Essential Guide to Vacuum Casting - Aria Manufacturing

Before a product is mass produced, manufacturers love to test their products and conduct market surveys. They therefore, need small batches of prototypes and sample products. One way of getting this small batches of prototype is Vacuum Casting.

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Vacuum Casting, also called urethane casting involves creating a mold of the desired product and then injecting liguid plastic or metal into it, this is done under a vacuum, to prevent air from getting into the mix. Once it solidifies, the mold is taken apart and the product revealed.

This article explains what vacuum casting is all about by giving a step-by-step process. This article will also consider the various applications of vacuum casting, its advantages and disadvantages, and alternatives to vacuum casting will also be stated.

Casting is a manufacturing process where liquid material, either metal, resin or plastic is poured into the hollow space of a mold, the mould is then taken apart after the liquid material has solidified. This shows that, the result of casting will only be as good as the mould. What is this mold and what is it made from?

For the vacuum casting process, the shape that forms the mold that holds the liquid material is usually made from silicone. This silicone mold is so important that it is one of the first steps in vacuum casting. It is flexible, heat resistant, and tear resistant. The process of making the mold will be futher explained when explainig the process of Vacuum casting.

Apart from the mold, another factor instrumental to the casting process is the vacuum. What does pouring liquid under vacuum mean and how does it work?

We can agree that the word vacuum means space. However in the context of casting, a vacuum is a space where the air pressure has been significantly reduced, it is a sealed environment where air has been removed by a vacuum pump. For vacuum casting, we generally have a vacuum casting chamber.

You see, without the vacuum, air can be trapped inside the liquid material creating air bubbles and air pockets, resulting in a poor surface finish. The more disastrous effect of this is that it will create weak spots in the material, resulting in structural defects.

For vacuum casting, the most commonly used material is urethane.

Here a metallic mold is used. The metallic mold is called a vacuum die.The liquid material use in die casten is usually from aluminum, zinc, and magnesium. Die casting has the advantage of high precision and fast production. It is however quite expensive, this is because the die is costly to make. It is therefore not suitable for small scale production runs.

This is done in a centrifugal casting box, a kind of rotating mold. Liquid metal is poured into this mold as it rotates. This type of casting is used for metals like steel, iron, and bronze. It produces strong parts and is ideal for cylindrical materials like pipes. The downside to this type of casting is that it can only be used to make symmetrical shapes. It is also more expensive than Vacuum casting.

More modern ways of casting, or at least prototype manufacturing, include 3D printing and Digital Light Processing.

The very first step is to create the silicone vacuum casting mold, this is done by creating a master mold. It is called the master moldel because it is the mold which in turn helps to form the silicone mold which then forms the final material.

The master mold is usually made by 3D printing or CNC machining. The final product will be the exact replical of the silicone mold, it is extremely important that the master moldel for the silicone mold be made with great precision and critical detailing.

The master model is then placed in a mould box, and the liquid silicone is poured over it. Air bubbles are removed by placing it in a vacuum casting chamber. The silicone is left to cure, the curing usually takes 8 to 24 hours.

Once the silicone has cured, the silicone mold is then cut carefully to remove the master model, this reveals a hollow cavity which must be the exact negative of the material to be made. If properly made, the silicone mold casting can be reused to make multiple identical and perfect replicals of the original material.

The Silicone is highly detailed and heat resistant. It has a flexible texture and is sometimes transparent. the Silicone mold is then carefully joined back together leaving the hollowed cavity accessible for the urethane to go through for the final production process product.

Injection molding is a process where molten material usually plastic, is injected into a mold under high pressure. It is then cooled down to form a solid material. The mold used in this type of manufacturing is usually a metal mold.

The injection molding process includes:

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1. Heat and melt the plastic pellets

2. Inject the melted plastic into a metal mold

3. wait for it to solidify

4. open and eject the finished product

Injection molding is more suited for mass production as against vacuum casting. The image below describes the differences between THe Vacuum Casting and the Injection molding manufacturing process.

3D printing is also known as additive manufacturing. A digital image of the material to be printed is first created, the printer then prints the digital file layer by layer. This manufacturing process is extremely fast.

However, the machine can be slow in making multiple copies. This means it is also not suitable for mass production. 3D printing is perfect to make materials with complex geometry and require a great dimensional accuracy

Vacuum casting is the process used to manufacture high-quality plastic parts that are comparable to injection molded parts. Vacuum casting technology has been developed for more than half a century, and it is a processing technology with high cost performance and very low cost and time cost for low-volume manufacturing parts. An-Prototype has more than 15 years of experience in Vacuum casting process, this blog aims to introduce the development history of vacuum casting, advantages and disadvantages, processing process, type and other information, to provide constructive advice for you to choose the right vacuum casting processing service.

Vacuum casting refers to the process of using a two-component polyurethane resin in the manufacturing process to produce high-quality plastic prototypes.Sometimes it is called a silicone mold or a soft mold. The difference between vacuum casting and injection mold is that vacuum casting uses a soft silicone mold, and injection mold is made of steel or aluminum and other materials. Because the process is carried out under vacuum, high-quality castings are produced without bubbles, with light surface texture, fewer defects, and lower cost of construction and time. 

It can be machined and delivered in a few days, and in most cases, dozens of parts can be delivered in a few weeks. Therefore, it is widely used in industrial production, especially for functional test processing, and some prototypes that require small batches and have no special requirements for product materials.

As early as , the first silicone resins had been developed. However, it was not until the s that vacuum casting technology was developed and presented to the world by the Technical University of Dresden and the Technical University of Cottbus in the German Democratic Republic. Since the Europeans did not systematically learn this processing technology at that time, they sold it to Japan in the s, mainly for the automotive industry. Within a few years, Europe re-introduced the technology and began to pay attention to it, and now almost all large production companies are using the technology in the research and development department, because the technology can reduce production costs and increase production efficiency.

Vacuum casting has long been widely used, and some flexible molds, such as natural rubber and other materials, have also been used by sculptors to make relief or sculpture molds for many years. Vacuum formed plastics were originally developed from vacuum casting and molding techniques in the s and s, and plastics such as polyethylene terephthalate (PET) were developed, which is now one of the most commonly used packaging products and the material for plastic water bottles. In the s, thermosetting plastics were developed and used for vacuum casting. Until now, these plastics perfectly mimic the appearance and properties of materials used in mass production, leading to a breakthrough in vacuum replication using silicone molds.

Step 1: First determine the main mold.
As with most modern manufacturing processes, the first step is to create a 3D model of the shape required when machining the part, and since the master model is used to cast the part, the master model must have the perfect size and appearance, which can be made using CNC machining or through 3D printing machining. The main die is then sanded, painted and primed to achieve the desired surface texture and quality. Because any flaw in the main mold will affect the appearance of the final vacuum cast part, when the appearance requirements of the parts are high, the main mold must be perfect.

Step 2: Casting mold.
Usually the master models are made of plastic or metal, and the only requirement is that they can withstand temperatures of 40°C and cure for a long time. The master model is positioned and secured into place in a custom sized wooden box, and liquid silicone is poured around it and baked for 8-16 hours to cure. After the silicone dries and solidifies, the box and riser can be removed. Finally, carefully cut the mold with a knife to reveal the negative cavity of the assembly. Any error will usually result in mold damage, and with careful selection and use of release agents, stickiness and surface blemishes can be avoided.

Step 3: volume production
The vacuum casting resin and pigment are mixed and poured into the silicone mold, where they are fully mixed and degassed in vacuum for 50-60 seconds during the automatic pouring process. Next, by eliminating air pockets inside the tool, vacuum technology allows gravity to do all the work of filling the mold. The parts are then transferred to the oven for curing. Curing time also depends on part size. When it hardens, the casting can be removed from the mold. Once the casting is complete, the sprue and risers can be removed, and the final process is to trim the edges of the raw material and apply additional sanding and polishing with grit sandpaper. The first machined part after completion must be inspected for quality. If the first piece is qualified, production can continue. This process can be repeated 20 to 30 times on average. Exceeding these times will cause the mold to gradually deform and affect the dimensional accuracy.

If you need a fast, easy and cost-effective way to produce high-quality plastic parts, vacuum casting is an excellent choice. Unlike injection molding, vacuum casting does not require DFM, thus saving project time. When choosing a manufacturing process, it is important to consider yield requirements, tolerance requirements, surface finish requirements, and lead time. Vacuum casting is an excellent choice for low-volume manufacturing, with tight tolerances and a smooth surface finish. Injection molding is most suitable for mass production, tolerances are large, and it is not suitable for the production of parts with higher precision.

Machining accuracy and recommendations: It is recommended that the wall thickness of the machined parts be between 1.5 mm and 4 mm. Use a radius greater than 3 mm. Avoid using 90-degree walls to increase part strength. The best depth for carving is 0.25 mm. The tolerances of the vacuum injection type are required to be machined according to ISO International Standard M.

1. Low cost: Compared with CNC machining and 3D printing, the initial production cost of vacuum casting is much lower.
2. Save production: vacuum forming does not require much time, greatly improving production efficiency, suitable for low-volume manufacturing.
3. Fine details: exquisite workmanship, high reducibility, low scrap rate. If the master mold is 3D printed, complex details can be produced when printing, and these details can be replicated in the final casting.
4. Dimensional accuracy: Parts produced by vacuum casting will fit perfectly together without post-processing steps such as grinding or drilling.
5. Excellent surface finish: Color and surface finish can be easily added, making the process ideal for aesthetically conscious industries.
7. Shorter lead time: Making silicone molds (which takes a few days) is much faster than making steel molds or aluminum molds (which takes weeks).
8. Material flexibility. Vacuum casting resins are available in a variety of colors to meet the needs of different applications.

Production limits: Vacuum casting is inherently suitable for low volume manufacturing, usually in the range of 1-30 parts. The process is not suitable for high temperature applications. At the same time, the main mold will produce shrinkage and round edge phenomenon. As the number exceeds this limit, more master models need to be made.
Dependence on master mold: The vacuum casting process pays great attention to detail. If there is a defect in the main mold at the beginning, it will affect all the parts that are machined later.