*Review* **Potential of New Sustainable Green Geopolymer Metal Composite (GGMC) Material as Mould Insert for Rapid Tooling (RT) in Injection Moulding Process**

**Allice Tan Mun Yin <sup>1</sup> , Shayfull Zamree Abd Rahim 1,2,\*, Mohd Mustafa Al Bakri Abdullah 2,3, Marcin Nabialek <sup>4</sup> , Abdellah El-hadj Abdellah <sup>5</sup> , Allan Rennie 6, Muhammad Faheem Mohd Tahir 2,3 and Aurel Mihail Titu <sup>7</sup>**


**Abstract:** The investigation of mould inserts in the injection moulding process using metal epoxy composite (MEC) with pure metal filler particles is gaining popularity among researchers. Therefore, to attain zero emissions, the idea of recycling metal waste from industries and workshops must be investigated (waste free) because metal recycling conserves natural resources while requiring less energy to manufacture new products than virgin raw materials would. The utilisation of metal scrap for rapid tooling (RT) in the injection moulding industry is a fascinating and potentially viable approach. On the other hand, epoxy that can endure high temperatures (>220 ◦C) is challenging to find and expensive. Meanwhile, industrial scrap from coal-fired power plants can be a precursor to creating geopolymer materials with desired physical and mechanical qualities for RT applications. One intriguing attribute of geopolymer is its ability to endure temperatures up to 1000 ◦C. Nonetheless, geopolymer has a higher compressive strength of 60–80 MPa (8700–11,600 psi) than epoxy (68.95 MPa) (10,000 psi). Aside from its low cost, geopolymer offers superior resilience to harsh environments and high compressive and flexural strength. This research aims to investigate the possibility of generating a new sustainable material by integrating several types of metals in green geopolymer metal composite (GGMC) mould inserts for RT in the injection moulding process. It is necessary to examine and investigate the optimal formulation of GGMC as mould inserts for RT in the injection moulding process. With less expensive and more ecologically friendly components, the GGMC is expected to be a superior choice as a mould insert for RT. This research substantially impacts environmental preservation, cost reduction, and maintaining and sustaining the metal waste management system. As a result of the lower cost of recycled metals, sectors such as mould-making and machining will profit the most.

**Keywords:** rapid tooling; geopolymer metal composite; additive manufacturing; injection moulding process

**Citation:** Yin, A.T.M.; Rahim, S.Z.A.; Al Bakri Abdullah, M.M.; Nabialek, M.; Abdellah, A.E.-h.; Rennie, A.; Tahir, M.F.M.; Titu, A.M. Potential of New Sustainable Green Geopolymer Metal Composite (GGMC) Material as Mould Insert for Rapid Tooling (RT) in Injection Moulding Process. *Materials* **2023**, *16*, 1724. https:// doi.org/10.3390/ma16041724

Academic Editor: Andrea Sorrentino

Received: 19 November 2022 Revised: 13 February 2023 Accepted: 16 February 2023 Published: 19 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

Time to market is a crucial aspect of a product development strategy, and speed is frequently compared to other factors such as functionality, creativity, or performance [1–3]. With numerous new technologies, worldwide rivalry for product creation is soaring. Furthermore, companies are always looking for cutting-edge technologies that are costeffective, capable of manufacturing goods in tiny quantities while maintaining excellent performance, and able to meet sustainability goals. This has driven the development of rapid tooling (RT) techniques, which are needed in today's market to replace traditional techniques with rapid product innovation and improve manufacturing processes, particularly mould-making [4–6].

As shown in Figure 1, RT provides quicker manufacturing for completing tests and starting final production, minimises costs, and reduces project time [7].

**Figure 1.** A Review of the Application of Rapid Tooling in Manufacturing [8].

Every industry, regardless of size, experiences a time when rapid tooling is required to address particular problems. Additionally, an improved tooling system is required for creating a limited number of functional prototypes to assess the product development cycle [8–10]. A small quantity of items is often utilised as a marketplace trial, evaluation need, and manufacturing process design [9,10].

Before mass manufacturing, functioning tools or prototypes must be launched for every scientific study [11–14]. These are not made available in large quantities to consumers but rather in limited amounts to researchers. RT is highly advantageous in this circumstance since it allows for the rapid introduction of items. Furthermore, the uses of production tools allow mass production to be obtained at a lower price because manufacturing costs are cheap. For this reason, many brand-new businesses and even big organisations prefer this technology to boost their profits and obtain a market advantage over their rivals [1–3].

Prototype companies or mould producers typically employ mild steel or aluminium for the mould inserts in RT. Production toolmaking is time-consuming and costly, and machining involves the same computer numerical control (CNC), electrical discharge machining (EDM), and electric discharge machining (wire EDM) procedures [15,16]. Recently, additive manufacturing (AM) has been employed to create mould inserts for RT [13,16]. For a limited number of prototypes, RT often uses models or prototypes made by AM as templates for manufacturing mould inserts or uses the AM process directly [4–6]. Numerous RT technologies are available on the market, such as a hybrid technique combining RT and AM to shorten RT production time.

RT can be categorised as either an indirect or direct technique and differs from traditional tooling in that the amount of time needed to create the tooling is significantly reduced [17,18]. Automated manufacturing methods use the AM process to generate mould inserts without the requirement for values to be predicted. Direct tooling includes processes such as additive manufacturing (AM), stereolithography (SLA), jet photopolymerisation (PolyJet), fused deposition moulding (FDM), and selective laser sintering (SLS) [5,6,19]. Al-

ternatively, the AM project as a master model is considered a secondary approach to create moulds for casting or plastic moulding processes. This technique combines the 3D KelTool process, metal casting, plastic casting, elastic moulding, and other comparable procedures to create injection moulding inserts [5,6,15,20]. Inserts constructed from epoxy-acrylate and utilising material for injection moulding by homo-polypropylene are used for quick tooling applications for 3D-printed injection moulds [21]. The mould insert constructed from steel and copper for hybrid prototype mould applications is created using a mix of laser powder bed fusion (L-PBF) and casting. In a study, L-PBF printed the steel shell with conformal cooling channels, and the shell was cast with copper [22]. For a material jetting (PolyJet) mould, a mould insert constructed from epoxy-acrylate resin is used and its in-mould behaviour is compared to a guidance mould insert fabricated from aluminium [23].

Failure in the moulding process in generating RT mould inserts is common for technologies with poor thermal and mechanical quality, such as SLS, FDM, SLA, PolyJet, and AM [5,6,19]. Zakzewski et al. [24] presented the bucking π-theorem, which was modified to analyse and characterise the poor surface quality Ra issue of SLS/SLM-processed samples, as well as the existence of porosity, a material structure defect. Furthermore, the use of SLA models is physically restricted. These constraints can be solved by developing procedures that use SLA parts as the "master blueprint" for the silicone mould process. In comparison to mechanical techniques, the DMLS process is inefficient for the design of basic plastic components. Furthermore, DMLS imposes a few constraints for a specific feature design for complicated components. According to previous studies on RT mould inserts, the stress applied to the mould insert during the injection cycle has a significant influence on the mould life [11,15,19,20]. Nowadays, a combination of RP technique and production tooling helps produce RT more quickly but faces dimensional accuracy and surface finish issues [17,18]. Moreover, the injection moulding process faces a cooling time issue where most of the mould inserts fabricated using RP techniques have very low thermal conductivity; thus, increasing the cooling rate will undoubtedly influence the cycle time for producing the components [4,16,25,26]. One of the RT options to increase competitiveness is using metal epoxy composite (MEC), which provides greater heat conductivity as mould inserts in RT application and lowers tooling production costs and lead times by 25% and 50%, respectively [3,6]. Using optimisation methods to determine the optimal composition for materials, as recommended in the linked literature, can be considered for future research, such as determining the best amount of Al or Cu to mix with epoxy resin for desirable mechanical properties [27–37]. The use of MEC mould inserts for RT in the injection moulding process, which uses pure metal filler particles combined with epoxy resin, has attracted the attention of many researchers [20,29,38–40].

The use of MEC materials as mould inserts offers better thermal and mechanical properties as compared to mould inserts produced using AM technologies [40–43]. However, dimensional accuracy and surface quality still need to be improved, so after the fabrication of mould inserts using MEC material and casting techniques, the mould inserts need to go through a secondary process (machining) to improve mould dimensional precision and surface quality in terms of cavities, especially for precision of plastic products (±0.05 mm).

Secondary (recycled) materials compete with primary materials in the metals business. Primary materials require the use of finite resources. Producing scrap materials or processed secondary metals can sometimes be more cost-effective than producing new primary materials, provided that the cost of collecting the waste is not prohibitively expensive [42]. Due to its spherical morphology and manageable particle size dispersion, gas-atomised (GA) powder is the most frequent feedstock for AM. However, much energy and inert gas are required to make GA powders [43]. Water-atomised powder is another option when increased powder solidification rates and reduced manufacturing costs are the priorities [29,44,45]. In contrast, melting the metal before ejection from the atomisation nozzles is energy-intensive because of the significant enthalpy difference between the liquid and solid states [45]. In metal AM, the feedstock powder is often remelted. This repeated melting is a costly and inefficient process [46]. As it can reduce materials of varying

sizes to powder, mechanical milling offers a chance for environmentally friendly powder production [31,47–49]. For mechanical milling, ambient or cryogenic temperatures are typically used [49,50]. The aforementioned considerable energy input is no longer required to reach atomisation temperatures [50]. Due to these potential benefits, mechanical milling is being used to reduce metal machining chips to powders that can be used in AM [46].

On the other hand, Davidovits' geopolymer technology is one of the groundbreaking innovations resulting in an affordable and greener binder alternative. The silica and aluminium in geosource materials such as metakaolin (calcined kaolin), and maybe techniques such as fly ash and bottom ash, are combined with the alkaline liquid to generate a geopolymer, an alkali-activated binder [1,11]. As a result, it reduces not only CO2 emissions but also recycles industrial waste, specifically using an aluminium–silicate mix to create products of higher value [9,10]. MEC using pure metal filler particles is beginning to be used by some researchers to investigate mould inserts in the injection moulding process [4,5,7]. However, a type of epoxy that can withstand high temperatures (>220 ◦C) is hard to find and still costly.

Additionally, besides municipal solid waste, coal combustion production (CCP) has been identified as the second-largest pollutant in the world. In 2011, about 130 metric tonnes (MT) of CCP were generated, with only 56.57 MT (43.50%) effectively used [51].

The four forms of solid waste created in substantial amounts by the CCP are boiler slag, bottom ash, fly ash, and flue gas desulphurisation (FGD) material [40–42]. One hundred and thirty metric tonnes of CCP included around 59.9 MT of fly ash. Fly ash was disposed of in surface impoundments covered with compacted clay soil, a plastic sheet, or both for the remaining 22.9 MT (38.36%) in landfills or surface impoundments [51–53]. The Environmental Protection Agency (EPA) of the United States (US) is now investigating the positive uses of fly ash [53–57]. This eliminates major health concerns associated with heavy metals and radioactive elements accumulated from fly ash disposal over time. Geopolymers derived from environmentally friendly materials, such as slag, or industrial by-products and used as a binding material are known as "green material".

One interesting property of geopolymer is that it can withstand temperatures up to 1000 ◦C. Nevertheless, geopolymer only has a compressive strength of 60–80 MPa (8700–11,600 psi), while epoxy has a compressive strength of 68.95 MPa (10,000 psi) [29,58]. However, employing geopolymer material has similar issues to using epoxy resin, which necessitates determining the optimal strength, accuracy, acceptable surface finish, and good thermal characteristics.

Early strength of geopolymer can be obtained as early as 1 day, with compressive strength up to 15 MPa, and continues increase up to 40–50MPa within 7 days, which is comparable with the strength offered by epoxy. Nevertheless, the optimum strength of geopolymer material can be obtained by 28 days (80 MPa) and the strength will keep on increasing over time [59].

It was recognised that the filler's interlaminar strength controls the bond strength of geopolymer reinforced with filler. The fact that filler with a bigger particle size has a lower binding strength is also well known. In addition, compared to epoxy resin, geopolymer showed high bond strength for both wet and dry interface surface conditions [59].

On the other hand, as the need for an environmentally friendly society grows, the quantity of waste material must be continually decreased. Hence, in order to achieve zero emissions, the idea of recycling metal waste from factories and workshops needs to be examined (waste free) [60–63]. Metal recycling helps to conserve natural resources while requiring less energy for manufacturing new products than would be required for virgin raw materials. Waste-free recycling reduces the emission of carbon dioxide and certain other harmful gases while also saving money and enabling industrial companies to reduce their production costs [64,65].

Through a Google Patents search (https://patents.google.com/ accessed on 10 February 2023), six patents granted/published that make use of (1) metal composite and composites made of (2) geopolymer and (3) metal were located. Table 1 lists the search terms for

this review's related field, rapid tooling. Fibre-reinforced metal composites (aluminium matrix composites) were developed by Yamamoto et al. [65] using aluminium alloy with 6–11 wt. % nickel as the metal matrix and reinforcing fibres. To mass produce complex parts with near-net shapes, Behi et al. [66] proposed using steel tooling in an injection moulding machine. In comparison to more traditional methods, this approach to producing complicated metal tooling is relatively cost-effective, making it possible to rapidly fabricate complex shaped parts using normal metal, ceramic, and plastic processing. According to the metal matrix composite introduced by Shaikh et al. [67], the fibre to metal or alloy ratio ranges from about 9:1 to less than about 1:1, and the fibres have an average diameter of approximately eight micrometres with a coating. Amaya and Crounse [68] discovered rapid manufacturing of mould inserts by employing blank die inserts formed from material typically used in the metal injection moulding process of complex shaped components to achieve high machinability rates, time and cost savings, extended tool life, and material savings. The dry-mix composition, as proposed by Nematollahi and Sanjayan [69], includes (a) an aluminosilicate material rich in silica and alumina and (b) a powdered alkali activator. Moreover, the dry-mix composition is chosen so that (i) the SHGC may be generated at ambient temperature without liquid activator, and (ii) strain-hardening behaviour and multiple cracking behaviours are observed. A strain-hardened, ambient temperature-cured geopolymer composite (SHGC) is generated by adding water and using a method of manufacturing an ambient temperature-cured SHGC. Qiang et al. [70] proposed a geopolymer composite material that is a type of 3D print as well as their preparation technique and applications, which included blast furnace slag powder accounting for 20~25% of the total composition weight, steel-making slag powder accounting for 10~15%, fly ash accounting for 0~5%, mine tailing machine-made sand accounting for 33~45%, exciting composite agent accounting for 3~5%, high molecular weight polymer accounting for 2.5~3%, volume stabiliser accounting for 1~3%, thixotropic agent accounting for 1~2%, defoamer accounting for 0.05~0.1%, and mixing water accounting for 13.9~12.45%. Each component is stirred, and subsequently pumped into the 3D printer applications for construction. The present invention's geopolymer composite material demonstrates good caking property, strong stability, good go-out pump from holding capacity and adhesive property, excellent form, and volume stability, resulting in the construction of buildings with good overall stability and safety during use. The six patents granted/published from 1990 to February 2023 are listed in Table 1.

RT is a cost-effective solution in the transition phase from new product development to mass production in the manufacturing industry [71,72]. RT, often referred to as bridge tooling, prototype tooling, or soft tooling, is a fast way to preproduce hundreds or even thousands of plastic parts prior to mass production, for design optimisation, functional testing, or preproduction verification, which can be a bridge between rapid prototyping (RP) and mass production. Shape, fit, and function prototype components are frequently made using RP technology, such as additive manufacturing [71,73,74]. Recycled metal waste such as mild steel, aluminium, copper, and brass after machining processes are as shown in Figure 2.

However, since 3D material qualities vary from those used in injection moulding, 3D-printed samples cannot provide a thorough evaluation of an injection-moulded part's functional performance [18,59], making RT extremely crucial for the manufacturing industry.

*Materials***2023**, *16*, 1724


**Figure 2.** (**a**) Metal scraps from turning; (**b**) metal scraps from grinding; (**c**) metal scraps from milling.

Using fly ash (waste from coal combustion) as the raw material, the metal scraps from the machining process are ground using a ball mill machine into a small and uniform size and mixed with geopolymer material to create green geopolymer metal composite (GGMC) material as in Figure 3. Then, this material can be used as mould inserts for RT applications which is expected to reduce tooling production costs and lead periods by up to 25% and 50%, respectively. The effect of GGMC material as mould inserts for RT in an injection moulding process and its relationship with compressive strength and thermal conductivity should be examined accordingly. Therefore, this research aims to determine whether geopolymer material may be used as RT mould inserts in the injection moulding process. The process by which GMCs are used as a new material for mould inserts is depicted in Figure 4.

**Figure 3.** Planetary Mono Mill Pulverisette 6 can be used in the ball mill process.

**Figure 4.** Graphical representation of GGMC as new material for mould inserts.

A power plant is a structure that produces waste geopolymer and generates electric energy from another form of energy. The geopolymer material is then combined with filler particles (waste from machining after a ball milling process to form a powder filler). The ratio of geopolymer and powder filler is evaluated accordingly in terms of thermal conductivity and compressive strength. Next, the optimised ratio is used to fabricate the GMC mould inserts. Then, GMC mould inserts are machined accordingly to fit the insert size and assembled in the mould base. Following the examination of the GMC mould inserts, the GMC mould is assembled in the injection moulding machine to mould out the specimen for further evaluation of the mould parts' quality in terms of shrinkage and warpage, including the cooling time required, which is definitely influenced by the thermal conductivity of the GMC mould inserts. The reliability of the GMC mould inserts is evaluated accordingly in terms of the number of shots (specimens) that can be produced before the mould starts to crack or wear.

#### **2. Injection Moulding Process**

#### *2.1. Important Processing Parameters in the Injection Moulding Process*

Processing parameters are essential to produce good-quality moulded parts in the injection moulding process. Previously, the trial-and-error approach to determine processing parameters relied upon a plastic injection moulding process. However, the trial-and-error approach is ineffective for complex manufacturing processes [75–77]. Therefore, many studies had been carried out over the years to minimise shrinkage and warpage defects by optimising the processing parameters [77–80]. In addition, it has also been observed that various critical processing factors, including packing pressure, melt temperature, packing shrinkage duration, mould temperature, and cooling time, have an impact on the quality of the moulded components produced (warpage) [77,81–83].
