Polymer Injection Molding – The Effect of Pressure on Viscosity

Hello and welcome to a new blog post. 

How much does pressure really affect polymer viscosity in injection molding?

When we talk about melt viscosity, most of us immediately think about temperature and shear rate. But pressure also matters — and for some polymers, it matters a lot more than many people expect.

The infographic below highlights a key point: the viscosity of thermoplastics depends not only on temperature, shear rate, and pressure, but also on chemical structure and physical conditions.

A good example is the difference between amorphous and semi-crystalline polymers:

• Polystyrene (PS): at 200 bar, viscosity can increase by about 22%
• Polyethylene (PE): at the same pressure, viscosity increases by only about 3–4%

Calculation in detail

The pressure dependence of polymer viscosity, often analyzed using methods like those proposed by Dudvani and Klein [2,3], is significant for polystyrene (PS) at high pressures.

Exponential Model: The effect of pressure P on the viscosity Eta of PS is generally described using the exponential formula: Eta = Eta_0 exp (P + Alpha).

Pressure Coefficient (Alpha): For atactic and syndiotactic polystyrene, studies show the average pressure coefficient Alpha is in the range of  1–3 x10^(-8) Pa^-1.

At 200 bar (200 x 10^5 Pa) with Alpha of 10^-8 Pa, viscosity increases to 1.2214 (22%). 

Mechanism: Increased pressure decreases the free volume available for polymer chain movement, increasing the intermolecular friction and thus the viscosity.

Conclusions

That is an important reminder for injection molding, where pressure effects can strongly influence filling behavior and process stability. In extrusion, by contrast, the effect of pressure on viscosity is often much less relevant.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

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Literature: 

[1] Rao Natti - Design Formulas for Plastics Engineers

[2] https://www.researchgate.net/publication/285636168_Comparison_of_Measurement_Techniques_for_Evaluating_the_Pressure_Dependence_of_the_Viscosity

[3] Dudvani I.J. and I. Klein: Analyis of Polymer MeltFlow  in  Capillaries  Including  Pressure  Effects,SPE Journal (1967) 41-45

[4] https://link.springer.com/chapter/10.1007/978-3-662-41458-3_31

🎯Getting Tolerances Right in Injection Molding: Why Standards Matter! (ISO 20457 & ISO 3302)

Hello and welcome to a new blog post. When it comes to plastic and rubber parts, achieving the right tolerances is key to ensuring quality, fit, and function. But did you know there are specific ISO standards that guide this process?

🔹 ISO 20457 for Plastics:

This standard provides clear guidelines for defining tolerances and acceptance conditions for dimensions of plastic parts produced by processes like injection molding, extrusion, or thermoforming.

Different tolerance groups (TG) are defined:

TG 6 for packaging parts

TG 5 for housing parts

TG 4 for precision parts (like gears)

Note: ISO 20457 applies only to unfilled thermoplastics.

🔹 ISO 3302 for Rubber:

For molded rubber parts, ISO 3302 is the go-to standard. It divides molded parts into four tolerance classes, from M1 (fine) to M4 (coarse). For most technical molded rubber parts, M3 (medium) is typically used.

Figure 1: Overview of ISO 20457 for thermoplastics and ISO 3302 for rubber injection molded parts.

In conclusion

Understanding and applying the right tolerance standards helps ensure your parts meet performance expectations and manufacturing requirements—saving time, reducing costs, and boosting customer satisfaction. 

As a rule of thumb, for precision injection molded parts such as gears, ISO 20457 TG 4can be applied. For housing parts TG 5 and packaging parts TG 3. For rubber molded parts,  ISO 3302 is key, and in particular M1 (fine) to M4 (coarse) (Figure 1).

Check out this post too, where tolerances are important to turn product requirements into plastic & plastic part specifications: read full post.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://maxnext.io/de/blog/allgemeintoleranzen-im-spritzguss-nach-din-iso-20457-ehemals-din-16742-2/

[2] https://www.nh-technology.de/download/Design-Construction-Guide-Plastic-Parts.pdf

[3] https://www.zorge.com/gummi-lexikon/din-iso-3302-1/

[4] https://www.toleranzen-beratung.de/unternehmen/aktuelles/ansicht/iso-204572018-toleranzen-fuer-kunststoff-formteile-informationen-zur-neuen-norm/

Estimating Maximum Injection Pressure in Plastic Injection Molding (Rule of Thumb Plastics Processing)

Hello and welcome to a new blog post in which we’re sharing a practical rule of thumb for estimating the maximum injection pressure in plastics injection molding. 

It’s straightforward: take the minimum necessary injection pressure of your chosen polymer and simply multiply by 1.2. 

The result? Your maximum injection pressure.

Examples applying this rule

Let us look at a few examples with standard parts and medium-viscosity polymers:

• Polyamide: Minimum pressure = 110 MPa → Maximum = 130 MPa
• Polyethylene: Minimum = 100 MPa → Maximum = 120 MPa
• Polycarbonate: Minimum = 120 MPa → Maximum = 150 MPa

Benefits:

• It is a quick and easy way to estimate directly at your machine
• Improves the process stability
• Protects the machine and the mold

This simple approach helps you quickly estimate your injection molding parameters and ensures smoother processing—every time!

More Rules of Thumb can be found in my "start here" section. 

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] Natti Rao: https://www.hanser-elibrary.com/doi/pdf/10.3139/9783446431492.fm

Estimation of Clamping Force in Injection Molding (Rule of Thumb)

Welcome to our latest blog post! 

If you’ve ever wondered how to choose the right injection molding machine for your project, understanding clamping force is key. 

Today, we’ll break down how to calculate the clamping force required for parts with wall thicknesses both below and above 1.5 mm. Mastering this calculation not only ensures high-quality parts but also helps you make smarter, more efficient equipment choices. 

Let’s dive in!

Case 1: part wall thickness below 1.5 mm

Clamping force F [kN] = projected area A [m2] * filling pressure pf [bar] / 100 

Filling pressure pf [bar] = Flow factor kf [bar/mm] * wall thickness factor kw [/] * flow length [mm]

Note: 1 bar = 1x10^5 Pa; Pa = N/m^2 = kg/(m x s^2) 

Flow factors kf depending which material one is using:

ABS: 2.0

PA: 2.0

PA+GF: 2.4

PBT: 2.0

PBT+GF: 2.3

PC: 2.0

PC+GF: 3.3

PE: 1.8

PESU: 3.7

PMMA: 1.8

POM: 2.4

PP: 1.2

PPO: 1.9

PPO+GF: 3.2

SAN: 1.8

PK (Polyketone): 1.2

Wall thickness factor kw:

0.4 mm: 11.9

0.5 mm: 9.2

0.6 mm: 7.3

0.8 mm: 4.5

1.0 mm: 3.0

1.2 mm: 2.1

1.4 mm: 1.5

1.6 mm: 1.1

Case 2: part wall thickness above 1.5 mm

Clamping force F [kN] = projected area A [m2] * pressure p [bar] / 100

Insert for pressure p:

Thermoplastics = 350 - 400 bar

Thermosets = 450 - 500 bar

Rubbers = 500 bar

Figure 1: Overview on clamping force estimation in injection molding.

Check out other Rules of Thumb posts here. 

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://manuelkuehner.de/wp-content/uploads/2015/04/formelsammlung_polymertechnik_kunststofftechnik_hhn_manuel_kuehner.pdf

[2] https://firstmold.com/de/tips/clamping-force/

[3] https://prototool.com/de/injection-molding-formulas/

[4] https://www.justerexpertwitness.com/case-directory

[5] https://polymermaterialselection.com

Cleaning of Injection Molding Tools I Dry Ice Blasting I Processing Hack

Hello and welcome to a new processing hack blog post, in particular on how to clean you injection mold in an optimal way. 

Dry ice cleaning (Dry ice blasting) is an efficient, non-abrasive method for cleaning injection molding tools that allows for faster cleaning cycles and reduced production downtime. The process involves propelling solid carbon dioxide (CO2) pellets at a high speed toward the mold surface, which removes contaminants through a combination of kinetic energy, thermal shock, and gas expansion (sublimation). 

Key Benefits

Minimal Downtime: Molds can be cleaned in-place (in-situ) and at their operating temperature, eliminating the need for cooling, disassembly, reassembly, and reheating. This can reduce cleaning times by up to 75% or more.

Non-Abrasive: Dry ice is a soft medium that sublimates into a gas upon impact, so it does not damage or erode delicate mold surfaces, intricate details, or critical tolerances (Class A-D finishes). This helps prolong the lifespan of valuable tooling.

No Secondary Waste: Because the dry ice turns directly into CO2 gas, there is no water, chemicals, or blasting media residue left behind. The only cleanup required is the removed contaminant itself, which can often be simply swept or vacuumed away.

Improved Product Quality: Cleaning molds more frequently and effectively ensures consistent venting and cavity shape, which helps prevent defects like flash, short shots, and splay, leading to lower scrap rates and higher part quality.

Environmentally Friendly and Safe: The process reduces or eliminates the need for harsh chemical solvents, improving workplace safety and environmental compliance. The CO2 used is often a reclaimed byproduct from other industrial processes.

Cleans Complex Geometries: The process can reach into hard-to-access areas, crevices, and fine vents that are difficult to clean with manual methods or other media. 

Apart from dry ice cleaning, there are three more cleaning technologies which can be utilized. I have listed them in Table 1. 

Table 1: Comparison of injection tool cleaning technologies. 

How It Works

The cleaning action is based on three main principles: 

• Kinetic Effect: High-velocity dry ice pellets physically impact and dislodge contaminants.
• Thermal Shock: The extreme cold temperature of the dry ice (-78.5°C or -109.3°F) causes the surface residue to shrink and become brittle, breaking its bond with the warmer mold substrate.
• Gas Expansion: Upon impact, the dry ice pellets instantly sublimate (turn into gas). This rapid expansion of CO2 volume creates microscopic "mini-explosions" that lift and carry the dirt particles away from the surface. 

Implementation

Manufacturers can either invest in their own portable dry ice blasting equipment (machines and a supply of dry ice and compressed air are needed) or utilize third-party contract cleaning services. Leading equipment manufacturers like Cold Jet offer a range of machines and accessories tailored for delicate to more aggressive cleaning applications. 

Example Cleaning Methods for PPS-GF / PPS-GF+MD Mold Deposits

Cleaning mold deposits from a Polyphenylene Sulfide (PPS) reinforced with glass fiber (GF) and mineral (MD) and/or impact modification often involves a combination of chemical and physical methods. The high service temperature and chemical resistance of PPS require specific cleaning considerations. 

• Chemical Cleaners (During Production): Specific solvent-based, fast-evaporating mold cleaners are effective for use while the mold is still in the injection machine. 
• Apply the cleaner to the top of the open mold and let gravity help the product flow down, flushing away contaminants. These cleaners work by dissolving greases, oils, and gas deposits, which are common byproducts of the molding process. 
• Products like Slide Resin Remover are used during production to "mold off" stubborn deposits in subsequent cycles.
• Physical Assistance: Chemical cleaning is often best when combined with a physical cleaning action.
• For light deposits, a clean, lint-free shop rag can be used for wiping.
• For strongly attached or heavy deposits, use a soft, non-damaging tool like a bamboo, copper, or brass spatula or brush to avoid scratching the mold surface. Metal abrasives can also be used with caution.
• Offline Cleaning (Tool Room): When the mold is removed for thorough cleaning, stronger, fast-evaporating cleaners (often hydrocarbons and naphthas) may be used to dissolve resins and greases. These can be followed by a mold polish/cleaning compound with mild abrasives for a final sheen.
• Advanced Method (Dry Ice Blasting): As explained above, dry ice blasting is a highly effective method that can remove deposits without disassembling the mold from the machine. It works by freezing and air-blasting the contamination off the surface. 

Best Practices for cleaning PPS-GF / PPS-GF+MD mold deposits

• Frequency: Clean frequently. Small amounts of deposits are easy to wipe away; once they accumulate, they can become corrosive and require more aggressive measures.
• Post-Cleaning Protection: Because PPS outgassing can be acidic, always follow cleaning with a rust preventive if the mold will be idle for more than a few hours.
• Solvent Selection: Ensure the cleaner is compatible with your mold temperature. High-evaporation solvents (like hydrocarbons) are best for offline cleaning, while slow-evaporating ones provide better "soak time" for thick deposits.

More processing tips can be found here: Optimizing your injection moulding production – my 5+ How’s

Thanks for reading & #findoutaboutplastics

Greetings,

Herwig 

Literature: 

[1] https://blog.coldjet.com/dry-ice-blasting-vs.-alternative-cleaning-methods

[2] https://www.findoutaboutplastics.com/2016/12/optimizing-your-injection-moulding.html

Wombat: Not Just an Animal—A Lesson for Plastics Operations

Hello and welcome to a new blogpost. When you hear “wombat,” you might picture a sturdy little marsupial from Australia. But in the world of plastics engineering, Wombat stands for something else entirely: 

Waste of Money, Bandwidth, and Time. 

And it’s a concept every polymer professional should keep top of mind.

Wombat in the world of plastics engineering.

Why Wombat Matters in Plastics

In our fast-paced industry, it’s easy to get sidetracked by shiny new technologies, endless data, or meetings that lead nowhere. But every minute and euro spent on non-essential activities is a minute and euro not spent on what really matters: making perfect plastic parts.

Wombat is a reminder to ask ourselves:

• Are we investing in tools and processes that actually improve quality and efficiency?
• Is this meeting, report, or project moving us closer to our production goals?
• Are we focusing on root causes, or just treating symptoms?

Applying Wombat Thinking to the Plastics World

Let’s look at a few practical examples:

• Material Selection: Don’t over-specify or under-specify. Choose the right polymer for the job, not the most expensive or the one with the flashiest datasheet.
• Process Optimization: Focus on parameters that truly impact part quality—like melt temperature, cooling time, and pressure—rather than chasing every minor variable.
• Troubleshooting: When defects arise, use data-driven root cause analysis. Don’t waste time on guesswork or “just try it” fixes.

The Payoff: Perfect Parts, Less Waste

By keeping Wombat in mind, you’ll streamline your operations, reduce scrap, and deliver higher-quality parts—without burning through resources. It’s about working smarter, not harder.

So next time you’re faced with a decision, ask yourself: Is this a Wombat? If it is, steer clear and refocus on what truly drives success in plastics manufacturing.

Let’s leave the wombats in the wild—and keep our operations lean, focused, and efficient!

Thanks for reading & #findoutaboutplastics

Greetings,

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

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Ratio Projects - A Story Beyond Plastic Material Prices

Hello and welcome to a new blog post in which I want to share a story that changed the way of thinking about cost-saving in manufacturing:

Ratio projects (fictional story for illustrative purposes only)

Not long ago, I was sitting across the table from a customer who was laser-focused on one thing: lowering material costs for an injection-moulded part. He was convinced that if we could just supply a cheaper material, his problems would be solved.

But as our technical team and I examined the part and the moulding operations together, something caught our eye. The runner and sprue—the channels that guide the molten polymer into the mould—were enormous. In fact, they were much bigger than the part itself! Imagine pouring a glass of water and spilling twice as much on the table as you actually get in the glass. That’s what was happening here.

Instead of just talking about material prices, we rolled up our sleeves and worked side by side with the customer. We reimagined the entire gating system, redesigning the part and the tooling. It was a true collaboration, blending expertise and creativity.

The result? We didn’t just shave a few cents off the material cost. We cut the total cost by 50%. Half! Not by using a cheaper material, but by using our knowledge to optimize the design and the process (Figure 1).

Figure 1: Optimizing the sprue and gating of an injection mold as part of a ratio project.

The customer was amazed. He realized that the real savings came not from squeezing suppliers for lower prices, but from looking at the bigger picture—design, tooling, and production.

This experience taught us all a powerful lesson: sometimes, the answer isn’t in the price tag of the material, but in the ingenuity we bring to the table. When we focus only on material costs, we risk missing out on much greater opportunities for improvement.

So next time you’re tempted to chase the lowest material price, remember—true value comes from partnership, innovation, and seeing the whole picture.

Thanks for reading & #findoutaboutplastics

Greetings,

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Don't Mix Maximum With Optimum Plastic Material Selection & Plastic Processing I Rule of Thumb

Hello and welcome to a new Rule of Thumb post discussing why mixing maximum with optimum in polymer material selection and plastics processing is not the best thing to do. 

Understanding “Maximum” vs. “Optimum” in Plastic Material Selection

When selecting materials, it’s important to distinguish between “maximum” and “optimum.”

Maximum refers to the highest possible value of a single property (e.g., the highest tensile strength, the greatest heat resistance, or the lowest density).

Optimum means the best possible balance among several relevant properties for a specific application. The optimum is not always the maximum of any single property, but rather the material that meets all key requirements most effectively.

Why is this distinction important?

Choosing a material based solely on a maximum value (such as the strongest or most heat-resistant polymer) can lead to unnecessary costs, processing difficulties, or other trade-offs. The optimal choice is the one that delivers the best overall performance for your specific needs—even if it doesn’t have the highest value in every category.

Example: “Optimal vs Maximum” in Plastic Material Selection

When selecting a plastic material for a specific application, engineers often seek the “optimal vs maximum”—that is, the best possible balance between competing requirements, such as mechanical strength, cost, processability, and chemical resistance.

Case Study: Gear Wheel for Automotive Application

Requirements:

• High mechanical strength and stiffness
• Good wear resistance
• Dimensional stability at elevated temperatures
• Cost-effectiveness for mass production

Material Candidates:

• Polyamide 6 (PA6)
• Polyoxymethylene (POM)
• Polyetheretherketone (PEEK)

Selection Process:

1. Define Key Properties:
   The gear must withstand high loads (tensile strength), resist wear, and maintain shape at temperatures up to 120°C.

2. Score Materials:
   Each candidate is evaluated for tensile strength, wear resistance, heat deflection temperature, and cost.

Find the “Optimal vs Maximum”:

• PA6: Good strength and cost, but absorbs moisture (affecting dimensions).
• POM: Excellent wear resistance and dimensional stability, moderate strength, good cost.
• PEEK: Outstanding properties, but very high cost.

After scoring, POM emerges as the “optimal vs maximum”—it offers the best compromise between performance and cost for this application, even though PEEK has higher absolute properties.

The “optimal vs maximum” is not always the material with the highest individual property, but the one that best meets all critical requirements for the application. In this case, POM is the optimal choice, delivering reliable performance at a reasonable cost.

Takeaway:
When selecting plastics, always look for the “optimal vs maximum”—the material that provides the best overall fit for your application, not just the highest value in a single property.

Optimum vs Maximum in injection molding

For example, in injection molding, the optimum flow rate is not necessarily the fastest possible, but rather the rate that minimizes shear stress, ensures proper mold filling, and avoids defects like air traps or weld lines. Similarly, the optimum temperature profile for a molding process might balance melt temperature for flow with cooling time for cycle time, while minimizing residual stress. 

Figure 1 presents the concept of "maximum vs optimum" for plastics processing. In essence, while maximum settings define the boundaries, optimum settings represent the sweet spot within those boundaries for achieving the best possible results in plastics processing.

Figure 1: Difference between optimum and maximum in plastics processing. 

More Rule of Thumb posts can be found here.

Thanks for reading & #findoutaboutplastics

Greetings,

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://youtube.com/shorts/q_KfChgTwdM

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

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

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

Mastering the Melt: Your Guide to Shear Rate Limits in Injection Moulding (Rule of Thumb)

Hello and welcome to a new Rule of Thumb post on plastics processing. In my previous post we discussed how to locate the maximum shear rates by using injection moulding fill simulations. Now we explore what shear rate limits we need to consider to not harm the processed polymer.

Rheology of polymers

Plastics exhibit non-Newtonian fluid behavior, where viscosity is dependent on the applied shear rate. In certain polymers, shear rate exerts a more significant influence on viscosity than temperature.

Under high stress conditions, such as during processing, polymer molecules align, leading to a substantial reduction and stabilization of the resin's viscosity. This phenomenon is known as shear thinning.

Injection moulding and shear rate / stress limit of polymer melts

In injection moulding, the injection rate or fill time directly correlates with the shear rate experienced by the plastic material. Fill time is a critical process parameter that affects shear heating and shear thinning.

Variations in fill time can alter the viscosity, pressure, and temperature of the polymer within the mould cavity, ultimately impacting the quality of the final part. Maintaining a consistent, optimized fill time is therefore crucial for process stability across different machines.

Excessive shear rates can induce polymer degradation, resulting in a decline in both the aesthetic and mechanical properties of the moulded component.

The shear rate within specific mould geometries, such as sprues, runners, and gates with a round cross-section, can be calculated using the formula: 

γ˙​=4Q​/πr^3, where γ˙​ represents the shear rate (1/s), Q (mm^3/s) is the volumetric flow rate, and r (mm) is the radius of the channel.

Shear stress and shear limit control table

Calculated shear rate values can be compared against established material-specific shear rate limitations to identify potential processing issues related to excessive shear. This data facilitates the mathematical determination of optimal flow rates and mould design considerations. Your calculated shear rate should not exceed the shear rate limit  for the material. Figure 1 shows the shear stress and shear rates limits of different plastics, based on empirical experiments and literature. 

Figure 1: Shear stress and shear rate control table. 

Conclusion

In plastics processing, maximum shear rates can reach over 10,000 s⁻¹ in injection moulding and 1000 s⁻¹ in extrusion, with even higher rates (exceeding 1,000,000 s⁻¹) occurring in specific applications like wire coating. Calculating the shear rates of the material during processing and checking if they are below the shear rate limit of the material will lower the risk of polymer damage. Furthermore risk of plastic part failure is reduced since the part will have the desired properties. 

More Rule of Thumb posts can be found in the "Start here" section. 

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://s3.amazonaws.com/entecpolymers.com/v3/uploads/pdfs/Rheology-vs-Shear-Rate-RGB.pdf

[2] https://www.findoutaboutplastics.com/2015/04/injection-molding-filling-simulation-my.html

[3] https://www.findoutaboutplastics.com/2022/05/6-benefits-of-injection-moulding.html

Polyarylamide (PARA; MXD-6) and Recycling: How much regrind is possible?

Hello and welcome to a new post in which we discuss the usage of regrind when processing the high-performance Polyarylamide (PARA; MXD-6).

Recycling Process:

Polyarylamide regrind (sprues, runners, etc.), containing reinforcements such as glass fibers, can be recycled with virgin PARA compound. Important is to exclude any contamination such as oil, release agents, and other additives.

Impact on Mechanical Properties and Color Variation:

Several studies and experiments have shown [1] that a split of 70/30 (70 wt% virgin PARA / 30 wt% regrind) is preferred. The compound used in each cycle was 70% virgin compound (containing 50 wt% glass fibers) mixed with 30% regrind from the preceding cycle. Very little change was observed in the mechanical properties (tensile strength, elongation at break) after recycling. Successive recycling may cause a slight change in color. 

Recommendations:

Important is to confirm the recycling rate experimentally to ensure the finished part meets specifications. Also, drying the regrind prior to injection unless directly grinded and reinjected at the moulding machine. Mark parts to facilitate after-use recycling (check out the part marking codes here).

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://content.solvay.com/ixef-para-processing-guide

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

Hello and welcome to another Rule of Thumb post. Today we discuss how to fast estimate the cooling time in injection moulding. 

Main influencing factor of the cooling time is the wall thickness, since it the wall thickness is included squarely in the cooling time calculation. Another factor is the tool temperature. For example, a 10°C increase of tool temperature will lead to a 30% elongation in cooling time. Increasing the melt temperature will lead only to a 3% increase in cooling time. 

Equation 1 shows the simplified cooling time calculation which can be used for a fast assessment (it is still recommend to calculate the cooling time with a more detailed equation whenever possible). We need to have the maximum wall thickness of your part and a multiplication factor between 1.5 and 2 (sometimes also higher e.g. 2.5 for PARA). 

Equation 1: Estimating the cooling time in injection moulding [1].

Check out the video on this topic on my YouTube channel too: 

Now that we have the cooling time, we can calculate the cycle time of the part with following relationship: 

t_cycle = 3/2 * t_cooling

More polymer engineering Rule of Thumb posts can be found under "start here".

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature:

[1] https://www.emsgrivory.com/fileadmin/user_upload/EMS-GRIVORY/documents/Brochures/English/5017_en_Long-fibre-reinforced-polyamides.pdf

[2] https://www.findoutaboutplastics.com/2018/04/plastics-part-design-10-holy-design.html

Injection Moulding of POM - Checking Thermal Degradation (Rule of Thumb)

 

Injection Moulding of POM - Checking Thermal Degradation

Hello and welcome to this new Rule of Thumb post, discussing how to check the thermal degradation of Polyoxymethylene (POM) during injection moulding.

Setting the optimal melt temperature during processing is important for POM since it cannot be thermally stretched too much. Polymers such as Polyamide and Polyesters can handle higher set temperatures better during processing compared to POM which develops gases as result of degradation.

What are signs for thermal degradation? 

Typical signs of POM degradation are: 

-melt shows a foamy structure when leaving the injection nozzle

-it has a strong odor

-the nozzle tends to spill material out

-black and brown specks

-black flow lines on moulded part

How to test for thermal degradation?

Once moulding with POM has started and has run a couple of cycles, the machine will be stopped at the end of the dosing time. Now the machine stands still for 10 minutes (natural POM) or 2 minutes (coloured POM). After the waiting time is over, injection of the melt into the open is done. During ejection, check if there is formation of foam. After cooling down the melt cake, the swimming test is done. If the melt cake swims, then there is too much formation of foam and the material is thermally damaged. 

For POM, the recommended melt temperature is 215 +/-5ºC at which the melt for standard grades will remain stable without degradation (residence time is around 30 minutes).

More Rule of Thumb posts can be found here. 

Thank you for reading and #findoutaboutplastics

Greetings, 

Herwig Juster

📚My Polymer Material Selection Book - Available Here

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Literature: 

[1] DuPont - Serie Teil 3: Spritzgießen von Polyoxymethylen (POM)

6 Major Benefits of Injection Moulding Simulation in Polymer Part Design and Material Selection

Hello and welcome to a new blog post. Today we have a closer look at how injection molding simulations support us in part design, polymer material selection and processing.

1. Injection point and gate placement

Finding the optimal injection point and gating is key to fulfill certain aesthetics or warpage requirements. Also, it helps to prevent flow line situations and as a consequence lower mechanical performance of your part. Nowadays most polymer injection molding simulations have gate placement tools integrated which can recommend you the optimal injection point. 

2. Placement and balancing of runners

Bringing the molten polymers towards the cavity, runners (thin channels) are needed. Aim is to ensure an even filling of your cavity. Runner analysis is hand in hand with the gating analysis from the previous point and most simulation software have a runner balance tool too. 

3. Warpage and shrinkage situation 

Analyzing the shrinkage and warpage situation is in particular needed when you use fiber-reinforced polymers which have an effect on the shrinkage and warpage of your part. Filling simulation can use the information of the velocity vectors to predict fiber behavior in the final part. And over this route, calculate the effect on shrinkage and warpage. 

4. Packing situation 

Analyzing the packing situation allows you to set the packing pressure and time for your part. There are several factors such as the material and mould shape which are influencing the packing. Packing analysis covers the prediction of the gate freeze time, clamping force needed in this phase, and predict areas where high volumetric shrinkage may appear. 

5. Cooling - mold 

There are injection moulding simulation tools which allow a design and optimization of the cooling channel layout of your moving and fixed moulding half. However, most tools simulate a uniform mould cooling at a set temperature.

6. Processing - identify critical shear rates

In case you work with polymers which are sensitive to mechanical stresses like shear rates then it is worth to have a plan of action how to locate critical areas and solve them by using simulation or in a simple way with analytic methods.

In the video I made you can see the perforated plate in the version of side gating and central gating. This applied method of shear rate tracer release is possible in the virtual molding package Sigmasoft.

In the following are the four steps of my procedure I use in the post-processing after I have done a process simulation:

1) Watch the shear rate contour plot to get the "big picture"

2) Activate the shear rate tracer

3) Analyze the release places and where the sheared material will end up in the part (to predict if there will be a decrease in the mechanical properties of the part)

4) Make geometry changes or process changes (melt temperature; inlet velocity profile)

The shear rate tracer method helps you to locate the punctual critical areas. So far, those are the advantages of such an approach. Another aspect is that the allover simulation will take more time and more memory as well as more working space.

In detail you can read here about my shear rate analysis. 

What are some of the most used injection moulding simulations?

There are several suppliers and often used are Autodesk Moldflow, SIGMASOFT Virtual Molding, Moldex3D, Vero VISI Flow, Simcon CadMould, and Solidworks Plastics.

Thanks for reading and till next time!

Greetings,

Herwig 

#findoutaboutplastics

📚My Polymer Material Selection Book - Available Here

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

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://www.findoutaboutplastics.com/2015/04/injection-molding-filling-simulation-my.html

[2] https://www.findoutaboutplastics.com/2018/01/data-is-new-plastic-data-algorithms-in.html

[3] https://www.findoutaboutplastics.com/2021/03/the-future-of-plastics-manufacturing.html

[4]https://www.3erp.com/blog/how-injection-molding-simulation-software-helps-you-design-better-parts/

[5] https://www.downloadcloud.com/injection-molding-software.html

Polymer Processing – Resin Moisture After Drying (Support Table)

 Hello and welcome to this blog post on resin drying before processing them. In this post I provide you with a drying table of the most used polymers which can support you in your processing operations.

A word on resin drying

If material is not properly prepared for processing operations such as injection moulding, the moisture will break down the polymer in the barrel at the processing temperatures. In general, this is referred to as hydrolysis.

This can occur for many different polymers, including:

-Polyamides (all types)

-Polyesters (PET, PBT)

-Polycarbonate

-Polysulfones

-Polyurethanes

 

There are five keys to remember which ensure good drying of resins [1]:

1. Pellet / regrind incoming moisture level

2. Air temperature

3. Dewpoint/ Desiccant

4. Residence time

5. Air flow

In this video below here, I discuss the topic of material drying and troubleshooting of moisture streaks in detail:

 

The table below shows the different maximum moisture levels after resin drying to ensure proper processing.

 

Table: maximum moisture content after resin drying of most used polymers

Analyzing the moisture level after drying can be done over a Carl Fischer titration (lab equipment needed) or over the Tomasetti’s Volatile Indicator (TVI). The TVI represent a more practical approach since you need only a hot plate and a glass plate to place the pellets after drying. The temperature of the hot plate is increased till the pellets melt and then it is important to check if there are bubbles. The bubbles are an indicator for moisture and there can be few bubbles (0.02-0.03% moisture), numerous bubbles (0.05-0.1% moisture), or many large bubbles (moisture above 0.1%) or no bubbles at all (dry material). If you do not have a hot plat you can also use the heater band of the injection moudling machine. 

Thank you for reading and #findoutaboutplastics

Greetings,

Herwig

 #polymerprocessing #herwigjuster

Interested to talk with me about your plastic selection and part design needs - here you can contact me 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.

New to my Find Out About Plastics Blog – check out the start here section

My YouTube channel

Literature:

[1] GE Plastics – Moulding Guide

[2] Saechtling Kunststoff Taschenbuch, Hanser-Fachbuch

Rule of Thumb Polymer Processing: Effective Injection Moulding Troubleshooting using the 4M Approach

Hello and welcome to a new Rule of Thumb blog post. Today we discuss the 4M approach for effective troubleshooting in polymer injection moulding.

Background of the 4M approach

In general, troubleshooting in injection moulding means to solve problems which are related to the part, mould, machine, material, or process. The 4M approach was coined by Mr. Kerkstra and Mr. Brammer and is a systematic way to solve injection moulding issues. As a troubleshooter you can influence 4 main areas:

1. Moulding process

2. Mould

3. Machine

4. Material

4M approach for systematic troubleshooting in injection moulding

During troubleshooting, potential root causes can be hidden behind each of the 4M’s. Using the 4M approach allows you to systematically work through one category at the time and create a list of potential root causes.

Always ask: ''which of the 4M’s could be the reason for the defect and why?” Getting to the root cause is key. Also, avoid processing around issues when in reality a change in the tool is needed.

Another trap in troubleshooting is trying to solve the wrong problem. For example, one is reducing the injection speed to avoid burns, however the real reason for the burn are blocked vents in the mould. Therefore, one may always ask what has changed in the mould, material, moulding process or machine setting.

Additionally, working to solve a potential root cause at a time and to do so collecting data is key. One can change a thing and then observe the impact. Changing several processing settings at the same time will make it impossible to solve any problem. And if there is no impact when you have done the change, then switching back to the original documented setting is fine.

What are some additional troubleshooting techniques?

There are several additional troubleshooting techniques, such as

-5 Why

-Scrap recording sheets

-Brainstorming

-Fishbone diagram

-Is/Is Not analysis

-Design of experiments (DOE)

Main takeaways

Asking questions and exploring answers is a main task in troubleshooting. Also it is important to check if the moulding problem is a new one or is already longer there. In case it is a new problem, one needs to dig in and find out what may have changed. Only change one thing at the time. The moulding process takes time to stabilize and to see the effect. Another point to pay attention to is that the medicine should not make you sick, meaning that the problem solved should not create new problems.

Successful troubleshooting combines knowledge, a systematic approach and experience

Additionally, I made a short training video on injection moulding troubleshooting, which you can find here.

Thank you for reading and #findoutaboutplastics

Herwig

Interested to talk with me about your plastic selection and part design needs - here you can contact me 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.

New to my Find Out About Plastics Blog – check out the start here section

My YouTube channel

Literature:

[1] Kerkstra / Brammer: Injection Molding Advanced Troubleshooting Guide, The 4M Approach