Product designers and engineers have a plethora of things to consider when working on a plastic injection mold for a project. While there are many thermoforming resins to choose from, a decision also must be made about the best metal to use for the injection molding tool.
The type of metal selected for the tool affects production lead time, cycle time, finished part quality and cost. This article lists the top five metals for tooling; we weigh the pros and cons of each one to help you decide which is best for your next plastic injection molding project.
Aluminum 7075 is a high-strength material with a large percentage of zinc, and it’s best used for medium-volume projects involving general-purpose resins, such as ABS, PP, PE and nylon.
Pro: Aluminum ranks number one in common tool metals when it comes to heat transfer—it can be heated and cooled very quickly. Al-7075 machines well, and although it may not be the least expensive material, some part of this cost can be recovered over larger production runs thanks to decreased cycle times.
Con: While aluminum ranks well on heat transfer and cost, it’s still one of the least durable tool materials. With this durability factor in mind, it shouldn’t be used for injecting corrosive resins, such as POM or PVC. Additionally, aluminum is not suitable for high clamping pressures.
P20 is the most widely used semi-hardened tool steel, good for volumes up to 50,000. It’s known for its reliability for general-purpose resins and abrasive resins with glass fibers.
Pro: P20 is used by many engineers and product designers because it’s more cost effective and tougher than aluminum in some applications. It can withstand higher injection and clamping pressures, which are found on larger parts representing bigger shot weights. Additionally, P20 machines well and can be repaired via welding.
Con: P20 is less resistant to chemically corrosive resins like PVC.
Similar to P20, NAK80 is a semi-hardened tool steel for more demanding applications. It’s a good choice for high-volume production, high clamping pressures and resins with glass fibers.
Pro: NAK80 is a tougher and faster machining semi-hardened tool steel than P20. NAK80 is corrosion resistant and doesn’t need to be stress relieved, even after 250,000 cycles.
Con: NAK80 does not transfer heat as well as some other metals. This means potentially longer cycle times that can increase processing costs, and it’s a more expensive material than P20 or Al-7075
An air-hardened tool steel, H13 is considered a “hot-work” steel and is a great choice for large-volume production orders with continuous heating and cooling cycles.
Pro: H13 can hold close dimensional tolerances after more than one million uses, and it’s also easy to machine prior to heat treatment when the metal is relatively soft. Another positive is that it can be polished to a mirror finish for clear or optical parts.
Con: Slightly better than NAK80, H13 has average heat transference but still doesn’t stand up to aluminum in the heat-transfer category. Additionally, it will be more expensive than aluminum or P20.
Finally, S136 is a high-purity stainless steel and is a great choice when surface finish is the primary consideration. S136 is ideal for PMMA, PVC, PP, PC and other optical-grade transparent resins.
Pro: Even more corrosion resistant than NAK80, S136 can be polished to the highest degree and is a top-of-the-line choice for product designers and engineers when optically clear surfaces, such as polycarbonates and acrylics, are a factor in production.
Con: S136 has the highest cost and lowest thermal efficiency compared with other materials.
From cost-efficient aluminum 7075 to poor-heat-transference NAK80, there are several options for designers and engineers to consider for their next plastic injection molding project. With the right manufacturing partner, choosing the right material will help meet project goals, expectations and deadlines.
Gordon Styles is CEO and President of Star Rapid, a provider of rapid prototyping, rapid tooling and low-volume production services. Utilizing his background in engineering, Styles founded Star Rapid in 2005 and under his leadership the company has expanded to 250 employees. With an international team of engineers and technicians, Star Rapid combines advanced technologies—such as 3D printing and multi-axis CNC machining—with traditional manufacturing techniques and high-quality standards. Prior to Star Rapid, Styles owned and managed the United Kingdom’s largest rapid prototyping and rapid tooling company, STYLES RPD, which was sold to ARRK Europe in 2000.
Image courtesy Khaligo/Adobe Stock.