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What’s Vacuum Casting? Process, Design, and Applications

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    A key technique in the fabrication of plastic parts is vacuum casting. Recognized for its efficiency and speed, vacuum casting stands out as a premier method for producing small batches of polyurethane products. This process is particularly valuable for creating preliminary samples of a product, allowing manufacturers to conduct market tests before full-scale production.

    This article aims to shed light on vacuum casting, a crucial process in plastic molding that merits attention. We will delve into the intricacies of the vacuum casting process, including the step-by-step procedure, the materials used, and its wide array of applications. By understanding the specifics of vacuum casting, stakeholders can better appreciate its significance in the development and testing of new products, ensuring that innovative ideas are feasible and market-ready.

    What’s Vacuum Casting?

    Vacuum casting is a soft tooling technique that employs soft silicone molds for casting polyurethane plastics and elastomers. That’s how it gets the names, silicone casting and urethane casting. The whole casting takes place in a vacuum chamber, which eliminates the risk of air bubbles within the cast, making final products accurate and flawless.

    vacuum casting molded object

    Compared to traditional casting, where metallic mold is permanent, costly, and takes time to produce, silicone mold-making is quicker. The lead time is around 15 days, and a silicone mold can make up to 25 parts. This makes it one of the most preferred methods for rapid prototyping.

    The Vacuum Casting Process: A Step-by-Step Guide

    The key element of vacuum casting is the silicone mold. However, there are some preliminary and post-molding steps involved in making and use of silicone mold.

    Here’s a brief look at all those steps:

    Step 1: Creating the Master Model

    First, prepare a master mold to make that silicone mold. It can be 3D-designed on any CAD software, like SolidWorks, CATIA, or AutoCAD.  To bring the mold to life, CNC machining or 3D printing are common choices. (Go through the next section for mold design tips.)

    The accuracy of the vacuum cast parts is largely dependent on the master mold, so it needs to be as precise as possible. For that, surface polishing or grinding is an option.

    Step 2: Making the Silicone Mold

    After completing the master mold, proceed to cast a soft silicone mold. Place the master mold in a casting box and pour liquid silicone. For curing, place the setup in a preheated oven at 40℃. It may take 10 to 16 hours for silicone to set and solidify.

    Remove the silicone mold carefully with a knife. You may use a mold release agent if having trouble removing the mold.

    Step 3: Casting the Parts Under Vacuum

    The final step is casting the polyurethane in the silicone mold. If you’re using a mix of resins, mix them first. Add color pigments now if you want any colors. Then, get the air out of the mold to create a vacuum and pour the resin in. This vacuum step is important because it stops air bubbles from forming and makes sure the resin covers every part of the mold.

    Materials Used in Vacuum Casting

    plastic resins for vacuum casting

    The silicone casting process largely suits polyurethane resins and elastomers. Some common material options are:

    ABS-Like Resins

    ABS resin has similar characteristics to ABS thermoplastic; it’s hard, dimensionally stable, and impact-resistant. Among these resins, ABS-like UP4280 stands out for its heat resistance and high strength, making it a popular choice for prototype manufacturing. Additionally, the PX250 resin offers high impact strength and UV resistance, qualities that are ideal for producing items like helmets. For rapid prototyping, PX100 presents another viable option, ensuring that there’s a suitable ABS-like resin for a wide range of applications. This structured approach highlights the specific benefits and applications of each ABS-like resin, maintaining logical consistency in content.

    Glass-Filled Nylon-Like

    Glass-filled nylon-like material significantly enhances the performance of products made through vacuum casting. By incorporating glass fibers into the nylon matrix, the material’s tensile strength and stiffness are substantially increased. This reinforcement allows it to handle higher mechanical loads and stresses, making it an ideal choice for applications that require durability and resilience. Its unique composition ensures that components made from Glass-Filled Nylon-Like material are not only stronger but also more capable of withstanding rigorous use.

    Elastomeric TPE-Like

    Thermoplastic elastomers such as rubber are also ideal for molding. UP8400 is a rubber-like elastomer, black or white in appearance, flows greatly, and has high elongation. It’s used for making gaskets, seals, and handgrips. T0387 is an example of a translucent rubber-like material, which is optimized for vacuum casting.

    PC-Like

    It’s a clear, lightweight, and high-impact material that’s easy to machine. Two PC-like materials, PX510 and UP6160 are common for vacuum casting. They excel in applications requiring durability and visual clarity, offering ease of machining and the ability to produce precise, detailed parts. These materials are ideal for creating components where strength and aesthetic appeal are paramount, making them indispensable in the realm of vacuum casting.

    Clear Resins

    Clear resins like PMMA/Acrylic are heat and shatter-resistant with medium strength. They’re great options for casting transparent castings and panels. X522HT is an example of a translucent plastic resin, which can be used in silicone molding.

    High-Impact Resins

    High-impact resins such as UP5690, which resembles polypropylene (PP), and PU8098, similar to polycarbonate (PC), are designed to replicate the desirable properties of their counterparts. These materials boast remarkable impact strength and resistance to chemicals, rendering them perfect for applications subjected to high loads. Their robustness and durability make UP5690 and PU8098 invaluable for producing parts that demand both resilience and reliability under stress.

    MaterialVariationsCharacteristicsHardness
    ABS-LikeUP4280, PX100, PX250Heat and impact-resistantD78-82
    Glass Filled NylonHigh Strength, rigidityD58 – 75
    TPE-LikeUP8400, T087High elongationD60-80
    PC-LikePX510, UP6160Lightweight, clear, high-impactD82-86
    Clear ResinsX522HT, PMMAHigh TransparencyD87
    High ImpactUP5690, PU8098High Impact StrengthD8575-86

    Design Guidelines for Vacuum Casting

    vacuum casting plastic parts

    These guidelines will help you design accurate vacuum casting products, and critically adjust details during the design phase.

    Tolerances

    Designing with tolerance in mind ensures your components fit together and meet the desired specifications. In silicone mold casting, there’s a shrinkage of 0.15%. Therefore, the design must be made following that.

    Wall Thickness

    With vacuum casting, there’s freedom to incorporate varying wall thicknesses within the same part. However, for optimal results, it’s best to maintain consistency and keep wall thickness at least 1.5 mm to ensure structural integrity.

    Ribs

    Adding ribs enhances strength and reduces warping, especially in large flat areas. Its thickness needs to be less than the adjacent wall thickness to prevent shrinkage and sinking. Ideally, rib thickness should be less than 60% of the nominal wall thickness.

    Bosses

    When designing housings or casing, bosses are common.  They add strength at attachment points. They should have a minimum diameter height of 1mm. Additionally, to minimize the risk of sinking, their wall thickness did not exceed 60% of the nominal part thickness.

    Undercuts

    Unlike in processes that require hard tooling, undercuts in vacuum casting do not necessitate the use of inserts and can be designed freely.

    Embossed Details

    For text and logos that are embossed (raised) or debossed (recessed), keep a minimum depth or height of 1mm and a minimum width of 1mm to ensure that these details are visible and crisp. Additionally, maintain a 1mm gap between characters to enhance readability.

    Design ElementDetail
    Tolerance+-0.05mm
    Min. Wall Thickness1.5 – 2.5 mm
    BossesHeight: 1mmDiameter: 1mm
    Ribs60% of wall thickness
    Embossed DetailsMin. Width 1mmMin Depth. 1mm
    Quantities20 – 25 per mold
    Lead Time15 to 20 days

    Applications of Vacuum Casting in Various Industries

    Here’s a glimpse of a wide range of industrial applications, where the polyurethane casting technique is used:

    IndustryApplications
    Automotive IndustryPrototype parts: dashboards, door handles, Functional components: light covers, air vents
    Consumer ElectronicsEnclosures for devices: smartphones, laptops, Prototypes for wearable tech, Buttons, and switches
    Medical and HealthcareMedical device prototypes: enclosures, components, Custom orthopedic devices, Equipment casings
    Aerospace IndustryPrototype components: interior cabin parts, Air ducts, and vents, Non-structural parts: covers, enclosures
    Food and BeveragePackaging prototypes

    Advantages and Limitations of Vacuum Casting

    To analyze if the vacuum casting process would be suitable for your desired applications, go through these advantages and limitations.

    Advantages

    Design Flexibility: The process accommodates a range of design elements, from intricate details like ribs, undercuts, and embossed logos to complex design geometries. It’s also cost-effective to make design changes in soft silicone, which is already to be discarded after a few uses.

     Low-Cost Tooling: The silicone molds, the key tool of the process, are relatively cheaper to construct compared to metallic molds. Silicone molds get prepared in a few hundred dollars where as molds for injection molding or metal casting can cost thousands.  

    High-Quality Finishes: Whether the requirement is for glossy, matte, or textured surfaces, the casting process can deliver results that meet or exceed expectations. One can mix resins, add pigments, and get the desired surface finish and quality.

    Quick Turnaround: The only time-tasking aspect is the master pattern design. But silicone mold preparation just takes hours. The lead time for the whole process is up to 15 days from start to finish. Whereas, a few other casting processes take weeks.

    Limitations

    Material Restrictions: The silicone molding process works best with polyurethane resins and elastomers, but it’s not suitable for molding hard industrial materials, for instance, metals. For metal vacuum casting, there’s a specialized similar process called vacuum die or vacuum investment casting.

    Size Constraints: Silicone mold has some size constraints. In most cases, it allows molds somewhere between 500 to 900mm.

    The Durability of Silicone Molds: These molds have a limited lifespan. They are capable of producing 20 to 25 high-quality parts before degradation in precision and detail begins to occur.

    Vacuum Casting vs. Other Manufacturing Processes

    plastic vacuum casted parts

    Finally, let’s compare vacuum casting with relatively similar molding processes.

    Vacuum Casting vs Vacuum Forming

    In vacuum casting, manufacturers pour liquid resins into a silicone mold under vacuum to produce high-quality parts free from air bubbles. This method suits prototypes or small to medium production runs best.

    On the other hand, vacuum forming involves heating a sheet of plastic until it’s pliable, then using vacuum pressure to form it over a mold. This process is best for larger and thinner items, such as packaging and product housings.

    Vacuum Casting vs Centrifugal Casting

    Centrifugal casting primarily molds cylindrical parts such as pipes, bushings, and rings by pouring molten metal into a rotating mold. The centrifugal force then evenly distributes the material along the sides, creating a hollow cylindrical object. On the other hand, the vacuum casting technique involves pouring molten polyurethane resin into a static silicone mold.

    Compared to vacuum casting, centrifugal casting is better suited for metals and results in parts with superior mechanical properties. However, vacuum casting offers more versatility in design and material selection for non-metallic parts, which makes it suitable for prototypes and non-structural components.

    Vacuum Casting vs Injection Molding

    Injection molding is a more precise, and advanced version of silicone vacuum casting. Injection molding forces molten material into a metal mold under high pressure to make parts with tight tolerance. It’s ideal for high-volume production, but the initial costs for tooling are significantly higher and take longer to produce than silicone molds used in vacuum casting.

    Whereas, vacuum casting uses a silicone mold for casting. It provides flexibility in design changes with lower upfront costs for mold making.  

    Vacuum Casting vs 3D Printing

    3D printing is an additive manufacturing process that builds parts layer by layer from a digital file. That means, it allows for any kind of intricate and complex geometries one can digitally design. It’s highly adaptable and can produce parts relatively quickly, but making multiple copies takes time.

    Vacuum Casting does take longer as it requires a high-quality master model (often produced by 3D printing or CNC machining). However, once a mold is made, it’s quicker to make multiple copies of a part with consistent material properties and surface finishes.

    Is Vacuum Casting Expensive?

    If we compare the vacuum casting process with two similar application processes, for instance, injection molding or 3D molding, it’s relatively cheaper. Injection molds are metallic and cost more to make than silicone and master molds. Secondly, the injection molding machines themselves are expensive compared to simple vacuum molding machines. Lastly, materials costs are also less compared to other processes.

    Choosing the Right Vacuum Casting Service

    Vacuum casting is a need of every industry – be it automotive, packaging, medical, or aerospace. It is also ideal for making a range of prototypes, that, if manufactured in-house, could significantly inflate costs.

    If your project is in the pipeline and you’re eager to see your new product concept materialized with precision and fidelity, Rapid Direct is your premier choice for vacuum casting services.  We specialize in crafting exact prototypes at a fraction of the usual cost and time.

    Our offerings accommodate a diverse range of requirements, whether you’re aiming to prototype a groundbreaking design or conduct market tests with a limited batch of products.

    RapidDirect holds ISO 9001:2015 and ISO 13485 certifications. This assures that every product we deliver meets the highest standards of quality. We have an engineering team that facilitates you from the initial design to the development of the final product.  

    Take the next step with Rapid Direct – Let us bring your ideas to life with unmatched precision and efficiency!

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