2.1.3. Packing Pressure

Packing pressure is the pressure used to inject and compress the molten plastic material into mould cavities until the gate freezes [85]. According to previous research, packing pressure is a crucial processing parameter that impacts the accuracy and quality of the moulded components produced. In addition, packing pressure is also a significant processing parameter after packing time which has a significant impact on shrinkage and flexural strength of the moulded parts produced [80–82]. Any changes in packing pressure will cause degradation of the mechanical properties of the parts moulded from virgin and recycled plastic material in various compositions. Inappropriate settings of packing pressure may result in high shrinkage defects in the moulded parts [85].

## 2.1.4. Mould Temperature

Mould temperature is known as the temperature of the mould that needs to be controlled in order to solidify the molten plastic material that flows into the mould cavities towards the ejection temperature. Previous studies showed that mould temperature is one of the significant processing parameters that affects warpage and shrinkage defects [83,87]. Kamaruddin et al. [86] examined mould temperature using the Taguchi methods, and reported that the shrinkage of moulded parts affected by mould temperature is a critical factor. This supports the findings of a study by Chen et al. [89] which found that the temperature of the mould plays a role in the shrinkage of the resulting moulded products in both the transverse and longitudinal axes. In addition, mould temperature cannot be set directly but it can be controlled by controlling the temperature of coolant used in the injection moulding process.

#### 2.1.5. Packing Time

The packing time is known as the time required to fill the mould cavities without pressing the mould or flashing the finished parts entirely with additional material [90]. The packing time is generally determined by the freeze time of the gate [91]. When gates freeze, the material is not permitted to flow into the mould cavities. Nevertheless, if the packing time is shorter, the molten material returns to the feeding system and causes a backflow phenomenon [89,92].

It can be seen that, in terms of material used as mould inserts for injection moulding, the thermal conductivity (which influences the melt temperature, mould temperature, packing time, and definitely cooling time) and compressive strength (which influences packing pressure and reliability of mould inserts) are important parameters that require the attention of the mould fabrication industries.
