Lubomír Zeman: Thermoplastic injection molding technology and weight reduction of injection moldings, part 1

INTRODUCTION

At present, one of the most widely discussed topics worldwide in the automotive industry is, simply put, the “battle” between advocates of cars with internal combustion engines – ICEV, Internal Combustion Engine Vehicle, vehicles with an internal combustion engine – and advocates of cars with fully electric propulsion, powered solely by energy from a battery – BEV, Battery Electric Vehicle, battery electric vehicles – or hybrid electric vehicles powered not only by electricity but also by an internal combustion engine, without the option of charging from the electricity grid – HEV, Hybrid Electric Vehicle, hybrid electric vehicle, or Plug-in Hybrid Electric Vehicle – PHEV, a combination of the characteristics of an electric vehicle and an internal combustion engine with the option of charging from an electrical socket using a cable. .

The two camps appear highly irreconcilable, but from an outside perspective they have one thing in common – an effort to reduce emissions, substances that pollute the air, and an effort to reduce the carbon footprint (the carbon footprint is a measure of the impact of human activity on the environment and on climate change. The carbon footprint focuses on the amount of greenhouse gases we produce in our everyday lives, for example by burning fossil fuels to generate electricity or heat, through transport, etc. It is expressed in carbon dioxide equivalents /CO₂/ and given in units of mass – kilograms and tonnes. Simply put, the carbon footprint is the amount of carbon dioxide and other greenhouse gases released during the life cycle of a product we use – emissions released by the operation of the product, for example driving a car – the direct footprint, and the indirect footprint from its use, including its manufacture and the like.), including sustainable development (Sustainable Development – development that preserves the ability of present and future generations to meet their basic needs while not reducing the diversity of nature and preserving the natural functions of ecosystems – a way of developing human society that reconciles economic and social progress with the full preservation of the environment).

One of the common denominators of the above efforts is reducing the weight of products entering the life cycle – Product Life Cycle. The term “product life cycle” is used in two senses:

- the life of a product from the extraction of raw materials, through its manufacture and use, to its disposal as waste – in this sense, it is used in connection with environmental impacts and life cycle assessment (LCA – Life Cycle Assessment)

- the life of a product from its introduction to the market to its withdrawal from the market

For this text, I will adhere to the first meaning – material, production, waste management.

Until recently, decarbonisation (the term decarbonisation encompasses limiting and reducing the carbon footprint and CO2 equivalents generated by the functioning of society. That is, the amount of greenhouse gas emissions released into the atmosphere, with the aim of achieving more sustainable development, using renewable energy sources and achieving greater sustainability in the supply chain) of the automotive industry focused primarily on reducing operational or exhaust emissions from energy or fuel consumption. Currently, operational emissions account for more than 80 % of the life cycle of ICE vehicles (Internal Combustion Engine, both petrol spark-ignition and diesel compression-ignition engines) and approximately 60 % for BEVs (battery-powered) with the current energy mix. There are reasonable grounds to expect this ratio to change gradually. The expansion of the electrification of road transport, powered by an increasing amount of energy from renewable sources, will eliminate exhaust emissions in the long term (emissions from brakes, tyre wear, etc. will remain) and, as a result, emissions embodied in the production phase (embodied emissions) will account for an increasingly dominant share. For BEVs using electricity from renewable sources, embodied emissions may account for up to 90 % of vehicle life cycle emissions in the future.

Weight is one of the main indicators that largely determines the extent of a particular car’s environmental impact. According to the European association of plastics processors Plastics Europe (www.plasticseurope.org), the carbon footprint left by a passenger car over its operational lifetime is reduced by 20 kilograms of carbon dioxide if its weight is reduced by just one kilogram. This is where one of the fundamental advantages of plastic parts comes into play − they weigh up to half as much as similar components made from other materials, and their use therefore reduces fuel consumption by 25 % to 35 %. Reducing weight can also, in many cases, lead to a reduction in the labour required for production (for example, reducing the wall thickness of an injection-moulded part can shorten its production time) and consequently also to a reduction in the cost of the part. One of the key concepts in the world of passenger car development and production is therefore "lightweight" − weight saving. However, this concept does not only encompass replacing metal with plastic. It also involves intelligent engineering approaches and design that uses only as much material as is needed to ensure the correct and safe functioning of the product.

We can no longer imagine our lives without the use of plastics. According to an estimate by the aforementioned European association Plastics Europe, total plastics consumption in Europe reached almost 60 million tonnes in 2022. The most significant sectors in terms of plastics consumption include the packaging industry (almost 40 %), construction (almost 20 %), and the automotive industry (almost 9 %), followed by the electrical and electronics industry and agriculture. Other sectors important for the use of plastics and the growth of their consumption include healthcare, where population ageing will drive the development of specialised medical devices and materials using polymer matrix-based materials. 

Although the trend towards population ageing will be evident in all regions of the world, it will emerge most rapidly in Europe, Asia and Latin America. For example, in Asia there are currently, on average, nine people of working age for every person beyond working age. By 2050, this ratio will fall to four people of working age. Europe can expect a particularly significant decline in its working-age population, which will be reflected in a decrease in the number of working-age people per person beyond working age from the current four to two.

Apart from the packaging and construction industries, where extrusion, blow moulding, thermoforming and vacuum forming are the most widely used plastics processing technologies, injection moulding is usually the preferred technology in other sectors. Let us therefore look at how thermoplastic injection moulding performs in the real world in terms of weight savings. 

PREREQUISITES FOR WEIGHT REDUCTION – MATERIALS FOR THERMOPLASTIC INJECTION MOULDING

The primary basis for reducing the weight of a part manufactured by injection moulding is the injection-moulded material itself. As already mentioned, compared with previously common engineering materials, such as steel with a density of 7,85 g.cm-3, non-ferrous metals (aluminium 2,7 g.cm-3, copper 8,9 g.cm-3), non-ferrous alloys (duralumin 2,8 g.cm-3, brass - average 8,8 g.cm-3, bronze - average 8,4 g.cm-3), etc., plastics have the advantage of a lower density (for comparison, among engineering materials, only wood performs better, with a dry density ranging from 0,270 g.cm-3 – spruce, through 0,313 g.cm-3 – pine, to 0,405 g.cm-3, oak), usually across their entire range suitable for injection moulding – amorphous materials - PC, PMMA, PS, HIPS, PVC, ABS, PC/ABS, SAN, ASA, PPO, CA, PPS, PEI, PES, PSU, PAI, semi-crystalline materials - LDPE, HDPE, PP, PA, POM, PET, PBT, PFA, LCP, PEEK, PEK, thermoplastic matrix composites with particulate fillers (M), reinforcing fillers (GF, CF), nanofillers, fillers of biological origin, etc., special types of thermoplastics – thermally conductive, electrically conductive, magnetic, liquid crystals (LCD), bioplastics – biodegradable from fossil raw materials, biodegradable from renewable raw materials, biodegradable and compostable, non-biodegradable from renewable raw materials, bio-derived blends combining engineering thermoplastics with bioplastics, blends, thermoplastic elastomers (TPE): TPE – O polyolefin types, TPE – S styrenic types, TPE – U polyurethane types, TPE – E copolyester types, TPE – A polyether block amide types, EVA ethylene vinyl acetate.

Plastics, as materials suitable for processing by injection moulding, have a wide density range (from approximately 0,9 g.cm-3 to approximately 2,2 g.cm-3, applicable to unfilled materials), which significantly helps – see the figures above - to reduce the weight of mouldings and assemblies of which thermoplastic mouldings form an integral part. An example from the packaging industry – a half-litre glass beer bottle (glass density – approximately 2,5 g.cm-3) weighs approximately 330 g/pc, while a one-and-a-half-litre bottle made of polyethylene terephthalate (PET, density 1,38 g.cm-3) weighs only approximately 38 g/pc; the difference in bottle weight is 88,5 %, not to mention the volume of the contents, which is 3x greater in PET.

Thermoplastics also offer highly variable properties, a very good ratio of weight to mechanical properties (elasticity, strength), toughness at low temperatures, and excellent insulating properties (they are thermal, acoustic and electrical insulators), but they can also conduct heat well, be electrically conductive, and be magnetic. They are resistant to water, corrosion and chemical influences, withstand biological agents (bacteria, moulds, fungi, insects), are hygienically safe, have low hydraulic roughness when used as pipes, and are resistant to deposit build-up – incrustation. They allow the use of a very wide range of processing technologies, are easy to colour, can be decorated very effectively – from a mirror-like gloss to various types of patterns and textures, are recyclable, are relatively inexpensive, etc.   

Having listed their positive properties, I must also point out that they have properties that limit their use. It can be said that the most widely discussed issue is their ecological impact, or rather the environmental (relating to the environment) properties of plastics. Under normal conditions, plastics are very stable, undergo very slow spontaneous degradation and are difficult to dispose of. Plastics usually have low heat resistance, their properties depend significantly on their service temperature and the duration of loading, their properties, especially those of composites with reinforcing fillers, are anisotropic (anisotropy = a property that describes the dependence of a given quantity on the choice of direction, i.e. they have different properties in the direction of polymer melt flow than in the direction perpendicular to the flow), their thermal expansion is one order of magnitude (approximately 10 x) greater than that of metallic materials, some plastics are hygroscopic and absorb water, they are partially flammable, some may release harmful monomers and decomposing additives, they exhibit weathering and physical ageing, are subject to stress corrosion, are notch-sensitive, permeable to low-molecular-weight substances, swell or even dissolve in selective solvents (selective solvent = a medium that acts as a solvent for at least one component of a polymer blend), the generation of static electricity can be a drawback, etc. Thermoplastic injection-moulded parts contain significant internal stresses, which can lead to their destruction in many applications.

Many of the above disadvantages of “pure” polymer materials can be reduced or even eliminated during their production through suitable additives, fillers, the preparation of material blends, chemical modifications, etc.

Before we look at the current material options for reducing weight, I will try to outline a possible path for future development in this area of producing injection-moulded parts with reduced weight.

Today's average passenger car contains approximately 120 kg (approximately 15 % to 20 % of its total weight) of all types of plastics processed using various technological methods. The trend towards further weight reduction is likely to intensify, also in view of the current pressure to move away from cars with internal combustion engines towards electric vehicles, which require high-capacity batteries to achieve driving ranges that customers consider reasonable, and therefore also batteries of considerable weight. In addition to range, this weight affects, for example, the ability to park in multi-storey car parks built under older regulations, i.e. with lower load-bearing capacities of the building structures, imposes greater mechanical loads on car components, increases the load on and wear of road surfaces, causes greater tyre abrasion, increases brake wear and abrasion, etc.

Battery capacity, which is directly proportional to the battery's specific energy and expresses the ratio between the energy and weight of the battery, has a direct impact on the range of an electric vehicle. Along with driving speed, the number of accelerations, climbing hills and the driver's driving style, including the use of electrical appliances installed in the car to enhance the comfort of the driver and passengers (especially air conditioning, which also draws energy from the battery, as heat energy recovery is insufficient to power the appliances), vehicle weight is one of the main factors limiting range. Weight comparison – the Hyundai KONA SUV – in its fully electric version with a 42 kWh battery and a theoretical range of 289 km has a kerb weight of 1 610 kg; in its diesel version with a 1,6 l engine, its kerb weight ranges from 1 390 kg to 1 504 kg, depending on equipment.

Despite the above requirements to increase battery capacity for BEV cars, it appears that BEV manufacturers' efforts to produce electric cars with the longest possible range – 1 000 km and more, which led to the development and production of batteries with capacities of 150 kWh and more, significantly increased vehicle weight but brought manufacturers greater profits – have encountered new customer requirements. According to the Chinese company NIO, which develops and manufactures BEVs with swappable batteries and also operates a network of battery-swapping stations, and also offers batteries with a capacity of 150 kWh (in 2023, a NIO ET7 model with such a battery travelled more than 1 000 km in 14 hours), depending on customer requirements, the most frequently swapped batteries are those with a capacity of 75 kWh (approximately 97 % of users), whereas previously it was 100 kWh. This points to the fact that battery capacity and range must be aligned with the car's positioning in the relevant segment and the way it is used. A city hatchback faces completely different requirements from a family SUV. They are also in different price brackets. All of this must be combined correctly, also taking into account dynamically evolving market priorities and trends. Despite a possible change in the trend – see the preceding text – the increased weight of battery-powered cars remains a fact.

According to ICIS (www.icis.com), polypropylene PP is the most widely used polymer in automotive construction, with a 35 % share, followed by polyurethane PU at 19 %, polyamides PA at 11 % and ABS at 8 %. More than half of the plastics consumed are used in automotive interior parts, followed by exterior parts, engine compartment parts and the electronic equipment of passenger cars.  

The development and use of new plastics with tailor-made properties is an important driver of development in a range of industrial sectors. It is a way of integrating companies engaged in compounding (from the English compound – to mix, a mixture) into the supply chain of major commodity plastics producers (a commodity is a product traded on the market without differences in quality; supplies from different manufacturers are interchangeable).

Additives for polymer systems offer a diverse range of polymer modifications (changes, adjustments), which are mostly aimed at improving their physical properties. These primarily include mechanical, end-use or processing properties. Modern additives are developed to increase the stability of the system in a given polymer environment and, at the same time, to obtain new functional properties. In this context, the following application areas in particular can be mentioned: protection against UV radiation, photoactive self-cleaning treatments (removal of contamination from various surfaces using catalysts that help bring about the change, activated by sunlight to break down organic substances into harmless components), conductive polymer systems and surface modifications.

Bioplastics

Polymer materials used in current designs are mainly produced from natural fossil substances – oil, coal, natural gas, i.e. substances that occur in nature but are non-renewable and can be depleted over time. Ways of replacing them are therefore being sought, and one approach is to use renewable, bio-based resources. Waste biomass (biomass is the totality of substances making up the bodies of all organisms, including plants, bacteria, cyanobacteria and fungi, as well as animals) and other renewable resources can be used, through thermochemical (thermal effects in chemical reactions and processes) and enzymatic processes (an enzyme is a simple or conjugated protein with catalytic activity; enzymes determine both the nature and the rate of chemical reactions and control most biochemical processes; almost all biochemical reactions are controlled enzymatically; enzymes are a special group of catalysts that increase the probability of a reaction occurring), to produce substances – bioplastics and bio-additives – capable of substituting for, replacing existing raw materials from fossil resources.

The term bioplastics encompasses an entire family of materials that are either bio-based or biodegradable, or both.

Biobased, or based on biomaterials, means that a material or product is (fully or partially) derived from biomass (from plants). The biomass used to produce bioplastics comes, for example, from maize, sugar cane or cellulose (the word originates from the Latin cellula = cell; it is a linear polymer of plant origin, an unbranched polysaccharide found, among other things, in fibre. The molecular formula of cellulose is C6H10O5).  

The term biodegradable describes a chemical process during which microorganisms commonly found in the environment convert materials into natural substances such as water, carbon dioxide and compost (without artificial additives). The biodegradation process depends on the conditions in which the biodegradable plastic is placed – temperature, pressure, microorganisms – as well as the specific material and its application.

European organisation European Bioplastics Assotiation,www.en.european-bioplastics.org presents a two-axis model encompassing all types of thermoplastics, including their possible combinations – see Figure 1. Thermoplastics are divided into four basic groups – the horizontal axis distinguishes biodegradability, while the vertical axis divides materials into those derived from fossil resources, most often petrochemical raw materials, or from renewable resources.

Figure 1 : Classification of thermoplastics according to European Bioplastics – four basic groups :

- plastics that are not biodegradable and are made from petrochemical raw materials – this category includes conventional, i.e. traditional plastics (although they represent only one group of plastics, they account for more than 90 % of global plastics production overall)

- biodegradable plastics from renewable sources – plastics that are made from biomass and are biodegradable

- biodegradable plastics from fossil raw materials – plastics that are biodegradable but are made from fossil resources

- non-biodegradable plastics from renewable sources – plastics made from biomass that are not capable of biodegradation

Note – caution: plastics made from fossil, petrochemical raw materials are not compatible with biodegradable plastics; they cannot be collected and recycled together and must be separated from one another and handled separately.

In 2021, bioplastics accounted for less than one percent of the more than 367 million tonnes of plastics produced worldwide, approximately 2.41 million tonnes, of which approximately 0.118 million tonnes of bioplastics were used in the automotive and transport industries. The packaging industry, as the largest user of bioplastic products, consumed 1.15 million tonnes, which represented approximately 48 % of the total bioplastics market in 2021 – see Plastic Europe. In 2026, bioplastics production is expected to reach approximately 7.59 million tonnes, and this share should exceed 2 % of the total quantity of plastics produced, with the packaging industry once again remaining the largest consumer.

In connection with the above figures on the quantity of bioplastics produced, we can ask ourselves – are bioplastics, and will they be, significantly beneficial to our future? Or rather, why have bioplastics, promoted as a major solution to the plastics problem (waste), fallen far short of their promising expectations? To begin with, I will quote a statement by Ramani Narayana, a professor at the School of Packaging in Michigan, USA, www.canr.msu.edu: “The concept that we could use it and throw it away, regardless of where we throw it, and it will safely disappear, does not exist”. In the following text, we will clarify Professor R. Narayana's statement using the example of PET bottles.

Single-use plastic packaging made from oil, usually PET, is the type in which most beverages and food are sold worldwide. In many respects, it is perfect packaging – strong, lightweight, versatile, clear and inexpensive. It protects products exceptionally well, keeps them fresh and even withstands the acids and pressure of soft drinks carbonated with CO2, without breaking or becoming permeable over months or years.

A competing bioplastic must replicate and imitate these functions and replace PET from fossil resources, which is successful only for certain products. The two most commonly used bioplastics are PHA, polyhydroxyalkanoate, generally made from sugars cultivated from algae, and PLA, polylactic acid, which is made from sugar found in crops such as maize or sugar cane. The price of PLA is one-tenth that of PHA, which is why PLA is used more widely, particularly in the packaging industry, for example. PHA is most commonly used as a coating on the inside of paper cups and in medical applications.

However, neither of these bioplastics has become widespread, for a simple reason - they do not match traditional plastics in strength and other properties, and they are also more expensive. The global plastics market is worth approximately 1.2 trillion dollars (trillion = 10¹² = 1 000 000 000 000), and bioplastics have a market share of approximately 9 billion dollars (billion = 10⁹ = 1 000 000 000).

Both of these bioplastics – PLA, PHA – can be broken down by microorganisms and become part of the natural world again within a relatively short period, but this will only happen if the bioplastic is collected as waste and composted in controlled and regulated industrial composting facilities at high temperatures, under a certain pressure and in the presence of suitable microorganisms. There are not many such facilities, particularly in developing countries, among which we too can be counted in this sense of the word, where the problem of plastic pollution is most serious. If bioplastics end up in landfills, as many do, without sufficient oxygen for them to break down, they can last for centuries and release methane, a potent greenhouse gas. If discarded into the environment, they pose a threat similar to PET plastic. Bioplastics can therefore be a false solution if the options for collecting and composting them are limited. One possible solution is energy recovery, while the environmental solution is to reduce the amount of single-use packaging we use.

Experts say that the challenges associated with the large-scale introduction of bioplastics show how difficult it will be to replace the billions of plastic bottles polluting the planet. Progress to date represents small steps compared with the growth in demand for plastic packaging, particularly in the developing world, where billions of bottles are used each year. Recycling conventional plastic bottles is an enormous challenge for low- and middle-income countries, many of which have virtually no recycling systems. Up to 95 % of the plastic waste carried by rivers into the world's oceans comes from 10 rivers in Asia and Africa. The same also applies to other industry segments that cannot do without plastic products.

Thermoplastics with Reduced Flammability

In connection with trends towards both reducing the weight of passenger cars and improving their safety, particularly BEVs, considerable attention is also being paid to the self-extinguishing properties of plastics used in the construction of such cars. Self-extinguishing behaviour is not a new requirement for cars, but with the introduction of battery-powered drivetrains it is becoming significantly more important, particularly from the perspective of fire safety.   

No one has yet been able to resolve all the factors that arise during the combustion of polymer-based materials. There is no comprehensive combustion model for these materials; some aspects are clear, while others, particularly when their effects are interconnected (as in real fires), allow only rough approximations.

As a general rule, all polymers are combustible under certain conditions. The flammability of polymeric materials as organic substances, which are inherently more or less combustible, is greatest when their macromolecules consist solely of carbon and hydrogen atoms. Their flammability can be reduced by additives, but even their action cannot eliminate flammability completely. To illustrate this, composite materials with a polymer matrix and fibrous reinforcing fillers can be cited - under the conditions that usually exist in a fire, glass fibres do not burn and therefore do not contribute to the combustion of the material, but at the same time they contribute to increasing the weight of the composite material concerned. Most inorganic fillers do not burn either, but they participate in the combustion process and may influence it.

Other fibrous reinforcements – carbon fibres, aramid fibres, man-made (polymeric) fibres, natural fibres, fibres of animal origin, mineral fibres, etc. – reduce the density of injection-moulding composites and will burn under fire conditions, but their contributions to the initiation and progression of combustion are generally much smaller than those of polymer matrices and are usually not given significant consideration. 

Polymers whose macromolecules contain a sufficient concentration of flame-retarding elements (retardation = delaying, slowing down, decelerating) stop burning once the flame is removed and are self-extinguishing (for example, polyvinyl chloride – PVC - and polytetrafluoroethylene - PTFE). 

Polymers that depolymerise (chemically complex substances break down into simple components) into a combustible monomer or decompose into combustible products have low resistance to fire, for example polyolefins (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polyoxymethylene (POM), rubbers, etc.

A polymer burns only at its surface, and therefore the burning rate also depends on the size of its burning surface, while a char layer forms on the surface, insulating the polymer from the flame and thus reducing the burning rate. 

Although no objective evaluation criteria are available for ranking polymers according to their resistance to fire, numerous methods are available for evaluating the flammability of polymeric materials. In thermoplastic injection moulding, the most commonly used are the so-called limiting oxygen index – LKČ (Limiting Oxygen Index – LOI) and the methods of the independent US safety science organisation with a global reach, Underwriters Laboratories, www.ul.org – UL 94 V (vertical test configuration) and UL 94 HB (horizontal test configuration). These tests use an open flame.

The limiting oxygen index – LOI = (O2 / ( O2 + N2 )). 100 /%/ - corresponds to the minimum concentration of oxygen, expressed as a percentage by volume, in a mixture with nitrogen at which the polymer continues to burn. A polymeric material is highly flammable when its LOI is less than 21 % by volume, slow-burning at an LOI between 21 % and 28 %, and self-extinguishing if its oxygen index is greater than 28 % by volume.

UL 94 is a widely used plastics flammability test that measures how a plastic part burns and how well it prevents flames from spreading. UL 94 flammability testing ranks plastics according to their ability to resist ignition, remain intact (cohesive, undamaged) at high temperatures and produce flammable plastic droplets. 

Since actual fire conditions are unpredictable, UL 94 flammability, like LOI, does not reflect how a material will react to fire. Rather, both the limiting oxygen index and the UL 94 standards determine how acceptable or suitable a particular thermoplastic injection moulding material is for a particular purpose in terms of its flammability.

The UL 94 flammability rating includes six flammability categories; for injection moulding granules, or for evaluating specimens made from them, four are used, ranked from the most flammable, HB, to the least flammable, V-0 :

- HB – horizontal burning test, in which burning stops before reaching a distance of 100 mm from the flame   

- V-2 – vertical, upright test (specimen positioned vertically) - burning stops within 30 seconds, allowing dripping, the formation of droplets from the vertical plastic specimen

- V-1 - vertical, upright test (specimen positioned vertically) - burning stops within 30 seconds, allowing dripping, the formation of droplets from the vertical plastic specimen in areas outside the flames

- V-0 - vertical, upright test (specimen positioned vertically) - burning stops within 10 seconds, allowing dripping, the formation of droplets from the vertical plastic specimen in areas outside the flames

Table 1 shows LOI values for selected polymer materials, including the correlation of LOI with UL 94 V-0.

Table 1: Oxygen index values for selected polymer materials

The flammability of polymer materials depends primarily on the chemical structure of the polymer and on certain physical factors affecting combustion, such as the amount of oxygen, surface area (fibres and films burn more readily than polymers in the form of products with thicker walls – UL 94 V ratings are usually stated together with the thickness of the specimen evaluated: 0.8 mm; 1.6 mm; 3.2 mm), etc. An important factor affecting the rate of decomposition and combustion of polymers is their thermal conductivity coefficient; those with a higher thermal conductivity coefficient are more difficult to ignite.

It follows from the above that, when processing them by injection moulding, we practically cannot cause them to burn by changing the processing parameters and thus reduce or increase their resistance to burning. Correctly selected processing parameters do not change the composition of the macromolecules of the material being injection moulded, nor that of the additives bound to or incorporated into them.

An inappropriate change in injection moulding processing parameters can influence (together with the design of the injection mould – the mould cavity venting system) the occurrence of the diesel effect, i.e. the formation of a localised area of burnt material, but the diesel effect does not involve a flame.

Thermal decomposition, particularly of additives in thermoplastics, can occur as a result of the polymer melt remaining at the processing temperature for a prolonged period in the plasticising barrel of the injection moulding machine or in the hot runner system of the injection mould. However, this is an indirect effect on flammability, in which individual polymer components, up to and including its macromolecules, may thermally decompose – but not burn. This is a gradual process that can begin at relatively low temperatures (usually above 100 °C). The thermal resistance of a polymer cannot be defined by a single temperature value, but can be defined by the length of time for which the polymer can be exposed to a certain temperature without a change in a specific property, such as strength. 

Similarly, we can thermally degrade a material through high shear stress, which is influenced by the composition of the material being injection moulded, the injection moulding parameters, particularly injection speed and the local temperature at a given point, and the cross-section of the mould cavity (during filling with polymer melt, the mould cavity in practice represents a flow channel in which shaped features, changes in thickness, etc., generally constitute hydraulic resistance that can locally increase the flow velocity of the polymer melt, thereby increasing friction and thus shear stress) or of the runner system (inappropriately selected - small – channel cross-sections relative to the volume of polymer melt being conveyed) at the given point in the mould. Again, however, this does not involve combustion.

The introduction to this chapter mentioned slowing down or retarding combustion, or the inhibiting effect, suppressing and preventing burning and combustion caused by heat and flame, and protecting plastic components against ignition and catching fire. Flame retardants (inhibitors) or self-extinguishing additives are substances that increase the thermal resistance of materials to burning and thus reduce or diminish the flammability of thermoplastics, or slow down, control and possibly also extinguish combustion. They usually become effective only at higher concentrations (5 % to 30 % or more, up to 80 %); their effectiveness depends on the flame-retarding capability of the substance concerned, and they therefore also affect processing properties (melt residence time at temperature, shear stress during plasticisation, shear stress during the filling of mould cavities with polymer melt) as well as service properties (they usually adversely affect the mechanical properties of polymers). Their potential decomposition can also affect the appearance of injection-moulded parts. Inorganic fillers also contribute to flame retardancy.

A number of flame-retardant additives prevent pyrolysis, which can cause fires. Pyrolysis is a process in which polymer molecules break down into hydrocarbon molecules and flammable gases, which then combine with oxygen to produce exothermic (heat-generating) chemical reactions. Reactive flame-retardant additives are incorporated into the polymer molecule itself, whereas active additives are blended into the compound with the polymer material. Both types of additives, or flame retardants, can suppress ignition in the vapour (gaseous) and condensed (solid) phases of a fire.

In general, three types of flame-retardant additives are available:

- radical quenching in the gas phase - brominated flame retardants (BRF). BRF are organohalogen compounds that undergo thermal degradation before releasing chemicals that reduce the oxidation reactions in flames. BRF are inexpensive and work with many polymers, but halogens are considered toxic and these additives are therefore banned in plastics

- endothermic degradation - uses mineral compounds known as antacids (a term borrowed from medical terminology – compounds that reduce the acidity of stomach contents), such as aluminium or magnesium hydroxides, as flame retardants for the vapour phase of a fire - the presence of gaseous oxygen or substances that release oxygen and thus support the combustion process. These compounds decompose when exposed to high heat, absorbing heat generated by endothermic reactions (energy in the form of heat is consumed). This stops combustion, cools the material and prevents flames from developing

- thermal shielding (char formation) - phosphorus-based flame retardants are non-halogenated compounds that form phosphoric acid when heated, which chars the solid material and creates a thick layer of char. This insulates the polymer and slows pyrolysis, making it more difficult to release gases that could trigger a fire. Phosphorus-based flame-retardant additives can be active (capable of responding to external stimuli) or reactive (the ability of a chemical substance to undergo chemical reactions).

Some plastics are inherently heat-resistant and have high melting points, which helps prevent them from catching fire. These plastics do not require flame-retardant additives, for example amorphous polyetherimide (PEI, melting temperature 219 °C), polybenzimidazole (PBI, continuous thermal resistance in air 343 °C), semi-crystalline polyetheretherketone (PEEK, melting temperature 371 °C), semi-crystalline polyphenylene sulphide (PPS, continuous service temperature up to 240 °C), amorphous PVC (melting temperature from approx. 160 °C to approx. 210 C), PTFE (melting temperature 327 °C), etc.

In addition to a number of advantages, such as suitability for a wide range of additives and especially for the production of composites in which, like PPS, they act as the polymer matrix – with reinforcing fillers (glass and carbon fibre), with particulate mineral fillers, modified elastomers, with high chemical resistance, high thermal resistance, resistance to electrical tracking, thermal and electrical conductivity, good sliding properties, etc. – they also have certain less desirable properties, particularly due to the high melting temperature of their crystalline fraction (above 280 °C) – high energy requirements during injection moulding - drying temperature 130 °C, dry-air drying time 3 to 4 hours, melt temperature 310 °C to 330 °C, mould temperature 130 °C to 150 °C.

The requirement for use, particularly in the construction of BEV vehicles, is closely linked to the requirement for durable colouring of injection-moulded parts (connectors, housings and other parts) associated with the battery (in a broader perspective, we must not overlook plastic parts in charging stations either) - orange colouring, which must comply with the standard for marking electrical conductors and must withstand the high processing temperatures used in injection moulding. In many cases, designers can use composites whose matrices consist of polypropylene homopolymers or copolymers, or polyamide composites, in both cases filled with mineral particulate fillers or reinforcing glass fibres. The materials have a UL 94 V-0 flammability rating.

End of Part One.

You can find Part Two here.
You can find Part Three here.
You can find Part Four here.