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What is Rapid Injection Molding:  Process, Materials, and Advantages

In the fast-paced world of modern manufacturing, the ability to quickly and cost-effectively produce high-quality prototypes and low-volume production parts is paramount. By leveraging advanced materials and streamlined processes, rapid injection molding offers an efficient solution for engineers and designers looking to bring their innovative ideas to life with precision and speed.

This article delves into the intricacies of rapid injection molding, exploring its process, materials, and the myriad advantages it brings to the table.

rapid injection molding

What is Rapid Injection Molding?

Rapid Injection Molding (RIM) is a unique injection molding technique designed specifically for the efficient production of small batches of plastic parts. It stands out due to its significantly shorter lead times when compared to traditional injection molding. Although the equipment used in RIM is similar to that in conventional production injection molding, the primary distinction lies in the mold design. This is largely accomplished by using mold materials like aluminum, which are more affordable and easier to machine than the standard steel molds used in traditional injection molding. While aluminum molds lack the longevity required for long production runs, they are adequately durable for smaller batches, typically up to at least 1000 – 500 parts.

How Does Rapid Injection Molding Work?

Rapid Injection Molding (RIM) operates through a streamlined process tailored for efficient production of plastic components. Here’s how it works:

Mold Design and Preparation

Designing parts for RIM involves adhering to Design for Manufacturing (DFM) principles specific to injection molding. RIM molds are crafted using standard materials for cost-effectiveness and rapid manufacturing. Typically, aluminum molds are preferred for their ease of machining and quicker fabrication, though they are not suited for high-volume production.

Mold Assembly and Setup

Once specialized RIM molds are fabricated, they are assembled into two halves—core and cavity—mounted on the fixed and moving plates of a standard injection molding machine. The assembly is secured using hydraulic cylinders to ensure precise alignment and stability during the molding process.

Injection Molding Process

To manufacture parts using RIM, thermoplastic or thermoset resin pellets are fed into a cylinder known as the barrel, where they are melted by external heaters mounted on the barrel, shear forces between plastic pellets, and the heat generated by the screw inside the barrel. Once the plastic is sufficiently melted, a reciprocating screw injects it into the mold through a nozzle, with a check valve preventing backflow into the barrel under pressure.

Molding and Ejection

High pressure fills the mold cavity with molten plastic, while hydraulic cylinders ensure the plastic does not leak at the mold’s parting lines. After a few seconds to allow the plastic to solidify, the mold opens, and the part is ejected using ejector pins. The screw retracts, and the cycle repeats.

Efficiency and Considerations

It’s important to note that while RIM optimizes production for small to medium batch sizes, the per-part production time is comparable to standard injection molding. RIM’s advantage lies in its ability to rapidly produce molds and efficiently manufacture parts without the extended lead times and high costs associated with traditional mold fabrication.

Rapid Injection Molding Equipment Components

Rapid Injection Molding (RIM) involves a range of specialized equipment components that work together seamlessly to produce high-quality plastic parts efficiently. Here’s a detailed look at these components:


The reciprocating screw is central to the RIM process. It has multiple functions, including metering, transporting, and melting the plastic pellets. As the screw rotates, it forces the pellets through an increasingly narrow space, generating heat through shear forces. This heat, combined with external heaters, melts the plastic. The screw then pushes the molten plastic into the mold through a nozzle. Some screws also incorporate mixing elements to ensure a homogeneous melt and even distribution of additives, such as colorants.


The mold in RIM consists of two halves: the core and the cavity. These are mounted on platens and brought together by hydraulic rams to form the mold cavity where the plastic part is shaped. Molds often have cooling channels through which a heat transfer fluid circulates, accelerating the cooling and solidification of the plastic, thus reducing cycle times. RIM molds are typically made from aluminum or other easily machinable materials, designed for shorter production runs but capable of producing high-quality parts.


RIM utilizes a variety of thermoplastics, chosen based on the specific requirements of the application. These materials range from standard commodity plastics to specialty grades with enhanced properties like increased strength, flexibility, or heat resistance. Thermoplastic materials can be repeatedly melted and reshaped, making them ideal for the RIM process.


The hopper is the entry point for the raw material into the injection molding machine. It stores the thermoplastic resin pellets and feeds them into the screw. Depending on production needs, the hopper can be fed from bulk bags or silos. It may also include preheating capabilities to reduce the time required to melt the resin, thereby shortening the overall production cycle. Additionally, level sensors in the hopper alert operators when more material is needed, ensuring a continuous supply.


The nozzle is the critical link between the barrel and the mold. It directs the molten plastic into the mold cavity with precision. Nozzles vary in diameter and design, depending on the machine’s capacity and the part’s requirements. They may also feature filters to prevent contaminants from entering the mold, which could cause defects in the final part. Gap filters are preferred over screen pack filters because they provide a larger filtration area without significantly impeding flow.


The barrel of the injection molding machine is equipped with electrical heaters that wrap around its exterior. These heaters help to melt the plastic pellets inside the barrel, supplementing the heat generated by the screw’s rotation. The combination of external heating and shear heating ensures that the plastic reaches the correct temperature and viscosity for injection.


The final product of the RIM process is the molded part, which must be designed with injection molding DFM (Design for Manufacturing) principles in mind. This includes considerations like optimal wall thickness, placement of reinforcing ribs, and appropriate hole sizes to ensure structural integrity and manufacturability. High-quality part design minimizes defects and improves the overall efficiency of the production process.

8.Nozzle Filters

Nozzle filters are used to ensure that the molten plastic entering the mold is free of contaminants. Gap filters are preferred for their ability to filter without significantly reducing flow rate or increasing pressure loss. These filters play a crucial role in maintaining the quality and consistency of the molded parts.

Additional Components and Considerations

Apart from the primary components, RIM presses may include other elements such as ejector pins, which are used to push the solidified part out of the mold once it has cooled and solidified. The use of Master Unit Dies (MUD) in RIM allows for modular molds that can be quickly altered or swapped out to accommodate design changes without the need for a complete mold remake, further reducing costs and production time.

Advantages and Disadvantages of Rapid Injection Molding

Rapid injection molding offers significant advantages in terms of efficiency, material versatility, and cost-effectiveness for production volumes. However, it also involves substantial upfront costs and lead times, making careful consideration of project needs and objectives essential when choosing this manufacturing process.

Precise injection reduces waste by injecting exact amounts of plastic into mold cavities, enhancing cost-effectiveness and environmental sustainability.Initial investment in specialized tooling (ranging from $3,000 to several thousand dollars) can be prohibitive for low-volume production runs.
High-pressure injection ensures uniform wall thickness and optimal ejection angles, resulting in stronger, pore-free parts compared to 3D printing.Complexity in mold design and fabrication can lead to higher costs per unit compared to processes like 3D printing for low-volume production.
Enables multiple part productions without needing new molds; utilizes durable, high-quality molds capable of withstanding injection pressures and temperatures.Requires expertise in mold design and material selection to optimize cost and performance; lack of experience can lead to higher initial costs and longer learning curves.
Capable of achieving high precision levels comparable to other manufacturing methods, depending on part complexity and material used.Limited flexibility in design modifications; significant changes may necessitate new mold fabrication, adding time and cost.
Reduces unit costs with increased production volume, making it cost-effective for mass production compared to CNC machining and 3D printing.Complex mold design and production can take 5-7 weeks for design and 2-4 weeks for manufacturing, longer than CNC machining and 3D printing lead times.
Achieves smooth finishes (up to Ra 0.8 mcm) through CNC milling and polishing, enhancing part aesthetics and usability.Design changes may require building new molds, which is time-consuming and expensive compared to 3D printing where modifications are simpler and cheaper.
Supports various materials (thermoplastics, elastomers, composites) with minimal system adjustments compared to 3D printing, facilitating versatility./
Minimal setup time reduces production lead times significantly compared to CNC machining and traditional prototyping methods./
rapid injection molding parts

Application of Rapid Injection Molding

Rapid injection molding is a versatile manufacturing process that finds applications across various industries. Its ability to produce high-quality parts quickly and cost-effectively makes it an ideal choice for numerous applications. Here are some key areas where rapid injection molding is commonly used:

  • Consumer Goods: Companies in the consumer goods industry use rapid injection molding to quickly launch new products, test market responses, and make necessary adjustments before committing to full-scale production. For products with shorter lifecycles, rapid injection molding provides a flexible manufacturing solution that can adapt to changing trends and consumer preferences.
  • Medical Device: Rapid injection molding is essential for producing medical device prototypes that comply with stringent regulatory requirements and can be rigorously tested for safety and efficacy. The medical industry often requires custom, patient-specific parts. Rapid injection molding allows for the efficient production of these tailored components without long lead times.
  • Automotive Industry: Automotive manufacturers use rapid injection molding to create prototype components for testing and validation, ensuring they meet performance and safety standards before mass production. It is also used for producing aftermarket parts, specialty components, and limited-run items for automotive enthusiasts and repair services.
  • Electronics: Rapid injection molding is ideal for producing enclosures and housings for electronic devices, offering a quick turnaround for new product launches and updates. The process is used to create precise, small parts needed for various electronic applications, ensuring high quality and reliability.
  • Aerospace: Aerospace manufacturers rely on rapid injection molding for producing high-performance components that meet strict quality and safety standards. The ability to use advanced materials ensures that parts are lightweight yet durable, crucial for aerospace applications.

Injection Molding Rapid Prototyping and Bridge Tooling

Injection molding rapid prototyping and bridge tooling serve critical purposes beyond just speed. Rapid prototyping allows for quick iterations and testing of designs using injection molding resins, rather than slower alternatives like 3D printing resins or machinable plastics. This method ensures that prototypes closely mimic final production parts in both material properties and performance.

Bridge tooling plays a pivotal role in efficiently transitioning from prototyping to full-scale production. It involves creating a temporary injection mold, less expensive and quicker to machine than a standard mold. This allows for the rapid production of initial batches of parts, bridging the gap between prototyping and full production without the cost and lead time of a permanent tool.

What About Wall Thickness in Rapid Injection Molded Parts?

The wall thickness of parts produced through Rapid Injection Molding (RIM) typically ranges between 1.5 and 2.5 mm. This range is consistent with standard injection molding practices, as the principles governing optimal wall thickness apply to both processes. The material used in the molding process can influence the ideal wall thickness, with different materials requiring specific thicknesses to ensure structural integrity and performance.

RIM does not impose additional limitations on wall thickness beyond those present in conventional injection molding. The chosen material’s properties dictate the necessary wall thickness to achieve the desired balance of strength, durability, and manufacturability. By adhering to these guidelines, manufacturers can ensure that rapid injection molded parts meet the required quality and performance standards.

Material Selection and Tolerances in Rapid Injection Molding

When designing parts for rapid injection molding, several key considerations come into play, influencing both material selection and part tolerances. Material choice is critical, ranging from standard polymers to specialized blends and grades such as those reinforced with glass or carbon fibers. Each material offers distinct properties and processing requirements that directly impact the final performance and manufacturability of the part. For example, abrasive materials like glass-filled nylon can accelerate wear on softer molds, leading to shortened tool life.

While traditional hardened steel molds excel in achieving tight tolerances over extended production runs, rapid molds—typically crafted from softer materials—may have limitations in dimensional precision and durability. This trade-off often favors cost-efficiency and faster production speeds, making it ideal for prototypes or smaller production volumes where exact tolerances are less critical.

Selecting the appropriate tooling material for rapid injection molding projects requires balancing these factors. Optimal decisions consider factors like required production volume, part complexity, material characteristics, and budget constraints. Partnering with experienced providers like BOYI ensures these considerations are integrated into the design and manufacturing process, facilitating efficient development and delivery of high-quality parts.

How to Coose a Rapid Injection Molding Tooling?

Rapid tooling for injection molding employs materials like aluminum, soft steels, and semi-hardened steels, each selected for specific advantages in terms of speed, cost-efficiency, and performance.

  1. Tooling Materials: Aluminum, soft steels, and semi-hardened steels (e.g., P20, NAK80).
  2. Advantages of Aluminum:
    • Fast machinability (twice as fast as steel).
    • No post-machining heat treatment required.
    • Good heat transfer properties for faster cycle times.
    • Limited polishability affects glossy or optically clear finishes.
    • Unsuitable for high clamping pressures or high-temperature plastics like PEI or PEEK.
  3. Advantages of Soft and Semi-Hardened Steels:
    • P20 steel: Quick machining, easy modifications for ECOs.
    • NAK80 steel: Excellent polishability, ideal for cosmetic parts.
  4. SPI Mold Classes:
    • Class 104: Hardened steel inserts, cost-effective base materials, suitable for low-volume production with tight tolerances.
    • Class 105: Often uses hardened steel inserts, economical choice for prototypes with minimal production cycles.
  5. Master Unit Die (MUD) Inserts:
    • Inserts within standard mold frames.
    • Cost-effective solution, faster turnaround compared to traditional methods.
    • Allows customization through removable inserts, reduces initial tooling costs.
  6. Considerations:
    • Production volume requirements.
    • Part complexity and design.
    • Surface finish specifications.
    • Budget constraints and cost-efficiency goals.

Comparing Rapid Injection Molding Prototyping with Other Methods

Rapid injection molding offers advantages in speed and material fidelity for prototyping, while 3D printing and CNC machining provide flexibility with complex geometries and quick setup times. Choosing the right method depends on your project’s requirements and manufacturing goals.

Here is a comparison table:

AspectRapid Injection Molding3D PrintingCNC Machining
Cycle TimesShorter due to faster mold creation and injection processLonger due to layer-by-layer printingLonger due to setup and machining time
Tooling RequirementsRequires temporary bridge tooling for initial prototypesMinimal setup time; no tooling requiredRequires specific tooling for each part
Material CompatibilityUses production-grade resins for accurate material testingLimited to specific printing resinsCan use a wide range of materials including metals and plastics
Complex GeometryCapable with the right partner and mold designLimited by printer capabilitiesCapable with advanced CNC machines
Design IterationsEnables rapid iteration with production-like partsLimited by material properties and printing technology/
Transition to ProductionSmooth transition as prototypes use same resins as productionPotential redesign needed for different manufacturing processes/

Rapid Injection Mold Tooling vs. Traditional Injection Molds

Rapid injection mold tooling offers speed and flexibility for prototyping and low-volume production, using materials designed for quick cycles and accommodating complex designs. Traditional injection molds, on the other hand, prioritize longevity, exacting tolerances, and performance consistency across high-volume runs.

Here is a comparison table:

AspectRapid Injection Mold ToolingTraditional Injection Molds
Tool LifeShorter due to faster machining and less durable materialsLonger lifespan due to high-quality materials and precision
TolerancesLess exacting, suitable for prototyping and low-volume runsVery precise, ideal for high-volume production
Materials UsedDifferent metals or combinations for balance of speed and qualityHigh-quality metals for durability and exacting requirements
Heating/Cooling CapabilitiesSuitable for rapid cycles with materials designed for quick heat transferOptimized for consistent performance over extended runs
Support for Design ComplexityCan handle complex designs with appropriate material choicesSupports intricate geometries with high precision
Process ComparisonsFaster setup and machining times; ideal for rapid iterationLonger setup times but optimized for continuous production

Equipment Used for Rapid Injection Molding

Rapid injection molding requires a variety of specialized equipment to produce high-quality parts efficiently. Here are some of the key pieces of equipment used in the process:

1. Injection Molding Machines

Injection molding machines are the core equipment in the rapid injection molding process. They consist of the following main components:

  • Injection Unit: Melts and injects the plastic resin into the mold.
  • Clamping Unit: Holds the mold closed under high pressure during injection.
  • Control Systems: Manages the machine’s operation, including temperature, pressure, and cycle times.

2. Molds

Molds are crucial tools in injection molding, designed to shape the molten plastic into the desired part. They can be made from various materials, such as:

  • Aluminum: Used for its fast machining times and excellent thermal conductivity, ideal for rapid tooling but with limited durability.
  • Soft and Semi-Hardened Steels: Offer a balance between durability and cost, suitable for rapid injection molding with higher volume requirements.
  • Master Unit Die (MUD): Features standard mold frames with interchangeable inserts, reducing cost and lead time for initial tooling.

3. Cooling Systems

Cooling systems are essential for maintaining the efficiency and quality of the injection molding process. They help regulate the mold’s temperature to ensure uniform cooling and solidification of the plastic parts. Components include:

  • Water Channels: Integrated into the mold to circulate coolant.
  • Chillers: Used to cool the water or other cooling fluids.
  • Temperature Controllers: Maintain the desired temperature within the mold.

4. Material Handling Equipment

Efficient material handling is vital for the rapid injection molding process. This includes:

  • Dryers: Remove moisture from the plastic resin to prevent defects in the molded parts.
  • Hoppers and Feeders: Store and feed the resin into the injection molding machine.

5. Automation and Robotics

Automation enhances the speed and consistency of the injection molding process. Key components include:

  • Robotic Arms: Remove parts from the mold and perform secondary operations such as trimming or inspection.
  • Conveyor Systems: Transport finished parts to the next stage of production or packaging.

6. Quality Control Equipment

Ensuring the quality of the molded parts is critical. Equipment used includes:

  • Coordinate Measuring Machines (CMMs): Measure the dimensions of the parts to ensure they meet specifications.
  • Optical and Laser Scanners: Provide detailed surface scans to detect defects or deviations from the design.

Summary Table

Injection Molding MachinesCore machinery with injection, clamping units, and control systems to manage temperature, pressure, and cycle times.
MoldsTools made from aluminum, soft/semi-hardened steels, or MUD inserts to shape the plastic parts; selected based on balance between cost, speed, and durability.
Cooling SystemsWater channels, chillers, and temperature controllers to regulate mold temperature, ensuring uniform cooling and part quality.
Material Handling EquipmentIncludes dryers to remove moisture, and hoppers and feeders for storing and feeding resin into the injection molding machine.
Automation and RoboticsRobotic arms and conveyor systems to automate part removal, secondary operations, and transport, enhancing speed and consistency.
Quality Control EquipmentTools like CMMs, optical, and laser scanners to measure dimensions and inspect surfaces for ensuring parts meet specifications and are free from defects.

Rapid injection molding utilizes a combination of advanced machinery, precise tooling, efficient cooling systems, and automation to produce high-quality parts quickly and cost-effectively.

Your Rapid Injection Molding Partner – BOYI

At BOYI, we are dedicated to delivering top-tier injection molding services tailored to meet your exact specifications. Our state-of-the-art facilities and expert team ensure that every project is executed with precision, speed, and exceptional quality.

Partner with BOYI to experience the perfect blend of innovation, precision, and efficiency in injection molding. Let us help you bring your ideas to life with unparalleled excellence and cost-effective solutions.

Contact us today to learn more about how BOYI injection molding services can drive your success.



How Quick Is Rapid Injection Molding Really?

Rapid injection molding is known for its speed. It offers a significantly faster lead time from design submission to obtaining sample parts compared to traditional full-scale production injection molds. This efficiency makes it an excellent choice for quickly iterating on designs and bringing products to market faster.

What Materials Are Used for Rapid Injection Molding?

Rapid injection molding utilizes a range of materials, including common thermoplastics like polypropylene and polyethylene, and specialty materials such as nylon and polycarbonate. These materials are chosen for their suitability in various applications, offering properties like strength, flexibility, and resistance to heat and chemicals. The ability to test multiple materials in the RIM process ensures that the optimal choice is made for the final product.

How to Choose the Best Materials for Rapid Injection Molding?

Choosing the best material for Rapid Injection Molding (RIM) involves considering the end use of the product. Since RIM is used for prototypes and short-run productions, materials should match those intended for full-scale manufacturing. Key factors include cost, mechanical strength, UV resistance, electrical properties, and thermal resistance.

What Are the Benefits of Using Rapid Injection Molding?

Rapid injection molding offers reduced lead times, increased design flexibility, and the ability to test multiple materials. It also supports low-volume production runs for on-demand manufacturing, making it ideal for quick prototyping and small-scale production.

Catalog: Injection Molding Guide

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