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Rapid Injection Molding: A Comprehensive Overview

bottle cap tool injection molded

What is Rapid Injection Molding?

Rapid injection molding (RIM) describes a process that is increasingly used for delivering injection molded parts on a short time cycle and often (but not always) at a lower tooling cost than more typical scheduled processes.

 

This rapid cycle is achieved by one or several of these approaches, collectively termed bridge tooling in that they are not generally mass-production capable but act as a bridge from prototyping to pilot build:

  • Soft tooling – cavity parts cut from Aluminum, un-hardened steel or Beryllium-Copper to ease the cavity machining.
 
 
  • Additive manufactured tooling cavities that require limited (or no) machining work to be put into low volume production
 
 
  • Simplified tooling designed for speed rather than endurance and high volume production.
 
 
  • High speed and rapid turnaround machining to accelerate tool production.
 
 
  • Modular tooling, where a pre-made ‘bolster’ (the bulk of the tool parts and actions) is used as a receiver of custom cavity components that are fast and simple to produce. This typically only allows for line of draw parts only, i.e. no undercuts. It also sets definitive size limits based on the size of the cavity inserts, so it’s more common for small parts.

Importance and applications of Rapid Injection Molding

Rapid Injection Molding (RIM) is crucial in Industry 4.0 manufacturing processes for various reasons:

  • Speed to market:

RIM enables quick production of molds, significantly shortening the time from design to early generation finished product in real production materials.

It allows much more representative prototyping and testing, facilitating greater design certainty and enabling finesse iterations without risking mass production and tooling schedules so greatly.

  • Cost efficiency:

RIM uses less expensive materials and processes compared to traditional mold making but still delivers production-quality validation parts, especially beneficial for prototyping and pre-production runs that confirm real product performance before high volume production commences.

It helps identify design flaws early, reducing costly mass production tooling iterations, recalls and waste in mass production.

  • Flexibility:

Deepens validation and refines the product performance before committing to full-scale production.

Ideal for fast-custom parts, limited scale production and products that require frequent updates or variations.

  • Quality assurance:

Finished and real-material products from parts that are functionally identical to those made by mass production methods allow for accurate testing of fit, form, and function. This establishes stronger QA processes earlier and allows HALT (highly accelerated life testing) evaluation before high volume production.

Supports a wide range of materials and allows altered material choices for evaluation, ensuring the end product better meets specific performance requirements.

RIM helps product schedules, budgets and outcomes in various ways.

Highly effective in creating functional prototypes quickly, enabling thorough testing and refinement before mass production. This is valuable insurance against product failures, recalls and mass production ramp-up headaches.

  • Market testing:

Enables small, early production runs of user-tolerant products to test market response and make adjustments based on feedback.

Provides high-quality prototypes for consumer testing, helping to gauge interest and gather real-customer and channel insights.

  • Bridge production:

Acts as a bridge between prototype and full-scale production, allowing manufacturers to begin selling products while final production tools are being made. Time to market and market entry risks are reduced, often considerably.

Supports initial product launches where speed and flexibility are critical.

  • Low-volume production:

Ideal for industries requiring rapid, high performance custom parts or small batches, such as medical devices, aerospace and automotive.

Suits products with limited or seasonal demand, reducing the risk of excess inventory.

Advantages of Rapid Injection Molding

RIM is in reality a relatively complex group of capabilities/services that fill a spectrum from advanced prototypes all the way to fast-entry to mass production.

High volume production capabilities

RIM has traditionally been associated with prototyping and low-volume production, but advances in materials, technology and processes have extended its capabilities to enable support of high-volume production effectively.

 

  • Advanced materials and tooling:

Current rapid injection molding uses advanced materials like age-hardened, high-grade Aluminum alloys, pre-hardened steel cavity inserts and additive manufactured components in advanced steels which can withstand higher production volumes.

Improved techniques in RIM mold designs and cooling approaches increase the durability and efficiency of the molds.

 

  • High-speed production equipment:

State-of-art machinery such as high-speed injection molding machines equipped with advanced controls and automation can maintain consistent production rates and quality, where RIM tooling is adapted to their capabilities..

Automation and robotics for insert loading, part handling and automated inspection systems reduce cycle times and labor costs, making high-volume production more feasible. Again, RIM tooling can be developed to accommodate these approaches.

 

  • Process optimization:

Using precise process control and monitoring to optimize injection parameters in real time, based on QA and vision systems ensures consistent part quality and reduces defects over large volumes.

Concurrent design and engineering processes streamline production and minimize lead times, allowing for rapid scaling from prototype to high-volume manufacturing. This is an ideal way to optimize scheduling through multiple generations of tooling that suit each development/production stage.

 

  • Scalable and modular production systems:

Modular tooling allows for easy scalability by using cavity inserts and interchangeability/commonality of tooling components, facilitating quick changes and adjustments for ramping-volume production.

Utilizing multi-cavity molds increases the number of parts produced per cycle, significantly boosting overall production capacity. RIM can bridge from prototypes to early production, allowing a more cautious approach to the high investment of high-speed production tooling.

 

  • QA and repeatability:

Implementing real-time monitoring and QA systems ensures that each part meets requirements, maintaining high standards through and between production runs.

Statistical process control (SPC) facilitated by continuous monitoring and adjustment from statistical data delivers consistent and compliant components.

 

  • Flexible and agile manufacturing:

Advanced mold changing systems and standardized tool interfaces enable quick transitions between different production runs, maximizing uptime and productivity. RIM tooling can be designed for compliance with these systems.

Streamlining processes and eliminating waste reduces processing costs and facilitates high-volume production demands effectively.

Applications in high-volume and bridging production

  • Consumer electronics: High-volume production of cases, functional components and connectors with tight tolerances and intricate designs.
 
  • Automotive industry: Mass production of interior and exterior plastic parts: dashboards, bumpers, controls, engine/fuel ancillaries and trim components.
 
  • Medical devices: Large-scale production of disposable/consumable medical products: syringes, vials, stents, sutures and diagnostic equipment.
 
  • Packaging: Manufacturing high volumes of plastic packaging, including bottles, caps and containers.
 
  • Industrial components: Production of high-strength and durable components for machinery and industrial applications.

Reduced cycle time

Reducing cycle times in rapid injection molding result from agile processes, simplified tool designs and structures and rapid manufacturing methods of several varieties. RIM tooling can be developed to accommodate state-of-the-art processes to be Industry 4.0 compliant:

 

  1. Advanced tooling materials: Materials like advanced Aluminum alloys and Beryllium-Copper used in rapid molds dissipate heat faster than steel molds, reducing cooling times significantly.

 

High-quality machining of molds ensures better fit and finish, minimizing the time needed for mold adjustments and maintenance. Spark erosion machining of pre-hardened cavity components can produce ready-to-use components.

 

  • Optimized mold design: Incorporating conformal cooling channels in additive manufactured tool parts enhances cooling efficiency and uniformity across the mold.

 

Modular tool designs allow a high proportion re-use rather than customization, shortening design cycles.

 

  • High-performance machines: Up-to-date molding machines can inject materials at higher speeds and pressures, increasing productivity.

 

DC servo controlled systems provide precise control over the injection and clamping processes, reducing cycle times and improving repeatability.

 

  • Process automation: Automated insert loading and part ejection and handling systems reduce the time between cycles.

 

Integrating secondary operations such as in-mold labeling or insert molding reduces the need for follow-on processing steps.

 

  • Optimized processing conditions through real time analysis: Utilizing data-driven techniques to optimize temperature, pressure and timing parameters reduces cycle times.

 

Continuous monitoring and automatic adjustments ensure optimal conditions are maintained throughout the production run, reducing downtime and cycle variations.

 

Multiple influences improve cycle times, and where the product is expected to ramp to high volumes these approaches are highly cost effective. Where RIM tooling is to be used in a project’s early stages, the opportunity presents to trial and bed-in these methods before mass production, increasing the production data and machine learning opportunities.

Utilization of various plastic resins

A major advantage in using RIM tooling is material flexibility, improving the representation of mass production performance earlier in the process:

 

  • Most plastic prototype materials are simulacra for real production and lack precision, strength or good representation of moldings:

 

FDM parts: close to real materials, but low precision and low strength parts that have poor anisotropy.

 

SLA and related processes: Moderate to poor strength resins that are brittle, weak and often creep badly. Part accuracy and anisotropy are fairly good.

 

Material jetting: Better quality materials and better anisotropy than SLA type processes. Potential for higher resolutions. Parts are still relatively weak compared with molded components.

 

CNC machined part: Moderate to good precision, close to real-materials properties but many features represent very high production costs so parts tend to be simplified to suit the process.

 

  • RIM tooling allows real production materials to be processed under real, or close to real mass production conditions – so parts will be fully representative of real production in all regards.

 

This opens up all molding materials as options, so the prototypes, field test products and pilot build products can be truly representative of mass production outcomes.

 

  • Cavity tools can be switched to alternate materials to achieve different properties. 

 

Where strength is found to be marginal in first parts, changing from a basic ABS to a high strength or impact resistant ABS simply requires minor parameter adjustments.

 

Stepping further along the strength continuum, switching to an ABS/PC blend is no more complex and can add 20%+ to load capacity.

 

In engineering materials, many options exist in the acetal (POM), nylon, PEEK, LCP etc ranges. Where shrinkage rates differ in a material change, this can be compensated to a degree by altering molding parameters (pressure, packing, dwell, cooling etc).

Ability to produce high-quality parts quickly

Every phase of the design-to-moldings progression is open to acceleration, where RIM parts are required:


  • Tool design: Re-use of modular tooling elements, such as blank bolster tools ready to receive custom cavity inserts, means that the tool design phase can be very much shorter.

 

Much of the design effort in high volume tooling is spent on the reliability and durability of the tool structure and the aggressive optimization of processing speeds.

 

In RIM tooling, performance over long term, high volume is not a major concern, so tool design can be much more generic and off-the-shelf, shortening tool design cycles considerably.


  • Tool manufacture: As with tool design, the difference between RIM and mass production tooling lies in the durability and processing speed of the tools.

 

Where the focus is on schedule rather than million-cycle endurance, materials can be less durable and therefore much easier/faster to machine.

 

This results in faster manufacture of tools that allows part manufacture to happen considerably sooner.


  • Tool trials and first parts: Suppliers who specialize in RIM supply operate on the basis of speed as their primary value proposition.

 

This leads to typically agile processes that put tools up onto molding machines the day they’re ready, so first-shots can be produced and evaluated.

 

After first shots, there is typically some adjustment either of cavities or tool function required, to improve blanking (face sealing), gate turbulence, gas venting etc. These can be very fast adjustments, within an agility-focussed (rather than mass production focussed) environment.


  • Production: Commonly, a second tool trial is a longer production run that will allow time for adjustment of molding parameters and changes of material to deliver best-possible T1 (first production trial) parts.

 

Where these are close to production standard, it’s common to run straight from T1 to a pilot run of plastics, to shorten the timetable by not taking the tool down from the molding machine.

Use of aluminum molds for rapid production

Various suppliers in the RIM capability spectrum have preferred methods for tooling manufacture. These range from normal production tooling, but done fast, through to specialist skills in RIM and a dedicated low-volume molding services:

Cavity elements for RIM tooling are often cut from Aluminum or Beryllium-Copper, offering reasonably hard wearing surfaces, good thermal conduction and the ability to machine the parts very quickly.

In an increasing number of cases, the cavity parts can be additive manufactured in a wide variety of materials – from Copper alloys to stainless steels. These parts will typically require some effort in finishing, so blanking (face sealing) and part surface textures can be controlled, as the printing processes cannot produce machined quality, flat surfaces. The economics of surface finishing make the additive manufacturing of cavity inserts a finely balanced decision that only offers cost-benefit from more complex parts.

 

Two forms of injection molding machine are prevalent in the RIM space:

  • Vertical axis molding machines offer some advantages in RIM service provision.

They tend to employ simpler format tools that can consist of two cavity parts with simple location pins that reference the two halves to each other. Clamping to resist the injector pressure is provided by the moulding machine. More complex and rapid action tooling can also be designed for vertical axis machines.

 

This simplicity can be beneficial for RIM processes in that the simpler tooling is fast to design and make. However, this process can be relatively slow to operate with such simplified tooling. It will typically not include many of the mass production tooling features such as optimized cooling channels and integrated and self operating ejector pins. It will not be suitable for typical levels of mass production of complex parts and it is ill suited to parts with multiple undercuts and limited draft angles.

 

These two part tools can be very effectively made in Aluminum, Beryllium-Copper and non-hardened steel and will provide production-representative parts in real materials, with most injection molding polymers being usable in this format.

  • Horizontal axis molding machines.

This is the more common mass production type equipment for more complex and larger parts. The tooling for these machines is considerably more complicated and typically optimized for durability and mass production.

 

For RIM applications, it is common to use a standard and ready-made tool (sometimes termed a bolster tool) that has internal cutouts prepared, ready to receive custom cavity insert pieces similar to those shown below.

 

These cavity inserts are often cut from Aluminum and mate to make the required cavity. Such inserts can use pre-existing gate and ejector features that are built into the bolster tool and require corresponding and standard features in the cavity parts.

 

This tool type is capable of modest production, but cannot be fully optimized as the cavity parts will generally not contain optimized cooling channels and so the cycle times will be longer, as conduction from the cavity inserts to the bolster is relied upon for cooling. However, Aluminum or Beryllium-Copper cavity blocks are highly conductive, so this can still deliver quite effective tooling.

rapid injection molded button
A basic cavity insert pair for a button molding, machined in Aluminum

Wide range of applications

RIM parts and service provision is a growth sector, with various competing methodologies and levels of service provision offered by a spectrum of suppliers.

Rapid Injection Molding Process

Rapid injection molding is essentially a streamlined version of the more generalized  injection molding process, intended to produce high quality, real production material prototypes and low-volume production parts quickly. These are the process stages, assuming the parts are already designed and rapid prototyped to confirm their readiness for tooling:

  • Material Selection: In hand with part design, resin selection must be based on the part’s requirements, the appropriate plastic resin is chosen. There may be limitations imposed by the RIM service provider, however the material differentials in difficulty for 1most components/projects pose little challenge in terms of availability/feasibility.

 

  • Tooling design: Simplified and minimized tool designs are developed, delivering functional and detailed designs in 2D and 3D CAD.

These tool designs can be: for whole injection molding tools that are moderate volume production ready: or they can be for simplified tooling cavities that can be mounted into pre-existing generic tool-sets: or they can be simple tool cavity blocks that can be slow-operated for a very limited number of parts, for experimental or design validation purposes.

 

  • Toolmaking: Exploiting both traditional toolmaking skills/equipment and advanced manufacturing techniques such as CNC machining, EDM and metal additive manufacturing, rapid tools are built quickly. These molds can be made from a range of materials from Aluminum, Beryllium Copper, tool steels and pre-hardened steels, depending on the volume and durability expectations. Material choices impose hardness-related costs, so these decisions must balance cost, speed and durability.
 

Incorporating conformal cooling channels in the mold design will improve cooling efficiency and reduce cycle times, but this cost is only justified when tools are to be used for moderate volumes and require short cycle times.

 

  • Molding process: The basic elements of the molding process are identical for all tool types, tool materials and injected polymers:

The tool is mounted to the machine type it was designed to operate on, molding parameters are set to a reasonable starting point and the barrel of the molding machine is heated to be ready to inject. The tool is closed and the machine clamping engaged to resist the injection pressure.

The selected resin is injected into the mold under high pressure. The mold cavity is filled, and the material takes the shape of the final part.

The part is allowed to cool and solidify within the mold. The use of conformal cooling channels ensures even and efficient cooling, reducing cycle times. However, simplified bolster tools carrying custom cavities without direct cooling can still process effectively, albeit more slowly.

Once the part has solidified, it is ejected from the mold. Automated systems or ejector pins are typically used to remove the part without damaging it, however, manually operated ejection can be used to additionally simplify the tooling, if production of parts being quite slow and low volume is acceptable.

 

  • Post-Processing: Any excess material (flash) is trimmed, and the part is finished to meet the desired specifications. Flash is more prevalent in the more basic and low cost approaches to tooling, as its obviation relies on high precision at the blanking (closure) faces of the tool which are liable to be of lower grade in lower cost executions.

Post processing may include surface treatments, polishing, or painting.

RIM is a versatile and efficient range of processes that bridge the gap between prototyping and full-scale production. The ability to produce high-quality parts quickly and cost-effectively makes it an essential tool across most manufacturing sectors.

How does Rapid Injection Molding compare with traditional injection molding?

Comparing rapid and traditional molding services/capabilities

Both rapid injection molding and normal injection molding offer advantages and they are suited to distinct stages of the product manufacturing life cycle. Rapid injection molding excels in speed and flexibility for low to medium volumes and prototyping. In contrast, normal injection molding is the best choice for high-volume production, offering superior part quality and durability. By understanding these differences, manufacturers can choose the appropriate method based on their specific project needs and production goals.

Significantly reduced lead time

As a general guide, the development of a volume-capable injection mold tool set for a multi-component plastic product will have a timeline that is at least 40 days from acceptance of quotation to delivery of T1 parts. It is not uncommon for subsequent adjustments and operational/cosmetic corrections to the tooling will take an additional 20-40 days and 3 or more trials to deliver production standard parts.

 

T1 is the common description for the second trial components run from a tool – T0 being the ‘expected failure’ parts that identify functional or manufacturing issues that are corrected immediately. The main difference between the two is surface finish. If the tools operate exactly as planned and make dimensionally accurate and fully formed parts at T0, then surface finishes/textures will be applied to the cavity so that fully production parts are molded at the T1 trials.

 

RIM tooling schedules are very variable, depending on the nature of the approach taken. The most extreme form is the additive manufacture or CNC machining of cavity inserts for a pre-existing tool, and these can have a schedule of as little as 3 days to T0 trials, from the most agile suppliers. More typically, this basic process takes 5-10 days to T0 and it can be longer, for fast-made but full-production capable tooling.

Rapid tooling techniques

RIM is essential for accelerating the development cycle, particularly in prototyping and low-volume production.

  • CNC Machining – uses computer-controlled tools to cut the mold material to the required forms. It can handle all materials, deliver good surface finish and is suitable for complex geometries and fast processing.
 
 
  • Metal additive manufacturing – creates molds or mold components directly from digital models by lithographic construction. It can be used for stainless steel, titanium, hard Copper alloys, Aluminum alloys and more. It is capable of producing complex  and intricate geometries with integrated cooling channels. This method offers particularly quick turnaround times but may require finish machining to deliver good molding finishes and tool components that close fully.

 

Processes include direct metal laser sinter (DMLS), selective laser sinter (SLS), powder bed fusion (PBF), powder bed additive manufacturing (PBAF), direct energy deposition (DED), material jetting, binder jetting and the list is growing.

 

  • Casting – in some cases, it is practical to cast cavity components by means of 3d printed wax masters or similar techniques. This can be a great way to create highly complex parts that pose schedule or process difficulties by other methods. This process is more commonly used for making Aluminum or Beryllium-Copper cavity tools, rather than steel.
 
 
  • Vacuum casting – a master pattern is 3D printed and a silicone mold is formed around it. This silicone mold is then used to cast urethane or other resins. This is a low cost, fast mold production method and good for complex geometries. It works well with a limited range of plastics, but can produce good quality and rapid parts.
 
 
  • Soft tooling – using more easily machined materials like Aluminum, Beryllium -Copper  or softer steels for mold construction. These tools are cheaper to produce than hard tooling, suitable for low to medium production volumes, and good for iterative design processes. The capability, cost and schedule of the tooling will depend heavily on the complexity and cycle time requirements.
 

 

Speed vs. durability: Rapid tooling techniques prioritize speed and flexibility at the expense of long-term durability and cycle efficiency. 

 

Complexity and precision:  Metal additive manufacturing approaches are ideal for complex geometries and high precision, while CNC machining provides better surface finish and accuracy for less intricate parts.

 

Cost:  Rapid tooling techniques generally reduce initial tooling costs, making them potentially attractive for prototyping and small batch production

 

 

Rapid tooling techniques offer significant advantages in speed, flexibility, and cost-effectiveness, making them essential for modern product development cycles, especially in prototyping and low-volume production. By leveraging advanced methods such as CNC machining, 3D printing, and DMLS, manufacturers can quickly produce high-quality molds and tools, accelerating time-to-market and enabling iterative design improvements.

Role of CNC machining in rapid injection molding

CNC machining is widely employed in the manufacture of components for rapid injection mold tools, as it offers precision, repeatability and high quality surface finish while delivering completed parts quickly.

 

It serves both in making structural components such as top and bottom plates, ejector and stripper plates and for cutting waterways and cavity features into tool elements.

Materials and Tooling

Selection of plastic resins for rapid injection molding

The process of selection of polymers (and necessary additives/modifyers) for use in rapid injection molding processes is a direct result of the product design materials selection. Select for functional needs in:

 

  • Tensile strength, impact resistance, and flexural strength are critical for parts subjected to mechanical stress.
 
  • Flexibility, for applications recruiting shock resilient or cyclic load tolerant parts.
 
  • Heat resistance, consider the maximum operating temperature expected.
 
  • Tolerance of chemical exposures, select resins that can withstand exposure to expected chemical contacts without degrading.
 
  • Color and surface  finish, where cosmetic needs must be met.
 
  • Transparency, meaning polycarbonate, polystyrene, ASA or acrylic are good options.
 
  • Melt-flow properties, since resins with low viscosity fill intricate molds more effectively.
 
  • Shrinkage must be allowed for, by oversizing the cavity.
 
  • Cost, balancing the resin’s cost with the required performance characteristics.
 
  • Tooling influences, as some resins may require more expensive molds due to wear and tear.

Considerations for end-use applications

When planning for the end-use of rapid injection molded parts, various factors are important in meeting desired performance:

 

  • Confirm the selected material, as implemented in the design, can withstand the mechanical stresses of use.
 
  • The part may need to be flexible (e.g., living hinges) or resilient (e.g., structural components).
 
  • The material must tolerate the temperature peak of the application without deforming, degrading over time or losing structural integrity. It must withstand the minimum temperature without serious embrittlement.
 
  • The part should resist attack from exposure to chemicals, solvents or other harsh substances involved in the application.
 
  • Consider the risks and consequences of exposure to moisture, UV light and other environmental factors.
 
  • The surface quality of the resultant part should meet the aesthetic standards required for the end-use application.
 
  • The material should be able to be colored in ways that suit it without influencing the molding, or translucent/transparent polymers can be used as required.
 
  • The molding process must deliver sufficiently high accuracy for fit and function.
 
  • Material shrinkage during the cooling process must be allowed for in making cavities, to ensure final dimensions.
 
  • Decide whether the part will require additional post-processes such as painting, plating or assembly. 
 
  • Consider assembly stages that require bonding, welding, or mechanical fastening, and choose materials that can perform.
 
  • Balance material performance, product volume and process issues with cost constraints.
 
  • The material should provide an appropriate lifecycle.
 
  • Consider whether the material can be recycled at the end of its life.

Production tooling options for rapid injection molding

Where the transition to volume production is required from the RIM tooling, a more refined and capable process must be used. Consider options such as:

 

  • Rapid production tooling, paying a higher price for faster service, without quality/capability compromises in the resultant tools.
 
  • Partial cavity completion in multi-cavity tools to reduce production times.
 
  • Build high quality cavity inserts in hardened steel for use in bolster tools – but expect them to be transferred to permanent and high volume tooling after the RIM process is satisfied.

Benefits and limitations of aluminum molds

Benefits:

  • Aluminum molds are less expensive to produce than steel, making them ideal for prototyping and low to medium volume production.
 
  • They’re easier and faster to machine, leading to shorter delivery schedules to T1 parts.
 
  • They’re easier to handle and install, reducing setup times and labor costs.
 
  • They provide efficient cooling even when lacking optimized cooling channels.

 

Limitations:

  • They are much less durable than steel molds, so they are unsuitable for high-volume production.
 
  • They are not suitable for producing parts with very tight tolerances or complex geometries that require high-pressure injection, as the tools can suffer distortion and blanking damage very easily.
 
  • You cannot achieve the same level of detail or finish as steel molds.

Role of steel rapid tooling in the process

Steel rapid tooling plays a crucial role in rapid injection molding, particularly when high durability and precision are required. Here are the key aspects:

 

  • Steel molds are more durable than softer material tools, making them suitable for higher-volume production, withstanding more cycles before showing signs of wear, maintaining consistent quality over longer production periods.
 
  • Steel molds offer better dimensional stability and can maintain tighter tolerances and better blanking.
 
  • They can handle more complex, precise and intricate part designs due to their strength and rigidity.
 
  • Steel molds can achieve higher-quality surface finishes, reducing the need for additional post-processing.
 
  • They possess better thermal/dimensional stability, which helps maintain consistent part quality.
 
  • Although of lower thermal conductivity than Aluminum, cooling galleries can be incorporated into steel molds to manage heat effectively.
 
  • Although initial costs are higher, steel molds become increasingly cost-effective with higher volumes because of their longevity, reducing per-part cost.
 
  • Steel molds can handle a wider range of plastic materials, including those with higher melting points, higher acidity in the melt and abrasive fillers.

Design guidelines for rapid injection molding

Designing parts for rapid injection molding is almost identical in methodology to best-practice for design for mass production. Common design goals are:

  • Select materials that offer appropriate mechanical, thermal and chemical properties for the application.
 
  • Design parts for uniform wall sections to relieve issues with warping, sinking etc.
 
  • If thicker sections are necessary, use coring or ribbing to reduce effective wall thickness, material usage and improve cooling times.
 
  • Apply appropriate draft angles (typically 1 to 3 degrees) for trouble-free ejection.
 
  • For parts with textured surfaces, increase the draft angle as required to accommodate for the texture depth.
 
  • Add ribs to strengthen the part without significantly increasing local thickness. Ribs should be around 60% of the wall thickness to prevent sinking.
 
  • Integrate bosses for screws or other fasteners, ensuring they have adequate support and are connected to the main walls with enough support.
 
  • Use filets to blend internal and external corners, reducing stress-concentration related distortion by improving material flow. Sharp edges can lead to stress concentrations and potential part failure.
 
  • Position gates to encourage laminar material flow and minimize weld lines.
 
  • Include sufficient venting to prevent air traps.
 
  • Define tolerances based on the capabilities of the molding processes, rather than hoping to exceed them.
 
  • Design for proper fit and assembly, taking into account the material’s shrinkage and dimensional stability.
 
  • Use rapid prototyping techniques to create and test early versions of the design, to identify potential issues before tooling. Conduct thorough testing of prototypes to ensure the part meets requirements.
 
  • Ensure the design considers ejection, to avoid areas of the molding ‘hinging up’.

Guidelines that apply to all moldings, but apply more strongly to candidates for RIM:

  • Design parts to avoid undercuts that complicate mold design and increase tooling complexity. Use these where you must, but recognize they may be barriers to RIM or add considerable time/cost.
 
  • Where possible, simplify complex geometries to reduce mold complexity and production time. This applies more to RIM than production tooling, where complexity is more tolerated.
 
  • Select materials that are compatible with the tooling processes. There are more restrictions in material for RIM processes, so select with care.

Applications and Production

Use of rapid injection molding for low-volume production

RIM processes are ideally suited to fast delivery of low volume components. Typical uses are:

 

  • Final design validation and trial-production components in order to confirm that the DFM outcome is as required, before committing to mass production tooling.
 
  • Final design validation to reduce the downstream cost/delay risks of design iterations in production tooling – or the disaster of product recalls.
 
  • ‘Golden sample’ product builds to make a more effective handover from design to production and allow optimization of production lines before the volume wave rolls through.
 
  • Market and field test ‘prototypes’ that are more full representative of function and durability than can be achieved by typical prototyping methods.
 
  • Early market entry for volume/demand testing and product reviews to ‘prime the pump’.

Cost considerations in rapid injection molding

The decision to commit to an RIM process requires apparently large costs and must be considered carefully:

 

  • RIM tools can be expected to range from 10% to 120% of the cost of production tooling. The most basic and simplified approaches are both fast and low cost, but lack anything approaching real-production capability. the most expensive approach is to simply make mass production tooling but push the timetable to deliver it faster.
 
  • The benefits of and RIM process can be very large, but they can also be hard to quantify in a cost-benefit analysis. If the design is assumed to be perfect and the mass production tooling schedule is acceptable, then RIM can seem like a waste of money.
 
  • The schedule and market impression risks involved in design iterations that most be performed on mass production tooling can be very damaging. RIM can offer insurance against these.
 
  • The market risk in delivering a product that requires replacement or recall can be catastrophic. RIM offers similar insurance against this.

 

As a rule, the accounts department will resist RIM costs unless either a) a strong case is made by the production/development team or b) the company has recent experience in the consequences of design iterations and recalls in parallel and related products.

 

Making a case for RIM generally requires a thoughtful presentation of the risk assessment based either on typical cases or recent experience. Insurance is best when it isn’t used, but in the case of RIM, buying the insurance is the ACT that prevents the downstream problems.

Benefits of rapid injection molding for small batch production

For small batch production that cannot justify mass production tooling – or where the batch outcomes become the catalyst for mass production demand – RIM is the go-to process.

 

It is simply not possible to make good product that will endure long term use, based on additive manufactured parts. Only molded parts will give molded part performance.

 

In the few cases where its possible to use CNC machined plastics to substitute for molded parts, this can work well – but it is very likely that when the batch size required exceeds a handful of sets, the cost of RIM will be competitive.

 

Finally, RIM opens up both material selection and processing options to allow fine tuning that simply cannot be achieved without molding.

Thousand-part production capabilities

Once thousand-set capability is required, the most basic approaches to RIM become impractical. This is primarily because of setup/processing issues such as manual unloading of simplified tooling, which can be a skilled and intricate process.

 

Typically, this level of demand limits the RIM process to custom cavity inserts to be mounted in pre-existing bolster tools. This process allows good cooling, relatively fast processing and integrated ejection.

 

If low effort in component finishing is sought, then these inserts should be CNC machined Beryllium-Copper or steel, or additive manufactured bronze, steel or stainless steel. Recognize however that 3D printed cavity parts are liable to require post-print CNC finishing to deliver surfaces that seal and cavities that have controlled finishes.

Quality standards in rapid injection molding

It is inevitable that some relaxation of quality standards must apply to components that result from the most basic forms of RIM. While cavity precision may well be excellent, molding parameter options such as packing pressure and controlled cooling are harder to regulate/select.

 

This will typically result in the need to accept these parts as advanced prototype rather than mass production, altering the QA process markedly.

 

However, when more advanced (and slower, more costly) RIM processes are applied, parts can be expected to be of, or very close to mass production standard in dimensional accuracy, straightness and material properties.

Conclusion

By leveraging rapid injection molding, manufacturers can accelerate product development and production schedules, reduce costs and schedule risks and facilitate design flexibility.

 

RIM offers:

 

  • Speed, with reduced production lead times.
 
  • Lower initial tooling costs, to meet final product evaluation and low to medium volume production.
 
  • More rapid and lower risk design changes and iterative testing.
 
  • High-quality, precise parts with good surface finishes that beat prototypes in all regards.
 
  • Support for a wide range of thermoplastic resins.
 

 

RIM is optimal for:

 

  • Late stage prototypes before full-scale production.
 
  • Producing small batches of parts for pre-mass production purposes.
 
  • Fast development of new products to meet evolving market demands.
 
  • Rapid production of custom and short-run medical, aerospace and other high value market sector components.

Future prospects and developments in the field

Advancements in the RIM field will continue to drive efficiency, flexibility, and sustainability in the rapid injection molding industry. the demand is unlimited, once the supply can meet the economic needs of increasingly ordinary products:

Near future prospects include:

 

  • Development of new thermoplastics and composites to enhance part performance and expand application possibilities is happening at a steady pace.
 
  • Increased use of robotics and AI to streamline production processes, reduce costs, and improve precision. Specialist RIM services are constantly increasing their capabilities and chasing cost reductions.
 
  • Combining additive manufacturing with rapid injection molding for more complex and customized mold designs is nascent and not yet cost effective in many applications, but the printing processes are improving rapidly.
 
  • Innovations in eco-friendly materials and processes will reduce environmental impact.
 
  • Utilization of digital twin technology for real-time monitoring and optimization of the molding process is a fact of advanced production, and its cost is falling so its broader application is imminent in Industry 4.0 settings.
 
  • Enhanced cooling methods will further reduce cycle times and improve part quality. This is in part driven by the increased access to additive manufacture and its effect on waterways.
 
  • Incorporation of sensors and IoT will result in better process control and predictive parameter adjustment for quality.
 
  • New RIM methods and materials are delivering quicker, more durable tooling that is better suited to the beneficial outcome of reduced cost and increased volume capability.
Jon
Jon
Jon is a dynamic and accomplished professional with a rich and diverse background. He is an engineer, scientist, team leader, and writer with expertise in several fields. His educational background includes degrees in Mechanical Engineering and Smart Materials. With a career spanning over 30 years, Jon has worked in various sectors such as robotics, audio technology, marine instruments, machine tools, advanced sensors, and medical devices. His professional journey also includes experiences in oil and gas exploration and a stint as a high school teacher. Jon is actively involved in the growth of technology businesses and currently leads a family investment office. In addition to his business pursuits, he is a writer who shares his knowledge on engineering topics. Balancing his professional achievements, Jon is also a dedicated father to a young child. His story is a remarkable blend of passion, versatility, and a constant pursuit of new challenges.

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