**3. Summary and Future Works**

A combination of RP technique with production tooling helps carry out RT more quickly but faces dimensional accuracy and surface finish issues. Moreover, the injection moulding process faces an issue with cooling time where most mould inserts fabricated using RP techniques have very low thermal conductivity, thus increasing the cooling time, which will definitely affect the cycle time to produce the parts.

Many researchers have started to explore the use of metal epoxy composite (MEC) as mould inserts for RT in the injection moulding process by using pure metal filler particles. However, epoxy that can withstand high temperatures (>220 ◦C) is hard to find and costly. Therefore, there is a potential opportunity for epoxy to be replaced by geopolymer materials, especially fly ash as raw material. Geopolymer material can withstand temperatures up to 1000 ◦C. Similarly, the compressive strength of epoxy is 68.95 MPa (10,000 psi) as compared to geopolymer which has strength of 60–80 MPa (8700–11,600 psi). The challenges of using geopolymer material are similar to those of epoxy resin in that optimal strength, good accuracy, acceptable surface finish, and good thermal characteristics must be determined. Based on the gaps found from the literature, recommendations for future studies are as follows:


This review has provided a clear reference for future development of mould inserts for RT using GGMC material. Thus, initiative needs to be taken to conduct an analysis on the effect of incorporating metal particles in geopolymer material as mould inserts for RT and its relationship with compressive strength and thermal conductivity. Moreover, the integration of metal scraps from machining with geopolymer formed from waste makes this research more interesting. GGMC material should be examined for metallurgical parameters such as corrosion rate, coefficient of expansion, surface roughness, and additive manufacturability. Furthermore, the machinability and the reliability of GGMC mould inserts should be explored and evaluated accordingly. At the end of this research, the discovery of new sustainable green material will benefit moulding and rapid prototyping industries, including with its environmentally friendly attributes.

**Author Contributions:** Conceptualisation, A.T.M.Y., S.Z.A.R., M.M.A.B.A., A.E.-h.A. and A.R.; data curation, A.T.M.Y., S.Z.A.R., M.M.A.B.A., M.N., A.E.-h.A. and A.R.; formal analysis, M.F.M.T. and A.M.T.; investigation, M.F.M.T. and A.M.T.; methodology, S.Z.A.R., M.M.A.B.A., A.E.-h.A. and A.R.; project administration, S.Z.A.R., M.M.A.B.A., A.R., M.N., A.E.-h.A. and A.M.T.; validation, A.T.M.Y., M.N., M.F.M.T. and A.M.T.; writing—review and editing, A.T.M.Y., S.Z.A.R., M.M.A.B.A., A.E.-h.A., A.R., M.F.M.T. and A.M.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Center of Excellence Geopolymer and Green Technology (CEGeoGTech) UniMAP and Faculty of Technology Mechanical Engineering, UniMAP. The authors wish to thank the Ministry of Education, Malaysia, for their financial support of this study through the Fundamental Research Grant Scheme (FRGS), FRGS/1/2020/TK0/UNIMAP/03/19.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to acknowledge the reviewers for the helpful advice and comments provided.

**Conflicts of Interest:** The authors declare no conflict of interest.
