# Optimisation of Shrinkage and Strength on Thick Plate Part Using Recycled LDPE Materials
Optimisation methods such as the Taguchi process and RSM have been used over the years and have shown exceptional performance in the production of better quality of moulded parts [6; 7; 12; 14; 15]. However, only a few researchers have applied these optimisation methods to reduce shrinkage and improvise the tensile properties of a recycled material [16; 17]. Most researchers have used the optimisation technique solely to test the tensile properties of recycled plastics [16; 17; 18; 19; 20]. [PERSON] et al. [16] studied the input parameters on parts moulded using recycled ABS materials. The Taguchi method was employed, whereby four input parameters were studied. The parameters were melt temperature, packing pressure, packing time and injection time. The output observed comprised shrinkage, elongation at breaking point and tensile strength. Recycled material that was used for manufacturing these parts was reported to be better than virgin material when the optimum input parameters were employed. Additionally, it was seen that the hoop tensile strength, elongation at break and shrinkage improved by 2.33%, 23.52% and 93.3% respectively, after optimisation. [PERSON] et al. [17] studied the optimal setting of processing parameters by using the Taguchi method on a plastic tray made of recycled high-density polyethylene (HDPE). The quality characteristics investigated were shrinkage, tensile strength and flexural strength. It was reported that the most significant processing parameter affecting the shrinkage and flexural strength was packing time, while melting temperature was reported to be the most significant processing parameter affecting tensile strength. Similarly, [PERSON] et al. [21] reported the potential use of recycled low-density polyethylene (LDPE), HDPE and polypropylene (PP) to improve tensile strength. Sets of virgin and recycled materials were mixed in a number of different compositions, and later, their tensile strength was examined. The results showed that tensile strength was reduced by 15% when the virgin LDPE content was reduced. The tensile strength was reduced to 5% when the proportion of new HDPE was reduced. It was also claimed that the tensile strength decreased to 3% when the proportions of new HDPE and LDPE in the content mixture were decreased. [PERSON] et al. [22] investigated the impact of the recycled content mixed with virgin material, whereby mechanical properties and the safety factor of the moulded component were set as the outputs. Recycled content was mixed into different percentages; the findings showed that stress at yield for 100% recycled HDPE material decreased by 20% compared to the fully virgin material. Meanwhile, the material blended with 40% recycled material had a minimal effect on safety factor as compared to the stress module. The tensile modulus of 100% recycled HDPE material was reduced by 40% compared to that of virgin HDPE. Additionally, the effect of mixing recycled material below 60% was found to be less significant on the tensile modulus (which was only 15%). Literature reviews have indicated that the optimisation of injection moulding parameters can be employed to achieve the desired performance [17; 23]. So far, this optimisation method has only been applied by a few researchers to increase part quality (in terms of tensile properties) using recycled plastic material [24; 25]. Meanwhile, most research has only focused on changes in material properties due to variations in the ratio of recycled materials mixed with the virgin materials [18; 19; 20; 21; 22; 25; 26; 27; 28; 29]. Therefore, it is necessary to conduct deep research in order to determine the full potential of reusing recycled material, especially post-industrial waste, which is easy to control from mixing with other types of materials. In this way, manufacturers can fully utilise raw materials in injection moulding processes. LDPE is a polyolefins material which is widely used in injection moulding process [30]. It is used to mould many types of products such as laboratory apparatuses and household products.