A Comprehensive Analysis of Vacuum Casting: Principles, Processes, Materials, Parameters and Industrial Applications

1. Introduction

With the rapid development of modern manufacturing and rapid prototyping technologies, enterprises across aerospace, automotive, consumer electronics and medical industries are increasingly pursuing low-cost, high-precision and short-cycle component manufacturing solutions. Against this industrial background, vacuum casting, also commonly referred to as urethane casting or silicone mold casting, has emerged as one of the most indispensable rapid manufacturing technologies. As a flexible indirect molding technology, vacuum casting perfectly bridges the gap between one-off prototype manufacturing and large-scale injection molding production, specializing in small-batch production ranging from 10 to 100 parts per mold and high-fidelity prototype replication.

Different from traditional casting and injection molding processes that rely on rigid metal molds, vacuum casting adopts flexible silicone molds and completes material filling and curing in a vacuum sealed environment. The core advantage of this technology lies in its capability to eliminate air entrapment during the molding process, produce bubble-free parts with exquisite surface quality and high dimensional consistency, and simulate the physical properties of various engineering plastics at a much lower mold opening cost. Compared with CNC machining and 3D printing, vacuum casting shows prominent superiority in batch production efficiency and surface finish of complex structural parts; compared with injection molding, it requires shorter production preparation cycles and lower upfront investment, making it especially suitable for product verification, trial production and customized small-batch manufacturing. This paper systematically elaborates on the working principle, complete technological process, applicable raw materials, critical process parameters and typical industrial application cases of vacuum casting, and analyzes its technical advantages and development constraints to provide a comprehensive reference for related manufacturing practices.

2. Working Principle of Vacuum Casting

The fundamental working principle of vacuum casting centers on negative pressure degassing and pressure-assisted cavity filling, combined with the thermosetting curing characteristics of polymer materials, to realize high-precision replication of parts based on master models. The entire molding logic can be divided into two core modules: mold preparation principle and vacuum pouring molding principle.

In terms of mold manufacturing, the technology takes a high-precision master model as the replication benchmark. The master model can be fabricated via 3D printing, CNC precision machining or manual finishing according to actual production demands. Liquid silicone rubber is evenly coated and poured around the pre-treated master model. After vacuum degassing to remove internal tiny bubbles, the silicone material is heated and cured to form an integrated flexible mold. The flexible characteristics of silicone enable the mold to adapt to complex part structures such as deep grooves, undercuts and curved surfaces, and simplify the demolding process without damaging finished parts.

During the pouring and molding stage, a vacuum chamber serves as the core working environment. The two-component thermosetting resin raw materials (base resin and curing agent) are mixed in a fixed mass ratio, and the mixed liquid material is placed in a vacuum chamber for secondary degassing to eliminate bubbles generated during mechanical mixing. Relying on the negative pressure environment inside the closed chamber, the air inside the prefabricated silicone mold cavity is completely extracted. The atmospheric pressure difference between the inside and outside of the mold pushes the low-viscosity liquid resin to quickly fill every corner of the mold cavity, including tiny microstructures and narrow gaps that are difficult to fill in atmospheric pouring processes. After the cavity filling is completed, the external vacuum environment is released, and the resin material undergoes cross-linking chemical reaction under constant temperature conditions to complete curing and solidification. Finally, finished parts consistent with the master model in size, structure and surface texture can be obtained after mold opening and demolding.

The vacuum environment is the key to determining the quality of vacuum casting products. It fundamentally solves the common defects of traditional atmospheric casting such as internal porosity, surface pinholes and incomplete filling caused by trapped air. Meanwhile, the flexible silicone mold can buffer the volume shrinkage of resin materials during curing, effectively reducing the warpage and cracking probability of finished parts and improving the overall molding yield.

3. Complete Technological Process of Vacuum Casting

The vacuum casting manufacturing process is a standardized closed-loop operation system, which consists of six sequential core stages: master model fabrication and pre-treatment, silicone mold manufacturing, resin material preparation, vacuum pouring, thermal curing and post-processing. Each stage has strict operating specifications, and any improper operation will directly affect the dimensional accuracy and surface quality of the final parts.

3.1 Master Model Fabrication and Pre-treatment

The master model is the original prototype of the cast part, and its precision directly determines the replication accuracy of all subsequent products. In industrial production, 3D printing (SLA resin printing) is the mainstream processing method for master models due to its high precision and low cost for complex structures; for ultra-high-precision functional parts, CNC machining of ABS, aluminum alloy and other materials is adopted. After the master model is formed, surface pre-treatment is required, including polishing to remove printing layers or machining burrs, cleaning surface dust and oil stains, and spraying release agent evenly on the surface. The release agent can effectively reduce the bonding force between the master model and liquid silicone, facilitating the subsequent mold splitting and model demolding work.

3.2 Silicone Mold Manufacturing

Mold making is the most time-consuming link in the vacuum casting process. First, fix the pre-treated master model in a customized mold frame, and design pouring gates and exhaust grooves according to the structural characteristics of the part to ensure smooth resin filling and air discharge. Then mix the silicone rubber and matching curing agent in a specific proportion, place the mixture in a vacuum degassing machine to remove internal bubbles, and pour the degassed silicone into the mold frame to completely wrap the master model. After standing for preliminary shaping, put the entire mold frame into a constant-temperature oven for curing at 40–60℃ for 4–8 hours. After the silicone is fully cured, split the mold along the preset parting line, take out the internal master model, trim the excess silicone edges, and complete the production of the two-piece or multi-piece combined silicone mold. A single high-quality silicone mold can support the production of 10–80 finished parts, and the service life varies according to the hardness of the poured resin material.

3.3 Resin Material Preparation

Select the appropriate polyurethane resin material based on the functional requirements of the product (hardness, toughness, high temperature resistance). Most casting resins are two-component materials composed of base resin and curing agent. Operators need to weigh the two components strictly in accordance with the manufacturer’s recommended ratio, and stir them mechanically at a constant speed to realize uniform mixing. After mixing, place the resin mixture in a vacuum tank for degassing treatment to eliminate micro-bubbles generated during stirring, so as to avoid pore defects inside the finished parts after curing.

3.4 Vacuum Pouring

Transfer the prepared silicone mold to the vacuum pouring chamber and close the sealing door to start vacuuming. When the internal vacuum degree reaches the preset threshold, tilt the feeding device to inject the degassed liquid resin into the mold cavity through the reserved pouring gate. The negative pressure environment guides the resin to fill the entire cavity rapidly and uniformly. After the filling operation is completed, keep the vacuum state for 2–5 minutes to ensure no residual air exists inside the cavity, then slowly release the vacuum and take out the filled mold from the chamber.

3.5 Thermal Curing

Place the poured silicone mold into a constant-temperature curing oven for heating curing. The curing temperature and time need to be adjusted dynamically according to the resin material type and part wall thickness. For conventional thin-walled parts with a wall thickness of 2–5mm, the curing temperature is usually set at 60–80℃ with a holding time of 1–3 hours; for thick-walled parts over 5mm and high-strength functional resin, the curing time needs to be extended to 3–6 hours to ensure complete cross-linking reaction of the internal resin and stable mechanical properties of the parts.

3.6 Post-processing

After curing, open the silicone mold and take out the finished parts. The post-processing work includes cutting off redundant pouring gates and exhaust channel residues, polishing the parting line burrs, and performing surface treatments such as sandblasting, spraying and electroplating according to customer requirements. For parts with assembly requirements, secondary fine trimming and size detection are also needed to ensure compliance with design tolerance standards.

4. Main Materials for Vacuum Casting

The materials used in vacuum casting are divided into two major categories: mold materials and casting materials. The selection of materials depends on production cost, part service environment and functional performance indicators, and different material combinations bring distinct molding effects and application scenarios.

4.1 Mold Materials

Silicone rubber is the only mainstream mold material for vacuum casting, which can be further divided into condensation-type silicone and addition-type silicone. Condensation-type silicone features low price and simple curing conditions, suitable for low-cost prototype manufacturing with low precision requirements, but its mold surface finish is poor and service life is short, only supporting less than 30 times of pouring. Addition-type silicone is the preferred material for industrial batch production; it has the advantages of high surface smoothness, low shrinkage rate (less than 0.1%), stable chemical properties and long service life, and can meet the replication demands of high-precision complex parts. In addition, addition-type silicone can be customized into different hardness grades from Shore A 20 to Shore A 80 to adapt to parts with different structural complexities.

4.2 Casting Materials

Polyurethane (PU) resin series materials occupy more than 95% of the vacuum casting casting material market, and can simulate the performance of almost all common engineering plastics, covering rigid, flexible, high-temperature resistant and transparent types:

Rigid PU Resin: This type of resin has high hardness and structural stability, with mechanical properties similar to ABS and PC plastics. It has excellent tensile strength and impact resistance, and is widely used to manufacture structural parts for electronic equipment shells and automotive interior accessories. Its Shore hardness ranges from D60 to D85, and the shrinkage rate is controlled within 0.15%.

Flexible PU Resin: Flexible resin simulates the characteristics of TPU and rubber materials, with Shore A hardness ranging from 30 to 90. It has good elasticity, wear resistance and folding resistance, and is mainly used for manufacturing sealing gaskets, shock-absorbing accessories and flexible protective casings.

High-temperature Resistant Resin: Modified polyurethane resin specially developed for high-temperature working scenarios, which can maintain stable physical properties in the temperature range of -40℃ to 120℃. It is commonly applied to automotive engine peripheral parts and high-temperature resistant components of aerospace equipment.

Transparent Resin: Low-impurity high-transparency PU resin, with a light transmittance of over 90%, comparable to acrylic and transparent PC materials. It is suitable for manufacturing optical accessories, instrument display covers and transparent appearance parts.

5. Critical Process Parameters of Vacuum Casting

The molding quality of vacuum casting is affected by multiple correlated process parameters. Reasonable parameter matching is the core to reduce defective rates and improve part precision. The key controllable parameters in actual production include vacuum degree, curing temperature, curing time, resin mixing ratio and mold service times.

5.1 Vacuum Degree

Vacuum degree is the most critical parameter determining the filling effect and internal quality of parts. The optimal vacuum range for mold degassing and resin pouring is -0.08 MPa to -0.095 MPa. If the vacuum degree is lower than -0.08 MPa, the residual air inside the mold cavity cannot be completely discharged, resulting in bubble defects on the surface and inside the parts; excessive vacuum degree (higher than -0.095 MPa) will cause the low-viscosity resin to boil instantaneously, leading to material overflow and uneven density of finished parts. For ultra-fine micro-structural parts, the vacuum degree needs to be adjusted to -0.092 MPa to -0.095 MPa to ensure full filling of micro gaps.

5.2 Curing Temperature and Time

Curing temperature directly affects the cross-linking reaction speed of polyurethane resin and the shrinkage degree of parts. The conventional curing temperature range for commercial resins is 50–85℃. Low-temperature curing (50–60℃) can effectively reduce the thermal shrinkage of materials and improve dimensional accuracy, but the curing cycle will be prolonged; high-temperature curing (70–85℃) accelerates the molding speed and improves production efficiency, yet it is easy to cause tiny warpage of thin-walled parts. Corresponding to the temperature range, the curing time of conventional parts is 1–6 hours, and the time needs to be increased by 20%–30% for parts with wall thickness exceeding 6mm to avoid incomplete curing of internal materials.

5.3 Resin Mixing Ratio

The mass ratio of base resin to curing agent varies with different resin types, and the common ratios in the industry are 1:1, 2:1 and 4:1. The mixing ratio directly determines the curing degree and mechanical properties of parts. Excessive curing agent will lead to brittle parts and easy cracking; insufficient curing agent will cause incomplete cross-linking reaction, resulting in permanent deformation and poor surface hardness of parts. In actual operation, the weighing error of raw materials must be controlled within ±1% to ensure stable product performance.

5.4 Mold Service Life Parameter

The service life of silicone molds is restricted by resin hardness and curing temperature. Under standard process conditions, a single addition-type silicone mold can produce 60–80 rigid resin parts or 70–100 flexible resin parts. When the number of pouring times exceeds the service limit, the silicone mold will experience irreversible deformation and surface aging, which will reduce the dimensional accuracy and surface finish of replicated parts, and the mold needs to be replaced in a timely manner.

6. Industrial Application Cases

Thanks to its low cost, high precision and strong structural adaptability, vacuum casting has been widely used in prototype verification and small-batch trial production in multiple industrial fields. The following are typical application cases in mainstream industries:

6.1 Automotive Industry

The automotive industry is the largest downstream application field of vacuum casting technology. Automakers usually adopt vacuum casting to complete the trial production of interior and exterior parts and engine peripheral functional parts in the product research and development stage, so as to verify the assembly rationality and service performance of parts before large-scale injection molding mold opening. A domestic new energy vehicle manufacturer applied high-temperature resistant PU resin to cast automobile air-conditioning duct parts. The parts have complex curved structures with multiple branch pipelines and thin-walled structures of 2–3mm. The manufacturer completed the production of 50 trial parts within 3 working days via vacuum casting. Compared with CNC machining, the production cost was reduced by 45%, and the surface finish and overall assembly performance were closer to mass-produced injection molding parts. In addition, flexible resin is also used to produce automobile door sealing strips and shock-absorbing rubber pads, which effectively shortens the vehicle R&D cycle.

6.2 Consumer Electronics Industry

In the consumer electronics industry, product iteration speed is fast, and the appearance and size tolerance requirements of parts are extremely strict. Vacuum casting is mainly used for the prototype manufacturing of smart phones, wearable devices and household appliances. A European wearable device brand used SLA printed resin as the master model, and adopted high-precision addition-type silicone molds and transparent PU resin to produce customized smart watch transparent protective shells. The dimensional tolerance of the finished parts was controlled within ±0.05mm, and the surface did not need secondary polishing treatment. The technology helped the brand quickly complete market demand testing of new products with a small batch of 80 samples, avoiding the high risk of massive investment in untested products.

6.3 Medical Equipment Industry

Medical equipment parts have high requirements for material safety, biocompatibility and dimensional precision. At present, special medical-grade polyurethane resin has been able to meet the biocompatibility testing standards of ISO 10993, and can be used to produce disposable medical auxiliary parts and rehabilitation equipment accessories. A medical technology enterprise used medical-grade flexible resin to cast customized rehabilitation finger fixing brackets for patients with finger injuries. According to the personalized size data of different patients, the enterprise quickly completed the production and adjustment of parts. Compared with traditional customized machining, vacuum casting greatly reduced the production cycle and manufacturing cost of personalized medical parts, and provided convenient solutions for precision rehabilitation treatment.

6.4 Aerospace Industry

Aerospace lightweight parts have the characteristics of complex structures and small batch demand. Vacuum casting is often used to manufacture non-load-bearing auxiliary parts such as instrument shells and wiring protection sleeves for aircraft and satellites. Due to the harsh working environment of aerospace equipment, high-temperature resistant and low-density modified PU resin is selected for casting. The parts manufactured by this technology have the advantages of low density, strong environmental adaptability and low cost, which can replace expensive composite materials for non-core structural parts, effectively reducing the overall R&D and manufacturing cost of aerospace prototypes.

7. Conclusion and Outlook

Vacuum casting, as a mature rapid indirect molding technology, realizes high-fidelity replication of complex parts by virtue of negative pressure degassing filling principle and flexible silicone mold matching with thermosetting polyurethane resin. This technology has obvious comprehensive advantages in small-batch production and prototype verification: low mold opening cost, short production cycle, excellent surface quality of finished parts, diverse optional materials, and the ability to simulate the performance of most engineering plastics. This paper systematically sorts out the working principle, six-stage complete process, mold and casting materials, core process parameters and typical industrial application scenarios of vacuum casting, and clarifies the parameter matching standards and material selection logic for actual production.

Nevertheless, vacuum casting still has inherent limitations. Restricted by the material characteristics of silicone molds and thermosetting resin, this technology cannot support large-scale mass production, and it is not suitable for manufacturing ultra-large-size parts and high-load-bearing structural parts. In the future, with the continuous upgrading of modified silicone materials and high-performance composite polyurethane resins, the service life of vacuum casting molds and the comprehensive mechanical properties of casting parts will be further improved. Meanwhile, the combination of vacuum casting and intelligent automatic feeding and curing equipment will realize the intelligent control of process parameters, reduce the dependence on manual operation skills, and further expand its application boundary in high-end manufacturing fields such as precision medical devices and intelligent wearable equipment. It is predictable that vacuum casting will still occupy an irreplaceable core position in the rapid manufacturing industry for a long time in the future.