How many cavities should you choose for your injection molding tool? I Rule of Thumb Polymer Processing

Hello and welcome to a new post. In today's post we discuss a community question I received:

How many cavities should you choose for your injection molding tool?

It’s a question that can make or break your project’s budget. Go too low, and you’re missing out on efficiency. Go too high, and tooling costs skyrocket.

Figure 1 [1] compares the cost of the injection mold, material, and injection molding as function of the mold cavities.  The sweet spot is at eight cavities as the optimal cavity number before costs start to climb. It is a classic "bathtub" cost curve and allows one to balance between tooling investment and production savings. 

Figure 1: Choosing the optimal umbers of mold cavities [1].

Conclusions

This curve serves as a first orientation. Important is that you collect all your costs and create such a total cost curve on your own. It depends if you are molding a packaging part, where more than eight cavities are beneficial, or if you are molding an engineering part such as a connector with pin overmolding, where fewer cavities may lead to an optimum already. 

Update - I received an interesting feedback: Prof. Jozsef Kovacs from University of Budapest highlights that László Sors developed a comprehensive analytical method for cavity number optimization as early as 1966, including equations, practical examples, and a nomogram. Sors’s work also addressed prototype molds, tool cost-efficiency, and the integration of thermal, rheological, and electrical calculations into mold design—well before these became industry standards. Sors is recognized as a pioneering figure in polymer tooling and design, leaving a significant legacy in the field. He published his know-how in the 1966 book: Műanyag-alakító szerszámok.

More "Rules of Thumb" posts can be found under "start here".

Thanks for reading & #findoutaboutplastics!

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature: 

[1] A. Pouzada: Design and Manufacturing of Plastics Products: Integrating Traditional Methods With Additive Manufacturing

[2] H. Juster: Optimizing your injection moulding production – my 5+ How’s I Plastics processing tips

Discover the Future of Polyketone Selection: Introducing the Aliphatic Polyketone (PK) Grade Screening App

Hello and welcome to a new blog post in which I introduce to you my new created Polyketone selection app. 

Polymer Material Selection and Plastic Part Design with Polyketone

In the fast-paced world of polymer material selection and plastic part design, finding the right material can be a daunting task. With the growing demand for high-performance, sustainable materials, aliphatic Polyketone (PK) is gaining renewed attention among engineers and designers—especially in Europe. That’s why I’m excited to introduce my newly developed app, designed to revolutionize the way you scout and screen commercial aliphatic Polyketone grades!

A Brief Journey Through Polyketone’s History

Polyketone’s story is as unique as its properties. Originally launched in the 1990s by Shell under the brand name Carilon, aliphatic Polyketone quickly attracted interest for its remarkable balance of mechanical and chemical properties. However, the material was discontinued in 2000, leaving a gap in the market. Fast forward to 2013, and Hyosung Corporation breathed new life into PK with the launch of Poketone, making this versatile polymer available once again.

What makes Polyketone so special? 

Its semi-crystalline molecular structure, alternating between carbon monoxide (CO) and olefin, imparts a unique set of properties. PK is available as both a copolymer (ethylene and CO) and a terpolymer (ethylene, propylene, and CO). The terpolymer variant, in particular, offers enhanced processing for extrusion and injection molding applications. Thanks to its excellent mechanical strength, chemical resistance, and low permeability, PK is a strong candidate to replace traditional engineering polymers like Polyamides, POM, and PBT.

Your New Go-To Tool for PK Grade Selection

Selecting the right Polyketone grade just got easier! My new app (try out here) is tailored to help engineers, designers, and material specialists efficiently navigate the landscape of commercially available aliphatic PK grades—focusing on European providers.

Two Powerful Ways to Search

1. Direct Grade Selection:

Choose from a comprehensive list of commercial Polyketone grades. Instantly access key properties such as:

• Tensile strength
• Tensile modulus
• Elongation at break
• Heat Deflection Temperature (HDT) at 1.8 MPa
• UL rating
• Charpy notched impact

2. Property-Based Search:

Have specific requirements in mind? Simply enter your desired property values, and the app will recommend matching commercial PK grades that fit your needs.

This dual approach streamlines the material selection process, saving you valuable time and ensuring you find the best fit for your application.

Figure 1: Overview of the Polyketone Selector "PK Selector" V1.

Built for the Community—And Growing!

This is just the beginning. As the first version, the app currently covers the most critical properties and a robust selection of grades. Over time, I plan to expand the database with more properties and additional grades, making the tool even more powerful and versatile.

Get Involved!

I invite you to try out the new Polyketone selection tool here and experience a smarter way to choose your next engineering polymer. Your feedback is invaluable—please share your thoughts, suggestions, or feature requests to help shape future updates.

Are you a supplier or manufacturer with a Polyketone grade you’d like to see listed? Reach out to me directly—I’m eager to collaborate and make this resource as comprehensive as possible for the entire community.

Ready to streamline your Polyketone selection process? Try the app today and join the conversation!

Thanks for reading and trying out the app!

Greetings and #findoutaboutplastics

Herwig 

Literature: 

[1] https://www.poketone.com/en/index.do

[2] https://www.polyketon.de/

Choosing the Right Polymer: Why the Cheapest Isn’t Always the Best (Example Rib Plate)

Hello and welcome to a new blog post. When it comes to creating a successful product, the material you choose can make or break your design—literally! Imagine you’re building a plate that needs to be stiff enough to withstand bending (Figure 1). You have a few plastics in mind: unreinforced Polyamide 6.6 (PA 6.6), Polypropylene (PP), and High-Density Polyethylene (HDPE). Which one should you pick? If you’re thinking, “Easy! Just go for the cheapest per kilogram,” think again.

Let’s dive into the fascinating world of material selection, where things aren’t always as simple as they seem.

The Price Tag Trap

At first glance, HDPE looks like a winner. It’s often the least expensive plastic by weight. But here’s the twist: not all plastics are created equal when it comes to stiffness. To achieve the same bending stiffness as PA 6.6 or PP, you’d need to make your HDPE plate much thicker. Why? Because HDPE has a lower modulus of elasticity—it’s just not as stiff.

Figure 1: Example material selection of rib plate - HDPE vs PP vs PA 6.6 [1]. 

The Domino Effect of Thickness

Making the plate thicker doesn’t just mean using more material (and thus, more cost). In injection molding, thicker parts take longer to cool. In fact, the cooling time increases with the square of the wall thickness! That means your production slows down, your output drops, and your costs go up. Suddenly, that “cheap” HDPE isn’t looking so cheap after all.

The Surprising Winner

In our example, the most expensive material per kilogram—Polyamide 6.6—turned out to be the most cost-effective overall. Its higher stiffness meant we could use less material and keep production fast and efficient. Sometimes, paying more upfront saves you money in the long run. Also, you can decrease the wall thickness even more, by using a PolyArylAmide (PARA; MXD6) instead a PA 6.6. 

Beyond the Numbers

Of course, cost isn’t the only factor. When choosing a material, you also need to consider physical, chemical, and thermal properties, as well as how easy it is to process. The “right” material is the one that balances all these needs for your specific application.

The Takeaway

Next time you’re selecting a material, remember: the cheapest option per kilogram might not be the cheapest solution for your product. Think about requirements such as stiffness, processing time, and all the other requirements (chemical resistance) your product needs to meet. A little extra thought at the beginning can save a lot of headaches—and money—down the line.

Let’s Talk About Your Design!

Do you have a plastic part design or a material selection challenge? I’d love to hear about it! Share your project or questions with me here, and I’ll be happy to provide feedback and help you find the best solution for your needs. Let’s make your next product a success—together!

Thanks for reading and #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature:

[1] Konstruieren mit Kunststoffen - IKKV, University Leoben

[2] https://www.findoutaboutplastics.com/2025/04/nature-is-built-on-5-polymers-modern.html

[3] www.polymermaterialselection.com

Plastic Part Failure Analysis - Using Thermal Analysis (DSC) to Estimate the Anti-Oxidant Level in Polymers

Hello and welcome to a new blog post. Let me start today with the following question: How do you ensure the long-term performance of polyolefin materials in demanding applications?

Understanding and measuring oxidative stability is key. The following post explores why oxidative stability matters for polyolefins like polypropylene, and how Differential Scanning Calorimetry (DSC) provides valuable insights into material durability, processing effects, antioxidant performance, and ultimately prevent plastic part failure. Dive in to learn how this classic yet often overlooked test method can help you make informed decisions about material selection and process optimization.

DSC Testing for Oxidative Stability

DSC measures the heat absorbed or released by a material as temperature or time changes. While commonly used for phase transitions (melting, recrystallization, glass transition), it is also effective for detecting exothermic events like oxidation.

How does a typical test procedure for oxidative stability look like?

• A sample (raw material or molded part) is placed in the DSC.
• The sample is heated in a nitrogen atmosphere to a set temperature (commonly 200°C/392°F, which melts PE or PP).
• After reaching the target temperature, air or oxygen is introduced.
• The antioxidant in the polymer protects it until it is depleted; then, oxidation occurs, shown by a sharp increase in the DSC baseline.
• The time from oxygen introduction to oxidation onset is called the Oxidation Induction Time (OIT).
• Also, the test can be used to access the oxidation onset temperature (OOT).

Example PP raw material with standard antioxidant package vs. PP raw material with improved antioxidant package

Figure 1 shows the result for a tested polypropylene (PP) raw material and the OIT was measured at 2.29 minutes. A second PP raw material, which contains an improved antioxidant package, showed a higher OIT (6.42 minutes), indicating better resistance to oxidation under the same test conditions [1].

Figure 1: Using DSC to estimate the anti-oxidant level in Polyolefins - Example PP [1].

Interpreting OIT Results

The OIT value alone is not meaningful, but comparing OITs of materials with similar antioxidant chemistries provides a relative measure of oxidative stability. AS a rule of thumb, higher OIT indicates better oxidative stability and, typically, higher antioxidant content.

DSC offers a quick and practical way to assess oxidative stability compared to more complex antioxidant quantification methods.

Applications of OIT measurements

Raw Material vs. Molded Part:

• Processing (molding) consumes some antioxidants, so molded parts usually have a lower OIT than raw materials. 
• Changes in processing conditions (temperature, screw speed, backpressure) affect OIT and thus the remaining antioxidant content.

Post-Processing and Environmental Effects:

• Sterilization (gamma or E-beam) can significantly reduce OIT, leading to loss of material toughness.
• Long-term heat aging also reduces OIT over time.

Conclusion

DSC-based OIT testing, despite its limitations in perfectly simulating real-world conditions, remains a valuable and practical method for comparing the oxidative stability of polyolefins. It is particularly useful for evaluating the impact of processing and post-processing on antioxidant depletion and for comparing materials with similar stabilizer chemistries.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

🔎I review your polymer material selection and plastic part design - contact me here

🔥 There is a problem keeping you awake - My personal website for Polymer Consulting

📊 Discover your Polymer Material Selection score by taking quick test here

Literature:

[1] https://www.ptonline.com/articles/the-importance-of-oxidative-stability-in-polyolefins-part-2