Optimizing the Injection Molding Parameters of Mask Waste Recycling into Door Knob Products

. The COVID pandemic has prompted people to adopt new facemask usage habits, which has led to an increase in the waste generated following facemask disposal. Recycling could be considered as a way to minimize facemask waste's adverse environmental impact because of its polymeric component. In this study, the recycling of face mask waste was carried out using injection molding to create a commercial door knob product. The experiment performed using Taguchi L4 orthogonal array was used to optimize injection molding parameters of the barrel and molding temperature as well as the inclusion of the ear loop part. Analysis of variance (ANOVA) ascertains the percent contributions to have minimal shrinkage and maximum product weight. Finally, grey relational analysis (GRA) was performed to obtain optimal parameters in multi-objective optimization. The optimal parameter combination was raised to ensure the successful recycling of waste facemasks into a commercial product.


Introduction
The incident in Wuhan City, Hubei Province, China in December 2019 was the beginning of the emergence of the coronavirus disease (COVID-19) [1] which then spread throughout the world.To reduce transmission of the virus, people were encouraged to use personal protective equipment such as medical masks, nonmedical masks, face shields, and gloves [2].Currently, the use of facemasks in everyday life is a new habit after the pandemic [3].It is said that even though the vaccine has been applied, the use of health protocols will still be applied for at least the next four years [4] since wearing masks could effectively reduce infections [5].
The increasing consumption of disposable facemasks raises more concerns about their impact on the environment due to the microplastic content in their filtering material.The plastic content of the facemask could pose a major threat to terrestrial and marine ecosystems [6].Several techniques have been initiated to deal with the increased use of facemask waste.The combustion method is considered to have an adverse effect that can pollute the environment due to the release of harmful and toxic gases.Another technique of collection is stockpiling which could lead to the wastage of land.The third method is recycling, which is considered the most efficient because of its costeffectiveness and creation of added value to the mask waste [7].
A facemask consists of three general parts with different types of materials as filtering part, nose wire, and ear loop [8].The first outer layer is a layer designed to purify large particles.Middle layer to block foreign particles.The inner layer is designed to inhibit dust and odors.The ear straps are made of a combination of polyester and spandex threads and the nose band is made of iron wire covered with PP wrap [7].A challenge to simultaneously recycling facemasks is the variety of physical characteristics within each type of content.Crespo et al., (2021b) said that the sorting of ear straps and nose wires should be manually performed which makes it inefficient in processing.Battegazzore et al., (2022) examined the recycling of masks with ear straps and found that the addition of ear straps could increase stiffness but slightly decrease viscosity and reduce the strength and toughness.Since material properties would be also affected by the manufacturing process parameters, it is necessary to determine optimal parameters for achieving specified material performances.
Injection molding is a process in producing a product by injecting liquid plastic material [11].The manufacture of products using injection molding depends on various attributes such as the geometry of the product, the materials used for manufacturing, and machining parameters (process parameters) [12].The manufacturing process using injection molding is much preferred because it can produce a wide variety of products with shapes and sizes that have complicated geometries [12].It is also suitable for mass production with complex geometry and consistent quality [13].However, the complexity of the process in injection molding makes it difficult to achieve the specified product quality during the manufacturing process [14].
Shrinkage is a serious problem causing a change in product dimensions [15] that leads to product variability in mass production.Inconsistency of product dimension would affect the customer preference toward the product quality.
Using a case of facemask waste recycling into a door knob product, this study aims to optimize the use of ear loop and injection molding parameters to have minimum shrinkage and optimum product weight.

Material and methods
This study evaluated an optimization of a door knob manufactured using an injection molding process.The mold design of the door knob is shown in Figure 1.

Material
The used facemasks were collected from the disposal of students on a campus of a state university in Surakarta, Indonesia.Before manufactured, the facemask waste was first cleaned to perform disinfection and to separate the mask from the contamination of dirt attached to the mask [16].The masks were washed using soapy water and then exposed the mask to sunlight for virus inactivation [17].After the cleaning process, the mask waste was separated between the filter mask, ear loop, and noise wire manually.The filtering and ear loop parts were cut into smaller pieces of about 10 mm to be processed at a later stage [10].

Manufacturing method
Injection molding was used to recycle the waste of facemasks into door knob products.The barrel and molding temperatures used were set as the experimental parameter for this study.The process of making products is carried out with cooking time in barrels for ±20 minutes and cooling in water for 30 seconds.

Design of experiment
The design experiment used in this study was an L4 Taguchi orthogonal matrix consisting of 3 parameters with 2 levels in each parameter.The parameters used are shown in Table 1.The values of injection molding parameters were determined based on previous studies [9].

Melt Temperature
The melting temperature is the temperature in the barrel where the material is melted before being injected into the mold [18].Smelting in the barrel uses an electric heater and a thermocouple is used so that the temperature in the barrel can be controlled with constant heat [19].

Mold Temperature
The mold temperature has a lower heat level than the temperature in the barrel.This has the aim of maintaining an appropriate temperature and can be evenly distributed plastic melt so as to avoid product defects [20].The temperature of the mold also needs to be controlled to regulate the cooling rate of the plastic [21].There were three door knobs manufactured in each experiment to have the measurement for shrinkage and weight as the variable responses.Product weight plays an important role in shrinkage, where shrinkage is reduced when product weight is maximized without overfilling the production process [28].The shrinkage values were derived from the measurement of the length and diameter of the holes using a digital caliper with an accuracy of 0.02 mm.A digital scale with an accuracy of 0.001 grams was used to measure the product mass.Shrinkage was measured after the product was removed from the mold.The product was allowed to stand at room temperature for 48 hours before measurement.The equation derived from [28] was used to calculate the shrinkage.
Where /: shrinkage in length or diameter (%) : mold dimension (mm) : part dimension (mm) Figure 2 shows the measurement for shrinkage in length and diameter directions.

Analysis
The Taguchi approach used in this study is a qualitybased technique to optimize process parameters for better results [29].Analysis of Variance (ANOVA) was performed to determine the contribution of each parameter in the experimental research design [30].
Analysis of variance in the orthogonal matrix is performed by calculating the total square of the control parameter with all control parameters, based on the calculation of the sum of squares of each column, variance analysis of the orthogonal matrix is performed.
Further, the S/N (Signal to Noise Ratio) value was calculated.In this study, the response variable of shrinkage used "lower the better", while the variable of product weight used "the greater the better" criteria.Since a multi-objective problem exists, this study also employed Grey Relational Analysis (GRA) including sequential processes of data normalization and calculation of deviation sequence, grey relational coefficient, and grey relation grade [31].The ANOVA for Grey Relation Grade (GRG) was performed to determine the contribution of each parameter for two different characteristics of shrinkage and product weight.

Result and discussion
This study evaluated the effect of barrel temperature, mold temperature, and the presence of ear loop on product shrinkage and weight.
Two different measures of shrinkage in length and diameter directions were performed.Table 2 shows the mean of three replications for each variable response.

S/N Ratio
The S/N ratio for each variable response is presented in Table 3.

Analysis of variance
The contribution for each factor derived from ANOVA in each variable response is presented in Table 4

Grey relation analysis
GRA in this study was used to determine the optimization results of shrinkage and product weight.
Using shrinkage and product weight test data, optimal experiments were obtained  Table 5 shows that the optimal experiment on shrinkage and product weight is the fourth experiment with 220C barrel temperature, 165C mold temperature, and the presence of an ear loop.
Figure 3 shows the plot for means and a plot for the S/N Ratio in each parameter.The most optimal parameter level could be determined from the largest S/N ratio chart which is tabulated in Table 6.The table shows that barrel temperature with a value of 220C, molding temperature with a value of 165C, and the presence of an ear loop are the optimum parameters to achieve the multiobjective of minimizing shrinkage and maximizing product weight.
Product shrinkage is closely related to viscosity in the injection molding process.Low viscosity materials will flow more easily through the mold but can result in greater shrinkage.The significant effect of ear loops is in accordance with the findings of Battegazzore et al., (2022) [10] which also revealed that the presence of ear loop in recycling process would provide low-viscosity materials to remain stable.The addition of ear loops could stabilize the viscosity which results in a minimum shrinkage.
The combination of barrel temperature and molding temperature parameters found in this study is also in line with the study of Crespo et al., (2021b) [9]  ANOVA in Grey Relation Grade is used to determine the value of influence and contribution to parameters on shrinkage response and product weight door knob.Table 7 shows that mold temperature is a parameter that contributes the most to attaining minimum shrinkage and maximum product weight.This is in accordance with the finding of Annicchiarico et al., (2013) [32] stated that mold temperature is a significant factor affecting shrinkage in parallel with the direction of flow and part of the mass independently.

Conclusion
In the recycling process of facemask waste into door knob using injection molding, the percent contribution of process parameters influencing shrinkage and product 66.43% of mold temperature, 27.51% of ear loop presence, and 6.43% of barrel temperature.The optimum injection molding process parameters are 220 C of barrel temperature, 165 C of mold temperature, and mix ear loop with the filtering part.

Table 1 .
Parameter setting

Table 2 .
Mean of variable responses

Table 3 .
S/n ratio for each variable

Table 4 .
Parameter contribution from ANOVA

Table 5 .
Grey relation analysis