Vacuum Casting vs Injection Molding for Low-Volume Production

Choosing between vacuum casting and injection molding for low-volume production isn’t just a technical call—it’s a schedule, budget, and risk decision. This guide compares vacuum casting (also called urethane casting) with injection molding (including rapid/aluminum tooling) for 10–10,000 parts, focusing on lead time, total cost, tolerances/surface finish, and scalability. All numbers and ranges are as of Jan 2026.

Key takeaways

• Need parts in about a week? Vacuum casting is typically faster; rapid/aluminum injection molding usually lands in 10–15 days at the quickest and more often 2–6 weeks for validated runs (2025–2026 provider data).

• For ≤100–300 parts, total cost often favors vacuum casting thanks to minimal tooling; beyond a few hundred to 1,000+ parts, injection molding amortization drives the per‑unit cost down sharply.

• Typical vacuum casting tolerances are around ±0.15 mm/100 mm with best‑case ±0.05 mm on small features; rapid injection molding commonly achieves ±0.08–0.2 mm, with much tighter tolerances in specialized micro‑molding contexts.

• Cosmetic quality is strong in both, but the route differs: vacuum casting mirrors the master model’s finish; injection molding offers SPI-grade finishes and repeatable textures.

• Materials/regulatory: vacuum casting uses polyurethane resins simulating thermoplastics; injection molding supports production thermoplastics (ABS, PC, PA, PEEK, PEI, LSR) including medical grades.

• Scalability: vacuum casting excels at prototypes and bridge tests; injection molding is the natural path to pilot runs and production.

  
  

What changes in a low-volume context

Low-volume production (10–10,000 parts) lives at the intersection of speed and economics. The first decision is time‑to‑first‑parts. The second is how quickly tooling cost amortizes. In early design gates—EVT/DVT/PVT—teams often need a handful of iterations under tight deadlines, then a stable pilot run with production‑intent materials. That sequence is why so many programs start with vacuum casting to validate design and cosmetics, then move to rapid/aluminum injection tooling for pilot and bridge manufacturing when unit economics and material certification matter.

Put simply: vacuum casting gets you “looks‑like/works‑like” parts quickly with low risk to your budget. Rapid injection molding takes longer to spin up but gives you production‑grade repeatability and a path to scale.

Vacuum casting — fast iteration with cosmetic quality

Vacuum casting uses silicone molds to replicate a master pattern, producing polyurethane parts that simulate common thermoplastics. Providers commonly deliver first articles in about a week for straightforward geometries. 

Material options are polyurethane resins that simulate ABS, PC, PP, PMMA, and various elastomers (Shore A 30–95). Many providers list clear, UV‑resistant, flame‑retardant, or food‑contact grades, but availability and documentation vary; confirm requirements at RFQ.

Low upfront cost and quick mold builds make vacuum casting attractive for short runs and frequent design changes. The trade‑off is higher per‑unit costs and limited mold life.

Best for: fast iterations, appearance and demo parts, market tests, and functional prototypes where the resin’s thermal/mechanical limits are acceptable. Not ideal for thousands of parts or harsh service environments where engineering thermoplastic performance and certification matter.

Injection molding — pilot runs that scale

Rapid/aluminum injection molding creates production‑grade parts from thermoplastics using machined metal tooling. Simple rapid tools can be produced in roughly 10–15 days in favorable cases, with many providers stating 2–6 weeks as a typical range depending on complexity, validation steps, and queue.

On tolerances, rapid injection molding commonly lands around ±0.08–0.2 mm for many features, with resin‑specific per‑mm factors. Specialized micro‑molding environments achieve micrometer‑level precision (single‑digit microns), but those are niche setups with stringent process control.

Materials cover the full spectrum—ABS, PC, PA, PP, PEEK, PEI, LSR, and more—with regulated medical grades and established validation paths (IQ/OQ/PQ) under ISO 13485 when required. This makes injection molding the more straightforward route for regulated industries once designs stabilize.

Tooling cost is higher than vacuum casting but lower with aluminum than steel. Public, provider‑posted USD ranges are sparse; industry sources emphasize that aluminum tools are “tens of thousands less” than hardened steel with faster turnaround, and single‑cavity rapid tools for simple parts can start in the low thousands while complex or multi‑cavity tools reach into the tens of thousands.

Best for: pilot and bridge runs, programs requiring production thermoplastics, tighter repeatability, SPI finishes, and a clear path to higher volumes with steel tooling.

Vacuum casting vs injection molding for low-volume production

Below is a side‑by‑side comparison with indicative ranges and notes (as of Jan 2026).

Commentary: If your primary decision factor is speed, vacuum casting will usually win. If your main concerns are per‑unit cost at 1,000+ parts, regulatory materials, and repeatability, rapid injection molding becomes the better fit.

Scenario playbook — when to choose which

In very small quantities (≤50 parts) needed inside two weeks, vacuum casting is the practical choice. You’ll likely receive parts within about a week to 10 business days with strong cosmetic fidelity to the master. Confirm resin options if you need specific properties (UV resistance, flame retardancy) and remember that extended heat or load can reveal the limits of polyurethane.

For 50–300 pilot sets during EVT/DVT, vacuum casting often offers the best total landed cost and fastest iteration, especially if you anticipate design changes after the first article inspection. If you know your design is stable and you require production thermoplastics, consider commissioning a simple rapid aluminum tool.

At 500–2,000 bridge runs, lean toward rapid injection molding. The per‑unit cost advantage from tooling amortization becomes meaningful here, and you can lock in SPI finishes and DFM‑driven consistency. If you must ship samples sooner, use vacuum casting for early units while the tool is being built.

Beyond 2,000 pre‑production parts, injection molding is typically the economic and quality winner. Aluminum tools can meet this range; moving to steel is the natural step when volumes and wear demand it.

Cost and lead‑time models (as of Jan 2026)

These simplified examples show how tooling amortization and schedule pressure play out. Your actual numbers will depend on geometry, resin, cavity count, gating, and QA scope—always RFQ with drawings and tolerance notes.

Example A: 500 parts, cosmetic housing

• Vacuum casting: assume $1,250 total silicone tooling (several molds) and $35/part. Total ≈ $18,750; about $37.50/part. Lead time to first parts: ~1 week.

• Rapid injection molding: assume $25,000 aluminum tool and $5/part. Total ≈ $27,500; about $55.00/part. Lead time to T1 samples: ~10–15 days to several weeks depending on validation.

Example B: 2,000 parts, functional enclosure

• Vacuum casting: assume $1,250 silicone tooling and $25/part. Total ≈ $51,250; about $25.63/part. Lead time: ~1–2 weeks for first batches; multiple mold sets needed to complete volume.

• Rapid injection molding: assume $25,000 aluminum tool and $0.75/part. Total ≈ $26,500; about $13.25/part. Lead time: ~2–6 weeks including validation.

Rule of thumb: if time‑to‑first‑parts under two weeks is critical and your quantity is under ~300 units, vacuum casting usually wins. If your quantity is around 1,000 units or more and you need production thermoplastics with consistent cosmetics and tighter control, rapid injection molding tends to be the better long‑term value.

Quality, tolerances, and risk control

Inspection and validation typically include First Article Inspection (FAI) against your drawing, with options like CMM for critical dimensions.

Common risks and mitigations are well understood. For vacuum casting, mold wear can lead to dimensional drift, and poor degassing can cause bubbles or surface blemishes. Tight control of vacuum, temperature, and resin handling helps. For rapid injection molding, warpage, sink, short shots, and weld lines appear when DFM isn’t observed. 

Regulatory notes: For medical and other regulated applications, injection molding offers clearer routes to biocompatible materials and formal validations (IQ/OQ/PQ) under ISO 13485. Vacuum casting can support early functional/cosmetic evaluations but requires careful resin selection and supplier documentation if any regulated exposure is anticipated. Many procurement teams also reference ISO 9001 for general quality systems in both routes.

For relevant internal reading on tolerances and inspection flows in fast‑turn environments, see the CNC machining services overview: https://www.cnkaierwo.com/cnc-machining/

Rapid tooling and hybrid paths

Aluminum tooling, 3D‑printed inserts, and hybrid strategies can compress timelines while preserving a path to scale. Aluminum tools machine faster than steel and can be reworked quickly for design tweaks—one reason they’re favored for pilot runs. Providers note aluminum tools lasting into the tens of thousands of cycles in many cases, though part geometry, resin abrasiveness, and process conditions drive actual life.

Hybrid paths to consider include starting with vacuum casting for cosmetic sign‑off and early field trials, then commissioning a rapid aluminum tool once the design stabilizes. For highly time‑sensitive programs, you can launch vacuum‑cast samples immediately while the aluminum tool is in build, then switch to molded parts as soon as T1s are validated. 3D‑printed mold inserts can help trial texture or localized geometry changes before cutting a revised full tool.

For a broader overview of rapid prototyping and low‑volume strategies, see the Solutions page: https://www.cnkaierwo.com/solutions/

PM RFQ essentials — decide fast with the right inputs

State quantities by build stage and deadline (EVT/DVT/PVT), call out critical dimensions and tolerances (e.g., features requiring ±0.1 mm or better), and include a drawing with GD&T and FAI/CMM expectations. Specify resin family, acceptable equivalents, and any certifications (ISO 13485 system, USP Class VI, food‑contact). Define cosmetic targets (SPI polish, texture, or provide a master/reference sample). Finally, outline your risk posture: whether you prefer vacuum casting first for rapid iteration or are ready to commit to a rapid tool, plus how you’ll manage change orders.

Wrap‑up

If speed‑to‑first‑parts is paramount and your batch is under a few hundred units, vacuum casting delivers. If your program needs production thermoplastics, tighter and more consistent control, and a cost curve that improves with volume, rapid injection molding is the smarter bridge to scale. Define your quantities and deadlines, call out critical tolerances and cosmetics, and choose the path that gets you to validated parts on time without overspending.