Lubomír Zeman: Thermoplastic injection molding technology and weight reduction of injection moldings, part 3

PREREQUISITES FOR WEIGHT REDUCTION - THERMOPLASTIC INJECTION MOULDING TECHNOLOGY

Conventional thermoplastic injection moulding technology is based on thermal processes – it is always necessary to reproducibly prepare a melt with the greatest possible thermal homogeneity (homogeneity = uniformity, consistency) throughout the entire injection shot from the appropriate injection moulding pellets, which may have been pre-treated – dried to the residual moisture level specified by the manufacturer, mixed with additives, for example a colour masterbatch or secondary material, etc. -  and, using injection pressure and injection speed, convey it as gently as possible – without unnecessary shear stress - into the temperature-controlled cavity of the injection mould. In the mould cavity, through the holding pressure phase of the injection moulding process – the level and duration of holding pressure – we seek to eliminate, remove thermal volumetric contraction – shrinkage, so that after cooling and ejection from the mould cavity, the moulded part has the specified shape and dimensions, including all other required quality parameters. 

Virtually all modifications (variants, changes) of thermoplastic injection moulding technology are based on the principles of conventional injection moulding technology described above, but not all result in a moulded part with reduced weight. Technologies capable of producing moulded parts with reduced weight compared with conventional injection moulding technology include:

- multi-component injection moulding - producing moulded parts from multiple polymer materials with different properties

- gas-assisted injection moulding – GIT - Gas Injection Technology - production of hollow moulded parts, where the cavity is formed by the pressure of an inert gas, with the possibility of eliminating, for example, sink marks on visible surfaces or reinforced edges of parts, reducing moulded part weight, etc. 

- water-assisted injection moulding – WIT - Water Injection Technology - a technology similar to GIT that uses water to create cavities - a faster process, more uniform internal surface quality

- production of thin-walled moulded parts - whether parts are considered thin-walled is determined not by the absolute value of their wall thicknesses, but by the ratio of thickness to melt flow length through the mould cavity with the corresponding thickness 

- structural foam injection moulding of thermoplastics - using chemical or physical foaming to injection-mould parts with a solid surface layer and a core consisting of a cellular structure - the surface quality of the moulded parts is not suitable for visible applications

- MuCell injection moulding technology - the cellular structure is already formed in a special plasticising unit, and the moulded part has a cellular structure throughout its cross-section - the surface quality is not suitable for visible applications; if the appearance of the moulded part is a requirement, surface finishing is necessary

- technologies for producing foamed moulded parts such as CELLMOULD, FOAMMELT, etc.

- micro-injection moulding - production of precision moulded parts with small dimensions and low weights

- high-precision ultrasonic micro-injection moulding – the metered material is melted in the mould cavity by ultrasound, which also assists its flow

Modifications of the Standard Injection Moulding Process

Injection Moulding Technology Using Hot Runner Systems in Injection Moulds

Moulds for thermoplastic injection moulding can be classified into groups according to various criteria; one criterion is classification according to the principle – cold or hot - by which the polymer melt is conveyed into the mould cavity:

- moulds with cold runner systems - the cold runner system is at the mould temperature and usually consists of channels located in the main parting plane of the injection mould; after the polymer melt has been conveyed into the mould cavity, the material in the runner system solidifies, just like the moulded part, changing from the molten state to the solid phase, and is ejected from the mould together with the moulded part at the end of the cooling phase of the injection moulding cycle and prepared for further processing (for example, for mechanical recycling – granulation, possibly followed by compounding - mixing with other components and additives - and is ready for another processing cycle - conversion into a polymer melt, injection into the mould cavity, etc., or is treated as waste).

- moulds with hot runner systems, sometimes called wasteless systems, consisting of a heated manifold and hot nozzles at a temperature that ensures the shot of polymer material remains in the molten phase - the primary task of the injection mould's hot runner system is to supply polymer melt in each production cycle either directly into the mould cavity or into an auxiliary cold runner (a combined runner system) and keep it in the molten state within the runner system throughout the production of the required number of moulded parts. This eliminates or minimises runner scrap (cold runners), while significantly reducing the need for waste management facilities to handle runner scrap, reducing the need for subsequent removal of gates from moulded parts, and increasing the scope for optimising the holding pressure phase without the risk of the gate cross-section freezing off, as occurs in cold runner systems when their cross-sections are incorrectly sized.

Figure 2 shows diagrams of injection moulds for comparison: on the left, cold runner moulds of the so-called two-plate and three-plate types; on the right, a four-cavity mould with a hot runner system and balanced runner channels ensuring equal polymer melt flow paths to the individual mould cavities.

Figure 2 : Comparison of injection moulds with cold runner systems – on the left, so-called two-plate and three-plate moulds,on the right, a four-cavity mould with a hot runner system and balanced runner channels ensuring equal polymer melt flow paths to the individual mould cavities

One variant of injection moulding technology with hot runner systems is cascade injection moulding - cascade injection moulding (cascade = a set of devices connected one after another, in series, forming a single technological unit) - consists of a series of hot valve-gated nozzles controlled so that the mould cavities are filled sequentially, producing only one melt flow front. Cascade injection moulding is primarily used for long moulded parts or parts with a large surface area, where, among other benefits, it minimises weld line formation. It can also be used advantageously for moulded parts with a small wall thickness relative to the polymer melt flow length – saving material. A diagram of cascade injection moulding is shown in Figure 3:

Figure 3 : Comparison of multi-point injection moulding (right) with cascade injection moulding (left); when several gates feed a single moulded part, the polymer melt from the individual gates spreads through the mould cavity in circular wavefronts, whose fronts inevitably meet and form a weld line where two already cooled fronts come into contact (a local reduction in strength, a visual defect, which cannot be eliminated, only the risk of its formation can be reduced), where there is also a risk of air entrapment and a burn mark forming on the moulded part; cascade injection moulding technology (left) is used to eliminate the phenomenon described: unlike the standard system for filling the mould cavity with polymer melt (open gates), cascade injection moulding technology uses hot valve-gated nozzles, which open sequentially (from the edge or from the centre) through control of the valve pin movement; the polymer melt does not form wavefronts,but flows progressively without forming weld lines,the same principle can also be used for moulded parts with a high ratio of wall thickness to flow path length in the mould cavity (in simple terms, with a small wall thickness).

A comparison of the injection mould runner systems described reveals two facts – the use of a hot runner system increases the price of an injection mould fitted with this system compared with a cold runner mould, and moulds with hot runner systems require a higher level of knowledge from both mould setters and injection mould maintenance personnel, all while delivering greater or lesser savings in the material being moulded. The overall economics of using injection moulds with hot runner systems versus cold runner systems must always be carefully assessed from both an economic and an operational perspective.

Multi-component thermoplastic injection moulding technology – injection moulding of hybrid structural parts

In principle, this involves applying knowledge from standard thermoplastic injection moulding technology, typically using one injection moulding machine with several plasticising units and one injection mould designed for the particular technology, i.e. to receive several different materials or the same materials modified in different ways (for example, in different colours), producing a moulded part composed of different materials, most commonly materials differing in hardness, for example overmoulding TPE onto a “hard” supporting material.

These technological processes, in which the mould cavity is filled sequentially in stages using a dedicated runner system for the first material and, in subsequent stages, another material or materials, again using their respective runner systems, enable production times to be shortened and eliminate the need to manufacture separate parts and assemble them. The technologies also offer advantages in meeting requirements for the geometric properties of parts, including reducing their weight, for the mechanical properties of parts, for protective and decorative properties, etc.    

Multi-component thermoplastic injection moulding technologies can be divided into three main groups according to their underlying principle:

- multi-material injection moulding – the principle of these injection moulding processes is the injection of two or more materials into a single mould cavity; precise interfaces between the materials are not defined in the moulded part – this technological segment includes the following technologies: sandwich, interval, marble-effect and sequential injection moulding

- overmoulding, insert moulding, surface overmoulding – a pre-manufactured component or components are placed in the mould cavity, and during the injection moulding process the inserted part, which may be made of various materials – metal, plastic, textile, etc. – is overmoulded – these injection moulding technologies include, for example: Insert Moulding (IM), In-Mould Decoration (IMD), In-Mould Labeling (IML), Outsert Molding (OM), In-Mold Closing (IMC), In-Mould Assembly (IMA), hybrid injection moulding – thermoplastic x metal, thermoplastic x thermoplastic, thermoplastic x thermoplastic-matrix composite, Moulded Interconnect Devices (MID), Three Dimensional Moulded Interconnect Devices (3D MID), Total Integrated Manufactured (TIM)

- multi-component – multi-step – multi-material injection moulding – 2K, 3K, 4K, etc. – the process enables the injection moulding of differently coloured parts from the same material or parts from different materials in the same or different colours, or hardnesses, etc.; the joints are inseparable – moulding shrinkage is utilised, with one component joining the other when moulded onto it – the interface between the materials is defined, the materials must be miscible or at least compatible (compatibility = ability to combine, mutual compatibility) and must exhibit adhesion, bonding.

Of the three groups of multi-component injection moulding of thermoplastics listed above, overmoulding, insert moulding and surface overmoulding processes are used to reduce the weight of parts, with their principles being more or less the same – a pre-manufactured component or components are placed in the mould cavity, and during the injection moulding process the inserted part, which may be made of various materials – metal, plastic, textile, etc. – is overmoulded with polymer melt.

It has already been stated that one route to weight reduction is the use of thermoplastics in part design. However, because the use of thermoplastics alone in load-bearing parts is not possible due to their relatively low physical and mechanical properties, there is a shift towards hybrid designs, in which thermoplastics are combined with metal parts, thermoplastic mouldings are locally reinforced with fibre reinforcements, for example carbon fibres, and functional thermoplastic features are moulded onto parts that have been preformed and placed in the injection mould; these parts are preformed, for example, using  thermoforming or forming with heat or with heat and vacuum, and may be made of various materials, from composites filled with plant fibres to composite laminate systems with glass or carbon fibres and a thermoplastic matrix.  

The use of thermoplastic-matrix composites in hybrid structural parts combines the advantage of an efficient processing method – placing a preformed thermoplastic-matrix composite part in an injection mould and overmoulding it, in specified locations, with a functional polymer composite – with a reduced-weight design offering high strength and toughness, including energy absorption and the ability to subject the assembly to dynamic loading.

An example of such a solution is the Airbag Housing moulding, originally injection-moulded from PA 6, with the housing having a wall thickness ranging from 3 mm to 4 mm. The new solution, shown in Figure 4, is a hybrid design that combines preheated roving (a strand, a bundle of continuous untwisted fibres with high longitudinal strength),  47 % glass fibres impregnated with PA 6, placed in the mould, formed in the mould during closing and overmoulded with a PA 6/PA 66 copolymer containing 40 % glass fibres. This solution reduced the weight of the part by approximately 30 %, with a part thickness of 1 mm, which also made it possible to shorten the production cycle time thanks to the reduced wall thickness of the polymer material. 

Figure 4: Injection moulding of hybrid structural mouldings – this combines several manufacturing processes: forming (preheated roving in the form of a sheet is formed in the injection mould during mould closing into the shape defined by the mould's shaping components) and plastics processing (injection moulding of thermoplastic-matrix composites), integrated into a single manufacturing unit –   the Airbag Housing moulding is a hybrid design that combines roving made of PA 6 + 47 % glass fibres, placed in the mould, formed in the mould and functionally overmoulded with a composite PA 6/PA 66 copolymer containing 40 % glass fibres

Structural Foam Injection Moulding of Thermoplastics

The technology for producing structural foam thermoplastics – injection moulding of thermoplastics with foaming – TSG, Thermoplastische Schaum­Guss  – in all variants of foaming processes - is again an injection moulding technology aimed at reducing the weight of mouldings, as well as improving their insulating properties, minimising internal stresses, minimising sink marks and, where applicable, reducing the hardness of mouldings.

In terms of production technology, we distinguish several processes that use either a physical or a chemical blowing agent to create the foamed structure. Structural foam products most commonly have a solid skin and a foamed core. Plastics used for structural foam injection moulding include ABS, PA, PE, PPO, PC, PP, PS, PBT, PC/ABS, thermoplastic elastomers, etc. Structural foaming of polypropylene and its composites with talc and calcium carbonate is the most widespread.

Polymer foams, as a subgroup of cellular materials, can be defined as an artificially created dispersion (distribution) of gas in a polymer matrix. They are produced using blowing agents, i.e. additives that release gases or vapours during the manufacturing process. Blowing agents are divided into chemical and physical types. The first group includes chemicals that decompose under the effect of heat into gaseous products, which create a cellular structure, known as structural foam, during plastics processing by extrusion, blow moulding or injection moulding. These blowing agents, in quantities of up to 2 %, can be used to foam any plastic, including engineering plastics, but only to a density of 0,2 g.cm-3 to 0,8 g.cm-3. Physical blowing agents include chemical compounds that change their physical state during foaming, for example compressed liquefied gases that expand when pressure is released, and volatile substances that turn into gas – for example methyl chloride, freons, CO2, butane, pentane, low-boiling alcohols, ethers, etc. They are dosed in quantities of 5 % to 10 % and can foam the plastic used to a density of 0,008 g.cm-3  to 0,08 g.cm-3.

Chemical Foaming

The cross-sectional structure of the wall of a structural foam moulding produced using a chemical blowing agent – a solid skin and a foamed core – is shown in Figure 5. The sections taken from PC mouldings, which – from top to bottom – had mould cavity filling times of 0,3 s ; 0,6 s ; 1,2 s ; 1,5 s, show the transition from the solid surface layer – the skin – to the central foamed region – the core; with rapid filling, we can see that the skin layer is thin and the core consists of microcellular foam, and as the filling speed decreases, or the time taken to fill the mould cavity with polymer melt containing a chemical blowing agent increases, the skin becomes thicker and the central structure transitions to macropores. 

Figure 5 : Cross-section of the wall of a structural foam moulding produced with a chemical blowing agent; the core has an irregular, random cell structure

The solid layer on both sides of the mould cavity wall formed as a result of rapid cooling of the polymer melt and ensures the moulding's shape, stiffness and resistance to mechanical stress, chemical exposure and thermal effects. Across the cross-section, the mechanical properties decrease from the outside towards the centre of the moulding wall. The outer surfaces of the moulding do not have a high-quality appearance, for example because some of the blowing gases escaped into the surrounding environment and created irregular patches on the moulding surface. The core consists of a relatively irregular, randomly oriented cell structure, the volume of which ultimately determines the density of the moulding.

Structural foam mouldings foamed with chemical blowing agents can be produced on standard injection moulding machines with screw plasticisation and a sufficiently high injection speed.

The advantages of structural foam mouldings produced with chemical blowing agents, which in many respects are also shared by physical foaming, include, for example:

- easy production of mouldings with complex geometries

- weight savings at the same stiffness – a favourable strength / weight ratio

- reduced moulding weight

- reduced shrinkage and reduced tendency to warp

- improved thermal and acoustic insulation properties

- elimination of potential sink marks

- low residual stress in moulded parts

- possibility of designing injection moulds for lower strength requirements

- good machinability

- weldability using filler wire technology

- possibility of using a standard injection moulding machine

The disadvantages include:

- the visible surfaces of moulded parts are not completely smooth and glossy

- some fine surface features and textures of the mould cavity cannot be reproduced

- the external visible surfaces exhibit mottling and traces of gas flow and discharge

- longer cooling times are required compared with standard injection moulding

- a certain minimum wall thickness for moulded parts – usually more than approximately 3 mm

- large gate cross-sections must be used – laborious removal of gate vestiges

Physical Foaming Methods

The principle of physical foaming is not complicated - a liquid physical blowing agent is metered into the polymer melt in the injection and plasticising unit of the injection moulding machine. It decomposes thermally, producing vapours that disperse in the polymer melt. After the melt has been conveyed into the mould cavity, where the pressure drops, the gas phase separates and forms a relatively ordered and fine cellular structure of voids in the core of the moulded part walls (core). Where the moulded part, or rather its cooling polymer melt, comes into contact with the “cold” wall of the mould cavity, a solid layer, the skin, forms, just as in moulded parts produced using a chemical blowing agent – Figure 6. Figure 6 shows a cross-section through the wall of a structural foam moulded part produced with a physical blowing agent – unlike a chemical blowing agent, a physical blowing agent creates a relatively fine and ordered cell structure in the core.

Figure 6: Cross-section through the wall of a structural foam moulded part, physical blowing agent, with a relatively fine and ordered cellular structure in the core

Because a relatively fine cellular structure is formed, injection moulding with physical foaming  is referred to as microcellular injection moulding of thermoplastics. The most commonly used blowing gas is nitrogen (N2), with carbon dioxide (CO2) used less frequently. Unlike chemical foaming, this technology requires a special injection unit with a gas metering device. The principle of microcellular injection moulding is to create a single-phase mixture (polymer melt + gas) in the shot ahead of the screw tip in the plasticising and injection unit of the injection moulding machine, and to convey it into the mould cavity, where it expands. The foaming gas is metered into the plasticising unit in the form of a supercritical fluid (SCF – Supercritical Fluid).

The main representative of this technology is MuCell – Microcellular Foam Molding, in which the foaming gas is metered into the plasticising unit of the injection moulding machine in the form of a supercritical fluid (SCF – Supercritical Fluid). The technology is promoted by the US company Trexel Inc., which cooperates with injection moulding machine manufacturers, such as KraussMaffei or Negri Bossi.

A number of injection moulding machine manufacturers have developed and sell injection moulding machines for microcellular injection moulding based on MuCell technology under their own names, such as Engel foammelt, Arburg ProFoam, Battenfeld Cellmould, etc. A microcellular injection moulding solution always includes an injection and plasticising unit with a screw featuring various mixing zone designs and a shut-off nozzle. It also includes a metering nozzle for dosing inert gas and a source of pressurised gas. To ensure the foaming reaction occurs even at the end of the polymer melt flow path in the injection mould cavity, a high cavity filling speed must be ensured, and the injection moulding machine must therefore have a high injection capacity. MuCell technology and its variants can also be used for injection moulding with hot runner moulds.

In addition to the technologies offered by injection moulding machine manufacturers, technologies from other manufacturers can also be used; for example, the Swiss company Sulzer Chemtech offers its solution under the name Optifoam. The advantage of this solution is that it neither requires nor uses special screw and injection unit designs; instead, the pressure metering device for the liquefied gaseous blowing agent is installed as an intermediate component between the head of the injection unit and the shut-off nozzle of the injection moulding machine. The nozzle also contains a static mixer made of cemented carbides, and the polymer melt and liquid blowing agent are mixed in the static mixer. A similar technology is also offered by the German company Demag Ergotech.

Linde offers another structural foam injection moulding technology. Its  PLASTINUM Perfoamer equipment for microcellular foam injection moulding of thermoplastics consists of a dryer for the granules to be moulded, where the granules are dried to the specified residual moisture content and brought to the required temperature. The granules prepared in this way are transferred by a vacuum loader to a pressurised impregnation unit, where they are saturated under pressure with a blowing agent, carbon dioxide CO2. The amount of CO2 absorbed by the granules depends on the specific material being saturated, the saturation pressure, temperature and duration of “impregnation”. Carbon dioxide is drawn from pressure cylinders in gaseous form, or it can be delivered in liquid form and converted from liquid to gas in a compressor unit. Once the required volume of material has been saturated in the impregnation unit, the pressure in the unit is released and the saturated material is transported to a hopper, from which it is fed directly into the screw and plasticising unit of a standard injection moulding machine. Depending on the size of the equipment, saturated material can be supplied to several injection moulding machines. Carbon dioxide remains in the granules prepared in this way for approximately two hours. The method allows the weight of mouldings to be reduced by up to 60 %. One advantage of the method is that it does not use a mixing screw and is therefore also suitable for thermoplastics susceptible to degradation or damage as a result of high shear stresses, such as POM.      

The technologies used for injection moulding structural foam thermoplastic parts also include co-injection moulding (similar to 2K technology). Co-injection moulding uses two plasticising units: one prepares a conventional polymer melt and the other a melt containing a blowing agent. The injection moulding process is based on alternately injecting the material for the skin of the moulding and its core into an injection mould whose temperature is appropriate for the material being moulded. First, the polymer melt that will form the outer, visible part of the moulding is injected, followed by the foamed core of the moulding into the mould cavity, and then conventional melt is injected again. This injection moulding procedure produces a solid skin with a foam core. It is logical that the procedure can be reversed to produce a moulding with a structural foam skin and a solid core, or, with a suitable mould design, foams of different hardnesses can be injected into a single moulding.

Structural Foam Injection Moulding of Thermoplastics Compared with Standard Thermoplastic Injection Moulding  

I have already mentioned that one of the main benefits of producing structural foam thermoplastic mouldings is weight savings. Weight savings can be viewed from two perspectives, which in fact form a single whole. One perspective is the saving of material as the raw material for producing a given moulding, and thus a reduction in production costs; the other is the reduction in the weight of the moulding, which contributes, among other things, to increasing its competitiveness, including subsequent savings, for example in the automotive industry through reducing the weight of conventional, hybrid or fully electric cars. Last but not least, there is also the environmental perspective.

It is a generally known fact that it is impossible to produce a thermoplastic moulding that contains no  residual internal stresses. One of the causes of residual stresses being introduced into thermoplastic mouldings is the holding pressure phase  of the injection moulding process, which compensates for the effect of shrinkage on the moulding and influences, among other things, its dimensional accuracy. The pressure, or rather its profile during the holding pressure phase and the duration of its application, can contribute to  “locking” pressure into the moulding and, after its ejection from the mould cavity, to deformation of the moulding or the formation of small cracks on its surface (for example in combination with exposure to a surface-active environment), etc. In the case of structural foam injection moulding technologies, the situation differs from standard thermoplastic injection moulding. In these technologies, the holding pressure phase of the injection moulding process is reduced (it has little effect on the final product) or completely eliminated, with the injection moulding process using no holding pressure phase. The shrinkage influenced by the holding pressure phase is compensated for here by the expanding nuclei of the cellular structure within the internal structure of the solidifying injected material.

Surface quality – see Figure 7 - is a significant disadvantage of all variants of the technology for injection moulding structural foam parts. Surface streaking and the inability to replicate textured mould cavity surfaces are caused by the escape of foaming gas through the surface of the moulding before the polymer melt contacts the mould cavity surface, and by the formation of local intermediate layers between the mould cavity surface and the outer surface of the foamed moulding.

Figure 7 :Typical appearance of structural foam mouldings

To eliminate the problem described, it is possible, for example, to use co-injection moulding technology (see above) or a technique used in standard injection moulding to minimise weld line formation – preheating the cavity surface of the injection mould before injecting the polymer melt containing a blowing agent. Preheating can be carried out, for example, using infrared heaters mounted on a robot, which preheat the cavity surface while the mould is open, after the moulding has been ejected from the cavity (this preheating method extends the injection moulding cycle but does not require a special injection mould design). Another option is to use induction heating of the injection mould cavity surface, integrated into the mould. The advantage is the speed at which the cavity face is heated by induced eddy currents; heating begins immediately after the moulding is ejected from the cavity, regardless of mould movements.

In addition to the options mentioned above for improving the quality of the visible surfaces of mouldings containing blowing agents, MuCell injection moulding uses GGD -  Gasgegendruck technology, a technology for injection moulding against gas pressure (nitrogen, dry air) in the injection mould cavity. In principle, before filling of the mould cavity with the mixture of polymer melt + gas in the SCF state begins, the cavity is filled with pressurised gas; the gas pressure can be as high as 80 bar. The mixture is therefore injected against the pressure of the gas cushion in the mould cavity. Depending on the progress of cavity filling with the mixture, the pressure in the cushion is controlled and gas is vented from the cavity. The counterpressure prevents the blowing gas from penetrating through the upper surface of the moulding into the space between the mould cavity wall and the wall of the moulding, resulting in a smoother moulding surface.

The German company INCOE, a manufacturer of hot runner systems for injection moulds, offers GAS – CORE technology, which, like the GGD technique, aims to improve the surface quality of structural foam mouldings. When using the Gas – Core technique, physical foaming of mouldings with a smooth, solid surface, injected into the mould cavity through a polymer melt hot runner system, is carried out in three steps – in the first, a portion of polymer melt without a blowing agent is injected into the cavity; the hot runner system is equipped with a pressurised inert gas metering unit and a mixing chamber. In the mixing chamber, the polymer melt is mixed with nitrogen or carbon dioxide, and the resulting mixture is injected into the mould cavity through a hot runner nozzle; in the third step, the mould cavity is topped up again with polymer melt without a blowing agent.      

Gas- and Water-Assisted Injection Moulding – GIT and WIT Technologies

One of the fundamental approaches to designing thermoplastic mouldings, set out in the principles of design for manufacturability, is to avoid thick walls and walls of differing thicknesses in the design of a given moulding. In addition to increasing the weight of the moulding, failure to observe this principle usually leads to a significant increase in the production injection moulding cycle time (the wall thickness of the moulding is squared in the calculation of the moulding cooling time, i.e. this is an exponential relationship between moulding cooling time x moulding wall thickness), sink marks (material accumulation results in greater shrinkage, which may not be successfully compensated for during the holding pressure phase), dimensional instability and warpage (different wall thicknesses in the moulding cause different local shrinkage – different dimensions and therefore warpage), an unjustified increase in consumption of the material being injected (an increase in the cost and weight of the moulding), etc. 

If, for functional reasons, design reasons or other reasons, it is necessary to design injection-moulded parts with thick walls, the part designer should also bear in mind other options for designing and manufacturing injection-moulded parts. The basic approach is to create a ribbed part instead of accumulating material or, if the shape can be made into a shell, to manufacture the two halves separately (they can have thin walls or be ribbed) and subsequently join them (by welding, adhesive bonding, snap-fitting, screwing together, etc.). Another possible solution is a retractable core in the mould, moving along a straight or curved path, or the use of injection moulding with a sacrificial, fusible core (a core made from water-soluble EVA thermoplastic, from bismuth–tin alloys, melting temperature 138 °C, tin–antimony–lead alloys, melting temperature 187 °C, etc.). Last but not least, structural foam moulding technologies, see the preceding chapter, or gas-assisted or water-assisted injection moulding technologies can be used.

Gas-assisted injection moulding of thermoplastics (GIT – Gas Injection Technology), usually using pressurised nitrogen — an inert, non-flammable, chemically neutral gas with no risk of reaction with polymer melts and their additives — is a technology whose principle involves introducing a pressurised inert gas into specific areas of the cooling polymer melt or moulded part, creating internal cavities in the walls of the part. GIT technology has several variants, essentially comprising two approaches — the partial-fill GIT process, in which the mould on a conventional injection moulding machine is only partially filled with polymer melt, and pressurised nitrogen is introduced into the cooling melt either through the machine nozzle or through an injector on the mould, creating cavities and, thanks to its regulated pressure, also acting as holding pressure. The second approach is the GIT process with overflow — standard injection moulding machines are  again used, but the first step of the process is to fill the mould cavity with polymer melt, followed by a switch to holding pressure. Once the holding phase is complete, pressurised nitrogen blows the polymer melt in the core of the part out of the mould cavity into an auxiliary overflow cavity, from which it is ejected from the mould together with the part. During the cooling of the part, the pressurised nitrogen compensates for and offsets the shrinkage of the cooling melt and helps replicate the surface of the mould cavity. Parts manufactured using GIT technology are shown in Figure 8.

Figure 8 : Parts manufactured using GIT technology – left: GIT with overflow into an overflow cavity; right : GIT technology and its use to compensate for shrinkage and sink marks

In addition to gas-assisted injection moulding of thermoplastics, an injection moulding technology can also be used in which hollow parts are produced not with gas assistance, but by pumping pressurised water into the cooling polymer melt core at the centre of the part — water-assisted injection moulding (WIT – Water Injection Technology). The technological principles of the two thermoplastic injection moulding methods are similar.

As with gas-assisted injection moulding, in WIT too (and here even more necessarily), we must remove the water from the process before it is completed. Two approaches are used to remove the water, both involving filling the mould cavity with polymer melt and subsequently injecting pressurised water through the injection moulding machine nozzle or through a filling nozzle in the mould cavity, with the option of regulating its pressure and flow rate (velocity) — after the water has completed its role in the holding pressure and cooling phases of the injection moulding cycle and a valve has been opened, it is expelled from the part by compressed air, or, again after the holding pressure and cooling phases are complete, it is pumped back out. Since water is used in the WIT injection moulding process, thermoplastics susceptible to hydrolytic degradation (hydrolysis = breakdown of the polymer chain by water) must be processed in short production cycles, with complete removal of water from the parts.

When comparing the two technologies, the shorter injection moulding production cycle time of WIT technology can be highlighted (at a temperature of 20 °C and a pressure of 1 bar, water has a thermal conductivity of 0,604 W.m-1K-1 and a specific heat capacity of 4 182 J.kg-1K.1) compared with GIT (at a temperature of 20 °C and a pressure of 1 bar, nitrogen has a thermal conductivity of  0,0143 W.m-1K-1  and  a specific heat capacity of 1 038 J.kg-1K-1). Another advantage of using water is its beneficial effect on the quality of the internal surfaces of hollow moulded parts, which it smooths, while the walls are more uniform in thickness — see Figure 9.

Figure 9 : Comparison of internal cavity formation using GIT and WIT technologies – top, bottom: cross-section of a clutch pedal manufactured using WIT technology

Both of the technologies mentioned – GIT,  WIT – when appropriately applied -  help address or reduce:

- material consumption by creating targeted hollow sections

- production cycle time

- enable good replication of mould cavity surfaces, minimising sink marks

- are suitable for producing mouldings with different wall thicknesses

- minimise warpage of mouldings

- increase the dimensional stability of mouldings

- enable the production of thin-walled mouldings with long polymer melt flow paths

- reduce the need for high clamping forces

Production of Thin-Walled Mouldings

The main purpose of this thermoplastic injection moulding technology is also to save material and improve production efficiency, shorten the production cycle and reduce the cost of preparing the polymer melt in the injection moulding machine. When considering this injection moulding technology, it is necessary to clarify what is meant by the term “thin moulding wall”. When injection moulding thin-walled parts, we must not think in conventional terms such as - a thin wall is a wall thickness of 0.6 mm to 1.2 mm. Instead, we must always take into account the ratio of the polymer melt flow length in the injection mould cavity to the thickness of this flow path. Only this ratio provides meaningful information about what constitutes a thin moulding wall, and the assessment must always relate to the specific moulding and its material.

As with the plasticising of large volumes of polymer melt, the aim when injection moulding thin-walled parts is to prepare the appropriate volume – shot – of polymer melt in the shortest possible time. Barrier screws are used for this purpose in both cases. Unlike standard three-zone screws (feed zone beneath the hopper, central compression zone, metering and mixing zone), these screws have a different design intended to accelerate preparation of the polymer melt and meter it in front of the screw tip. A barrier screw has a feed zone and a double-flight barrier screw zone for rapidly conveying the melting pellets into a shear-mixing zone of various geometries, from which the polymer melt passes into the shot in front of the screw tip – see Figure 10.

Figure 10: Barrier plasticising screw - the barrier screw has 2 flights - in the blue section,  the melt remains separated from the unmelted or partially melted pellets – heater zone settings – blue curve, a standard three-zone screw has a rising curve from the hopper to the injection nozzle; barrier screw, red curve – the temperature settings of the individual zones are reversed, from a high temperature at the hopper to a lower temperature at the nozzle

Compared with standard injection moulding machines, the injection units also have higher injection parameters – a maximum injection pressure of up to approximately 3 500 bar and a forward screw injection speed of up to 2 000 mm.s-1.

Effective use of this injection moulding technology also requires a suitable moulding material with high flowability and, because high injection pressures and injection speeds are used, adequately dimensioned injection moulds, including the positions and surface areas of the ejectors (risk of deformation during ejection). High holding pressure values (short holding time, but a high holding pressure level) also affect material shrinkage; for example, PC/ABS Bayblend T 65 MN exhibits shrinkage of 0.77 % at a holding pressure of 300 bar, but only 0.3 % at a holding pressure of 800 bar.

END OF PART THREE. 

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