What are the specific processes for vacuum casting?

Vacuum casting is similar to injection molding in that it requires a tool with a cavity with the final shape of the component. The main difference lies in the use of "soft" molds made of silicone for vacuum casting, while "hard" metal molds processed by CNC are used for injection molding. Therefore, the disadvantage of vacuum casting is that the mold wears out faster. However, for small batch prototype plastic components that require production grade quality and surface finish, it is more cost-effective.

The vacuum casting process has three main steps:

Mother mold: In the first step, the mother mold of the final component is made using additive manufacturing technology. Polyjet 3D or Stereolithography (SLA) 3D printing processes are the most commonly used methods because they can produce high-resolution and naturally smooth surface components. The master mold is usually manually completed before creating the mold tool to achieve optimal surface details.

Forming: In the second step, the 3D printing master mold is wrapped in liquid silicone, which solidifies around the master mold and encapsulates all the features of the printing component. When the mold solidifies, cut it into different halves and remove the mother mold or 3D printed components. So as to leave an inner cavity that is exactly the same shape as the component.

Pouring: In the final step of the vacuum casting process, liquid polyurethane or silicone is poured into the silicone mold, and then the mold is placed in the chamber to help remove bubbles from the liquid material. For opaque components, the chamber is usually pressurized. For transparent components, the chamber is usually vacuumed to reduce bubbles and increase transparency. After curing, separate the silicone gel in half and remove the newly formed part. Repeat this process until the required quantity is reached.

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The difference between vacuum casting and Reaction Injection Moulding for prototyping method

Vacuum casting and Reaction Injection Moulding are common methods for hand board processing, and their principles may appear similar on the surface. The original can be made through CNC machining or 3D printing. For more complex structures, the original can be made through 3D printing, and then a silicone mold can be made using the original. Then, batch replication can be carried out using this silicone mold. However, there are still some differences between the two methods. Below, we will analyze the differences between vacuum casting and Reaction Injection Moulding in prototype processing.

1、 Vacuum casting

Vacuum casting is a common processing method that uses the original template to create a silicone mold in a vacuum state, and uses PU material for pouring in the vacuum state, in order to clone a replica that is the same as the original template. It is mainly used for small batch customization of plastic prototypes, usually in the tens to hundreds of plastic prototypes. If this method is used for processing, it is more cost-effective.

The material properties of vacuum casting products can simulate the performance of most existing engineering materials. Common materials include domestic PU, imported PU, transparent PU, soft PU, Saigang, ABS, PP, PC high temperature resistant ABS, etc. Transparent parts such as PC, organic glass, thermoplastic elastomers such as TPR, TPR, TPU, and various soft materials such as silicone and rubber can be replicated. At the same time, various colors can be added to the replicated product to meet various appearance requirements. Product replication is not affected by volume, complexity, or various structures, making it easy to mold, with a short cycle and fast results.

2、 Reaction Injection Moulding

Reaction Injection Moulding, also known as low-pressure reaction injection molding, is a new process applied to rapid molds. It mainly involves mixing two component polyurethane materials and injecting them into the rapid mold at room temperature and low pressure, forming products through chemical and physical processes such as material polymerization, cross-linking, and curing. It has the advantages of high efficiency, short production cycle, and low cost. Suitable for small batch trial production in product development process, as well as the production of small batch, structurally simple covering parts and large thick and uneven wall thickness products.

The European and American countries that use Reaction Injection Moulding the most are relatively more mature in terms of materials and infusion technology. China was the first to use Reaction Injection Moulding, which was suitable for the automotive industry. In recent years, there have been relatively more Reaction Injection Moulding processes used in the production of medical products and instruments in China. As the production quantity of instruments and medical products is not large, the annual sales quantity of a product is only tens or hundreds of sets. If steel molds are used for injection molding, the cost is relatively high. There are also many medical products with relatively thick or uneven wall thickness, which are not suitable for injection molding processes. The surface curvature is complex and not suitable for vacuum forming processes. However, Reaction Injection Moulding is different. Thick wall products can be injected, and the wall thickness can be uneven, which is normal. The maximum thickness of the perfusion wall (5-6mm) can be around 30mm, There are many materials used in the mold (such as aluminum mold, resin mold, plastic mold, gypsum mold, etc.), and there are many choices of injection materials, including imported materials, domestic materials, materials close to ABS, and materials close to PP.

Summary of differences: Vacuum casting is mainly used to replicate small volume prototypes (such as ABS, PC, acrylic hand boards, etc.) in small batches, while Reaction Injection Moulding is used to replicate large volume prototypes, such as car bumpers, medical instrument case shells, instrument panel hand boards, etc.

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Common problem solutions for vacuum casting

 1、 Dimensional deviation and product deformation

Cause and solution: It is closely related to the production process of re molding, especially the temperature control of the mold. Other reasons such as the scaling ratio of the prototype and material shrinkage should also be well controlled. Products with a length exceeding 1.5 meters need to be made using reinforced molds

2、 The products produced cannot be assembled

Cause and solution: The drawings of product engineers are set based on the injection molding process of the mold, often without considering the difference between the re molding process and injection molding process. Due to the vacuum adsorption molding process, the precision of the product can be controlled within ± 0.1mm. For assembly products with higher precision requirements or hand tightness requirements, small batch production engineers need to make adjustments to the replica prototype, combined with post-processing assistance, to meet the assembly requirements

3、 Finished product becomes brittle and cracked

Cause and solution: The main reason is the use of domestically produced inferior materials and defects in the replication process. The first solution is to ensure that the channels for material procurement are guaranteed. If there are no problems with the materials, improvements should be made in the process, and attention should be paid to controlling the oven temperature, heating time, cooling time, cooling method, etc.

4、 Bubbles and hollow spaces

Cause and solution: The generation of bubbles in the replica product is related to the reaction between the silicone mold material and curing agent. When vacuuming, it is necessary to control the time, as too long or too short a time can easily produce bubbles. Additionally, it is related to the product structure and the position of the feeding port. The generation of bubbles on the surface of a product can greatly affect its appearance and material durability, so it is necessary to strictly follow the manufacturing process to effectively avoid them

5、 Lost product details

Cause and solution: In the production process of some relatively slender products, it is easy to lose details. The main reason is that the silicone mold is not baked well and is prone to deformation, resulting in the loss of details. In addition, during the process of making molds, damage to the prototype of the product directly leads to the loss of details in the replicated product. In addition to improving the process, it is necessary to carefully inspect every step of prototype polishing, mold making, and finished mold making in order to find a solution.

6、 Rough and uneven surface

Cause and solution: The quality of the replica product is closely related to the prototype, and it is necessary to improve the surface quality of the finished product. Firstly, the production process and surface effect of the prototype should be improved, and the prototype can be made through CNC. Additionally, the use of domestically produced inferior silicone can also lead to rough and uneven surface of the mold. The solution is to use imported silicone molds for production, select the most suitable process to make prototypes, and ensure the hardness, surface, and compatibility between materials.

7、 Color fading and surface orange peel after coloring

Causes and solutions: The discoloration problem is related to the flatness of the product surface, the selection of primer and topcoat, and is a problem involving multiple reasons. The main solution is to choose a primer that is compatible with the material, such as a two-component primer and automotive specific paint. Good paint can not only improve the surface effect of the product, but also increase its aging and corrosion resistance functions.

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Vacuum casting: a time-saving and labor-saving way to produce small batch parts

 Vacuum casting,also known as Urethane Casting or Silicon Mold, is a process carried out in a vacuum environment. It involves defoaming, stirring, preheating, and injection molding of the castable, followed by secondary solidification molding in a constant temperature box. Through this method, the produced plastic parts can achieve similar strength and hardness requirements as ABS and other raw materials.

This process is mainly applied in the development of new products and small batch production scenarios, as it can produce plastic parts with performance similar to that of injection molded products. In addition, silicone replica also allows for color matching to meet diverse production needs according to customer needs

Using materials

ABS、PP、PC、 And soft glue, etc

Technological advantages

Silicone replica is an ideal choice for small batch high-quality plastic handboards. It can not only quickly verify the feasibility of product design, but also help customers achieve the fastest and most economical customization of small batch components when the quantity demand cannot reach the steel mold.

a、 Significantly shortened the product development cycle. By adopting replication technology, product prototypes can be rapidly formed, thereby accelerating the testing and validation process. This not only saves valuable time resources, but also improves research and development efficiency, enabling products to be launched to the market earlier.

b、 Effectively reducing the costs of product development and manufacturing. Compared to traditional manufacturing methods, re molding can reduce the consumption of raw materials and identify and correct potential issues in the early stages of product design, thereby avoiding expensive modification costs in the later stages.

c、 Helps design flexibility. By creating realistic product prototypes, designers can more intuitively evaluate the appearance and performance of the product, and then make necessary adjustments and optimizations to the design. This not only improves the accuracy of the design, but also enhances the market competitiveness of the product.

d、 High degree of restoration and high product accuracy. Can accurately replicate the structure, details, and texture of the prototype, providing high-quality and high-precision injection molded parts.

process flow

Create prototype parts

Based on the provided 3D drawings, prototypes are manufactured using 3D printing technologies such as SLA and DLP, which can preserve complex shapes, structures, and surface precision to the greatest extent possible, greatly reducing the time from design to prototype, enabling designers and engineers to verify the feasibility of the design more quickly, and iteratively improve based on feedback.

Making silicone molds

After manufacturing the prototype, manufacture the mold frame, fix the prototype, pour silicone, wait for drying, remove the prototype to obtain the silicone mold, and the silicone mold manufacturing is completed.

Note: If the mold is large or complex, it may need to be done carefully and slowly. Clean the silicone mold to ensure its surface is clean. If necessary, the edges of the mold can be trimmed or trimmed.

Vacuum casting

According to the needs of the produced product, prepare appropriate pouring materials such as resin, wax, or concrete, and slowly and evenly inject the pouring materials into the silicone mold to ensure that the entire mold is filled. Wait for a certain amount of time to cure or harden according to the material requirements. After the pouring material is completely cured, carefully remove the prepared product from the silicone mold.

Attention: After 30 to 60 minutes of curing in a constant temperature box at 60 ° -70 ° C, the mold can be demolded. If necessary, secondary curing should be carried out for 2-3 hours in a constant temperature box at 70 ° -80 ° C. Under normal circumstances, the service life of silicone molds is 15-20 cycles.

Process application

a、 Plastic prototype: Its raw material is plastic, mainly used as a prototype for some plastic products, such as televisions, monitors, drones, hair dryers, and so on. The most common photosensitive resin used in 3D prototype making belongs to the plastic prototype category.

b、 Silicone replica board: Its raw material is silicone, mainly used as a board to display the design appearance of products, such as cars, mobile phones, toys, handicrafts, daily necessities, etc.

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Vacuum Casting VS 3D Printing

Perhaps people may wonder: why do 3D printing products still need to be vacuum replicated after production?

The current mainstream processes for 3D printing are SLA (light curing) and SLS (selective laser sintering). The surface of products produced by SLA technology is smooth and delicate, but the strength is low and the toughness is poor; The products produced by SLS technology have good performance in terms of strength and toughness, but their surface is rough and lacks good surface smoothness.

At the same time, 3D printing adopts a layer by layer manufacturing method, which means its density is low and its strength is also low. However, the vacuum lamination process uses injection molding method, and its density is relatively high, so its strength is naturally high. Furthermore, there are various types of materials for vacuum replica products, such as transparency, ABS like, rubber like, color matching, etc., which means that one mold can be used to produce various types of products. Choosing 3D printing to produce products is just one way to make silicone molds faster. Often, 3D printed products cannot better verify their performance and structure. Therefore, we can choose vacuum replication technology to obtain our ideal components.

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Vacuum Casting VS CNC machining

 1. Low cost and relatively short product production cycle. There is less waste, and the processing cost is much lower than CNC machining. Due to its fast speed and low cost, it effectively solves the cycle and cost of new product development for enterprises.

2. Suitable for small batch product processing and production, once an original version is made, it can be copied based on the original version, while CNC machining requires machine tools to make hand boards one by one.

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Benefits of Urethane Casting

 Speed

The process of urethane casting is quicker because the silicone molds utilize to manufacture parts through this process are created very rapidly, particularly contrasted with the hard tooling utilized in injection molding. For low-volume production, it is perfect, when amounts do not explain investment in tooling of the injection mold.

If the strategy is to injection mold a huge amount of parts; however some are required instantly, at that point urethane casting is a quick technique to acquire first-run parts made before the creation tooling is prepared.

It’s splendid for testing item structure for one-off items or for testing consumer responses or business market to the new item. Urethane cast parts can take a project to its higher level with such a rapid improvement time short-run production or for end-user testing. It can give an organization a first-to-showcase advantage.

Cost

The cured and liquid poured silicone molds utilized for projecting urethane are less expensive to produce contrasted with making injection mold “hard” tooling.

To place it in context, a casting project may be many dollars, contrasted with the huge number of dollars it would be for a comparable project of injection molding.

Urethane casting, CNC machining or injection molding are the best way of producing parts that will be determined by part quantity. Urethane casting provides a great value for low-quantity parts and prototypes because the price is very less; however, the quality of the parts is superior.

Quality

The process of cast urethane can utilize a range of preparations to make parts that are hard or soft, transparent, or colored. The molds of silicone can accurately mimic manufacture textures and offer excellent surface finish detail.

Additionally, parts can be over-shaped, have post-process treatments like artistic creation, and have inserts fused into them.

With injection molding, Urethane casting makes pieces with physical properties on the same level, and the parts are physically higher to their 3D printed complements. Regardless of nonstop improvements of 3D printing, it still has structural and material integrity restrictions.

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Advantages of Vacuum Casting

 1. Fast turnaround

We can provide up to 20 parts in 15 days or less, depending on part specification and volume.

 

2. Affordability

Silicone molds are less expensive than the tooling used for injection molding, resulting in low prices.

3. Capable of producing large parts

Depending on the type of equipment used, vacuum casting can to create very large parts.

4. Superior surface finish to injection molding

The vacuum process removes air bubbles and allows the material to capture fine details.

 

5. Color options

Coloring pigments can be added to the resin for a variety of color options.

6. Repeatability

Silicone molds can be used around 20 times before they need replacing.

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The advantages of silicone mold

 ① Short Manufacturing Period

Once you provide the 3D CAD files of the parts, we can produce the silicon rubber mold in a few days, with which we can cast the polyurethane plastic products. Compared with the manufacturing period of the traditional steel mold (30~90 days), it can shorten 90% of the time.

② Easy to Demould

We usually need to set up the draft angle in the traditional metal mold, in order to demould easily after forming, especially for the parts with the deep grooves. Considering the silicone rubber has the good flexibility and elasticity, the parts with the complex structure, fine pattern, no draft angle or a deep groove, can be removed directly after casting, which doesn't have to be added the draft angle, so as to simplify the mold design as much as possible.

③ Good Replication Performance

The room temperature vulcanized silicone rubber has an excellent simulation and a very low shrinkage rate, which can well reproduce the fine features of the prototype parts and basically won't lose the parts’ dimensional accuracy. It is an excellent mould material.

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What is Silicon Mold?

Silicon Mold, often called Vacuum Casting, is a manufacturing process used for rapid prototyping and low production runs. The silicone mold is usually applied in the new product development stage, and a small batch (20-30pcs) of samples’trial-manufacture. 

The silicone mold with good toughness can manufacture the parts with complex structures, fine patterns, no or even negative draft angle. The mould manufacturing period is short while the products' quality is high .

We generally use the silicone rubber and the reactive polyurethane materials to make the silicone mold . It is a popular technology of rapid tooling manufacturing.

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What is the Urethane Casting Process？

 Urethane casting, often called Vacuum Casting, is a manufacturing process used for rapid prototyping and low production runs. The urethane casting process involves pouring liquid urethane material into a silicone mold, followed by curing in an oven, and then removing the plastic part from the mold once it has cooled. Secondary operations commonly used in production settings can also be performed on the cast parts if desired. Urethane casting is ideal for low-volume part runs and prototypes. The silicone molds do not last long and will typically only cast about 20 parts. The silicone mold is easy and relatively inexpensive to make, giving urethane casting a unique advantage over other plastic part manufacturing processes like production injection molding in low volume scenarios.

The first step in the urethane casting process is to create the mold from a model or master pattern. Master patterns can be created with CNC machining, 3D printing, or by using an already existing product. The pattern is encased in quick-curing liquid silicone. After the silicone has cured, the mold is cut in half and the model item is removed to reveal a negative. The negative formed by the master pattern is then used for casting the end product.

The silicone mold material is excellent at picking up surface detail and duplicating the surface finish of the master pattern onto the cast urethane parts. The finished dimensions of the urethane cast parts will depend on the accuracy of the master model, part geometry, and casting material.

Vacuum, heat, and pressure are applied during cast urethane molding, resulting in a strong, stable, void-free part. Cast urethane parts are a good choice for quickly producing high quality, precise, and consistent work pieces. Despite the advantages, the urethane casting process can only be used for producing low-volume or prototype parts, as the molds are not durable enough for repeated use.

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How Vacuum Casting Works

 The vacuum casting process has three steps: master pattern, molding, and casting.

Step 1. Master Pattern Making

The quality of the casting urethane parts is determined by the quality of the master pattern, which is best machined by CNC when you need to hold tight tolerance, glossy finish, or even optically clear. In addition, we also will consider making the master by SLA for those parts with more complex shapes and structures.

We can paint parts surface for texture or satin effect to simulate the final mold texture of the product. The silicone molds will copy the details and textures from the original master, so the cast urethane parts will come out the same as the master model without any extra finish on the surface.

Step 2. Silicone Mold Making​

Casting molds are made from liquid silicone resin, also known as RTV molds. The chemical stability, self-releasing properties and flexibility of silicone rubber are excellent in that it gives minimal shrinkage and carries fine details effectively from master to mold. The mold making process starts as follows:

Paste tape in a flat position around the master, so that it is easy to open the mold later, which will also become the parting surface of the final mold.

The master model is suspended in a casting box and placed with plastic rods to set gates and vents.

Liquid silicone is poured around the master and removal all air, creating a vacuum chamber to ensure the master mold is free of air bubbles. Then is cured liquid material in an oven, at a temperature of 40℃. It generally takes 8-16 hours depending on the volume of the mold.

Once the silicone rubber is cured, cut open the casting box, remove the master model from the silicone to form an empty cavity, and the silicone mold is made.

Step 3. Casting Process

Standardized operational procedures assure accurate part replication and consistent mechanical properties. Our vacuum casting technicians to control all operational parameters: mixing, de-gassing, stirring, preheating, casting, and de-molding, allowing for near-perfect copies of the master pattern to be created. The manufacturing processes of vacuum casting parts are as follows:

First, put the silicone mold into the oven and preheat it to 60-70°C.

It is very important to choose a suitable release agent before closing the mold and use it correctly to avoid stickiness and surface defects.

Prepare the polyurethane resins to preheat them to about 40°C before use. Mix two-component casting resins in exact proportions, and then fully stir and degas for 50-60 seconds under vacuum to avoid air pockets or bubbles

The resin is poured into the mold inside the vacuum chamber under computer control and cured again in the oven. Average cure time: 1-3 hours for small parts and 3-6 hours for big parts.

Remove the casting part from the silicone mold after curing. Prepare to repeat this vacuum casting work cycle.

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What is Vacuum Casting?

 Vacuum casting is a manufacturing technology that uses a vacuum to draw liquid material into a mold. It differs significantly from injection molding, which pushes liquid material into a mold using a screw. 

The process of vacuum casting offers significant advantages, and is particularly useful for parts that have undercuts or fine details. 

The process starts with a master model, which Leshine creates using one of its CNC machining centers — though 3D printing can also be used. This master model is then immersed in liquid silicone, which is cured and becomes the mold. 

Once it has been cut and the master model removed, the silicone mold can be put to use. This stage involves pouring casting resin into the mold, as the vacuum removes bubbles and air pockets to ensure a smooth finish. 

The resin part is then cured and removed from the silicone mold, which can be reused around 20 times. 

Each resin part is an exact copy of the original master model.

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