What is small batch soft plastic vacuum casting supplier and Why Do We Use Them?

Vacuum casting is an elastomer casting process in which liquid plastic or metal is injected into a silicone mold under vacuum, thereby producing plastic and rubber components.

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Despite its wide range of applications, many people have limited knowledge of this technology and are unsure how to use it effectively.

Therefore, this article will delve into the vacuum casting process, providing a comprehensive technical guide covering the basic principles, equipment selection, process flow, characteristics, and limitations of the process.

This guide will dive deeper into the specifics of the vacuum casting process, how it compares to other production techniques, the best urethane casting equipment, and how to produce 3D printed patterns for vacuum casting.

Vacuum casting is a common manufacturing method for creating small-batch production runs for market-testing prototypes, personalized products, made-to-fit medical devices, and more.

Note: Vacuum casting is better known as urethane casting or polyurethane casting in the US. This article will use these terms interchangeably.

The process replicates a master model by pouring a silicone mold around it. In a highly skilled procedure, the mold is sliced into two halves, the master model is removed, and then a gating and ventilation system is added. Subsequently, the model maker casts a urethane-based resin into the resulting cavity and cures it under a vacuum to prevent air bubble intrusion.

Vacuum casting is a manufacturing process similar to injection molding where urethane resin is cast in a cavity between two silicone mold halves.

Vacuum casting is also a common solution to copy existing objects without having to resort to a complicated reverse engineering workflow. These are often rare or discontinued items, such as sculptures, fossils, historical relics, antiques, collectibles, and craft models, for which no CAD model or other form of blueprint is available.

As vacuum casting creates extremely detailed models with a perfect surface finish and in a wide variety of materials with a vast range of mechanical properties, it is applicable for end-use products in low volumes, such as:

With only about 10 days lead time from the production of the master model to receiving the cast products, vacuum casting is not only useful for rapid prototyping, but also for bridge production, without requiring the substantial upfront investment that comes with industrial-level manufacturing processes like injection molding, thermoforming, and spin casting.

Instead of room-temperature vulcanization (RTV) silicone, mold makers can use high-temperature vulcanization (HTV) silicone or rubber. This results in a more durable mold, extending its lifetime to as many as 300-500 casts per mold.

The same master model can be reused for creating multiple silicone molds.

In case the project demands a higher quantity, clients have a few different options:

The process creates highly detailed models that are virtuallyindistinguishable from the final product. This makes urethane-cast models outstanding for investor-pitch presentations, trade shows, and commercial photography. Because one silicone mold can produce roughly 50 casts, vacuum casting also allows companies to launch the first series of their product for early market testing.

Vacuum casting is an ideal manufacturing process for creating low-volume batches of items that need to approach production-grade quality.

Let&#;s look at the pros and cons of different fabrication techniques:

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Another advantage of vacuum casting is that bigger parts can be created much faster and cheaper than 3D printing. It allows for mold sizes of roughly 50 cm (20 inches) for the longest dimension, and casting weight up to 1.5kg (3.3 lbs).

The main factor influencing a choice between 3D printing and vacuum casting is product geometry. Vacuum casting is explicitly intended to simulate injection-molded parts. As a low-investment alternative to full-scale injection molding, it can even combine flexible and rigid materials in an overmold or add metal components directly in the prototype through insert molding. For creating a small series of injection-moldable components, vacuum casting is more cost-effective than 3D printing.

It is a common misunderstanding that 3D printing is restricted to one-offs only. On the contrary, it is a viable method for low volume manufacturing of end-use parts . It is also increasingly popular for companies to invest in an in-house print farm. Such an array of 3D printers can handle low-volume batches of hundreds or even thousands of products.

In this webinar, we'll show you how to use stereolithography (SLA) 3D printed molds in the injection molding process to lower costs, reduce lead times, and bring better products to market.

The high fidelity between a vacuum-cast model and a final product that pops out of an injection mold allows engineers to optimize the design for manufacturing. It provides them with a high-quality product preview without having to burden upper management with the astronomical fixed costs that can come with contract manufacturing at an injection molding factory.

Vacuum casting results in nearly zero waste, whereas in injection molding up to 20% of the used polymer is lost due to scrap rates, residues, and overproduction.

A silicone mold allows for significant undercuts in the part design. However, when testing for injection molding, it&#;s recommended to stick to the injection molded design as much as possible and build in the required draft angles of the final product.

Vacuum casting handles variable wall thickness better than injection molding, because there is no heating-cooling cycle of the cast material. This results in less part distortion.

Because the mold is flexible, there are limits to part size and shape. Still, there are specific techniques to enable exceptions. For example, phased casting is the use of metal supports to counteract gravity for casting elongated parts in a three or four-part mold.

After oven-curing, parts still undergo some post-curing for up to a week and remain vulnerable to UV radiation and high temperatures.

In injection molding, molten polymer is pushed into the mold cavity under high pressure and high temperature using a hopper and lead screw. Vacuum casting operates at low temperatures. Instead of molten plastic, it works with liquid resins that are drawn into the mold under a vacuum, much like the capillary action inside a plant stem. So while there are many different resins available to mimic various polymers, prototyping in the final injection-molded material is not possible.

A silicone mold for urethane casting is constructed in a similar fashion to an injection mold in that it has two halves&#;one forming the core, the other the cavity, and the liquefied material is injected in between those to form the final product. However, there are a few differences:

However, product developers regularly overlook the fact that vacuum casting is often a smarter approach to cross the bridge from prototyping to production. Of all early-stage prototyping methods, the resin casting process bears the most similarities to injection molding. Additionally, it&#;s highly cost-effective; molds cost between $200 and $1,000, and a typical per-unit price is $10-$100.

An acrylic mold or low-volume injection molding with 3D printed molds can be an option, depending on the quantities and part quality needed. A 3D printed mold created with a Formlabs SLA printer typically costs under $100 and delivers 100+ pieces without significant loss of quality.

That&#;s why lower-grade tools are often used as an intermediate step to hard tooling. An aluminum mold still ensures up to 1,000-5,000 molding cycles at a much-reduced cost. Because soft tooling processes are much faster and allow for a simplified mold design, they also roughly halves the lead time when compared to injection molding.

Product designers and engineers often turn to 3D printing for design validation, and make the immediate leap to injection molding for production validation. But the problem is that the cost of an injection mold , made out of CNC machined high-grade steel and good for millions of parts, can cost between $5,000 and $100,000 depending on part size and complexity.

Vacuum Casting Process

Vacuum casting is a highly skilled procedure that consists of four steps.

1. CAD Design

A designer or engineer creates a digital representation of the object to be produced using a CAD software environment like Fusion 360 or SolidWorks. This should be done keeping in mind the following guidelines in designing for urethane casting:

• Wall thicknesses between 1.5 mm - 4 mm. The absolute minimum thickness is 1 mm.

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• Ribs can be thinner at 60% of wall thickness.

• Use generous radii > 3 mm.

• Avoid 90-degree walls to improve part strength.

• Build in a 0.5 mm groove between two shells that fit together to improve the look of the product.

• Screw boss wall thickness should be equal to hole diameter.

• Engraved letters work best at 0.25 mm depth.

When complete, the CAD model gets exported to STL format for producing the pattern.

2. Making the Master Pattern

The master pattern is the positive model from which the copies will be derived through casting. Because a silicone vacuum casting mold delivers the exact replica of the master pattern, it is paramount that the 3D printed pattern accurately embodies the original CAD model, especially in terms of ultra-smooth surface quality. Some secondary characteristics of a good master pattern are hardness, chemical and temperature resistance, as well as durability to prevent degradation over time. 

CNC-milled master patterns used to be the standard, but they are costly and time-consuming to produce. They also come with limitations regarding holes, channels, internal radii, and organic surfaces. 3D printing is the cutting-edge alternative and currently prevailing method for producing master models.

There are multiple 3D printer types available to both mainstream and commercial customers, but the most commonly used processes for urethane casting are SLA and SLS 3D printing.

SLA 3D printing produces parts with the smoothest surface finish, which is ideal for vacuum casting patterns. While SLA prints are already smooth with the right print settings, they will still require some sanding and polishing to remove supports and achieve an absolutely flawless finish. Some advanced materials like Rigid 10K Resin also offer superior mechanical properties, temperature resistance, and durability.

SLS 3D printing with nylon results in strong, temperature-resistant, and precise prints. As this technique doesn&#;t require supports, it&#;s also ideal for intricate features like thin walls, engraved details, snap fits, hinges, and nested or interlocking parts. The surface finish of printed parts is a bit too coarse for printing master patterns for urethane casting, so SLS 3D printed parts can benefit from media blasting or vibratory tumbling to reduce surface roughness.

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3. Making the Mold

In this most labor-intensive stage of the process, the model maker crafts a mold box out of melamine board and hot glue. They then secure the master pattern to the bottom with small drops of CA-glue. Sometimes, pockets are created using a rotary tool where lips designed onto the master model will fit into for extra fixation.

For a tight parting line between the two mold halves, all holes between the master pattern and the bottom panel need to be filled up. Styrene blocks and Plastalina modeling clay are preferred materials to do this. Then, add dowel pins and registration bars for perfect alignment with the other mold half.

After vigorously mixing the two-component silicone, it is poured and degassed inside a vacuum chamber. But before doing so, the model maker craftily fills up all tiny gaps in the master pattern with the silicone mixture using an acid brush in order to prevent air traps, and then back-pours the silicone to form the first half of the mold.

After curing for several hours, the mold half is released from the mold box and flipped upside down. The mold maker expertly installs acrylic vents, risers, and a fill port for the second half of the process. Spray mold release on top of the silicone and brush it inside deeper areas. Then, a new mold box is constructed for pouring the second mold half in a similar fashion.

To prepare the mold for vacuum casting, release it from the mold box and spray it with an even coat of E236 urethane mold release. Recombine the mold halves to form a single block, and reinsert the dowel pins and registration keys for alignment.

4. Vacuum Urethane Casting

In this step, the original master pattern is replicated through casting in a vacuum chamber. First, the mold is taped to a wooden board and vent straws plus a pouring funnel are added to it.

The two-component resin is measured and mixed with a coloring agent for pigmentation. Then, the casting material is degassed in a vacuum chamber for a few minutes.

The polyurethane blend is now ready for pouring. The riser straws will indicate when the cavity is completely filled up. After curing at 70˚C, the mold is opened to reveal a carbon copy of the original model with all details and surface textures preserved.

The finalization consists of trimming the edges of flash material, an extra polishing cycle with -grit sandpaper, and oven-curing to improve the part&#;s material properties.

Pro Tips

The above procedure applies mostly to thin-walled prototypes, such as housings for electronics, storage containers, and other cases where we would like to avoid a visible split line on the product&#;s A-surface.

For other items where this is less critical, it suffices simply to encapsulate the master pattern in a silicone block and manually cut it open. The cutting is done in a sine-wave manner, which requires some experience and dexterity.

Here are some best practices to enhance urethane casting results:

• Be careful with using spray release directly on the model, as it can imbue a slight surface texture that will be visible in the casting

• Preheat resins to 40˚C, preferably a day before use, and occasionally flipping the bottle to prevent crystallization.

• Prevent moisture from creeping into resin bottles, because this acts as a blowing agent for PU. One way to do this is to backfill bottles with argon gas immediately after use.

• When mixing two-component chemicals, always use the double-cup mixing method. Pouring directly from the first container is a surefire way to create unmixed streaks and ruin the casting.

• Use rubbing alcohol for loosening hot glue while disassembling the mold box for future use.

• Install the fill port at a corner of the product that provides the shortest average distance to different sections of the part. This ensures that all areas are filled up.

• To prevent air bubble formation, pour from a single point and around delicate areas with a lot of details, never directly on top of them.

• Position the gate low and the vent high.

• Add metallic powders for a spectacularly vibrant finish, or consider using polypyrrole or graphite powder for electrical conductivity.

• When mixing urethane, allow plenty of room above the surface, as the liquid mixture will foam up and rise inside the vacuum chamber like a marshmallow in a microwave.

• Use a professional vacuum casting machine to speed up the process and achieve more reliable results.

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