Injection molding and urethane casting are two classic processes for manufacturing plastic products, and are widely used in the daily production of various types of plastic products. When implementing creative plastic products, the first task is to weigh the compatibility of the two, because the choice directly affects the final presentation of the product.
To help you make scientific decisions, this article will sort out the core differences between the two, analyze the technical characteristics, scope of application and cost-effectiveness. By comparing the advantages and disadvantages, application scenarios and potential risks, it will help you accurately match the manufacturing plan according to product requirements, production scale and budget, and realize the transformation of ideas into products.
What Is Urethane Casting?
Urethane casting (also known as urethane casting) is a process for manufacturing products using liquid urethane resin through a casting process. The core process is to inject liquid polyurethane material into a silicone mold, and after curing, it forms a replica that is highly consistent with the prototype. This process combines the flexibility of rubber and the strength of plastic, and can produce products that are both rigid and flexible. It is widely used in fields that require high precision, complex structures or small batch production.
What are the advantages and disadvantages of urethane casting?
Advantages
1.Extremely low mold cost
The cost of silicone mold is only 1% of that of steel mold (about 500-2000 yuan), and the production cycle is shortened to 1-3 days, which is suitable for rapid trial and error.
2.Complex structure reproduction ability
It can form ultra-thin walls (0.5mm), hollow cavities and slightly undercut structures that are difficult to achieve by injection molding, and the surface texture resolution reaches 20μm (such as leather grain and wood grain).
3.Material flexibility
By mixing resin formulas, it can simulate the hardness (Shore A 30 to Shore D 80) and mechanical properties of materials such as ABS, PP, and rubber.
4.Fast delivery cycle
It only takes 48-72 hours from design to physical product, and supports high-frequency design iterations (such as 5 revisions of medical device handles in just 2 weeks).
5.Low-risk trial production
No high mold investment is required, suitable for verifying market demand or functional testing, avoiding capital sinking before mass production.
Disadvantages
1.Limited mold life
Silicone molds can only be reused 20-50 times, and are not suitable for mass production of more than 500 pieces.
2.Limitations in material performance
Conventional resins have weak temperature resistance (80-120°C) and lower mechanical strength than injection molded parts (tensile strength is about 30-50MPa, and injection molded ABS can reach 60MPa).
3.Low dimensional accuracy
Curing shrinkage is about 0.15%-0.3%, and the tolerance is controlled to ±0.3mm/100mm, which is lower than the ±0.1mm of injection molding.
4.Unit cost increases with quantity
Unit cost is low for small batches (such as about 38 yuan/piece for 100 pieces), but the cost is significantly higher than injection molding after more than 500 pieces.
5.Post-processing requirements
Manual trimming of gates and flash is required after demoulding, and surface treatment (such as painting) requires additional processes.
What is injection molding?
Injection molding is a process that injects molten plastic material into a precisely designed mold under high pressure, and forms a plastic product of the desired shape and size after cooling and solidification. Its core process includes: mold closing → injection → pressure holding → cooling → mold opening → product removal, and batch production is achieved through periodic cycles. This process is suitable for thermoplastics (such as PE, PP, PVC, etc.), and can achieve efficient molding of complex parts through physical state changes (solid state → molten state → solid state).
What are the advantages and disadvantages of injection molding?
Advantages
1.Extreme mass production efficiency
A single molding cycle takes only tens of seconds, and with multi-cavity molds (such as 64 cavities), tens of thousands of pieces can be produced per day, which is suitable for large-scale production of automotive parts, daily necessities, etc.
2.Ultra-high precision control
The mold steel hardness is HRC 45-55, and with mold temperature control technology, the tolerance can reach ±0.05mm (such as the PIN position of electronic connectors).
3.Excellent material performance
Supports special engineering plastics such as high temperature resistance (such as PEEK resistant to 260℃) and high strength (glass fiber reinforced plastic tensile strength exceeds 150MPa).
4.Extremely low unit cost
The mold life exceeds 500,000 times, and the unit cost can be as low as 0.1 yuan after mass production (such as disposable plastic tableware), and the marginal cost is close to zero.
5.Excellent surface quality
Mirror polished molds can produce Ra≤0.05μm finish, directly realizing spray-free appearance (such as mobile phone shells).
Disadvantages
1.High mold cost
Precision steel mold development requires 50,000-500,000 yuan, with a cycle of 4-8 weeks, which constitutes a financial threshold for start-ups or small-batch projects.
2.Difficult design modification
Once the mold is finalized, structural adjustment requires remaking or modifying the steel mold (cost increase of 30%-200%), and the iteration flexibility is poor.
3.Structural restrictions
Minimum wall thickness must be ≥1.2mm, and the undercut structure requires a complex slider mechanism (increase mold cost by 20%-50%).
4.Material waste
Gate and runner waste accounts for about 5%-15% (hot runner technology can be reduced to 2%, but the mold cost increases by 30%).
3.High energy consumption
Injection molding machines with a clamping force of 100-5000 tons consume 20-200kWh of electricity per hour, and the carbon emission intensity is significantly higher than that of flexible processes.
Comparison: Urethane Casting vs. Injection Molding
Although both methods fill a mold cavity with plastic,there are some differences between urethane casting and injection molding. Let’s check them out.
| Features | Urethane casting | Injection molding |
| Mould tools | Silicone rubber molds are used | Use hardened steel or aluminum molds |
| Material selection | Thermoset polyurethanes are commonly used | Suitable for thermoplastics such as ABS, polypropylene, TPU, etc |
| Throughput | Best for small batches | Best for large quantities |
| Tolerance | The average tolerance is about 0.01 inches | Achieve tight tolerances (up to 0.005 inch) |
| Wall thickness | The permissible wall thickness is 1 mm to 5 mm | Produces consistent wall thicknesses (0.5 mm to 4 mm) |
| Undercut | Easier to adapt to undercuts | Complex undercut die design |
| Apply | Suitable for small part volumes and rapid prototyping | Ideal for high-volume production |
| Delivery time | Shorter lead times facilitate rapid prototyping | Longer lead times, especially for complex designs |
| Cost | Low tooling cost, suitable for small to medium batch production | The cost of tooling is high, and high-volume production is cost-effective |
Which Has Lower Startup Costs?
The startup costs of urethane casting and injection molding differ by 10 times? LS uses data to reveal the difference in initial investment between the two processes, analyze hidden cost traps, and help you accurately choose a production solution.
Comparison of startup costs of polyurethane casting and injection molding
Key data of initial investment
| Process type | Mold cost range | Applicable batch size | Cost recovery critical point |
|---|---|---|---|
| Urethane casting | 5002,000 | 5 – 500 pieces | Immediate payback |
| Injection molding | 10,00080,000 | 5,000+ pieces | Requires mass production of 5,000 pieces |
Urethane casting: asset-light startup solution
Ultra-low mold threshold
The cost of silicone molds is only 1%-3% of steel molds, and mold production can be completed within 72 hours. It is particularly suitable for:
- Startup teams verify market demand
- Crowdfunding product trial production (such as Kickstarter hardware projects)
- Medical equipment functional prototype testing
Zero environmental additional cost
Operation at normal temperature and pressure, ordinary studios can carry out production, eliminating infrastructure investment such as dust-free workshops and heavy equipment.
Flexible iteration advantage
The cost of a single mold modification is <$200, supporting 5-10 version iterations of the same product (such as smart wearable device development).
Injection molding: heavy asset and long-term investment
High initial sunk costs
The cost of steel mold development is equivalent to the price of a family car (10k-80k), and needs to bear:
- 4-8 weeks mold production cycle (time cost)
- Minimum order quantity (MOQ) limit (usually 5,000 pieces)
- 30%-100% additional modification costs caused by mold design errors
Hidden cost traps
- Dust-free workshop construction: ISO 8 level standard requires $150k+
- Injection molding machine rental: 200-ton model monthly rental 3k-5k
- Raw material inventory: at least 10 tons of plastic pellets stockpiled (occupying $15k+ cash flow)
Case: Comparison of startup costs for production of smart home thermostat housings
1. Background
A start-up company develops smart thermostats, and plans to produce 300 enclosures for testing and validation in the first order, and the subsequent production may increase to 10,000 units. Now compare the two solutions of polyurethane casting and injection molding to assist in production decisions.
2. Plan details
(1) Scheme A: Urethane casting
- Mold development: 3D printed prototype costs $200, silicone mold making costs $800, can be reused 30 times, and only takes 3 days from design confirmation to delivery.
- Production inputs: $5 per piece of material, $3 manual pouring labor cost.
- Total start-up cost: $3400, calculated as 200 800 (5 3)×300.
- Delivery ability: the first piece will be issued within 24 hours after the mold is completed, and it will take 5 working days for 300 pieces to be delivered.
(2) Scheme B: injection molding
- Mold development: Precision steel mold (H13 steel, 2 cavities) cost $25,000, mold testing $2,000, and took 7 weeks.
- Production inputs: $0.8 per piece of material, $150 per hour for renting an injection molding machine (2.5 hours for 300 pieces), and $500 for a clean room to share the cost.
- Total start-up cost: $28,590, calculated as 25,000 2,000 (0.8×300) (150×2.5) 500.
- Delivery ability: After the mold is completed, it takes 2 weeks to test the mold to adjust the first piece, and 300 pieces of production can be completed within 3 working days after the mold is completed, but the mold takes 7 weeks.
Cost comparison conclusion
| Cost type | Polyurethane casting | Injection molding | Difference |
|---|---|---|---|
| Mold development | $1,000 | $27,000 | +$26,000 |
| Unit cost | $8/unit | $3.5/unit | -$4.5/unit |
| Total cost of 300 units | $3,400 | $28,590 | +$25,190 |
4. Decision-making suggestions
(1) Recommend polyurethane casting
- Save $25,190 on your first order of 300 units.
- The cost of modification is low, which is convenient for design iteration.
- The market verification failure was manageable at just $3,400.
(2) Injection molding application scenarios
- The initial investment is large, and 667 units need to be pre-sold at a profit of 43 US dollars / unit.
- The cost of mold modification is high when the design is defective.
- It is suitable for enterprises that have received large orders or have clear mass production needs.
How Do Material Options Differ?
Material selection is a crucial consideration when discussing urethane castingversus injection molding. Here are the main differences between the two molding processes in terms of material selection:
| Project | Urethane casting | Injection molding |
|---|---|---|
| Material Limitations | Only room-temperature curing resin, tensile strength ≤ 80 MPa, engineering plastics cannot be used | There are no obvious restrictions, and high-strength materials such as glass fiber reinforced nylon are supported, and FDA food-grade certified materials can be used |
| Material advantages | Good feel and texture effect, suitable for small batch production | A wide range of material options support high-strength materials and can be directly used with FDA food-grade approved materials |
From the perspective of material selection,urethane casting and injection molding have their own advantages. Urethane casting is suitable for those production projects that have special requirements for product feel and texture, and the output is not large. With a wide range of material options, especially for high-strength materials and FDA food-grade approved materials, injection molding is uniquely positioned to create products that are subject to high loads or come into contact with food. Therefore, when choosing a molding process, manufacturers must fully consider the actual needs of the product and the characteristics of various materials to make the most appropriate decision.
Which Delivers Faster Lead Times?
A customer needs to produce 100 plastic gears and requires delivery within 1 month.
Assuming that the materials meet the performance requirements (strength ≥ 80 MPa), analyze the difference in delivery time between urethane casting and injection molding.
1.Time comparison (taking 100 pieces as an example)
| Stage | Urethane casting | Injection molding |
|---|---|---|
| Mold making | 48 hours (silicone mold) | 4-8 weeks (steel mold CNC machining) |
| Single piece production cycle | 20 minutes/piece (manual post-curing) | 30 seconds/piece (fully automated) |
| Total production time for 100 pieces | ~33 hours (100×20 minutes) | ~0.83 hours (100×30 seconds) |
| Total delivery cycle | 48 hours (mold) + 33 hours ≈ 3.3 days | 4 weeks (mold) + 0.83 hours ≈ 28 days onwards |
2. Comparison of different production scale schemes
(1) Small batch (≤ 500 pieces)
Urethane casting has obvious advantages, mold development is only 3 days, and the cycle time from design to delivery is short. The injection molding mold is made in more than 4 weeks, and the mold waiting time is much longer than the former production cycle. For example, 100 gears are cast in polyurethane and can be delivered in 3 days.
(2) Large quantities (≥ 1000 pieces)
Injection molding is more efficient. After the mold is completed, it takes only 8 hours to produce 100,000 pieces. Like 100,000 pieces order, the total injection molding cycle is about 8 weeks (mold making) and 8 hours (production), a total of 59 days; If the polyurethane casting takes 20 minutes per piece, 100,000 pieces need to be produced without sleep for about 13.8 days, which is a long time and high cost.
3. Key decision-making rules
The key to the decision is the trade-off between tooling and production time. The production time of the product is shorter than that of injection molding and polyurethane mold production time, and the injection molding is better, which is suitable for 100,000-level large-scale orders; On the contrary, small batches of polyurethane casting.
4. Decision-making suggestions
- Urgent small quantity order: selected polyurethane casting, 3 days delivery, quick response.
- Long-term mass production: Invest in injection molds in advance, with large upfront investment, but the unit cost is low and fast, suitable for large-scale demand.
5. Risk Warning
- Injection molding risk: small batches (such as 100 pieces) for injection molding, waiting 4-8 weeks for molding, easy delivery delay, affecting reputation.
- Urethane casting risk: Polyurethane is easy to age for a long time, and products with high durability requirements, such as automobile parts, should be injection molded with engineering plastics to ensure quality.
What Are the Precision Gaps?
When discussing the accuracy gap between urethane casting and injection molding, we can conduct a detailed analysis from two aspects: tolerance comparison and surface quality.
Precision comparison analysis (urethane casting vs. injection molding)
| Dimension | Urethane casting | Injection molding | Difference multiple |
|---|---|---|---|
| Tolerance range | ±0.3 mm (fluctuation caused by silicone mold deformation) | ±0.05 mm (in accordance with ISO 2768-m standard) | 6 times |
| Surface roughness | Ra 3.2-6.3 μm (needs to reach 1.6 μm after manual polishing) | Ra 1.6 μm (direct molding) | 2-4 times |
| Dimensional stability | Low (greatly affected by temperature and humidity) | High (steel mold + automatic control) | Significant difference |
| Cost impact | +$8/piece (polishing cost) | No additional cost (directly meet the standard) | — |
Case: Medical device shell manufacturing (tolerance±0.1mm, surface sterility required)
Urethane casting
- Tolerance defects: The actual tolerance ± 0.3mm (error ≈ the thickness of 3 sheets of A4 paper), which cannot meet the needs of precision assembly, resulting in excessive component clearance.
- Surface treatment dilemma: manual grinding to Ra1.6μm is required, the cost of a single batch increases by 800 yuan, and there is a risk of microbial contamination in manual operation.
- Quality judgment: due to excessive tolerances and surface defects, the product needs to be reworked or scrapped, and it has not passed the medical-grade acceptance.
Injection molding
- Tolerance control: The actual tolerance ± 0.05mm (error ≈ half a hair diameter) to meet the needs of nesting assembly and ensure the stability of the equipment.
- Surface quality advantage: direct molding reaches Ra1.6μm medical grade standard, in line with FDA/ISO 13485 certification, no secondary processing is required.
- Efficiency results: pass the quality inspection at one time, save rework time and pollution risk control costs.
Key conclusions:
Process accuracy differences
- Urethane: Affected by the thermal expansion of silicone mold and artificial error, the tolerance is ± 0.3mm, which is difficult to adapt to precision parts.
- Injection molding: The steel mold rigidity and closed-loop temperature control system compress the tolerance to ±0.05mm, which meets the high-precision needs of medical/electronics.
Cost & Mass Balance
- Low-precision scenarios (e.g., trim, cushions): Polyurethane is low-cost, but it is subject to tolerances and post-processing costs.
- High-precision scenarios (e.g., gears, medical interfaces): Injection molding avoids the risk of failure and reduces long-term maintenance costs.
Industry-standard adaptability
- Automotive/Electronics: ISO 2768-m injection molding process for critical components such as sensor housings; Polyurethane is only used for non-load-bearing parts.
- Medical/Food: FDA/ISO 13485 certified injection molding process to manufacture parts that come into direct contact with the human body; Polyurethanes require additional microbiological validation, with a <50% success rate.
Conclusion
The core difference between urethane casting and injection molding lies in material properties, production efficiency and precision level. Urethane casting uses room temperature curing resin, which is fast and low cost (especially for small batches), but is limited by strength (≤80 MPa) and precision (±0.3 mm), and is suitable for flexible, low-load short-term parts (such as cushions, prototypes); injection molding relies on engineering plastics (such as glass fiber reinforced nylon) and steel molds, with significantly superior strength (≥200 MPa) and precision (±0.05 mm), and supports large-scale automated production of complex structures, but requires mold time (4-8 weeks) and high initial costs. The final choice depends on the priority of demand: choose urethane for urgent small batches or flexible needs, and injection molding for long-term large batches or high-precision structural parts.
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Team LS
This article was written by various LS contributors. LS is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding,metal stamping and more.
FAQs
1.What is the difference between casting and injection molding?
The difference between casting and injection molding is: casting is to pour liquid or semi-solid materials into a mold for cooling and curing molding, which is suitable for a variety of materials but the process is complex and the production efficiency is low, and the product accuracy and surface quality may not be as good as injection molding; Injection molding is to inject the molten plastic into the mold through high pressure to cool and solidify the mold, mainly suitable for plastics, with high process efficiency, automatic production, accurate product size, good surface quality, suitable for large-scale, standardized product production.
2.What are the disadvantages of urethane casting?
The disadvantages of urethane casting mainly include: the cost of the mold is high and the life is limited, which is not suitable for mass production; The silicone mold is easy to deform, resulting in unstable product dimensional accuracy; The surface quality of the product may not be as good as that of injection molding and requires additional treatment; At the same time, the performance of urethane materials may be reduced in high temperature or strong polar solvents, strong acid and alkali media, and the use scenarios are limited.
3.Can you inject urethane molds?
Yes, it is possible to inject polyurethane molds. Polyurethane (TPU) is a commonly used injection molding material with good abrasion resistance, oil resistance, low temperature resistance and elasticity. Injection molding polyurethane molds can realize automated production, improve production efficiency, and ensure the dimensional accuracy and surface quality of products. However, it should be noted that injection molding polyurethane molds require the use of specialized injection molding machines and molds, and the mold manufacturing cost is high.
4.What is urethane Molding?
Polyurethane molding is a molding process that processes polyurethane materials into the desired shape and size. The polyurethane molding process includes a variety of methods such as injection molding, casting molding, reaction injection molding (RIMPU), etc. Among them, injection molding is one of the most commonly used methods, which involves injecting a molten polyurethane material into a mold, cooling and curing, and then demolding to obtain a finished product. Polyurethane molded products are widely used in automobiles, electronics, medical, sports equipment and other fields because of their good physical properties and processing properties.
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