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

Hello and welcome to another Rule of Thumb post. Today we discuss how to handle weld lines in polymer injection moulding. 

Check out my other posts on this topic here.

In a lot of plastic part designs, welding lines are unavoidable, especially if your design has holes. The biggest downside of weld lines is that they impact the aesthetics and strength of a plastic part. However, we can minimize them, move them, and increase the strength by following the recommendations presented here: 

• Mould and Material Adjustments: Increasing mould or material temperature can improve flow and supports better entanglement of the poylmers at the weld position.
• Gate Placement: Strategic positioning of gates can influence the location and appearance of weld lines. Changing the gate position can results in moving the weldline to a more hidden position. filling simulations such as MoldFlow can support you to find the optimum gate location even before making the tool. 
• Part Design: Modifying wall thickness or creating a single flow path can help to eliminate weld lines. Changing the wall thickness will shorten or elongate filling time and thus moving the weld line. 
• Runner System Optimization: Reducing runner dimensions allows you to increase melt temperature and prevent weld line formation due to better polymer entanglement (see number 1).
• Injection Speed: Increasing injection speed will lower the filling time, prevent a too early cooling of the melt and potentially reduce as well as move weld lines.
• Material Selection: If possible, considering a lower viscosity or lower melting point polymer can enhance flow properties.
• Post-Moulding Machining: For critical areas, machining of the desired part features such as a hole after moulding, can be used to remove weld lines entirely.

Figure 1 summarizes the aforementioned points.

Figure 1: Overview of the considerations for minimizing, and/ or moving weld lines, as well as increase weld line strength.

While these methods can be effective, it is essential to consider the potential impact on other product characteristics and manufacturing costs.

Also, before implementing any of these techniques, consulting with a plastics expert helps to avoid unintended consequences.

Check out my other rule of thumb posts here.

Thanks for reading and #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 

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Literature:

[1] https://www.crescentind.com/blog/8-ways-to-fix-injection-molding-weld-lines#:~:text=Increasing%20the%20temperature%20of%20the,an%20opening%20called%20a%20gate.

6+ Inputs for Reducing Flash Formation of Polyphenylene sulfide (PPS) During Injection Molding

Hello and welcome to this post on injection molding tips with the focus on flash reduction of PPS and obtaining low flash PPS parts.

Background

Having worked in the field of high-performance polymers and specifically with PPS for the past ten years, I have often received the same question: How can flash formation be minimized and reduced during PPS molding? In this text, I will share more than six practical tips to address this issue, based on my extensive experience in the industry.

Let us get started. We will first discuss the possible causes of flash and then show possible solutions. 

What is flash formation in injection moulding?

In general, flash occurs when molten plastic flows out of the mould during injection and solidifies, causing a visible effect (Figure 1). 

Figure 1: Visible flash formation on an injection moulded part. 

Why flash can be an issue with PPS?

Since PPS is a high flow polymer, it is an excellent choice for thin walled injection moulded parts. Also, the good flow properties allow for high filling loads. The good flowability allows the PPS to flow during filling out of the cavities without too much effort. Figure 2 compares the spiral flow length of different PPS types to Polyamide 6 with 50 wt% glass fiber [based on 1,2]. The highly filled PPS (GF/MD 65 wt%) reaches 40% more in flow length compared to the PA 6 (GF 50 wt%).

Figure 2: comparison of spiral flow length (1 mm thickness) of different PPS compounds (incl. unfilled) vs. PA 6- GF 50 wt% [based on 1,2].

Reducing flash with PPS: possible causes vs solutions 

How to start: Check the Mold, Clamping Force, Packing Pressure

The first step to enable flash-free injection molding with PPS is to control the mold, specifically by cleaning and inspecting the venting and parting line. 

Next, the required clamping force should be verified. Short shots should then be produced to determine the switch-over point, followed by the application of the appropriate packing/holding pressure (height and time).

Optimize Molding Parameters

1) Low packing phase from 0.0 to 0.5 s and then start with regular packing phase. The 0.5 seconds in delay allow the melt to freeze the parting line (seal it off) and flash is reduced.  

2) Too high injection pressure -> decrease cutoff position, injection packing pressure, and injection time forward

3) Too high injection rate -> decrease injection rate

4) Too high polymer melt temperature -> decrease barrel temperature and lower backing pressure

5) Too high mould temperature -> decrease mould temperature (min. 135-140 °C)

6) Too low mould clamping force -> increase clamping force; alternatively check the possibly to move mould to larger injection moulding machine (higher possible clamping forces)

During molding operations

Mold Wear and Misalignment During Production: At the start of production, parts typically exhibit no flash; however, flash may develop as the production run progresses. It is important to verify that the mold steel has adequate hardness and that cavity edges are not experiencing excessive wear. Additionally, ensure correct mold alignment and keep parting lines clean and free from material buildup in the parting area.

Check PPS Drying Procedures

Mineral-filled PPS compounds can be hygroscopic, they should be dried at 135–163 °C (275–325 °F) for 2 to 3 hours to prevent moisture-driven "drool," which can cause flash.

Bonus Tip: Select Low-Flash Grades: 

Utilize specific PPS compounds designed to minimize flash, such as Ryton R-4-230BL (40% glass fiber reinforced), which provide improved processability.

Furthermore, there are PPS alloy compounds which have low flash properties and fast cycle times (f.e. Ryton PPS XK2340 or XK3340). 

Check out my YouTube training video on this topic too: 

Now I am curious, let me know - what are your tips to reduce flash when moulding polymers such as PPS?

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] http://mold-technology4all.blogspot.com/2011/07/wall-thickness.html

[2] https://www.plastics.toray/technical/torelina/tec_017.html

[3] https://www.solvay.com/sites/g/files/srpend221/files/2018-10/Ryton-PPS-Processing-Guide_EN-v2.1_0.pdf

[4] https://youtube.com/shorts/ci88l0FtSUM

[5] https://www.syensqo.com/en/brands/ryton-pps

[6] https://www.linkedin.com/pulse/6-inputs-reducing-flash-formation-polyphenylene-sulfide-herwig-juster-fzjhf

Guest Interview: Alexander Lehner-Jettmar – Technical Sales Engineer at Biesterfeld Interowa GmbH & Co KG

Hello and welcome to this guest interview. Today I present to you Mr. Alexander Lehner-Jettmar from the international plastics trading and distribution company Biesterfeld Interowa GmbH & Co KG. We have the chance to learn about the most common injection moulding defects and how one can avoid them.

Enjoy the interview!

1. Tell us about yourself, your current role, and your way into the polymer industry

 

Hello Herwig, thank you for interviewing me and having me on your blog! I am Alexander Lehner-Jettmar and work as a technical sales engineer at Biesterfeld in Austria. I have been working in the plastic industry for 8 years and before that I completed the study of plastics and environmental technology at the TGM in Vienna.

Biesterfeld is one of the leading international distributors in the plastics industry and a partner for innovative solutions in the field of high-performance plastics, engineering thermoplastics, thermoplastic elastomers, styrene copolymers as well as standard polymers and additives. In my role as technical sales engineer, I support customers from the very first idea to the finished application and provide technical advice in the areas of material selection, tool and component design, processing and failure analysis.

2. Based on your experience, what are the most common injection moulding defects and how can one avoid them?

There are different types of injection moulding defects which I help to resolve at my customers. Among the defects I am confronted with are: too small gate designs, holding time too short, bad venting, wrong melt temperature, wrong tool temperature, and moisture in the granules.

Let me discuss a few of these issues.

Small gate design

Polymer parts are designed by using complex methods such as computer-aided design, finite element analysis and mould flow calculations. While these methods are very useful, I often see that there is less focus on the importance of the correct design of the feed system. Semi-crystalline thermoplastics undergo a volume shrinkage during the transition from the molten state to the solid state. This shrinkage, which may be as much as 14 %, depending on the type of resin, has to be compensated during hold time by the supply of additional melt into the mould cavity. That can only be done if the gate cross-section is adequate to ensure the presence of a fluid centre during the holding phase. If the gating system is too narrow, the holding pressure cannot remain effective beyond the desired holding pressure time. In that case, volume shrinkage cannot be adequately compensated, resulting in the formation of voids and sink marks.

In designing the feed system, the first point to be considered is the wall thickness of the moulded part. The diameter of the runner should never be less than the wall thickness of the moulded part.

When moulding partially crystalline, unreinforced polymers, the minimum gate thickness should be 50 % of the wall thickness of the moulded part. This would also be adequate for reinforced compounds. To minimise the risk of damage to the fibres and also bearing in mind the higher viscosity of these compounds, the gate thickness should be up to 75 % of the wall thickness of the moulded part. How a self-separating sprue system is optimally designed can be seen in Figure 1.

Figure 1:  Optimally design of a self-separating sprue system [Biesterfeld/DuPont].

If the sprue cannot go direct into the cavity, the gate length is especially crucial. The gate length should be ≤1 mm to prevent premature solidification of the sprue, so that the mould will heat up near the gate, and the holding pressure is working most effective (Figure 2).

Figure 2: Optimal design of gate if the sprue cannot be placed directly into the cavity [Biesterfeld/DuPont].

Also the gate position should be in the region of the maximum wall thickness of the moulded part to avoid voids and sink marks. This is especially essential for semi-crystalline plastics.

Bad venting

A well designed venting system is crucial for the durability and maintenance intervals of an injection mould.  Especially when processing flame-retardant materials, the durability of a tool can be significantly extended by implementing a good ventilation.

Since flame-retardant modified compounds are demanded more and more often, the subject of proper ventilation is also becoming more and more important.

Nearly always, I see insufficient venting when I look more closely at tools in the field. In most cases, the vent is not placed close enough to the cavity which prevents it from working efficiently. Also, the width of the ventilation gap, the so called "vent land" is often not adequately designed. The recommendation is that the vent land is max 0,8 mm long before the vent channel gets bigger and leads the air outside. In most cases, the vent channel is more than 3 mm away from the cavity. This leads to defects such as burn marks, mould deposits, poor part surfaces and an increased mould tool abrasion. If the venting is insufficient, component quality also suffers. Besides fire marks at the end of the flow path, also the weld line strength is affected. A well placed vent near the weld lines can double the weld line strength by allowing air to escape more easily in this section.

In essence, the following rules are applicable when designing a venting system.

• Vent land must be as short as possible (max 0,8 mm)
• Vent land must be as wide as possible
• Moving cores/inserts should also be vented (parting line or add ejector)
• Relief channels must exit to atmosphere without restriction

The following Figure 3 illustrates the recommendations.

Figure 3: Design of proper venting around injection moulding tool cavity [Biesterfeld/DuPont].

It should not only be vented at the end of the flow path. The earlier the ventilation is intended, the more air can be pressed out of the cavity prematurely to avoid higher pressure at the end of the flow path.

In many cases the injection pressure can reduce by a good venting and energy costs can be saved, which is also a nice side effect.

Holding time too short

In practice many injection moulders, working from their experience of amorphous polymers, tend to use shorter hold pressure times and longer cooling times. Unfortunately, this approach also tends to be used for semi-crystalline polymers.

Once the mould cavity has been filled, the polymer molecules start to crystallise, i.e. the molecule chains become aligned with respect to each other, resulting in higher packing density. This process starts in the outer zone and ends in the centre of the part. As mentioned the shrinkage can be up to 14% of the volume and has to be compensated during holding time. I often see that the holding pressure time is too short to compensate for this shrinkage. Parts made in this way often show excessive shrinkage, warpage, sink marks, voids and, in some cases, enormous loss of mechanical properties. In addition, there may be considerable dimensional variations too.

The required holding pressure time depends on the polymer and the additives used. The following Table 1 shows the typical required holding pressure time per mm wall thickness for different materials.

Table 1: Crystallisation time in seconds per mm wall thickness.

Wrong melt temperature

Choosing the optimal melt temperature is vital for part quality when moulding semi-crystalline engineering polymers. As a rule of thumb we can state that the margin of tolerance for semi-crystalline polymers is less than when processing amorphous resins. The moulder at his machine directly influences the properties of the end-product.

Melt temperature can be too high or too low and both are wrong. In addition, even distribution of temperature in the melt is also a factor to be kept in mind.

Temperatures that are too high degrade the polymer, that is, destroying the molecular chains. Another consequence may be that additives in the melt, such as pigments, impact modifiers, flame retardants etc., also decompose or start reacting too early. The results are poorer mechanical properties as a result of the shorter molecular chains, surface defects and a bad odour in the production.

When the temperature is too low, the polymer melt fails to achieve the required homogeneity. This drastically reduces impact resistance and leads in most cases to considerable variations in physical properties.

The data sheets for engineering polymers indicate the optimum melt temperature range for each polymer. In general, the temperature setting of the barrel heating zones alone is not reliable because, apart from the temperature rise due to the heater bands, friction from the screw rotation also generates heat. How much heat is generated this way depends on screw geometry and rpm as well as on back pressure. For the correct melt temperature, you should therefore not only rely on the machine parameters, but also measure the actual temperature of the melt.

What I also often see is that the hot runner is set hotter than the plasticising unit. Since the granulate should already be completely melted before it reaches the machine nozzle, a higher temperature in the hot runner makes no sense. The material is only thermally damaged and the risk of black specs is increased if there are dead spots in the hot runner system, which leads to quality issues.

For semi-crystalline materials the nozzle and the hot runner system should run 5-10°C lower than the last zone of the cylinder, as long as the hot runner nozzles do not freeze. If the hot runner nozzles freeze, you should have a look at the thermal decoupling of them. 

3. Let us deepen the impact of residual moisture in various polymers such as Polyamides, Polyesters (PET, PBT), Polycarbonate and others. 

Many plastics absorb moisture from the atmosphere. How much they absorb depends on the type of resin. Moisture in the granules, even if it is only surface condensation, can cause problems in parts moulded with engineering polymers. Many types of undesired effects can occur, including processing problems, poor surface on moulded parts or loss of mechanical properties. It is seldom possible to tell if moisture is present by visual inspection alone.

The following Figure 4 shows the maximum moisture absorption of various plastics at room temperature and 50% humidity.

Figure 4: Maximum moisture absorption of various plastics at room temperature and 50% humidity [Biesterfeld/DuPont].

Most engineering polymers require the moisture content of the granules to be below a certain maximum level for a proper processing.

During the injection process, moisture in the granulate can act as a processing aid, which increases the flowability. A certain amount of moisture is therefore useful. However, if the moisture content is too high, surface defects on the part and hydrolytic degradation of the polymer chains can occur, which reduces the mechanical properties and is not reversible.

The need for drying depends mainly on how sensitive the raw material is to water. Naturally, the moisture content of the material as it is delivered, the type of packaging and the period of storage are also important criteria. For example, polyamide is generally packed in bags with a barrier layer of aluminium, so that it can be used straight out of the bag. However, most processors of PA prefer to dry the resin in any case, even though drying is not necessary if the material is used within one hour. PET and PBT, on the other hand, are far more critical regarding moisture uptake and must always be dried to ensure that impact strength of the moulded parts is not affected. Another factor is that these resins pick up moisture very rapidly after drying, so that moulders should exercise special care when handling open containers of PET and PBT, when they are in transport or conveyor systems, as well as regarding their dwell time in the hopper. Thus, in unfavourable climatic circumstances PET can absorb enough moisture in 10 minutes to exceed the maximum permitted moisture content for moulding of 0,02%.

Additionally. 

Table 2 shows the typical effects of high moisture uptake in engineering polymers.

Table 2: Typical injection moulding effects of high moisture uptake in engineering polymers.

It is important to follow correct drying procedures if you want good quality mouldings. Simple hot air dryers of various types are not suitable for drying polyesters, for example, however dehumidified air dryers are acceptable. Only these can provide the required constant and adequate drying, whatever the ambient climatic conditions. In addition to maintaining the correct drying temperature, it is important to ensure that the dew point of the drying air remains lower than ≤20° C. When operating multi-container systems with different fill heights and bulk densities, it is also important to ensure that the air throughput in each container is sufficient.

4. In recent years more and more Pre- and Post Consumer Recyclates are moulded - what do we have to consider with such materials in injection moulding?

Drying regrind e.g. in the case of containers which were left standing around open requires special care. In these cases the recommended drying times are usually not enough. Fully saturated polyamide may need more than 12 hours to dry. The yellowing associated with such treatment is practically unavoidable.

To minimize the moisture pickup when using regrind, the following points should be considered.

- Always store sprues and regrind in closed containers.

- Seal containers or bags that have been partially used.

- Leave a lid on the hopper.

5. Where can the readers find out more about you (LinkedIn, etc)?

If there are any questions or if anyone would like to dig deeper into these topics, I can be reached via my email address a.lehner-jettmar@biesterfeld.com, or via my LinkedIn profile.

That was the guest interview with Alexander Lehner-Jettmar from Biesterfeld Interowa – thank you Alexander for sharing your experiences and optimization tips in polymer injection moulding!

Thanks for reading!

Greetings and #findoutaboutplastics

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] Biesterfeld/DuPont - Konstruktion und Verarbeitung von Kunststoffen: https://www.interowa.at/website/file/konstruktion_verarbeitung_broschuere_a5-richtig_print.pdf

Guest Interview: Bianca Gubi – Product Manager Circular Economy at ENGEL Austria GmbH “Plastic waste is a valuable raw material. If we manage to keep it in circulation, we can protect our environment and our climate.”

Hello everyone and welcome to this guest interview. Today I present to you Bianca Gubi who is product manager Circular Economy at Austrian injection moulding and automation manufacturer ENGEL AUSTRIA GmbH. We have the chance to deep-dive into Circular Economy moulding, learn about the newly developed two-stage moulding process for recycled materials, and what to take care in design for recycling in injection moulding.

Enjoy the interview!

Guest Interview with Bianca Gubi, working as Product Manager Circular Economy at ENGEL Austria GmbH.  

1. Tell us about yourself, your current role, and your way into the Circular Economy and recycling.

I have been working intensively on the subject of the circular economy for five years now because it's also very important to me personally. Plastic waste is a valuable raw material. If we manage to keep it in circulation, we can protect our environment and our climate. As product manager for Circular Economy at ENGEL, I'm happy to be able to make an active contribution now. That’s something I really like about my work. Here in the Circular Economy Division, we focus on sustainable technologies and are committed to ensuring that recycled plastic waste can be reprocessed to create high-quality plastic products. As an injection moulding machine manufacturer, ENGEL is part of the plastics industry and takes its responsibility very seriously. We are continuously enhancing our machines and have a range of new technologies for the circular economy in our portfolio. As a product manager, I am responsible for the design and marketing of this technology portfolio. That means I analyse what our customers need and collaborate very closely with R&D to meet those requirements. 

2. What are the differences between regular injection moulding and Circular Economy moulding? And are there things which need to be handled with special attention?

For the injection moulding machine, it's more or less irrelevant whether you process granulate, that is, virgin material, or a regranulate, that is, recycled material. This is basically good news if you're looking to establish a circular economy, because it means that it's very easy to switch to regranulate in many applications. But two things, above all, are important if you want to achieve high product quality across the board. Firstly, the plasticising unit needs to be precisely matched to the properties of the material you want to process. ENGEL has a great deal of in-house plasticising expertise and produces all the plasticising units itself; this means that we can offer our customers fantastic support here. Secondly, when you are processing recycled material, you have to take into account that the material is exposed to more pronounced batch fluctuations than you are accustomed to from virgin material. We meet this challenge with smart assistance. For example, the iQ weight control smart assistance system can automatically adjust the injection moulding parameters responsible for quality to match the current conditions for each individual cycle and ensure consistently high component quality by doing so. For standard injection moulding, it is important for the regranulate to be single grade and free of impurities. And if there are impurities, our Circular Technology engineering helps to produce good parts anyway – with our new two-stage process for example. 

3. Can you tell us about the newly developed two-stage moulding process for recycled materials? And can you point to some of the benefits of processing flakes directly compared with ready-to-use recycled pellets? 

Thanks to the new two-stage process, ENGEL makes it possible to process plastic waste as flakes in injection moulding directly after grinding. Since this eliminates pelletising, as a separate and particularly energy-intensive, process step, this innovation significantly improves energy- and cost-efficiency in plastics recycling.

To be able to process flakes in injection moulding, we have broken down plasticising and injection into two independent, but mutually tuned, process steps. In the first stage, the raw material, for example plastic flakes originating from post-consumer or post-industrial collection, is melted in a conventional plasticising screw. The melt is transferred to a second plasticising unit for injection into the cavity. Depending on the material and application, a melt filter and a degassing unit can be integrated between the plasticising and injection units; this allows customers to create high-quality products from contaminated plastic waste. 

The two-stage process is far leaner than the legacy recycling process, where plastic waste is ground, compounded, filtered and pelletised after sorting and cleaning. Thus far, the plastic has had to be melted twice for reprocessing. Pelletising the recycled material is an energy-intensive process which typically also involves logistics overhead. The need for this step is removed completely in the two-stage process. Based on calculations by ENGEL, the energy required for manufacturing the product is reduced by at least 30 percent.

4. How do you see “Design for Recycling” in injection moulding? And what are the upcoming trends in sustainability for plastics moulding?

Design for Recycling is an absolute must-have, if we are serious about the circular economy. Later recycling needs to be taken into account and planned for as early as in the development of new products. One example is thin-wall packaging produced using in-mould labelling. The trend here is towards monomaterial solutions, which allow the containers to be recycled along with the label. Another field where a great deal is happening in terms of design for recycling is lightweight construction in the mobility sector. At ENGEL, we are focusing on thermoplastic composite solutions. In the ENGEL organomelt process, thermoplastic sheets or UD-tapes are formed and functionalised in an integrated process by moulding the functional elements directly onto the preform. Again, the aim is to use materials belonging to the material group of the fibre composite prepreg matrix material for moulding on. The parts created in this way can then be fed back into the material cycle at the end of their service lives.

When it comes to trends for greater sustainability in injection moulding processing, ENGEL is taking action at various levels. Among other things, this is about innovative technologies for processing recycled materials, about process stability thanks to smart assistance systems, but also about the energy efficiency of the processing machines. 

5. Where can the readers find out more about you and the Circular Economy offers?

Just contact me and my colleagues. We are on site at very many trade fairs worldwide and can be contacted at any time by phone, email or on LinkedIn. Our website also gives people great overview of our commitment to sustainability at ENGEL: ▷ Circular Economy & Plastics Recycling - ENGEL (engelglobal.com). We also have our own blog there to keep our readers up to date at all times.

That was the guest interview with Bianca Gubi from ENGEL AUSTRIA GmbH – thank you Bianca and the whole team for sharing such interesting insights of the Circular Economy moulding!

Thanks for reading!

Greetings and #findoutaboutplastics

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 

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2 Component Injection Moulding: How TPS (SEBS) Can Stick to Polyamide or Polycarbonate Without Using Additional Adhesive?

Overmoulding PA and PC with PC - 2 component moulding

Hello and welcome to a new blog post in which we discuss how TPS can stick to Polyamide or Polycarbonate in 2 component injection moulding. 

During polymer material selection, one scenario can be that a solid plastic housing, made out of Polyamide, needs to have a sealing moulded on it. The selected sealing material is a styrene-ethylene-butylene-styrene (SEBS) from the thermoplastic elastomer class. A two-phase block copolymer with hard and soft segments forms the basis of SEBS. 

SEBS (Figure 1)  is used in different applications such as footwear, and seals which have low requirements towards chemicals and (heat) aging. Furthermore SEBS has excellent weatherability making it useful for many outdoor applications. It is also known for its ability to uptake oil and soften its Shore hardness. SEBS is non-polar leading to a harder adherence on polar polymers such as Polyamide. 

Figure 1: Structural formula of SEBS.

How can we increase the adherence of SEBS towards PA and PC?

There is a chemical trick one can apply: grafting maleic anhydride (= acid anhydride of maleic acid) onto the ethylene butadiene mid-blocks allows for better connection. The functional groups enable an adherence to polar PA or PC. 2 component overmoulding applications can be done and no additional usage of adhesives is needed.

Thanks for reading!

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.hexpol.com/tpe/resources/tpe-academy/what-is-tpe/what-is-tps/

[2] http://plasticnotes.blogspot.com/2009/11/polymer-polarity.html?q=polar

Design Properties for Engineers: Hot Runner System Suitability of High Performance Polymers

 Hello and welcome to a new blog post!

Today I present to you the hot runner system suitability data of high performance polymers for injection moulding.

In my comparison I have selected three different hot runner systems:

-Nozzle gate, open with straight outlet  

-Nozzle gate, open with tip ("hot tip")

-Valve gate with needle

For every of the eleven selected high performance polymers the suitability to use the different hot runner systems was checked by literature review.

 Hot Runner System Suitability of High Performance Polymers

Allover, material degradation is more likely to occur when hot runner systems are used due to the longer residence times at higher temperatures and often not completely balanced flow/temperature gradients in the hot runner system itself. Therefore it is essential to lower the temperature of the hot runner whenever moulding is interrupted.

Additionally, it is beneficial to have a hot runner system which allows a separate heating zone by using different control units as well as using an electrical circuit which enables gradual heating (less risk of short circuiting due to moisture present in the heater cartridges).

What are your experiences with hot runner systems and high performance polymers? 

Let me know in the comments below. 

Greetings 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.

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My YouTube channel

Literature:

[1] https://docplayer.org/112974799-Heisskanal-systeme-fuer-technische-thermoplaste.html

[2] https://www.picoplast.nl/uploads/Grivory%20HT1V%205%20FWA%20black%209225%20-%20MDS%20-%20DE.pdf

[3] https://www.synventive.com/uploadedFiles/MK-PRM.BRM.DE-P.HRGD01.pdf

[4] https://multimedia.3m.com/mws/media/96718O/injection-molding-guide-for-dyneontm-pfa.pdf

Basics of Plastic Part Failure Evaluation in Injection Moulding

 In this Youtube video we cover the basics of part failure evaluation in injection moulding.

The video is divided into three parts: 1) Types of plastic part failure -Mechanical, thermal, chemical, environmental failures, combined with time 2) Areas of part failure in injection moulding -Premoulding, moulding, postmoulding, and Application (end-use) 3) Failure analysis in injection moulding In injection moulding, problems can occur in one of the four areas: - Raw materials - Additives involved in the material formulation - Injection moulding machine and tool in operation - Process control, settings, and monitoring Furthermore, there are two types of defects in injection moulding: - Moulding defects, occurring during injection moulding (short shots, air entrapment, etc) - Moulded part defects, which are identified after the part was moulded (jetting, weld lines, bubbles, etc). In conclusion, there are several root analysis tools which can be used to identify the problem and develop a solution to solve the moulding problem. Examples are the Pareto chart technique, the 5 Whys, the Fishbone diagram, and the Failure Mode and Effects Analysis (FMEA). Important is to have a systematic approach which helps to diagnose and solve part issues of the injection molding operation. Greetings and #findoutaboutplastics Herwig

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Rule of Thumb for Injection Moulding - " What happens in the cavity..."

 

"What happens in the cavity, stays in the part" - RJG Inc

"What happens in the cavity stays in the part" - this rule of thumb, coined by training engineers of company RJG Inc [1,2], I heard often from my colleagues during my time I worked in the Application Technology department of a global active material supplier. 

However, why it is such a useful advice and rule of thumb for part quality?

In general there are four polymer processing variables which impact the quality of moulded parts: temperature, viscosity, packing and holding pressure, and cooling [1]. 

Additionally, monitoring the cavity pressure helps to detect defective parts in an early stage. Cavity pressure monitoring pays off especially with parts which tend to create problems during moulding as well as high volume parts. When the pressure sensor locations are at the end of the cavity, then detection of short shots is possible. Furthermore, gate sealing detection is achieved in a similar way by adding a sensor at the gate end. In-mould sensor installation is around 20% of all injection moulds and is increasing rapidly due to automation of production using cyber-physical production systems in the past years. 

I hope you find this rule of thumb tip for injection moulding useful. 

Thanks for reading and #findoutaboutplastics 

Herwig Juster 

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 Literature 

[1] https://www.yumpu.com/en/document/read/39691706/scientific-molding-in-cavity-sensors-and-data-management-mapp 

[2] https://de.rjginc.com/

Polymer Processing: Tolerance and Roughness Charts

Hello and welcome to this blog post on tolerance and roughness charts for different polymer processing techniques. This post supports you in the polymer shaping selection as part of the material selection process.

1. Tolerance chart: A part will not be exactly shaped to a specified dimension. There will be a deviation Δx from a desired dimension x which is allowed by the specifier. This is in general referred to as tolerance T which is defined as e.g. x =100 ± 0.1mm, or as 0.01. The bar chart allows selecting different polymer processing techniques to achieve a desired tolerance.

2. Roughness chart: A part will have different surface roughness R, which is measured by the root-mean square amplitude of the irregularities on the surface. A rough surface will have an R < 100 μm and a high polished surface has a R < 0.01 μm. The bar chart allows selecting different polymer processing techniques to achieve a desired surface roughness.

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

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Literature: 

[1] Granta - Material and Process Selection Charts, 2010

How to Calculate the Residence Time in Plastics Injection Moulding [incl. online calculation tool]

The term residence time in injection moulding operations refers to the time that a plastic pellet takes from entering the injection moulding barrel until entering the injection mould. It relates to the amount of polymer material present in the cylinder of the injection unit, the shot weight and the total cycle time. Often, residence time is also referred to as Hold-Up Time (HUT).

Melting of plastics for processing is usually attained by bringing the plastics over a certain temperature, i.e., glass transition temperature for amorphous thermoplastic polymers and glass transition temperature as well as crystalline melting temperature for semi-crystalline thermoplastic polymers. For both types of thermoplastics longer than necessary heat exposure, especially in the presence of oxygen (air), may induce chemical degradation. Therefore, the residence time in injection moulding at polymer-sensitive melt temperatures needs to be optimal. In this context, residence time is especially important for polymers such as, for example, PVC, POM, ABS, PBT and PET.

Melt temperatures have to be chosen in a way that the material’s thermal stability during processing is ensured [1, 2]. Guidance about optimal residence time and residence time for different polymers is given by material manufacturers in processing and design guides.  In practice, tools for accurately calculating the melt residence time depending on the utilized machine and processing conditions are usually not available. This prevents processing engineers from making quick process assessments. For this reason, I have created an online tool to calculate the residence time of your injection moulding operation. This can be used online or downloaded. The calculation is based on the formula below [3].

Formula for calculating the residence time in injection moulding

Here, number 8 represents the volume of the molten polymer in the barrel. This is the ratio between flight height and screw length, which for most injection moulding machines is approximately 8. Part A gives the number of shots in the barrel and Part B represents the cycle time to produce the part.

Finally, keeping the residence time at an optimum level will help you keeping materials’ degradation to a minimum and, consequently, the mechanical properties of your final moulded part to a maximum.



Successful residence time injection molding calculation  and thanks for reading!

Till next time!

Greetings, 

Herwig Juster

Interested to talk with me about your polymer material selection, sustainability, 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

My personal website


Literature

[1] http://www.solvay.com/en/binaries/Sulfones-Quick-Molding-Guide_EN-227546.pdf

[2] GE Plastics - Injection Moulding Guide

[3] Christoph Jaroschek - Spritzgießen für Praktiker
  [4] https://www.wittmann-group.com/sites/default/files/2021-05/wiba_prnews_plasticizing-screws-article-series-part2_04-2020_en.pdf