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