How the two processes actually work
Vacuum casting and injection molding are both replication processes that turn a master pattern into many plastic parts, but they sit at opposite ends of the tooling-cost spectrum. Vacuum casting uses a soft silicone mold built around a master, while injection molding uses a hard steel or aluminum tool mounted in a clamping press.
Both methods bridge the gap between a one-off prototype and mass production, yet they differ sharply in setup cost, lead time, and how many parts the mold can produce before it wears out. Understanding that trade-off is the key to picking the right one for your quantity.
Vacuum casting process in detail
Vacuum casting starts with a master pattern, typically a CNC-machined or SLA/SLS 3D-printed part that defines the geometry. Technicians suspend the master inside a casting box, pour liquid silicone around it, cure the silicone, then cut it open to remove the master and create a flexible two-part mold.
A measured amount of two-part polyurethane resin is degassed under vacuum, poured into the mold cavity, and cured. The vacuum removes trapped air so the resin replicates fine detail. Each silicone mold typically yields 15 to 25 good castings before the rubber loses dimensional accuracy.
Injection molding process in detail
Injection molding begins with a precision-machined mold, usually from hardened steel for long runs or aluminum for shorter ones. The mold is mounted in an injection molding machine that melts plastic pellets and injects the melt under high pressure into the closed cavity.
After the part cools and solidifies, the mold opens and the part is ejected, ready for the next cycle that takes seconds. Because the tool is rigid and the process is automated, every shot is nearly identical, which is why injection molding dominates at high volume despite the expensive upfront tool.
When vacuum casting is the better choice
Choose vacuum casting when you need somewhere between about 10 and 200 realistic parts and you do not want to spend weeks or tens of thousands of dollars on a steel tool. It is ideal for pre-production validation, design confirmation, and market tests where the look, feel, and material behavior matter.
Vacuum casting also shines when the design is still evolving. Because a silicone mold costs little and takes days, you can rebuild the master, pour a fresh mold, and re-run without losing a major tooling investment. That flexibility is hard to match with injection molding.
When injection molding is the better choice
Move to injection molding once the design is frozen and you need thousands of identical, fully specified parts. The high fixed cost of the tool is spread across a large run, driving the per-part cost far below what casting or machining can achieve at scale.
Injection molding is also the right answer when you need the widest material selection, including glass-filled engineering resins, and when consistent mechanical properties, tight color matching, and certification matter. For production that will ship to customers for years, the steel mold is worth the investment.
Cost and the break-even point
For very low quantities, vacuum casting is almost always cheaper because the only tooling is an inexpensive silicone mold. You pay for the master pattern, the mold, and the resin per part, with no heavy capital outlay.
The crossover with injection molding usually falls between roughly 200 and 1,000 pieces, depending on part size, material, and complexity. Below that band, the steel tool's amortized cost keeps unit price high; above it, injection molding's speed and low per-shot cost take over. Always quote both before committing.
Tolerances, accuracy and materials
Vacuum-cast parts hold typical tolerances around +/-0.2 to +/-0.5 mm, with accuracy limited by silicone flexibility and resin shrinkage. The mold slowly deforms with each pull, so later castings may drift slightly from the first ones.
Injection molding holds tighter, more repeatable tolerances, often around +/-0.1 mm or better, because the steel cavity is dimensionally stable. On materials, casting mimics ABS, PP, PA, and rubber-like elastomers in polyurethane, while molding accesses the full commercial plastics catalog including high-temperature and reinforced grades.
Surface finish and post-processing
Vacuum casting can reproduce fine surface texture from the master, including matte, glossy, or textured finishes, and it supports pigmented resin for near-final color. However, each part has a casting gate and parting line that must be trimmed and sanded.
Injection molding delivers a clean, consistent surface straight from the tool, with the chosen finish molded in. There is minimal hand work, which keeps per-part labor near zero at volume. For visible consumer products, the molded finish is typically superior and far more repeatable.
Lead time and iteration speed
A vacuum casting project can go from approved master to first parts in about one to two weeks, with the silicone mold taking only a couple of days to make. This short loop lets teams validate several design revisions quickly.
Injection molding needs a multi-week tooling phase: mold design, steel cutting, sampling, and correction of molding defects can take four to eight weeks before stable production. The process is slow to start but extremely fast once running, so it rewards frozen designs and high volumes.
Typical applications by industry
Consumer electronics brands use vacuum casting for enclosure prototypes and small pre-launch batches shown to investors and at trade shows. Medical device makers cast anatomical models and limited device housings for clinical evaluation.
Automotive and industrial firms use injection molding for clip-on brackets, connectors, and housings produced by the million. Many companies follow the same path: cast to prove the product, then invest in a steel mold only after the design and demand are confirmed.
How FabProto supports both processes
FabProto builds the master pattern, runs vacuum casting for your low-volume bridge needs, and partners with molding suppliers when you are ready to scale, so you get one accountable source across the whole journey.
Our engineers will tell you the exact quantity at which injection molding becomes cheaper for your specific part, and help you avoid paying for a steel tool before the design is ready. That transparent guidance is why teams trust FabProto from first prototype to production.