Due to the saturation of industries, the main objective of manufacturers is not only to produce products, but also to use innovative methods to produce more products in less time and thus increase production. As technology develops, several rapid prototyping techniques are available for manufacturers to produce products in less time and with greater efficiency.
These methods not only save time but are also cost-effective. One such rapid prototyping technique used today is urethane molding, which is quite similar to injection molding with minor differences. This technique produces the best functional prototype with the desired properties.
1. Urethane Molding
Urethane molding is a rapid prototyping technique for producing products with soft rubber parts that cannot be machined with a CNC machine or other such machine. Urethane molding uses a 3D printed master pattern and silicone molds to create desired high-quality products up to 30 inches in size.
This process is very similar to injection molding, which uses a hard tool rather than a soft silicone tool. When designing urethane molding, precautions must be taken to ensure it has the desired properties. Urethane molding is a functional plastic part that can be presented as a model or used in a fully functional product.
The dimensions of finished products produced by urethane molding depend on the accuracy of the master model and casting material. In general, products manufactured using this manufacturing technique are expected to have a shrinkage rate of 0.15%.
2. Urethane molding steps
Generally, four steps are followed in urethane molding as listed below
Step 1
First, a master pattern is designed and printed using rapid prototyping or 3D printing techniques. A master pattern is basically the original piece or its representation. This master pattern is also used to create the shapes that can be used in casting. This is designed precisely because the dimensions of the products depend on this master standard. Additionally, there are several editing options that can be used in this case.
step 2
In the second step, the created master model is covered with liquid silicone to create a precise shape.
step 3
After the mold hardens, it is cut in half. The resulting cavity is used to shape the final product.
Step 4
In the final step, the manufacturer injects urethane or another resin to fill the voids. After the material has hardened, it is removed from the tool. This process is repeated until the desired product is produced.
3. Leakage system
A casting system is a system of elements necessary to correctly carry out the casting process. In urethane molding, the molding system consists of a hopper, runner, air channels, urethane release agent, and devices to hold the mold halves together. All these elements are arranged in a certain way to correctly carry out the casting process.
The sprue and hopper are plastic tubes connected to the mold opening. Material is dumped into this opening and flows through the channel and hopper. The larger the hopper, the greater the casting pressure, allowing the polymer to fill all areas of the model.
There are ventilation openings on the opposite side of the mold to allow air to escape when material is poured into the model to make the product. A urethane release agent is essentially a film of air that prevents the casting from sticking to the mold. The release agent is used to remove the protruding element caused by friction between the casting and the mold. It also covers some of the microscopic holes created during casting and makes the cast product smoother.
4. General casting tolerances
For castings, a tolerance of ±0.010 inch or ±0.003 inch per inch of the part is typically expected. Due to the thermal expansion of the liquid and the flexible form reaction, a contraction rate of +0.15% can be expected. Irregular or very thick geometries can cause deviations or deformations due to shrinkage.
The surface finish is externally smoothed to a material or matte finish. Growth lines may be present on internal or hidden features. Polishing or custom finishes must be clearly defined. The shrinkage or surface finish of the product depends on the master pattern design. Therefore, precautions must be taken when designing the master pattern.
5. Material used
Urethane molding is used to produce products with specific physical properties. The material used in the casting process therefore has a strong influence on the physical properties. Therefore, it is suitable for different types of polyurethane plastics, from rigid to flexible. The most common materials used in the process include:
- Rigid, impact-resistant urethane
- Rigid, heat-resistant urethane
- Flexible, clear urethane
6. Color Option
There are two color options in urethane molding, each with its own advantages and disadvantages.
1) Spilled colors
The infused color gives the product a special texture. Furthermore, spilled paint cannot peel off the surface and chipping does not occur. However, the number of poured colors is limited, so the specified color may not be available.
2) Color
When painting the object after casting, there are many colors available, so that the color meets the specifications. Furthermore, when paint is applied after casting, it can remove the lines that remain after the product is separated from the mold, giving the product a uniform surface appearance. However, painting after casting requires subsequent machining work, which increases production costs. In addition, paint can provide good protection against ultraviolet radiation, but it can peel off easily, which is another disadvantage of surface painting.
7. Comparison between injection molding and urethane casting
Injection molding and urethane are generally similar processes. The differences lie in the details of the manufacturing processes.
1) Tool making
The first difference between injection molding and urethane casting is the way the tools are manufactured. Injection molding utilizes a hard tool, as opposed to the soft tool used in urethane casting. Therefore, urethane castings are manufactured by molding a 3D printed model of the finished part, while injection molding tools are processed through CNC grinding, milling, and other processes.
2) Quantity and production volume
Urethane molding is used to produce only a few parts at a time, so urethane molding is preferable for small volumes. For large production volumes, the quality and durability of a tool steel mold generally yields a better return on investment. Therefore, it is more cost-effective to invest in an injection molding facility for high-volume production. If you want to keep initial costs low, urethane molding is preferable.
3) Material costs
The high-quality plastic used in injection molding is more expensive than the urethane or polyurethane resin used in urethane molding. If fewer prototypes are required, it is better to use urethane resins. For products intended for a long service life or for applications with certain hard physical properties, appropriate plastic must be used and the injection molding process must be used.
4) Delivery times
The production of injection molding tools requires more complex and extensive processing and is usually completed after several weeks. A typical lead time of almost two months can be required to produce an injection molding tool. Urethane castings, on the other hand, take relatively less time to produce as they generally only require a 3D printed model and the creation of a soft mold around it.
5) Cost per piece vs. total cost
Generally the cost per part is for cast Urethane are higher than in injection molding, mainly due to the smaller number of parts. However, the overall cost is typically lower with urethane casting because tooling and material costs are lower.
6) Tolerance
The tolerances for urethane are +/- 0.010 inches for the first inch and +/- 0.005 inches for each additional inch, while for injection molding the tolerances are +/- 0.005 inches for the first inch and +/- 0.002 inches for each additional inch. inch was.
7) Applications
Since prototypes are made with silicone molds using the urethane casting process, this is comparatively less durable and will not last more than 20 to 25 pieces. Therefore, this process is suitable for situations where production volume is low and fewer parts with high quality, precision and performance characteristics are required. Some of the applications of the urethane casting process are listed below:
- Bridge to production
- Parts for point of sale displays
- Exhibition pieces
- User rating
- Consumer tests
- Crowdfunding campaigns
- Conceptual models
- Sales Patterns
- Technical models
- Marketing Test Examples
- Pre-series
- Test prototypes
- Rapid prototypes
- Distribution centers
- Press
- Wheels for skateboards, robots and other rotating applications
- Conveyor systems
8. Advantages of Urethane Molding
This method is more effective in reducing tooling costs and production time. Manufactured parts are generally made from urethane or its resin, which improves their physical properties. Parts made from urethane moldings have the following advantages
Abrasion Resistant: Molded urethane products outperform rubber, plastic or metal in high abrasion and wear applications.
Chemical resistance: Products manufactured using the urethane molding process have excellent resistance to most solvents, chemicals, aliphatic-based oils and greases.
Coloring: There are two color options available for urethane molding, each with its own advantages and disadvantages. Peeling-resistant cast paints are generally used.
Dimensional stability : Urethane resins exhibit excellent dimensional stability over a wide range of pressures and temperatures. Products manufactured using this process are highly elastic and can be stretched significantly without deformation.
Environmental resistance: Urethane is largely inert in the presence of ozone and oxygen. It is more resistant to sunlight and general weather conditions than natural or synthetic rubber.
hardness : Urethane polymers can be formulated to cover a wide range of hardness, from 20 Shore A (the hardness of a rubber band) to 75 Shore D (the hardness of bone). The manufactured prototype will therefore have these properties.
Impact resistance: Although conventional plastics can become brittle as hardness increases, urethane maintains its elasticity and strength throughout the hardness range.
Machinability: Urethane products can be drilled, tapped and machined like metal products.
Resilience: Urethane polymers can be custom formulated to give hard polyurethanes elasticity similar to much softer materials, making urethane an excellent shock/vibration absorbing material.
9. Limitations in Urethane Molding
Urethane molding has some disadvantages. When producing complex parts, the designed master pattern may not cover all features. Additionally, it is labor-intensive to remove castings and excess material, which leads to errors. If air bubbles become trapped in the casting, the material will make the casting brittle or not completely fill the thin walls. Another limitation of urethane molding is that the mold is quite temperature sensitive and cannot withstand temperatures above 270 degrees Fahrenheit for long periods of time.
Final considerations
So Urethane molding is one of the most important rapid prototyping techniques used in low production volumes. It allows prototypes to be made and presented before mass production, saving costs and a lot of money on tooling. Additionally, the physical properties of urethane products described above provide an advantage over traditional materials that have less strength and hardness. Therefore, in low production volumes, urethane molding is one of the best manufacturing techniques, very similar to the injection molding process used in high production volumes, which uses a harder tool rather than a soft tool.
