Open Casting vs Injection Molding vs Compression Molding: Selecting the Right Process for Your Application

Choosing the right polyurethane manufacturing process determines both component quality and project economics. Open casting delivers premium properties and design flexibility at low tooling cost, making it ideal for custom parts and volumes under 5,000 units. Injection molding provides the fastest cycle times and tightest tolerances for high-volume production above 10,000 units—but demands significant tooling investment. Compression molding bridges the gap, handling medium volumes (500–25,000 units) with moderate tooling costs and good performance for simpler geometries.

This article compares these three polyurethane molding processes across the factors that matter most: volume requirements, tooling costs, part complexity, material properties, and lead times. Whether you’re producing prototypes or scaling to production quantities, understanding these trade-offs enables better decisions.

1. Open Casting (Gravity Pour Casting)

Process Overview

Open casting—also called gravity pour casting—involves mixing liquid prepolymer with curative and pouring the reactive blend into molds at atmospheric pressure. The material fills the cavity by gravity, cures over 16–48 hours, then undergoes post-curing at elevated temperatures to develop full mechanical properties. For detailed process steps, see our guide to the polyurethane casting process.

The process accommodates everything from simple open molds to complex closed molds with inserts. Production can range from entirely manual hand-pouring to semi-automated systems with metering equipment that maintains precise mix ratios.

Advantages and Limitations

Advantages:

Open casting offers the lowest tooling investment of any polyurethane manufacturing process. Silicone molds cost $500–$2,000; aluminum molds run $1,500–$5,000; even steel molds rarely exceed $15,000. Compare that to injection molds at $50,000–$150,000 or mor

The process delivers premium mechanical properties because thermoset polyurethane chemistry creates permanent crosslinks that thermoplastic processing cannot match. Typical cast polyurethane achieves tensile strength of 25–60 MPa (3,600–8,700 psi), elongation of 300–700%, and tear strength of 80–180 kN/m. These values exceed what injection-molded TPU can provide. For property comparisons, see our cast polyurethane vs TPU analysis.

Design flexibility represents another major advantage. Open casting handles large parts, variable wall thicknesses, encapsulated inserts, and complex geometries that would require expensive tooling modifications in other processes.

Limitations:

Longer cycle times (hours rather than minutes) make open casting labor-intensive at higher volumes. The process requires skilled operators to maintain quality. Above approximately 5,000 annual units, other processes typically become more economical.

Longer cycle times (hours rather than minutes) make open casting labor-intensive at higher volumes. The process requires skilled operators to maintain quality. Above approximately 5,000 annual units, other processes typically become more economical.

Ideal Applications

• Custom and prototype parts requiring material validation before production tooling investment
• Large components like industrial rollers, wheels, and pads
• Low-to-medium volume production (1–5,000 units annually)
• Premium performance requirements where mechanical properties cannot be compromised
• Custom polyurethane solutions with unique specifications

2. Injection Molding (Thermoplastic Polyurethane)

Process Overview

Injection molding heats thermoplastic polyurethane (TPU) pellets to 200–230°C (392–446°F), then injects the molten material under high pressure into steel molds. The material cools and solidifies within seconds to minutes. Fully automated production lines can run continuously with minimal operator intervention.

Reaction Injection Molding (RIM) offers a related approach using thermoset materials. RIM combines two liquid components under high pressure (1,500–3,000 psi) in a mixing head, then injects the reactive mixture into heated molds where it cures. RIM suits larger parts and can achieve some of the property advantages of thermoset chemistry while maintaining injection molding’s process efficiency.

Advantages and Limitations

Advantages:

Speed defines injection molding’s primary advantage. Cycle times measured in seconds or minutes enable production rates impossible with other polyurethane molding processes. High-volume applications—automotive parts, consumer products, industrial components—benefit most from this efficiency.

Consistency follows from automation. Once dialed in, injection molding produces thousands of identical parts with minimal variation. Tight tolerances of ±0.1mm are achievable, compared to ±0.5mm typical for open casting.

Limitations:

Tooling investment represents the largest barrier. Steel injection molds cost $50,000–$150,000 or more, with lead times of 8–16 weeks. This investment only makes sense at volumes sufficient to amortize the cost—typically 10,000+ units.

Thermoplastic polyurethane cannot match thermoset cast polyurethane’s mechanical properties. TPU exhibits higher compression set (permanent deformation under sustained load), lower tear strength (50–120 kN/m vs. 80–180 kN/m for cast), and reduced abrasion resistance. Material selection is also more limited than the extensive formulation flexibility available in casting.

Part size depends on machine capacity. Very large components may exceed practical injection molding limits.

Ideal Applications

• High-volume production runs (10,000+ units annually)
• Precision parts requiring tight dimensional tolerances
• Complex internal geometries that benefit from high-pressure mold filling
• Applications where material properties meet requirements without premium thermoset performance
• Products where recyclability of thermoplastic material offers value

3. Compression Molding

Process Overview

Compression molding places pre-measured material into a heated mold cavity, then closes the mold under hydraulic pressure while heat and pressure cure the material. Cycle times range from 5–30 minutes depending on part thickness and material system.

The process works with both thermoset polyurethanes and millable polyurethane gums. Unlike injection molding, compression molding doesn’t force material through gates and runners, which can degrade some materials.

Advantages and Limitations

Advantages:

Tooling costs fall between open casting and injection molding—typically $10,000–$50,000. This makes compression molding economically viable at medium volumes where injection molds cannot be justified but open casting becomes too labor-intensive.

The process excels at large cross-sections and thick-walled parts that would be difficult to fill or cure properly in other processes. Material waste runs lower than injection molding since there are no runners or sprues. Parts can be molded to near-net shape, reducing secondary operations.

Compression molding handles high-hardness materials (Shore 80A and above) effectively, including some formulations that would be difficult to process in other ways.

Limitations:

Cycle times exceed injection molding, limiting production throughput. The process is more labor-intensive during loading and unloading phases. Flash (excess material at mold parting lines) requires trimming.

Dimensional tolerances (typically ±0.25mm) fall between injection molding’s precision and open casting’s variability. Complex geometries with thin sections or intricate details are better suited to other processes.

Ideal Applications

• Medium-volume production (500–25,000 units annually)
• High-hardness components like wear plates, bumpers, and industrial seals
• Simpler geometries where compression molding’s limitations don’t apply
• Cost-sensitive projects where tooling budget constrains options
• Parts requiring thermoset properties at volumes below injection molding’s breakeven point

4. Process Selection Decision Matrix

Factor	Open Casting	Injection Molding	Compression Molding
Annual volume	1–5,000 units	10,000+ units	500–25,000 units
Tooling cost	$500–$15,000	$50,000–$150,000+	$10,000–$50,000
Cycle time	16–48 hours	Seconds to minutes	5–30 minutes
Lead time (first parts)	2–4 weeks	8–16 weeks	4–8 weeks
Tolerances	±0.5mm typical	±0.1mm achievable	±0.25mm typical
Part size	No practical limit	Machine-limited	Medium to large
Complexity	High	Medium-High	Medium
Material properties	Premium (thermoset)	Good (thermoplastic)	Good (thermoset available)
Prototyping cost	Low	High	Medium

Cost Crossover Points

The economics shift at predictable volume thresholds:

Below 500 units: Open casting almost always proves most economical. Low tooling cost dominates the calculation.

500–5,000 units: Evaluate based on part complexity, quality requirements, and property needs. Open casting may still win for premium applications; compression molding suits simpler parts at the higher end.

5,000–10,000 units: Compression molding typically offers the best balance. Injection tooling may be justified if volumes are growing.

Above 10,000 units: Injection molding usually provides the lowest per-part cost—if TPU properties meet requirements. Where thermoset performance is essential, compression molding or high-volume casting operations remain competitive.

5. Conclusion

The right polyurethane manufacturing process depends on matching production requirements to process capabilities. Volume drives the economics, but material properties, tolerances, part complexity, and lead time all influence the decision.

Open casting delivers unmatched flexibility and premium properties for custom applications. Injection molding provides efficiency and consistency at scale. Compression molding offers a practical middle ground for medium volumes and simpler geometries.

Manufacturers with multi-process capabilities can optimize each project independently rather than forcing applications into a single process. When evaluating suppliers, assess their process expertise alongside capacity and quality systems.

6. Frequently Asked Questions

Which process is most cost-effective for prototype quantities?

Open casting is nearly always most cost-effective for prototypes. Tooling can be produced in one to two weeks for $500–$2,000, allowing material validation in the intended formulation before committing to production tooling. Injection molding requires expensive steel molds that don’t make sense until volumes justify the investment—typically 10,000+ units.

What volume justifies investment in injection molding tooling?

Most applications require annual volumes above 10,000 units to justify injection molding tooling costs of $50,000–$150,000+. The breakeven point depends on part size, complexity, and how long the tooling will remain in service. Some high-volume programs justify the investment at lower annual quantities if the product lifetime extends many years.

Can different processes achieve the same hardness range?

All three processes can produce polyurethane across a wide hardness range—Shore 20A through 85D is achievable. However, the specific formulations differ. Open casting offers the widest selection of specialty formulations. Injection molding is limited to available TPU grades. Compression molding handles high-hardness materials (Shore 80A+) particularly well.

How do cycle times compare across manufacturing processes?

Cycle times vary dramatically. Open casting requires 16–48 hours for initial cure plus 16–24 hours post-cure—measured in days, not minutes. Compression molding runs 5–30 minutes per cycle depending on part thickness. Injection molding operates in seconds to minutes, enabling the highest production rates. These differences drive the volume thresholds where each process becomes economical.

Which process offers the best dimensional tolerances?

Injection molding achieves the tightest tolerances, with ±0.1mm achievable for precision applications. Steel molds don’t flex during processing, and automated control eliminates operator variability. Compression molding typically holds ±0.25mm. Open casting provides ±0.5mm standard tolerances, though post-machining can achieve tighter specifications when required.

Pepson has manufactured high-performance polyurethane elastomers since 1998, serving industries worldwide from our Dongguan, China facility. Our material science expertise and quality manufacturing deliver solutions optimized for demanding applications.