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Low-volume injection molding lets you leverage the benefits of plastic injection molding without incurring the costs associated with producing high volumes of parts. This article explains what low-volume injection molding is and considers its advantages and disadvantages. You’ll also learn about the relationship between part design and low-volume plastic parts production, and examine alternatives such as CNC machining and 3D printing.
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What is Low-Volume Injection Molding?
Low-volume injection molding produces volumes ranging from a few hundred plastic parts up to tens of thousands of components. This makes it different from high-volume injection molding, which can produce hundreds of thousands, or even millions of parts per year.
Tooling for Low-Volume Injection Molding
Low-volume injection molding uses different tooling than high-volume injection molding. Typically, high-volume injection molds are made from hardened steels. By contrast, low-volume molds are made of aluminum or soft or semi-hardened steels.
Aluminum molds require less than half the machining time of steel molds. In addition, because aluminum has excellent heat transfer properties, aluminum tooling can heat and cool readily. For shorter lead times during mold making and faster cycle times during injection molding, aluminum has some advantages.
Yet, aluminum molds come with strong disadvantages as well. Because aluminum is softer than steel, aluminum tools wear more quickly for fewer shots. Aluminum’s softness also means it can’t handle the high clamping pressures from larger injection molding machines. Consequently, aluminum tooling may not support large parts or parts that require many side actions.
In addition, some grades of plastic are not suitable for aluminum molds. Plastics that require high temperatures, corrosive grades of plastic, and plastics that contain glass fibers are not suitable for this type of tooling. Aluminum molds are also harder to polish and more easily scratched, a problem with parts that are supposed to be glossy or optically clear.
Although aluminum is a commonly used material, Fictiv seldom uses it for molds. Instead, we usually recommend soft and semi-hardened steel tooling for low-volume injection molding. Fictiv can produce this type of tooling efficiently and cost-effectively, and in less time than you might think. Download our injection molding timeline template to keep your projects on track and see how much time you can save on your next injection molding project with Fictiv.
Soft and Semi-Hardened Steel Tooling
There are two main types of soft and semi-hardened steel tooling for low-volume injection molding: P20 and NAK80. Fictiv also uses another grade of steel, 718H, which is a bit harder than P20 but has the same general advantages and disadvantages.
P20 steel is harder than aluminum and its tooling life is between five and ten times longer. Heat treatment isn’t required, P20 can be easily welded, and this general-purpose tool steel can be machined more quickly than harder grades of steel. Tooling lead times can be as short as one or two weeks, but that’s still longer than with an aluminum mold.
NAK80 is a semi-hardened tool steel that has a longer tooling life than P20 steel. It costs more than P20, however, and it typically takes 25% longer to machine the tooling. NAK80 polishes well, however, and this makes it a good choice for optically clear and high-gloss parts.
Low-Volume Injection Molding Advantages
Low-volume injection molding offers the following advantages.
- Less expensive molds
- Shorter lead times
- Reduced production costs
Less Expensive Molds
Low-volume injection molds that are made of aluminum or soft or semi-hardened steels cost less than hardened steel molds both in terms of their material costs and their machining costs.
Shorter Lead Times
The speed and ease of machining reduces the time between the start of mold making and the start of injection molding. Getting your tools made faster also accelerates the time to market for your injection molded products.
Reduction Production Costs
During injection molding, aluminum molds support faster cycle times because of aluminum’s greater thermal conductivity, or ability to transfer heat. Compared to a hardened steel mold, more parts can be produced in a unit of time.
Low-Volume Injection Molding Disadvantages
Despite its advantages, low-volume injection molding also comes with some trade-offs.
- Limited volumes
- Design constraints
- Reduced mold life
Sometimes, production volumes rise if the initial demand projections are inaccurate. This shifts the requirements from low-volume to high-volume injection molding, and higher volumes typically require a more robust and expensive tool. Ultimately, this increases the total cost of tooling since it’s more cost-effective to invest in a single, expensive hardened steel mold than both low-volume and high-volume molds.
Part design and material selection need to account for the softer nature of molds made from aluminum or softer or semi-hardened steels. For example, low-volume molds made of softer metals cannot achieve the tight tolerances and fine surface finishes of molds made from hardened steel.
Reduced Mold Life
Aluminum and soft and semi-hardened steels have a lower yield strength and fatigue limit than hardened steels. Consequently, these softer molds may require repairs. If mold maintenance is required, especially when it’s unexpected, this can add costs and delay projects.
Low-Volume Injection Molding Applications
Low-volume injection molding can be used for low-volume plastic parts production or pre-production prototyping.
Low-Volume Plastic Parts Production
Low-volume injection molding is best used when the number of parts required is relatively small and unlikely to increase dramatically. The crossover volume that separates low-volume from high-volume injection molding is not precisely defined, but it’s generally accepted that anything from a couple hundred to tens of thousands of parts is suitable for low-volume injection molding.
Low-volume injection molding can also bridge the gap between initial prototyping and full production through pre-production prototypes. Normally, these prototypes are produced in batches of up to 1000 parts to provide enough parts for testing or market research. Low-volume injection molding can also be a cost-effective way to find issues that require modifications to more expensive tooling or changes to the parameters of injection molding machines.
Design Considerations and Low-Volume Plastic Parts Production
When designing a part for low-volume injection molding, it’s important to follow proven best practices as described in Fictiv’s Injection Molding Design Guide — there are specific considerations for the mold materials.
For example, injection molds made of softer metals cannot match hardened steels in terms of tighter tolerances and finer surface finishes. Low-volume injection molds for prototyping may also need to support tooling modifications after test articles are produced. Therefore, the mold needs to be thick enough to support additional machining.
Typically, aluminum tools have fewer moving parts to reduce mold wear. Their higher thermal conductivity also means that thinner wall sections can solidify quickly. This underscores the importance of using a consistent wall thickness in your part design. When aluminum tooling is used, the aluminum grades are typically QC-10, and 7075.
Low-Volume Injection Molding Alternatives
Low-volume injection molding is not always the best option for lower-volume production. Although low-volume injection molds are cheaper than high-volume injection molds, any type of tooling requires an investment. These four manufacturing methods provide alternatives to plastic injection molding.
- CNC machining
- 3D printing
- Urethane casting
For lower part volumes, CNC machining is sometimes used instead of injection molding. Multi-axis CNC machines are required for more complex parts, however. Although CNC machines can be highly automated, they still can’t match the speed of plastic injection molding.
3D printers can produce parts that have complex geometries, including ones that are not possible with any other manufacturing technology. The production rate is relatively slow, however. Consequently, although 3D printing is often used for prototyping, it may not be able to produce parts quickly enough for low-volume production.
Vacuum forming, a type of plastic thermoforming, is ideal for thinner parts with simpler designs. Production rates are slow, however, and material choices are limited compared to injection molding.
Urethane casting can produce very robust components that look and feel like injection molded parts but without a significant tooling investment. The manual nature of this process is a significant cost driver, however, and urethanes can cost more than thermoplastics.
Fictiv Supports Low-Volume Injection Molding
Regardless of part volume, it’s important to get design for manufacturing (DFM) assistance as soon as possible during the design process. When you create a Fictiv account and request a quote, you’ll get DFM feedback that can help to ensure the success of your injection molding project. As your operating system for custom mechanical parts, Fictiv will guide you through the entire process and help you find the right tooling solution for your project.