Time to read: 8 min

If plastic injection molding is fast, how is rapid injection molding different? Traditional injection molding already has short cycle times, aka the amount of time it takes to produce parts. Depending upon a part’s size and complexity, a single cycle can take just a few seconds. 

That’s significantly faster than CNC machining, which can take anywhere from several minutes, to a few hours, to a few days. In terms of cycle times, traditional injection molding is also faster than 3D printing, which typically takes a minimum of several minutes per build.

What is Rapid Injection Molding? 

Rapid injection molding is considered to be rapid because the tools that it uses take less time to produce. With traditional injection molding, a complex tool can take eight weeks or longer to machine. With rapid injection molding, the tooling times are much shorter – and the mold materials are less expensive. 

Yet there are some tradeoffs to consider. Because of their materials of construction, rapid injection molds have a shorter tool life and can’t match the part tolerances of traditional high-volume molds, which are often made of expensive hardened steel.

So, how do you know if rapid injection molding is what you need? After all, terms like “fast” or “slow” are relative. If shorter lead times are critical for your project, when do you need your parts and which tradeoffs can you make? And can rapid injection molding support design complexity?  

Remember: rapid injection molds take less time to produce than traditional injection molds.

Injection Molding Rapid Prototyping and Bridge Tooling 

There are two main reasons to use rapid injection molding – and they’re not just about speed. One is about rapid prototyping and the other is about bridge tooling.

  • You need prototypes or production volumes quickly, and you want to use injection molding resins instead of 3D printing resins, machinable plastics, or other materials. 
  • You plan to use a traditional injection mold for high part volumes, but you need a less expensive “bridge tool” that is faster to machine so that you can get some parts quickly.

Rapid Prototyping Injection Molding Example

Unlike traditional injection molding, which has lead times measured in months, rapid injection molding can put parts in your hands in a few weeks. That’s what Alea Labs discovered when the developer of smart HVAC systems partnered with Fictiv on a project. 

In this rapid prototyping injection molding example, the plastic vents and grilles Alea Labs designed had many ribs at various angles. The company needed a partner that could support design complexity and shorter lead times to accelerate time-to-market. They found it in Fictiv.

Design engineers

Design engineers need to balance rapid injection molding’s speed against other requirements.

Rapid Injection Molding Prototyping: Some Comparisons

Before you decide to go with rapid injection molding for prototypes, it’s best to understand how it works in terms of part design, tooling, processing, and part samples. Then, make sure you have the right partner. This article is a good place to learn about rapid injection molding, and Fictiv is ready to help when it’s time to start molding parts.

3D printing and CNC machining have longer cycle times than injection molding, but you won’t have to wait for a tool to arrive before prototyping or production can begin. CNC machines use tools for cutting, milling, drilling, and other operations, and every shop has a suite of tools on hand, but milling a part takes much longer than molding one. So, it might take longer to 3D print or CNC machine a part, but prototyping that starts sooner can put parts in your hands more quickly, enabling you to refine your designs more quickly, too.

Yet there are other considerations. For example, will 3D printing support the same resin you plan to use during production? If not, rapid injection molding is a better choice for prototyping – and you’ll enjoy an easier transition to production since you won’t have to redesign your part for a new manufacturing method. CNC machining can produce parts with complex geometries, but so can rapid injection molding – if you have the right partner. 

Rapid Injection Mold Tooling vs. Traditional Injection Molds

Rapid injection mold tooling has a shorter tool life and less exacting tolerances than the tools used in high-volume injection molding. But that doesn’t mean you have to sacrifice part quality to get a mold more quickly. Fictiv machines rapid injection molds from different metals — or combinations of metals — to balance speed against other project requirements, such as support for design complexity. The materials we use are also suitable for rapid heating and cooling injection molding.

We can help you compare traditional injection molding to rapid injection molding, and we also provide 3D printing and CNC machining services if you need them. No matter which process you choose, you’ll get design for manufacturing (DFM) help along with your quote. Because there are key differences between traditional and rapid injection molding, the rest of this article covers part design, tooling, processing, and part samples.  

Rapid injection molding DFM

Fictiv provides DFM feedback along with your quote so that you get your parts made faster. 

Part Design and the Rapid Injection Molding Process

There are ten major considerations when designing a part for the rapid injection molding process.

  1. Material Selection
  2. Wall Thickness
  3. Transitions
  4. Corners
  5. Draft
  6. Ribs and Bosses
  7. Tolerances
  8. Parting Lines
  9. Gates
  10. Ejector Pins

Fictiv’s Injection Molding Design Guide examines each of these considerations in detail. They’re the same with both traditional injection molding and rapid injection molding, but rapid injection molding raises some special concerns with material selection and part tolerances.  

Material Selection

Injection molding materials range from commonly used polymers to specialty plastics and polymer blends. There are hundreds of different plastic resins available, and they each have different end-use properties and processing requirements. Plus, the same plastic material can come in different grades, including resin types with glass or carbon fibers.

Rapid injection molding supports many of the same plastics as traditional injection molding, but abrasive materials such as glass-filled nylon cause softer molds to wear more quickly, which shortens tool life. That doesn’t mean you can’t use these materials, however. In the case of Alea Labs, the company needed to use a special molding compound with a high glass content.  

Corrosive grades of plastic, like PVC or POM, can also cause an injection mold to wear more rapidly. Tool life usually isn’t a problem if you need prototypes or lower production volumes, but it’s worth considering how many parts you need. If a rapid injection mold requires repairs or replacement, it could affect your project timeline.

Part Tolerances

Rapid injection molded parts that will be used in larger assemblies need to have correct and consistent dimensions so that parts fit together. Because some degree of dimensional variation is expected in any manufacturing process, you’ll need to define part tolerances. These allowable variations are expressed as plus or minus dimensions and are a function of both the resin type and the mold material. 

Traditional injection molds made of hardened steel can produce parts with fine tolerances and withstand production of hundreds of thousands of parts. Softer rapid injection molds won’t last as long and might not be able to meet the same dimensional standards. The commercial tolerances that rapid injection molds can achieve are less precise, but you’ll get lower-cost parts using lower-cost molds at faster speeds. 

Let’s say that’s what you need — what’s the right type of tooling material for your rapid injection molding project?

rapid injection molded parts

Rapid injection molded parts need tolerances that support alignment and assembly.

Rapid Tooling Injection Molding

Rapid tooling injection molding uses tools made of aluminum, soft steels, or semi-hardened steels in various grades. They typically have a single cavity because multi-cavity tools take longer to produce. Hardened steel is sometimes used with rapid injection mold inserts, but hardened steels take longer to machine and are more expensive. Rapid injection molds can also use a master unit die (MUD) to deliver tooling quickly and less expensively, but at lower volumes and with less exacting tolerances.

Aluminum Molds

Aluminum can be machined twice as fast as steel and doesn’t require post-machining heat treatment — together, these two factors shorten tooling times significantly. Aluminum also has excellent heat transfer properties, which can support faster cycle times. Aluminum doesn’t polish as well as steel, however, and that can affect the appearance of rapid injection molded parts that need to be glossy or optically clear. And if you need tight-tolerance parts, an aluminum mold probably isn’t the best choice.

Typically, aluminum molds can handle 10,000 shots or less. That might be enough for your rapid injection molding project, but aluminum molds can’t handle the high clamping pressures from large injection molding presses. PEI, PEEK, and other plastics that require high processing temperatures aren’t recommended for aluminum molds either because aluminum doesn’t maintain high temperatures well.       

Soft and Semi-Hardened Steels

That’s why Fictiv generally recommends soft and semi-hardened steels instead of aluminum for rapid injection molds. Although aluminum tools can be cut in just a few days, P20 steel tooling has lead times of two weeks or less – and P20 can be easily welded to support engineering change orders (ECOs), which are common during prototyping or early-stage production. For glossy or optically clear rapid injection molded parts, NAK80 semi-hardened tool steel is the best choice.     

The Society of the Plastics Industry (SPI), the trade association now known as PLASTICS, defines five classes of injection molds. These classes provide a convenient way to compare plastic injection molding tooling in terms of production volume and cycles, but they’re also useful when considering rapid injection molds.

SPI Mold ClassVolumesCycles
101HighOver 1 million
102Medium-to-High500,000 to 1 million
103MediumUnder 500,000
104LowLess than 100,000
105PrototypeLess than 500

Based on volumes and cycles, SPI classes 104 and 105 might provide what you need. SPI Class 104 molds have hardened steel inserts for tighter tolerances, but have bases made of less expensive cast metal. SPI Class 105 molds sometimes use hardened steel inserts and are the least expensive SPI mold classification.

Multiple Materials and Master Unit Dies

Rapid injection molds can also use master unit die (MUD) inserts with a standard mold frame and customized, removable inserts. MUD molds, as they’re called, can be produced relatively quickly while lowering the cost of initial tooling by as much as 66%. If cost is a consideration, a MUD mold may provide the right combination of advantages.

Processing, Part Samples, and Plastic Injection Molding

Ultimately, it doesn’t matter how fast a mold maker can machine a tool if an injection molder can’t use it to process your parts without defects. That’s why Fictiv’s global Quality Management System (QMS) ensures our network of carefully vetted manufacturing partners keep quality standards high, regardless of whether you need traditional injection molding or rapid injection molding.  

And Fictiv can provide you with T1 samples for functional testing and dimensional measurements in just a few weeks. You can also get T2 and T3 samples if your design changes — and if your design changes require mold changes, then a steel mold that supports welding could make the difference between keeping your tooling or starting over.

There’s a lot to consider with rapid injection molding, and it pays to have expert guidance to make the right decision. 

With Fictiv, it’s simple to create a free account, upload your part drawing, and get a quote along with detailed DFM feedback. And our in-house injection molding experts will guide you from prototyping through production, making recommendations that will help you leverage the speed of rapid injection molding and meet your other requirements.

Create your account to get started.