Casting and injection molding are two prevalent manufacturing processes employed in the fabrication of numerous everyday products. Polyurethane, a thermoplastic material, finds common use in crafting various components such as gaskets in automotive applications, casters, and rollers for medical carts, as well as diverse hardware items like pulleys, bushings, shock absorbers, and scraper blades.
To ascertain whether vacuum casting or injection molding is more suitable for your specific application, it is essential to evaluate whether the part is intended for prototyping or mass production. Additionally, determining the quantity of parts you plan to produce is crucial in making an informed decision.
Vacuum casting is a method of producing parts by pouring liquid plastic into a mold under a vacuum. Typically, the mold is crafted from a pliable material like silicone. Molds filled with liquid plastic are placed in an oven for curing, although not all plastics require heat for this process. Some plastics can cure at room temperature over time. After curing, the mold is opened, allowing the casted part to be extracted.
This process is commonly employed to manufacture finalized plastic components for small production runs or for rapidly prototyping a product destined for eventual injection molding.
The soft molds used in vacuum casting are fashioned from a "master pattern." This master model serves as a 3D-printed or CNC-machined replica of the part to be cast. The master pattern is immersed in liquid silicone, which then cures and solidifies. Once set, the master pattern is divided in half, and the 3D-printed or CNC-machined section is removed. This leaves behind two mold halves with internal cavities that, when combined, mirror the part to be cast.
The lead time for production-level vacuum casting typically ranges from 1 to 2 weeks. While the completion of the master pattern and mold for vacuum casting can be achieved in three to seven days, achieving production volumes takes longer due to the required curing time for the plastic in the mold, which cannot be expedited.
The production output for a single vacuum casting mold generally ranges from 1 to 10 parts per day, with the actual output influenced by factors such as the type of plastic and the geometry of the part.
The volume of products generated through vacuum casting is relatively small compared to other manufacturing methods. This is primarily attributed to the labor-intensive, manual nature of the vacuum casting process. In contrast to highly automated processes like injection molding or CNC machining, which require minimal manual intervention, casting plastics in vacuum casting demands meticulous preparation.
Before casting, plastics must be appropriately prepared, with some available in a liquid state while others need to be melted in an oven if purchased in solid form. Once liquefied, plastics must be mixed at the correct ratios with different chemicals to ensure proper curing and achieve desired characteristics such as specific colors. These ratios can be adjusted to attain properties like abrasion resistance or increased compressive strength.
While some fast-setting plastics may cure within 30 minutes, others could take several hours. These factors make the vacuum casting process suitable for prototyping and small-scale production runs but less practical for large-scale manufacturing operations.
Injection molding is the process of creating highly precise products by forcefully injecting molten plastic material into the cavities of a mold at extremely high pressure. Unlike casting, where gravity assists in filling the mold cavity with plastic resin, injection molding relies on pressurized filling of cavities and rapid cooling of plastic parts. This makes injection molding a significantly faster and more repeatable process, particularly well-suited for large-scale manufacturing operations.
Molds for plastic injection molding are crafted using methods such as CNC machining and electric discharge machining (EDM). These processes can create cavities with tolerances as tight as ±0.1 mm on average and, in some cases, may achieve values as low as ±0.025 mm depending on the application.
Due to the high pressures and temperatures involved in injection molding, molds are typically constructed from hardened steel or aluminum.
The lead times for creating the injection molding tooling are typically lengthy due to the precision required in crafting the metal molds. However, once the molds are perfected, the production phase of injection molding requires less lead time compared to casting. Injection molding can swiftly produce large quantities of plastic parts.
The high pressure and rapid cooling inherent in injection molding contribute to a much faster cycle time. While some die sets for injection molding may feature only one or two cavities, others can have well over 100. This capability allows injection molding to produce hundreds, if not thousands, of parts per day to meet production orders.
The volume of parts achievable through injection molding far surpasses that produced by casting. Injection molds can incorporate multiple cavities for identical plastic components, and the overall cycle times are notably short.
When deciding whether vacuum casting or injection moulding is the right manufacturing technique, consider the items listed below:
Vacuum casting is better for prototyping or small production runs, while injection moulding is better for medium-to-large production runs. Casting can produce 1-10 parts per day while injection moulding can produce hundreds, if not thousands, depending on the number of cavities in your mould.
- Vacuum Casting: The tooling costs for vacuum casting are significantly lower than those for injection moulding. The materials required for vacuum casting, including plastic resin, curing agents, and silicone moulds, are more economical compared to the hardened steel or aluminum die sets essential for injection moulding.
- Injection Moulding: Injection moulding involves higher tooling costs due to the precision and durability required in creating the metal molds.
Compared to vacuum molds, injection molds cost more, but last longer. For smaller production runs, such as 250 to 300 parts per year, vacuum forming is especially cost-effective. Because they’re simpler, vacuum molds also take less time to produce.
- Vacuum Casting: Parts produced through vacuum casting tend to be more expensive than injection-moulded parts. This is attributed to the manual labor involved in casting and the typically lower volumes produced.
- Injection Moulding: Injection-moulded parts are more cost-effective, especially for large production runs, owing to the automated and efficient nature of the injection moulding process.
- Vacuum Casting: Vacuum casting is limited to a specific set of curable plastics.
- Injection Moulding: Injection moulding offers a broader material selection, allowing for the use of various thermoplastics and thermosets.
- Vacuum Casting: Silicone moulds used in vacuum casting are not as durable as the hardened steel or aluminum injection-ready moulds used in injection moulding. The lifespan of vacuum casting moulds is comparatively shorter.
- Injection Moulding: Injection moulds, made from robust materials like hardened steel or aluminum, can last for decades, providing a longer lifespan compared to vacuum casting moulds. Consider the expected duration of mould usage based on your specific needs.
The table below shows the differences, advantages, and disadvantages of vacuum casting and injection moulding:
| Vacuum Casting | Injection Moulding | |
| Commonly Used For | • Small quantities: Prototyping, bridge tooling, and small production runs. | • High volume production: Medium-to-large production runs. |
| Advantages | • Resins and silicone moulds are cheap and manufacturing has a low cost. • Rapid tooling: Tooling lead times are faster since moulds can easily be made using 3D printing or CNC machining and liquid silicone. | • Highly automated process means the price per part is much lower compared to cast parts. • Large volumes of parts can be made in a short time. • Parts are ready to be used after moulding. • Moulds are durable and long-lasting since they are fabricated from hardened steel or aluminium. |
| Disadvantages | • Labour-intensive process which drives the price per part up. • Production output is small compared to injection moulding. • Long production lead times. • Silicone moulds are not as durable as metal moulds. • Parts require post-processing and trimming after removal from the mould. | • Long tooling lead times due to the extensive manufacturing processes needed to fabricate moulds. • Expensive upfront costs. |
This article presented vacuum casting and injection moulding, explained what they are, and discussed when to use each process best.
GD-HUB provides a wide range of manufacturing capabilities, including vacuum casting, injection moulding, die casting, compression moulding and other value-added services for all of your prototyping and production needs.
