2.3.1. Alternative Materials for the Mould Insert

Small numbers of functional plastic parts that range from five to one thousand units are usually needed during the product development stage to confirm the development stage before mass manufacturing. An alternative material is required for mould inserts to reduce the cost, time, part quality, and production number [48]. Currently, the alternative material that is used in mould insert fabrication is epoxy resin. The different types of epoxy resin with their properties are listed in Table 4 [101].


**Table 4.** Different types of resin and their properties [102].

Nevertheless, there are some restrictions when using epoxy as a mould insert in RT for injection moulding. Epoxy has limitations that must be overcome, such as its low hardness and strength [103]. Geopolymer can be used to replace epoxy since it is robust and strong and is now utilised in building concrete. In addition, it preserves the environment, reduces cost, and supports sustainability of waste management systems [81,94,95,104,105]. As an implication, industries related to mould-making will benefit the most due to the reduced cost when using recycled materials.

### 2.3.2. Rapid Tooling (RT) Mould Inserts

Rapid tooling is an example of how rapid prototyping is used in the manufacturing industry. It enables the rapid and low-cost construction of moulds for small batches of manufacturing goods. Tooling may be either harsh or soft, and can be classified as direct or indirect [9]. An efficient method of direct tooling involves the use of soft materials in a rapid prototype process such as stereolithography material [96,106–108]. Numerous different tools, such those made of powder metal [96,103], are made of tough materials, and in the indirect tooling method, a casting pattern is made by rapid prototyping and then used to manufacture the proper tool. Due to its simplicity of usage in producing mould inserts, aluminium-filled epoxy resin [109,110] is becoming a popular soft material. Silicone rubber is mostly utilised in the manufacture of indirect tools [107].

The most significant factor for injection moulds made utilising the RT process is tool life. RP technology has improved to the point where tools directly generated by RP machines should represent all of the model's various elements and features accurately and precisely. On the other hand, many soft rapid prototyping materials are unable to tolerate sufficiently high pressure and melt temperature owing to their poor heat conductivity [107], resulting in shortened useful life of the instrument. Although other methods, including metal laser sintering, may be employed to apply metal coatings on pliable materials [108] to enhance their hardness, it will raise the manufacturing process's difficulties. Alternatively, epoxy resin is often used in indirect moulds due to its plasticity or compatibility with casting models. The use of metal powder may greatly boost its hardness and heat conductivity, prolonging tool life even more. However, this does not prevent the epoxy resin from

hardening within the mould chamber and becoming brittle. Indirectly crafted tools thus wear very quickly [104]. Some of the studies concentrating on RT are listed in Table 5.

Tomori et al. [110] investigated how changing the material formulation and determining the validity of composite tooling boards affected mould efficiency and component quality. An example of the method for setting up a tooling board is illustrated in Figure 5. The boards were constructed using three materials: RP4037 (fluid), RP4037 hardener, and silicon carbide (SiC) filler (powder). For the six moulds, two cutting speeds (1.00 and 1.66 m/s) and three tooling board formulations (28.5%, 34.75%, and 39.9% wt. % SiC filler) were used. The surface roughness of the moulded components served as the study's response variable, while cutting speed served as the study's independent parameter. As there was no visible mould damage, the physical structure of the mould was unchanged by SiC concentration and cutting speed. This discovery indicated that the SiC content in the mould has a significant impact on the surface roughness of the moulded items. Additionally, the flexural strength rose with the SiC filler concentration (from 58.75 to 66.49 MPa), following a pattern comparable to the heat conductivity of the mould material. The influence of filler concentration primarily on the direction of welding for moulded components was not examined in this research.

Senthilkumar et al. [111] studied the effects of epoxy resin on the mechanical characteristics of aluminium (Al) particles. The sample was cast utilising Al filler mixed into epoxy resin at various concentrations. Optical microscopy revealed that the Al particles were uniformly dispersed throughout the epoxy resin matrix. These results show that increasing the amount of Al particles inside the epoxy resin matrix significantly raises both the thermal conductivity (3.97 to 5.39 W/mK) and the hardness value of the composite (69 to 89 RHL). The sample's fatigue life decreased from 15,786 cycles to 734 cycles as the Al content of the epoxy resin increased. The best percentage of Al filler particle for enhancing mould performance and durability was found to be between 45 and 55 wt.% There was an improvement of 72 RHL in durability, 10,011 cycles in fatigue resistance, and 4.06 W/mK in thermal conductivity. However, the hardness value increased by 4.34% for every 5% increase in Al filler particles, which might reduce the fatigue life by 36.58%. Nevertheless, there has been no further research on the moulded components' flexural strength, compressive strength, tensile strength, or surface appearance.

Srivastava and Verma [27] attempted to determine how the addition of Cu and Al particles to epoxy resin composites altered their mechanical properties. Epoxy resin was mixed with Cu and Al particles (1, 5, 8, and 10 wt. %) to create a variety of filler compositions. The results of the mechanical tests showed that the epoxy resin with Al reinforcement has excellent tensile properties, with a tensile strength of 104.5 MPa at 1 wt. %, while the epoxy resin composites with Cu filler was optimal in the hardness test (22.4 kgF/mm2 at 8 wt. %) and had a compressive strength of 65 MPa at 10 wt. %. In addition, epoxy resin composites filled with Cu demonstrated better performance than those filled with Al despite having a lower hardness. This finding demonstrated that the tensile strength, wear loss, and hardness of the material all decreased steadily with increasing filler content, whereas the compressive strength, friction coefficient, and hardness all showed an increase. However, the impact of the welding direction on the surface of the moulded components is yet to be determined.

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


 RT.


**Figure 5.** Method for setting up a tooling board.

Fernandes et al. [26] studied the dimensions and mechanical characteristics of epoxy resin/Al insert-moulded PP injection components for RT. A 140 mm diameter sphere was made up of five chambers with 2 mm thick walls that formed the work's central geometrical component. The length of the test was 60 mm, the diameter of the entrance was 6.5 mm, and the draught angle was 2◦. To test the suggested mould, a novel hybrid mould comprising epoxy resin and Al was employed in this work to insert polypropylene (PP) pieces. In addition, comparable pieces were inserted to use an AISI P20 (conventional) steel mould, the same as in the genuine application. Epoxy resin/Al insert-filled components had slightly higher tensile strength at yield (22.0 ± 5.0 MPa) than steel AISI P20 insert-filled components (20.0 ± 4.5 MPa), but the difference was not statistically significant. Epoxy resin/Al-injected parts had lower values for ultimate tensile strength, elongation at break, and modulus of elasticity than steel AISI P20-injected parts. Furthermore, the Shore D hardness of objects formed by AISI P20 steel inserts increased by 8.5% in comparison to goods moulded by epoxy/Al inserts. When compared to components injected using an epoxy/Al mould, those injected using an AISI P20 steel mould showed less shrinkage. Based on these findings, epoxy/Al moulding blocks may be a high-quality alternative to fast tooling for producing single units or small series. Furthermore, this research did not investigate whether the orientation of welding on the moulded components was affected by the impact.

Khushairi et al. [112] investigated various epoxy compositions using Al, Cu, and brass fillers which were tested for their mechanical and thermal properties. In Al-filled epoxy, different combinations of brass and Cu filler (10, 20, and 30% wt. %) were used. Brass and Cu densities were 2.22 g/cm3 and 2.08 g/cm3 at the optimum filler content, respec-

tively. When 30% Cu fillers were added to an epoxy matrix, the total thermal diffusivity (1.12 mm2/s) and thermal conductivity (1.87 W/mK) were the maximum, but adding brass had no effect on thermal properties. When 20% brass filler was added, compressive strength increased from 76.8 MPa to 93.2 MPa, whereas 10% Cu filler raised compressive strength from 76.8 MPa to 80.8 MPa. As a result of porosity, multiple metal fillers diminished the compressive strength. According to this research, fillers boost mechanical, thermal, and density properties of Al-filled epoxy. Nonetheless, a careful evaluation of the surface characteristics, notably the welding line of the moulded components, is necessary to determine the moulded parts' quality.

Kuo and Lin [113] examined the quick injection moulding of Fresnel lenses from liquid silicone rubber. The experiment was conducted utilising RT and liquid silicone rubber (LSR) parts to build a horizontal LSR moulding machine (Allrounder 370S 700–290, ARBURG, Loßburg, Germany). Injection moulds for LSR injection moulding could be manufactured using Al-filled epoxy resin. The total microgroove depth and width of the Al-filled epoxy resin mould were 90.5% and 98.9%, respectively. LSR-moulded components exhibited typical microgroove depth and width transcription rates of roughly 91.5% and 99.2%, respectively. LSR-moulded components' microgroove depth as well as width may be modified to within 1 m. The mean surface polish of the Al-filled epoxy resins increased by around 12.5 nm following 200 LSR injector test cycles. However, further testing on tensile strength, compressive strength, hardness, and density, as well as weld line observations, is essential to understand the impact of quick injection moulding on the recommended mould in terms of moulded component quality.

From the review that has been carried out, it can be seen that numerous elements such as flexural strength, hardness, thermal conductivity, tensile strength, compressive strength, density, thermal diffusivity, and surface roughness of the new material introduced are important factors that need to be considered prior to its use as mould inserts for RT in the injection moulding process.

#### *2.4. Geopolymer*

A geopolymer is formed by combining a dry solid containing high aluminosilicate content, called a precursor, with alkaline solution and other ingredients if needed [114]. It is a semicrystalline, three-dimensional structure made of the tetrahedral structures of silica and alumina that share oxygen [115]. Geopolymer precursor can be obtained in two ways: from geological origin or industrial by-products. Examples of geological origins are kaolinite and clay, while industrial by-products are fly ash (FA), wheat straw ash, and furnace ash. Geopolymers are activated using high-alkali solution for the polymeric reaction to occur by using sodium hydroxide (NaOH), potassium hydroxide (KOH), or a mixture of sodium oxide (N2O) and silicon monoxide (SiO) [116].

The geopolymer concrete curing process has a significant impact on mechanical characteristics and microstructure development [117,118]. Excellent mechanical strength, reduced creep, improved acid resistance, and minimal danger of shrinkage are all characteristics of geopolymer concrete [41,119,120]. The durability of waste pozzolan-based geopolymer concrete that is cured at high temperatures has been extensively studied [121–124]. By curing the geopolymer at a higher temperature, one may enhance the geopolymer's mechanical properties, polymerisation level, microstructure density, and overall strength [117,125–127].

Geopolymers come in a variety of unique shapes, and each type has certain properties. Geopolymers are an alternative material in the tooling industry. However, changing the geopolymer composition will change the qualities of the geopolymer, where selecting the correct geopolymer precursor will give the tooling industry greater advantages.
