Vacuum casting and injection moulding are two distinct manufacturing methods, each offering unique advantages and limitations. The choice between these techniques depends on production volume, material requirements, cost considerations, lead times, part complexity, and surface finish requirements.
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When it comes to plastic parts in engineering products, choosing the right manufacturing process can significantly affect the part quality and cost. Injection moulding is a popular method for plastic parts as it can produce large volumes. However, due to the high tooling cost, it is unsuitable for initial prototyping or small-batch production.
Overview of Injection Moulding
Injection moulding is a highly efficient and widely used manufacturing process for producing large volumes of plastic parts with high precision and repeatability. Most plastic components in mass-produced consumer products, such as Cars, toys, CCTV cameras, smart speakers, earbuds, and phones, are made using Injection moulding. According to Grand View research, the global Injection moulding market will be worth USD 423.75 billion by 2030.
The Injection moulding process involves injecting molten thermoplastic or thermosetting polymers into a mould cavity under high pressure. Once the material solidifies, the mould opens, and the finished parts are ejected, ready for further processing or assembly.
Benefits of Injection moulding
- High production rate – As mentioned above, high production rates are possible, making Injection moulding suitable for mass production of identical parts. The process can be either fully automated or semi-automated.
- Material range – There are over 70000 thermoplastics and 18000 thermosets with varying properties and characteristics.
- Precision and Repeatability – If designed to avoid the common Injection moulding defects, due to their excellent repeatability and precision, Injection moulding machines can produce consistent part quality and dimensional accuracy.
Limitation of Injection Moulding
- High tooling cost—Injection moulding tools are expensive and time-consuming to manufacture. Although some manufacturers offer aluminium alloy tooling for smaller volumes, the most common material is steel, which offers significantly more life than aluminium alloy tools.
- Long lead time – Tools typically have a 4-5 week lead time, which might be too long for prototypes or smaller batch production.
- Limited design flexibility – Modifying injection mould tooling can be time-consuming and costly, which limits design change flexibility during the initial stages of production development.
Overview of vacuum casting
Vacuum casting, or Urethane casting or Polyurethane casting is a flexible, cost-effective, and quick method of producing low—to medium-volume plastic parts. The process involves making silicone moulds from a master pattern using 3D printing or CNC machining. Once the moulds are ready, liquid polyurethane resin is poured into the mould cavity under vacuum. After curing, the parts are removed from the mould, producing high-quality replicas of the original master pattern.
Since the casting takes place in a Vacuum, it produces high-quality, bubble-free parts.
Benefits of Vacuum casting
- Cheap tooling – Compared to Injection moulding tooling, vacuum-casting silicon moulds are inexpensive, especially if 3D-printed master patterns are used. This significantly reduces the development cost if multiple iterations are required during the initial stages of product development.
- Quick turnaround—3D printing has enabled rapid vacuum casting, enabling rapid prototyping of parts for visualisation, testing, and validation.
- Low to medium production volume: Vacuum casting is ideal for small batch production due to its low cost and quick turnaround time.
Limitation of Vacuum casting
- Limited material options – Compared to Injection moulding, vacuum casting material choices are limited in terms of strength, flexibility, and heat resistance.
- Small-scale production – It is less suitable for large-scale production due to longer cycle times and higher per-unit costs than injection moulding.
- Surface finish – Quality may vary depending on the part’s complexity, the surface quality of the 3D-printed part, and the casting technician’s skill.
Vacuum Casting and Injection Moulding – Comparative analysis
The choice between these techniques depends on production volume, material requirements, cost considerations, lead times, part complexity, and surface finish requirements. By conducting a thorough comparative analysis and evaluating your specific project needs, you can determine the most suitable approach to effectively and efficiently achieve your desired outcomes in plastic part manufacturing.
| Feature | Injection Molding | Vacuum Casting |
| Tooling Time | Long (weeks to months) | Short (days to a few weeks) |
| Investment Capital | High (expensive moulds) | Low (affordable silicone moulds) |
| Production Volume | High (thousands to millions) | Low to medium (up to hundreds) |
| Part Size | Typically, small to medium | Small to medium, with some flexibility |
| Material Availability | Extensive range of thermoplastics | Limited to certain resins and polyurethane |
| Cost per Part | Low for high volumes (economies of scale) | Higher per-part cost, especially for low volumes |
| Application | Mass production of plastic parts | Prototyping, low-volume production, complex geometries |
| Design Flexibility | Lower flexibility and requires detailed planning and considerations for mould design | High, suitable for complex and intricate designs |
| Part Complexity | Moderate to high, but with limitations on intricate details due to mould constraints | High, excellent for detailed and complex shapes |
| Surface Finish | Excellent, smooth surfaces with fine details possible | Good, can achieve fine details but may require post-processing |
| Overall Project Time Scale | Long (due to tooling and setup) | Short to medium (quicker setup and adjustments) |
Lead times
Vacuum casting offers shorter lead times for tool manufacture and production setup, making it ideal for rapid prototyping and low-volume test batches. Silicone moulds are quicker to produce, especially with a 3D-printed master pattern.
In contrast, Injection moulding is time-consuming due to the precision machining required to manufacture the metal mould tools. Once the mould tool is ready, Injection moulding offers faster production cycles than Vacuum casting.
Cost
Vacuum casting generally has lower upfront costs for tooling and setup, making it more cost-effective for small batches or prototypes. However, due to the manual process and shorter mould lifespan, the price per part is higher than that of injection moulding.
The most significant setback of injection moulding is the initial high cost of metal moulds. To withstand the pressure and have a longer lifespan, the mould tool is typically machined from hard steel, making it expensive. However, with a higher production rate and longer tool life, injection moulding significantly reduces the cost per part compared to Vacuum casting.
Production volume
Vacuum casting is ideal for low to medium volumes, while injection moulding is more suitable for high volumes due to its efficiency and economies of scale. Injection moulding can produce millions of parts with consistent quality, while Vacuum casting mould can only make about 50 -100 parts from a mould. This also depends on the part complexity and size.
Part size
Both Injection moulding and Vacuum casting can produce parts of various sizes, typically small to medium. In Injection moulding, size is restricted by the machine size. In Vacuum casting, Vacuum chambers are usually small, and the part size is also limited by material and mould design.
Material Options
Injection moulding offers a broader range of material options, including commodity plastics, engineering plastics, and high-performance polymers, catering to diverse application requirements. However, vacuum casting is limited to specific resins and polyurethanes, which might not offer mechanical properties for load-bearing and functional production parts.
Design flexibility and part complexity
Both methods can accommodate a wide range of part geometries, but injection moulding is better suited for complex shapes and intricate features due to its high precision and consistency. High pressures in Injection moulding also help the material flow into nooks and crannies. Although it can produce complex parts, product designers should remember that some features, such as undercuts, might require sliding tools and manual intervention, which will increase the tooling cost.
Injection moulding offers less design flexibility due to rigid mould design, and even minor changes are time-consuming and costly. Vacuum casting, on the other hand, offers high design flexibility because of the lower cost of silicone moulds and how quickly moulds can be reworked or remade.
Surface finish
The surface finish of injection moulding can be excellent, and various standard surface finish grades are offered. Vacuum Casting has a good surface finish and can achieve fine details, but it may require additional post-processing for a smoother finish.
In Vacuum casting, the surface finish can be adjusted using the master pattern surface finish since the master pattern surface is replicated inside the silicone mould.
How to choose between Vacuum casting and Injection moulding
When deciding on the manufacturing process, it is essential to determine where you are in the design and development process of the product. Ask yourselves the following questions to make an informed decision.
- What is the expected production volume?
- High volumes: Injection Molding
- Low to medium volumes: Vacuum Casting
- What is your budget for tooling and initial investment?
- High budget: Injection Molding
- Low budget: Vacuum Casting
- How quickly do you need the parts?
- Longer timeline: Injection Molding
- Shorter timeline: Vacuum Casting
- How complex are the parts?
- Moderate complexity: Injection Molding
- High complexity: Vacuum Casting
- What materials are required?
- Wide range of thermoplastics: Injection Molding
- Specific resins and polyurethanes: Vacuum Casting
- What is the importance of surface finish?
- Excellent finish required: Injection Moulding
- Good finish with potential post-processing: Vacuum Casting
In conclusion, vacuum casting and injection moulding are two distinct manufacturing methods, each offering unique advantages and limitations. Understanding their differences allows you to make informed decisions that drive success and innovation in product development.
In our experience, vacuum casting can be a great rapid prototyping technique for injection moulding parts. Vacuum casting parts are also great for product prototypes, small production batches for pre-production tests, and products for exhibitions.
Once the part has undergone the required iterations, the injection moulding can be a good path to mass production.