as soft and hard and many other variations. When the cavity of the mold is filled with material, the mold is placed in a urethane chamber in which the vacuum is created.

The silicone tool and the 2 components of the desired material are placed in the vacuum chamber. The chamber is then placed under a vacuum and the two components are stirred together.

 The liquid plastic is then pressed into the mold by the negative pressure, which enables the exact reproduction of the original shape.

The mold, including the cavity (provided cavity), is then placed in a curing oven, in which the produced part is cured.

The dimensions of the molded part depend on the precision of the original type, the plastic and the part geometry. A shrinkage rate of 0.15% is considered normal.

By varying the mold temperature, the silicone mold can be made larger or smaller depending on the shrinkage of the plastic part (silicone grows or shrinks under temperature)

urethane casting material Shore D and Shore A - colored through

When choosing the material, the surface structure and the hardness of the later form or component play a decisive role. Depending on the application, transparent, translucent, or opaque parts can be manufactured.

There are basically two different materials, each of which has its own advantages. The Shore A material, an amber-colored rubber that is usually colored black, has an elongation at break of up to 400% as a cast component. Typical areas of application include models and prototypes, hoses, pipes or seals.

The second material Shore D is a transparent material that can be colored in a targeted manner. It can also be used to produce transparent components. The disadvantage of Shore D is its poor elongation properties.

With a hardness of 35 to 100 Shore A, all types of soft tissue can be produced in 5 Shore steps. From vehicle seals to bellows to housing seals, all component variants can be produced with this. Flexible parts such as seals or hoses are also optimally produced with the Shore A material and can be produced in a wide variety of colors.

Plastic urethane casting

In principle, many different materials can be used for the production of urethane cast parts. Depending on the material, the production of unbreakable to flexible and rubber-like parts is possible. Food-safe and chemical-resistant molded parts can also be produced. The temperature resistance ranges from -50 ° C to 200 ° C.

The material most commonly used in urethane casting is plastic. The reason for this is the positive properties of the synthetic resin. In principle, different cast resins are available for the process. 

This enables the production of:

• Hard
• Soft
• Transparent
• Clear
• colored
• rubber-like and glass fiber reinforced part. 

The materials are heat-resistant, food-safe, and chemically resistant, among other things. Complex and filigree models can be produced particularly well with vacuum casting.

Geometries that cannot normally be demolded by means of conventional manufacturing methods such as injection molding or RIM due to undercuts can be demolded using the flexible, rubber-like silicone mold.

After the component has been manufactured, the surface can be post-processed in a variety of different ways. From painting to polishing and drilling - milling to screen printing, there are numerous options with which the vacuum casting can be optimized in order to ultimately meet the requirements of the customer.

Products and uses of urethane casting

The areas of application for the urethane methods are broad and are used in almost every industry. From small series or pre-series technical components to samples for trade fairs for customer presentations to the post-production of components for vintage cars. 

Due to the large variety of materials and the numerous options for surface finishing, precise and targeted low volume production is possible with urethane casting. If plastic or rubber parts are required that should be of high quality, the casting process is used. In summary, the urethane method is used for the following areas:

• Prototypes
• Pre-series as a market test
• Small series for the first marketing
• Spare parts that are difficult to obtain
• Design models and exhibition models
• Functional models
• Precise prototypes
• Components for Aviation (Aviation Materials)
• Multi-colored components

Pilot, pre-production, and small series

The urethane casting process is particularly suitable for the production of test, preliminary and small series. These are produced, for example, for marketing purposes or trade fairs and get early feedback directly from the customer first step to avoid hardware startup failure. But prototypes are also often used in the technical area for assembly, construction, or function tests.

Serial production

Especially in the food industry, aircraft construction, automobile construction, and medical technology, products are manufactured in series using the urethane process. Typical products made with vacuum casting include:

• Seals
• Hoses
• Displays
• casing
• Disguises
• Covers

Rapid prototyping with urethane casting

Rapid prototyping means something like "quick prototype construction". Rapid prototyping plastic parts that are manufactured with the vacuum cast process are characterized by very high quality and can be manufactured within very short production times.

 When creating a plastic prototype, air is extracted from the mold with the aid of a vacuum, which creates the model that was previously created on the computer inside the mold.

Such prototypes are used as a test or test object before series production or as a model for illustrative purposes and a variety of other areas of application.

In rapid prototyping, the urethane casting process enables cost-effective and fast production of plastic components. In order to be able to produce a prototype using the vacuum process, a master pattern model must first be produced. 

The basis for the master model is formed by 3D CAD data or printed models from stereolithography. First, the prototypes are designed on the computer. 

The production of large quantities of unusable molded parts and components is no longer an issue.

 Low Volume with urethane casting

Once the prototype has been produced, and inexpensive preliminary or small series can be produced using the casting urethane process. The prototype is reproduced true to the original. Production companies can have cast parts produced in small batches of around 100 parts and carry out appropriate visual or mechanical tests. 

Fine details can be worked out and the molded part optimized. With coatings, lacquering and much more, the parts can be viewed, tried out, and also presented at trade fairs or in market surveys not only on the computer but also in its real shape. 

This gives the company valuable suggestions and suggestions for improvement. Pre-series are also required for product certification. Due to the high economies of scale of prototypes, casting urethane is the first choice among the various production processes for quantities over five. From a number of five molded parts, the relative costs for the tooling steadily decrease.

Conclusion on urethane cast

If a new product is to be tested for its appearance or function, urethane cast is the first choice among the various processes. While the injection molding process is associated with relatively high production costs for small quantities.

Prototypes and small series can be produced cheaply and still in high quality with the urethane casted process. 

In addition to the low production costs, the possibility of short-term production, the high production accuracy, and the option to produce components with high complexity or integrated functions are among the most important advantages of the process.  

Almost anything is possible with the urethane casted process, from transparent to coloring in a wide variety of colors. 

“3D printing has been talking about it for years now. However, if stereolithography or sintering no longer hold any secrets for many people, vacuum cast is less well known to manufacturers and the general public, although it brings many advantages over these technologies.”

Vacuum casting is a fast manufacturing technology, admittedly less imposing than 3D printed and all of its machines. The first step in vacuum casting is the manufacture of a model, usually printed in stereolithography to obtain a detailed part. Rarer, it is also possible to obtain this model via an already existing part. 

From a single mold, it is possible to produce up to 25 highly detailed parts in record time. In the long term, this, therefore, allows a significant reduction in costs.