Monday, 13 January 2025

Injection Moulding of PolyArylAmides (PARA; PA - MXD6) - Avoid this important mistake with the mould temperature

Hello and welcome to a new post. Today we discuss the crystallization behavior of PolyArylAmide (PARA; PA MXD6), a semi-crystalline high performance Polyamide. Learn the ABCs of PARA here and the differences between PA and PARA here

PARA combines excellent surface finish with high glass fiber loadings, outstanding stiffness (over 38 GPa possible), outstanding strength (>300 MPa possible), low creep, excellent flow (thin walls), and a CLTE which is similar to that of metal. 

Crystallinity is Key

PARA's performance heavily relies on the level of crystallinity achieved during processing. Higher crystallinity leads to improved physical properties, dimensional stability, and high-temperature performance.

Semi-crystalline materials like PARA require temperatures above their glass transition temperature (Tg = 85°C for PARA) to crystallize effectively. Mould temperature directly influences the material's temperature during solidification. Studies indicate that a mould temperature around 120°C is necessary, especially for thin-walled parts, to maximize crystallinity. Figure 1 shows the relative crystallinity of PARA (PA MXD6) as a function of mould temperature [1]. The measured part had a thickness of 3 mm and samples from the core and from the skin where checked for crystallinity. 

Figure 1: Relative crystallinity of PARA (PA MXD6) as a function of mould temperature [1].

DSC analysis as an effective tool to check crystallinity of semi-crystalline polymers

Differential Scanning Calorimetry (DSC) can assess crystallinity in an effective way. An exothermic peak in the DSC curve around 80-120°C signifies incomplete crystallization during injection moulding. Figure 2 shows a DSC curve of a fully crystallized and incomplete crystallized PARA. In the area of the Tg (85°C), an exothermal peak is visible, indicating that the tool temperature during moulding was too low. Sometimes lower tool temperatures are applied to decrease cycle time or to decrease shrinkage and warpage effects. However, this comes at the cost that the part which uses a high performance polymer, will not perform like a high performance polymer. 

Figure 2: Comparing a DSC curve of a fully crystallized and incomplete crystallized PARA [1].

The maximum crystallinity of PARA can vary depending on factors like processing conditions, specific grade of PARA, and additives. PARA typically exhibits crystallinity levels ranging from 40% to 60%.

What are factors influencing crystallinity in general?

  • Moulding temperature: As discussed, high mould temperatures (> 120°C) are crucial for achieving higher crystallinity.
  • Cooling rate: Slower cooling rates generally allow for more complete crystallization.
  • Nucleating agents: Adding nucleating agents can significantly enhance crystallinity rates.

What are the consequences of low mould temperature with PARA?

There are five issues related with low mould temperature which I come across over and over again when moulding PARA: 
  • Increased water absorption: Higher amorphous content leads to increased water absorption, affecting dimensional stability.
  • Post-Crystallization and distortion: Incomplete crystallization can lead to post-crystallization after moulding, causing part distortion.
  • Higher creep: Parts moulded at lower temperatures exhibit increased creep behavior.
  • Poor surface finish: Irregular surfaces and fiber appearance can result from insufficient mold temperature.
  • Ejection issues: Very low mould temperatures can lead to extremely low shrinkage, hindering part ejection.

Conclusion

High mould temperatures (>120°C) are crucial for processing PARA (PA MXD6) compounds. They ensure optimal crystallinity, leading to superior mechanical properties, dimensional stability, and overall part performance. Also, check from time to time the mould surface tempature to ensure that there is a match between set temperature vs. real temperature. 

I hope this blog article has been helpful. If you have any questions, please feel free to leave me a message.

Thanks for reading & #findoutaboutplastics

Herwig Juster

Literature: 

[1] https://www.syensqo.com/en/brands/ixef-para/documents

[2] https://www.findoutaboutplastics.com/2023/11/design-properties-for-engineers-abcs-of.html

Thursday, 9 January 2025

10+ Key Considerations for Selecting the Optimal Polymer Material I Polymer Material Selection Tip

Hello and welcome to a new post on polymer material selection. In the world of plastics, selecting the right polymer material for your project is crucial. The wrong choice can lead to costly mistakes, product failures, and even safety hazards. To help you make the right decision, I have outlined 10 key considerations to keep in mind (Figure 1):

1. Cost: Polymer materials come in a wide range of prices. It is important to balance price and performance when making your selection. Consider factors such as processing and tooling costs, as well as the cost per volume rather than weight.

2. Environmental Exposure: The end-use environment of your product will play a major role in determining the suitability of a polymer material. Consider factors such as temperature, UV light, and chemical exposure.

3. Material Property Data: Gather as much data as you can about the material's properties, including short-term, long-term, and cyclic properties. This information can be found in technical data sheets, material suppliers, and databases such as CAMPUS (Check out my collection of databases here).

4. Processing Data: Ensure that the material you select is compatible with your chosen manufacturing process, such as injection molding or extrusion. Also consider factors such as part design and compatibility with other materials.

5. Appearance: If the appearance of your product is important, be sure to select a material that meets your aesthetic requirements. This includes factors such as surface haptics, finish, and color. Using glass-fiber reinforced compounds will have lower surface aesthetic compared to unfilled polymers. 

6. Agency Approvals: If your product will be used in a regulated industry, such as food, medical, or automotive, you will need to ensure that the material complies with all relevant regulations and standards.

7. Industry Specifications: Some industries have specific requirements for polymer materials. Be sure to check for any relevant industry specifications before making your selection.

8. Actual Requirements and Load Cases: It is important to map out the true load cases and material requirements for your specific application. This will help you select a material that can meet the demands of your product.

9. Prototype Testing: Before making a final decision, it is always a good idea to test prototypes of your product to ensure that the material you have selected performs as expected.

10. Choosing Suppliers: When selecting a polymer supplier, be sure to choose a company that has a broad product portfolio, strong customer support, and a global presence.

Figure 1: 10 key considerations for selecting the optimal polymer - polymer material selection tip.


Update: based on the community feedback (thanks !!), I added sustainability/circularity considerations, as well as checking for Intellectual Property (IP) protection (Figure 2).

Figure 2: Extended key considerations for selecting the optimal polymer - polymer material selection tip.

By following these 10 key considerations, you can make sure that you select the optimal polymer material for your project. This will help you avoid costly mistakes and ensure that your product is successful.

In addition to the above, it is also important to consult with a polymer expert. They can help you understand the different types of polymer materials available and make recommendations based on your specific needs.

Conclusion

Selecting the optimal polymer material for your project is an important decision. By following the tips in this article, you can make sure that you choose a material that is both cost-effective and suitable for your application.

I hope this blog article has been helpful. If you have any questions, please feel free to leave me a message.

Thanks for reading & #findoutaboutplastics

Herwig Juster

Literature: 

[1] https://www.polymermaterialselection.com/start-the-book

Thursday, 2 January 2025

Rule of thumb: Water Lilies and Polymer Melts - Both Show Exponential Behaviours

Hello and welcome to the first post of the new year 2025! I hope you had a great Christmas break and I welcome you all back to a new exciting year of polymer engineering topics. 

Let us start with a Rule of Thumb post (check out my other Rules of Thumb posts here). We are living in non-linear, exponential times, which can be seen daily by the rapid advancements in technology, from Artificial Intelligence (AI), to the stock market. 

For example, if you take the compounded interest of the S&P 500, we see an exponential growth over time. Compounded interest is for Warren Buffet the key for wealth building.

For our brain it is harder to imagine exponential relations, since for us humans, linear thinking is the main "operating model"

There are lots of examples of exponential behaviour: 

  • Population growth
  • Growth of cells
  • Spread of a disease in a pandemic
  • Financial - compounding interest rates

Water lily and exponential growth

A good way to represent exponential behaviour is by looking at a pond where water lilies are growing. Water lilies double in area each day, resulting in an exponential growth. Let us imagine the following: the water lilies take 30 days to cover the whole pond. When will they cover half of the lake? 

Exactly, on the twenty-ninth day. 

Example from polymer engineering: power-law of polymer melt viscosity

Also in the plastics industry and polymer engineering, a well known example of exponential behaviour is the viscosity of polymer melts (Figure 1). 

Figure 1: Water Lilies and Polymer Melts - Both Show Exponential Behaviours.

In general, the viscosity of plastics is a function of shear rate, temperature, pressure, and chemical composition. 

At low shear rates, polymer melts show a linear behaviour (= Newtonian behaviour). In the lower shear rate region, the viscosity is independent of the shear rate.

The viscosity reaches a level which is referred to as zero-shear viscosity. 

At higher shear rates, the Power law behaviour takes over and the viscosity decreases with increasing the shear rate following a negative slope of (n-1), where n is the Power-law index.

This quantitative relationship is represented by the Power-Law model, where n is the Power-law index and k is consistency (Pa*s); Typical n values of polymer melts are between 0.2 and 0.6.

Conclusions

Exponential behaviours are dominating not only our daily life, however also the plastics industry and polymer engineering topics. It is important to have a certain awareness about such behaviours to not be suppressed and use the exponential times in our favours. 

More Rule of Thumb posts can be found under “Start here”

Latest Rule of Thumb posts:

Rule of Thumb in Polymer Injection Moulding: Fast Estimation of Cooling Time

Rule of Thumb in Polymer Engineering: How Economy of Scale Can Lower Costs

Rule of Thumb: Dealing with Weld lines in Polymer Injection Moulding

Thanks for reading & #findoutaboutplastics

Herwig Juster

Literature: 

[1] https://www.forbes.com/sites/bill_stone/2024/08/25/warren-buffetts-secret-formula-for-wealth-creation/

[2] https://www.azom.com/article.aspx?ArticleID=19175



Monday, 30 December 2024

How to win in Polymer Engineering in 2025

Hello and welcome to the last past of 2024. Today I will share with you how to win in polymer engineering in 2025 using the following three steps: 

1. Define categories where you want to grow in Polymer Engineering - for example part design and material selection

2. Set objectives for each category, what you want to achieve in 2025 - for example: be able to systematically select the optimal plastic material for my part

3. Define quantifiable key results for each objective - for example: making a material selection example once a month by using examples from real life and literature


Happy new year and all the best for 2025! 

Greetings & #findoutaboutplastics

Herwig Juster