Occupational ergonomics research and applied contextual design implementation for an industrial shop-floor workstation

International Journal of Industrial Ergonomics 72 (2019) 188–198

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International Journal of Industrial Ergonomics journal homepage: www.elsevier.com/locate/ergon

Occupational ergonomics research and applied contextual design implementation for an industrial shop-floor workstation

T

J. Sanjoga,b, Thaneswer Patelc, Sougata Karmakarb,∗ a

Department of Mechanical Engineering, Vaugh Institute of Agricultural Engineering and Technology, Sam Higginbottom University of Agriculture, Technology and Sciences (SHUATS), Prayagraj (Allahabad), Uttar Pradesh, 211007, India b Department of Design, Indian Institute of Technology (IIT), Guwahati, Assam, 781039, India c Department of Agricultural Engineering, North Eastern Regional Institute of Science and Technology (NERIST), Nirjuli, Arunachal Pradesh, 791109, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Human modelling Manufacturing Posture Psychosocial work environment Work study Workload

The aim of this original research article is to identify the occurrence of work-related ergonomics risk factors, in order to implement context specific human centered design interventions in the injection molding shop-floor workstations of plastic furniture manufacturing factories within the framework of industrially developing countries. Questionnaire study, postural assessment tools, computer aided design, digital human modeling and simulation, and basic work study techniques were used. Plastic processing industry is highly fragmented, consisting of small, medium scale enterprises with tremendous growth potential. Occupational design ergonomics research in the injection molding plastic furniture manufacturing shop-floor workstations is very scarce in industrially developing countries. Shop-floor workers are affected by prevalent awkward working postures and consequent body part discomforts. Useful and easily implementable accessories/fixtures with convenient design features were conceptualized. Virtual ergonomics evaluation of the workstation with proposed accessories/ fixtures showed significant reduction of awkward working postures. Physical prototypes of the proposed fixtures were constructed and real human trials were performed in the factories. Time study indicated reduction in operator cycle time when compared with time taken before design modifications. Research methodology, results and design solutions described from an ergonomics perspective would definitely serve as a helpful guide for existing as well as upcoming factories in the injection molded plastic furniture manufacturing industry of industrially developing countries and further similar research endeavors.

1. Introduction Small and medium enterprises offer significant employment opportunities in industrially developing countries like India. Plastic processing industry in India is highly fragmented, consisting of small, medium scale enterprises and has been predicted with tremendous growth potential driven by the establishment of petro chemical plants with generation of huge employment opportunities (Central Institute of Plastic Engineering and Technology, 2010). Molded plastic furniture manufacturing industry primarily employs injection molding process. Injection molding process is capable of making articles at high production rates and low labor cost per unit (Central Institute of Plastic Engineering and Technology, 2007). Huge number of injection molding machines is expected to be installed in India (Central Institute of Plastic Engineering and Technology, 2010). However, practical occupational design ergonomics research in shop-floor of Indian plastic processing

∗

industry is very scarce (Sanjog et al., 2016). The same can be said of industrially developing countries also. Occupational work occupies an important part of our daily activities and it is the responsibility of employees, managers and supervisors to create and maintain a safe workplace (Dodge, 2012). Identification of workplace risk factors like occurrence of musculoskeletal disorders are one of the initial steps to be performed in recognizing problematic jobs (Pao and Kleiner, 2001). Psychosocial factors/needs are also very important for employees/workplace wellbeing (Dickson-Swift et al., 2014; Bone, 2015; Lima and Coelho, 2018). Subjective measures of operator workload are used in human-machine system evaluations due to ease of application and non-intrusiveness (Rubio et al., 2004). Work study (term used for method study and work measurement) is beneficial for the assessment of human work in all its contexts, involving systematic investigation of all factors which affect efficiency, economy of the work being investigated for making suitable

Corresponding author. E-mail address: [email protected] (S. Karmakar).

https://doi.org/10.1016/j.ergon.2019.05.009 Received 1 February 2018; Received in revised form 25 October 2018; Accepted 16 May 2019 0169-8141/ © 2019 Elsevier B.V. All rights reserved.

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improvements (Bureau of Indian Standards, 2002). Further, the practice of using Digital Human Modeling (DHM) software, the state of art technology for human centered design applications (Karmakar et al., 2012) is also found to be very limited in India in spite of numerous advantages (Karmakar et al., 2014). DHM has been found to be useful, reliable in postural assessment and for virtual workstation invesgtigations (Binoosh et al., 2017). It is interesting to observe that, worldwide, the application of DHM is found to be the largest in vehicle design (Reed and University of Michigan, 2017). The tremendous growth potential of plastic processing activities using the injection molding machines demands an urgent comprehensive work-related design ergonomics research in various shop-floor workstations of the injection molded plastic processing industry for proposing context specific human centered design solutions for the benefit of all stakeholders. The aim of this research article is to systematically evaluate the existing injection molding shop-floor workstations in plastic furniture manufacturing factories from an occupational design ergonomics perspective for conceptualizing and implementing human centered design interventions for humanizing work activities. Carefully selected appropriate research methods were utilized, featuring questionnaire study (for investigating the prevalence of symptoms of musculoskeletal ailments, psychosocial work environment, and workload), postural assessment tools, statistical analysis, DHM, and basic work study techniques.

Fig. 1. Selected working postures in the injection molding workstation.

activities in injection molding workstation were photographed for investigations. Recording of work activities by means of videography was not permitted by factory managements. Major work activities (represented by selected working postures of an individual worker) in the injection molding workstation are as shown in Fig. 1.

2. Methodology 2.1. Selection of injection molding workstation

2.4. Working posture assessment A survey was conducted to observe work activities in shop-floor workstations of four injection molded plastic furniture manufacturing factories in Assam, a northeastern state of India. Injection molding workstation (found in all factories surveyed) was selected from a particular factory to perform ergonomics research.

Ovako Working Posture Assessment System (OWAS) (Karhu et al., 1977; Mattila and Vikki, 1998) and Rapid Entire Body Assessment (REBA) (Hignett and McAtamney, 2000) were utilized for evaluating the selected working postures. Since continuous video recording was not permitted by the factory management, evaluation of relative proportion of postures in terms of time (Matitila and Vilkki, 1998) in OWAS was not possible. Therefore, the classification in OWAS methodology to identify poor posture, determination of action categories for harmful effect on musculoskeletal system was performed. OWAS technique was employed first to understand overall scenario of awkward posture among workers during various activities. As there were some limitations (absence of neck and elbows/wrist assessments, no separation of right and left upper extremities) in OWAS technique (Ovako Working Posture Assessment System, 2009; Battini et al., 2011), postural assessment using the commonly used REBA technique (Schwartz et al., 2019) was used subsequently for drawing clear inferences. OWAS action categories (adopted from, Mattila and Vikki, 1998) for prevention of musculoskeletal disorders and REBA methodology (adopted from, Hignett and McAtamney, 2000) for determination of risk level, action to be taken were utilized as guidelines (Table 1) for evaluating the working postures.

2.2. Selection of workers towards questionnaire study Male workers were employed in the injection molding workstations of all the factories surveyed. Standardized Nordic Questionnaire (Kuorinka et al., 1987), the commonly used tool for investigating the existence of musculoskeletal disorders in any occupation (Khan and Singh, 2018) was used to investigate the occurrence of musculoskeletal troubles. The procedure involved showing a body map to workers/volunteers and elucidating responses to various queries. Healthy (not having any medical report of suffering from chronic diseases) adult male workers of similar age, height (stature) and weight; and having more than one-year experience were selected as participants/responders in agreement with pre-set inclusion criteria. Forty-six operators (46) from injection molding workstations volunteered as participants/responders. Injection molding workstation operators were considered as the experimental group. Fifteen (15) individuals with similar demographic characteristics but involved in different activities (administrative/supervisory occupations, having more than one-year continuous work experience) were selected to constitute the control group for comparison of troubles (ache/pain/discomfort) in body parts with the experimental group. None of the respondents included in the survey suffered from discomfort/pain/ache in body parts as a result of accidents/injury in the past. Information regarding age, experience was gathered from an interview followed by direct measurement of stature and weight.

2.5. Creation of digital human models and rendering of comfort angles Non-availability of Indian anthropometric data base of factory workers employed in plastic processing industry was observed during the review of literature. Therefore, civilian anthropometric database (Chakrabarti, 1997) of adult Indian male population was utilized to build digital human models for assessments. Smallest, average and largest dimensional adult Indian male population were represented by 5th p, 50th p and 95th p virtual male human models respectively. All body dimensions of a particular individual may not fall under the same percentile category, but, such an ideal condition may be considered for generating human models for research purposes to solve associated constraints (Karmakar et al., 2011). Comfort angles were imparted on digital manikins. Comfort angles for different body segments were

2.3. Recording and categorizing of work activities in injection molding workstation Contextual knowledge is essential for virtual ergonomics investigations by means of DHM (Dukic et al., 2002). Hence, work 189

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Table 1 OWAS and REBA action categories. OWAS Action Categories

REBA Action Categories

Action category (score)

Explanation

Score

Risk level and action to be taken

1

Normal and natural postures with no harmful effect on the musculoskeletal system - no action required Postures with some harmful effect on the musculoskeletal system - Corrective actions required in near future Postures have harmful effect on the musculoskeletal system - Corrective actions should be done as soon as possible The load caused by these postures has a harmful effect in the musculoskeletal system - corrective actions for improvement required immediately

1

negligible risk, no action necessary

2 or 3

low risk, change may be needed

4 to 7

medium risk, further investigation, change soon high risk, investigate and implement change very high risk, implement change

2 3 4

8 to 10 11 +

adopted and suitably adjusted from published literature (Rebiffé, 1966; MacLeod, 1999; Henry Dreyfuss Associates, 2002). Uncomfortable range of movement was given ‘red’ color while ‘green’ color was used to signify the comfortable range with respect to body segments. 2.6. Creation of virtual model of injection molding workstation Mechanical design feature of DELMIA DHM software was used to generate virtual model of injection molding workstation. Actual dimensions of injection molding machine and work accessories were measured at the factory site. 2.7. Interfacing digital manikins with virtual injection molding workstation DHM helps to improve physical attributes in the manufacturing work cell by enabling designer/engineer to generate a digital human (with specific population attributes) which may be inserted into threedimensional graphic renderings of work environments (Chaffin, 2007). Proper interfacings of digital manikins with virtual injection molding workstation model, featuring selected working postures (Fig. 1) was done using ergonomics design and analysis feature of DELMIA software. Computer generated model of injection molding workstation and digital human model were interfaced on a 1:1 scale. Interfacing on 1:1 scale helped to impart accurate investigations as workstation dimensions were in exact proportion to virtual human body dimensions. Existing workstation and work activities were evaluated using digital human models representing 5th p, 50th p and 95th p Indian adult males for identifying postural comfort. Visual representation of the virtual evaluation is shown in Fig. 2 (example of 50th p male digital human model).

Fig. 2. Virtual representation of working postures a - h, 50th p male.

work environment among the workers in the injection molding workstation workers was investigated. The findings were also compared with a control group (administrative/supervisory employees) for arriving at suitable conclusions. 2.10. Selection of questionnaire for subjective workload assessment National Aeronautics and Space Administration-Task Load Index (NASA-TLX) (Hart and Staveland, 1988) provides an overall workload score based on a weighted average of rating on six subscales (Jung and Jung, 2001). Therefore, NASA-TLX questionnaire was selected to investigate the subjective workload of the workers in the injection molding workstations and the control group. The following formula was used to calculate the mean weighted ratings (for each scale title) in the experimental and control group.

2.8. Spinal load analysis Mechanical load on lumbar spine is considered to be a contributing factor to many disorders of the back (Chaffin and Andersson, 1991). ‘Biomechanics single angle action analysis’ feature in DELMIA software, giving details about compression and joint shear values were utilized to evaluate spinal load of L4-L5 segments in lumbar region of digital human models for the selected working postures.

2.11. Design of initial concept injection molding workstation accessories and interfacing with digital human models

2.9. Selection of questionnaire for investigating the psychosocial work environment

Several people were found to be working in different work shifts in injection molding workstations. Geometric design of equipment, work places and product should be based on anthropometry of user population in order to ensure man machine compatibility and avoiding awkward working postures (Bureau of Indian Standards, 1991; MacLeod, 1999). The common procedure is to design for a range of population from 5th percentile (small operator) to the 95th percentile (large operator) (Helander, 1995). Extreme group in the population encompassing smallest and largest body dimensions are represented by 5th

The Copenhagen Psychosocial Questionnaire (COPSOQ) was developed considering prominent theories and concepts (Nuebling et al., 2013) and is also a validated questionnaire (Copenhagen Psychosocial Questionnaire II, 2007). Therefore, COPSOQ II (medium version) was selected for investigations. Appropriate guidelines (Copenhagen Psychosocial Questionnaire II, 2007; Kristensen, 2001) were followed in calculating the scores for each individual scales. The psychosocial 190

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percentile female and 95th percentile male respectively. However, it is fitting to design for the predominant gender if the workforce is dominated by a particular gender (Helander, 1995). Workers in shop-floor of injection molded plastic furniture manufacturing were male. Hence, workstation and work methods were assessed and modified/designed suitably for male gender. Initial concept models were interfaced with 5th p, 50th p, and 95th p digital human models representing adult Indian male anthropometry. Practical low-cost improvements, validated with application of basic ergonomics principles in workstation design should be attempted in the context of small workplaces in industrially developing countries (Kogi, 2012). Hence, workstation accessories which can be easily built and low in cost were conceptualized for ergonomics assessments.

Table 2 Comparison of demographic characteristics of experimental and control group. Variables

Age Weight Stature Experience

2.12. Method study

Units

years kg cm years

Experimental group

Control group

Comparisons (Mann Whitney U test)

Injection Molding Workstation (IMW) (n = 46)

Other Employees (CG) (n = 15)

IMW Vs. CG

Mean

SD

Mean

SD

25.1 57.9 165.5 2.3

3.3 7.1 6.3 1.3

25.4 58.7 162.7 1.8

2.5 5.3 7.4 0.7

NS NS NS NS

‘NS’- No significant difference (p > 0.05); IMW – Injection Molding Workstation; CG – Control Group.

Method study aims to eliminate unwanted motions, minimize fatigue and enable better synchronization of efforts (Bureau of Indian Standards, 2002). Operation chart (left and right hand) is a simple and effective aid for scrutinizing work activities if the work activities can be visualized in terms of elemental hands motions (Barnes, 1980). Two symbols are used for constructing operation charts, namely, small circle for indicating transportation (moving hand to grasp an object) and a larger circle to signify actions like grasping, positioning, using, or releasing an object/article (Barnes, 1980). Method study using left and right hand operation chart was used to record and analyze work activities before and after work station, work method design modifications.

elements. Injection molding machine cycle time for a product (armless chair; Fig. 1) was observed from three different factories. Observed operator work cycle times (time taken for processing individual finished products) was timed for two individual workers per factory engaged in processing a similar product (armless chair). Sixty injection molding machine cycle times and observed operator work cycle times were considered. Statistical analysis for comparison of injection molding machine cycle times (between factories), observed operator work cycle time (before and after design modifications) between workers (within factory and between factories) was performed using SPSS software.

2.13. Work measurement 3. Results Time study technique can be employed as a tool for facilitating method improvement (Mundel and Danner, 1994; Bureau of Indian Standards, 1995). Occurrence of work related musculoskeletal disorders are influenced by work pace and other factors like design of workstations (Carayon et al., 1999). Observed operator cycle times before and after suitable ergonomic investigations and design interventions can serve as indicators for reduction in work pace without compromising on productivity. A qualified worker (one who has acquired the skill, knowledge and other attributes to carry out the work in hand to satisfactory standards of quality and safety) should be timed while performing the job (Kanawaty, 1992). If more than one worker is doing same operation, time study analyst should time one or more operators, whereas if all the workers are found adopting same method for a specific job and a difference in task completion time is observed then operator performing the job closely to normal pace should be timed (Barnes, 1980). For simplicity in calculations, most companies adopt a conventional method for determining number of cycles to be timed depending on total number of minutes per cycle (Bureau of Indian Standards, 1995; Kanawaty, 1992). The conventional method for determining number of cycles to be timed using stopwatch (for timing individual operator work cycle time) adopting direct time study (intensive sampling) procedure was followed. Three factories were included in time study investigations for ease of comparisons. Two operators from each factory who were experienced having necessary skill and knowledge were selected and cycle time was noted. The present research work is primarily focused on ergonomics design interventions for minimizing awkward working posture. Setting of standard times for particular work activity is not the objective of present study. Hence, individual work element (reaching for finished product, de-flashing finished product, reaching for product label etc.) were combined and the entire operation of retrieval of an individual finished product and subsequent activities were taken as a single work task for recording time. In other words, time was observed for completing a particular work cycle instead for individual work

3.1. Demographic, body parts discomfort in experimental and control group Statistical analysis using SPSS software was performed to compare the experimental group with control group with respect to workers’ characteristics (age, weight, stature, experience) as shown below in Table 2. Incidence of symptoms of musculoskeletal ailments (during last 12 months) in different body parts is shown below in Table 3. 3.2. Visualization of body segments in uncomfortable range of motion Proper interfacings of digital manikins with CAD model of injection molding workstation and colour coding helped in visualizing body segments occupying uncomfortable positions (within their range of Table 3 Number of participants (% of total number) suffering from symptoms of musculoskeletal ailments in different body parts during the last 12 months. Body parts

Shoulder (right) Shoulder (both) Elbow (right) Elbow (Both) Wrist (right) Wrist (both) Fingers (right) Upper back Low back Knees (one/ both) Ankle (both) Neck

191

Experimental group

Control group

Injection molding Workstation (n = 46)

Administration and supervisory employees (n = 15)

5 (10.9%) 26 (56.5%) 3 (6.5%) 6 (13.0%) 24 (52.2%) 15 (32.6%) 2 (4.3%) 2 (4.3%) 38 (82.6%) 16 (34.8%)

0 0 0 0 0 0 0 1 (6.7%) 2 (13.3%) 1 (6.7%)

3 (6.5%) 0

0 2 (13.3%)

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Table 4 OWAS, REBA and spinal load analysis of selected postures (a to h). Posture

a b c d e f g h

REBA Right Body Side

REBA Left Body Side

Action Category

Score

Score

3 1 2 1 1 1 1 1

6 5 10 2 7 4 8 6

OWAS

8 2 10 5 2 2 8 1

Table 5 Status of psychosocial work environment among the injection molding workstation workers and comparison with control group.

Digital Human Model L4 - L5 Spine Limits Range

Percentile

5th p 50th p 95th p

Compression (Newton)

Joint Shear (Newton)

357–1441 859–3780 576 - 3068

24–91 12–153 17–152

Dimensions

Quantitative Demands Tempo, Work Pace Emotional Demands Influence At Work Possibilities For Development (Skill Discretion) Meaning Of Work Commitment To The Workplace Predictability Rewards Role Clarity Role Conflicts Quality Of Leadership Social Support From Colleagues Social Support From Supervisors Social Community At Work Satisfaction With Work – Job Satisfaction Work Family Conflict Horizontal Trust Vertical Trust Justice And Respect Self-Rated Health Sleeping Troubles Burnout Stress

movement) while performing various work activities (Fig. 2). 3.3. OWAS, REBA scores and spinal load of selected postures Observational techniques are mostly preferred for postural assessment in industry as they do not interfere with work activities, production process and no requirement of any additional equipment (which may cause operator discomfort) to be placed on worker's body parts (Genaidy et al., 1994; Juul-Kristensen et al., 1997). OWAS and REBA scores for working postures were computed (Table 4) for an individual worker as shown in Fig. 1. Spinal load analysis (at L4 - L5 segments) with respect to selected working postures (Fig. 2) was performed using DHM software. The range of compression, joint shear values obtained are given in Table 4.

Granulator Workstation (GW, n = 10) (Mean)

Control Group (CG, n = 15) (Mean)

Comparisons (Mann-Whitney U test) BW Vs. CG

34.2 85.3 42.1 45.9 69.0

33.0 45.0 44.6 56.7 78.7

NS * NS * *

72.3 57.9

84.4 73.3

* *

69.3 59.6 85.7 25.1 58.3 57.9

77.5 77.5 80.0 49.2 75.0 70.0

NS * NS * * *

62.9

75.6

*

77.5

80.0

NS

59.2

67.8

*

37.5 66.1 71.3 56.8 55.9 19.6 32.6 9.8

30.6 53.9 67.5 69.2 60.0 33.7 33.3 28.8

NS * NS * NS * NS *

3.4. Psychosocial work environment assessment and comparisons

NS – No significant difference (p ≥ 0.05) * - Significant difference (p ≤ 0.05).

The results of psychosocial work environment survey relating to workers in the injection molding workstations and control group employees and comparisons are given below in Table 5.

3.9. Work measurement Injection molding machine cycle time observations (Table 10) and observed operator work cycle times (Table 11) before and after design modifications, suitable comparisons are shown below.

3.5. Subjective workload assessment Subjective workload scores with respect to workers in injection molding workstations and control group employees and comparisons are given below (Table 6).

4. Discussion Experimental group as well as the control group was of similar age, stature, weight and work experience (Table 2). Statistical analysis was performed following a similar method adopted by Gangopadhyay et al. (2007). It was observed that number of participants suffering from discomfort in body parts while working was 5 in control group; whereas number of sufferers was 43 in experimental group. Chi square test of data revealed that experimental group was significantly different (χ2 20.945; P ≤ 0.000) from control group in terms of percentage of responders suffering from discomfort during work. Although participants of experimental group were of similar characteristics (age, stature, weight and work experience) as of control group; higher prevalence of symptoms of musculoskeletal ailments in various body parts among participants of the experimental group were due to differences in occupational activities which in turn were influenced by working posture, work methods, workstation design/layout etc. REBA showed presence of risky work postures (Table 4) which needed to be changed. Work station layout, design of equipment's and tools, anthropometric characteristics of worker, work methods are the main reasons for inducing awkward working posture at trunk, neck and shoulders and further determines positioning of workers' body when performing a task (Keyserling et al., 1992; Pheasant and Haslegrave,

3.6. Visualization of body segments in comfortable and uncomfortable range of motion Proposed injection molding workstation accessories was interfaced with 5th (Fig. 3), 50th (Fig. 4) and 95th p (Fig. 5) digital human models for virtual ergonomics evaluation. 3.7. OWAS and REBA scores OWAS and REBA scores for working postures in proposed workstation for 5th p, 50th p and 95th p male digital human models are shown in Table 7. 3.8. Operation chart Left and right hand operation chart for work activities observed in existing workstation is shown in Table 8. Left and right hand operation chart for work activities observed in the proposed workstation is shown in Table 9. 192

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Table 6 Subjective workload assessment of the injection-molding workstation workers and comparisons. Scale title

Mental demand Physical demand Temporal demand Performance Effort Frustration level

Mean raw rating

Mean weighted rating

IMW (n = 46)

CG (n = 15)

Comparisons (Mann-Whitney U test) IMW Vs. CG

IMW (n = 46)

CG (n = 15)

Comparisons (Mann-Whitney U test) IMW Vs. CG

21.7 79.9 94.6 27.7 70.4 7.1

72.3 9.6 14.6 10.3 30.0 15.3

* * * * * *

34.3 80.8 94.6 27.7 71.3 13.0

72.4 20.7 15.9 10.4 30.8 15.8

* * * * * *

NS – no significant difference (p > 0.05). * - significant difference (p ≤ 0.05). IMW – Injection Molding Workstation; CG – Control Group.

Fig. 5. Virtual representations of working postures (a–h), 95th p male, in the proposed model.

Fig. 3. Virtual representations of working postures (a–h), 5th p male, in the proposed model.

deviated work postures are also encompassed (Finneran and O'Sullivan, 2010). It is widely accepted that extremes of posture are associated with upper-extremity musculoskeletal disorders among workers (Gerr et al., 2014). Physical mismatch between workers' anthropometry, worktable and product dimensions, work methods resulted in the prevalence of awkward work postures (Fig. 1; Table 4) with certain body segments in uncomfortable range of motion (indicated by red color; Fig. 2). Compressive forces at L4-L5 lumbar spine (due to mass of body, load acting on hand and trunk) have an allowable limit of 3433 N and maximum permissible limit of 6376 N as recommended by National Institute of Occupational Safety and Health (Leyland, 2008). Safe limit of 500 N with 1000 N as maximal permissible limit has been suggested by University of Waterloo ergonomic research group for joint shear (Leyland, 2008). Spinal load analysis using DHM software indicated that compressive forces exceeded safe limits (but within maximum permissible limits) for 50th p digital human model while evaluating selected working postures. Shear forces were within safe limits (Table 4). Compression and shear forces generated at L4 - L5 segments, in selected working postures were within safe limits (Table 4) for 5th p and 95th p digital human models. Significant differences in mean values were observed between workers in the injection-molding worker's workstations and the employees in the control group for various psychosocial factors (Table 5). The injection-molding shop-floor workers and administrative/supervisory employees perceived psychosocial stress differently (in some aspects) in their respective workplaces. The psychosocial investigations reveal that there are no serious problems with respect to the

Fig. 4. Virtual representations of working postures (a–h), 50th p male, in the proposed model.

2006). Work activities (Fig. 1; placing retrieval tool, de-flashing keeping product on floor, reaching activities) characterized by extensive reach also contributed to risky work postures. Among various risk factors towards development of musculoskeletal disorders, 193

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psychosocial work environment in the factories considered except for tempo (work pace) factor among the injection-molding shop-floor workstation workers. Therefore, design interventions should also focus on reducing the tempo (work pace) of the workers in addition to reducing incidence of awkward working postures. The mean raw ratings for various scale titles for the subjective workload assessments were significantly different between the experimental group and control group (Table 6). Temporal demand, physical demand and effort are rated very high by the experimental group. Mental demand was rated very high by the control group. The performance of both the experimental group and control group were good as they were found to be successful in completing the tasks associated with their jobs. The mean weighted ratings for the scale titles were also significantly different (p ≤ 0.05) between the experimental group and control group (Table 6). The weighted ratings were also found to be high for temporal demand, physical demand and effort among injection-molding workstation workers. Therefore, attention should also be given to reduce the perceived temporal demand, physical demand and effort with respect to the injection molding shop-floor workstation workers while attempting human centered design interventions. Prevalence of distinctive body part discomfort was found among injection molding workstation workers. Working table in injection molding workstation was found to be designed arbitrarily thus aiding in the occurrence of risky, uncomfortable work postures. Postural stress on account of various body postures is helpful and should be considered in designing or redesigning workplaces (Olendrof and Drury, 2001). Further, tempo (work pace) was rated to be very high during investigations of the psychosocial work environment. Similarly, workers in the injection molding workstations experienced very high temporal demand,

Table 7 OWAS and REBA postural assessment for working postures in the proposed workstation. Manikin

5th p (Fig. 3)

50th p (Fig. 4)

95th p (Fig. 5)

Posture

a b c d e f g h a b c d e f g h a b c d e f g h

OWAS

REBA (right body side)

REBA (left body side)

Action category

Score

Score

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 3 1 2 3 1 3 2 1 1 1 2 3 2 3 1 1 1 1 1 1 1 3 1

2 3 1 2 3 1 3 2 1 1 1 2 3 1 3 1 1 1 1 1 1 1 3 1

Table 8 Operation chart for work activities observed in the existing injection molding workstation. Left hand

Symbol

Symbol

Right hand

– – – – – – Reach for finished product Grasp finished product – – – – – Lift finished product Transport finished product to work table Position finished product on work table Grasp finished product – – – – – – Reach for product label chart 1on work table Hold product label chart 1 Release product label chart 1 Reach for finished product Grasp finished product Release grip on finished product Reach for product label chart 2 on work table Hold product label chart 2 Release product label chart 2 Reach for finished product Grasp finished product – – Remove finished product form work table

– – – – – – o O – – – – – o o O O – – – – – – o O O o O O o O O o O – – O

O O O O O O – O O O O O O – – O O O O O O O O O O – O O – O O – O O O O O

Hold de-flashing hand tool Reach for retrieval tool on work table Grasp retrieval tool Transport retrieval tool towards product Position retrieval tool on product Retrieve finished product – Transport retrieval tool to work table Place retrieval tool on table Release grasp on retrieval tool Reach for finished product Position cutting tool on finished product De-flash finished product – – Reach for finished product Position cutting tool on finished product De-flash finished product Grasp cut runner Reach for dropping runner Release cut runner into collector Reach for finished product De-flash finished product Reach for product label 1on work table Remove product label 1 – Reach for finished product Stick product label 1on finished product – Reach for product label 2 on work table Remove product label 2 – Reach for finished product Stick product label 2 on finished product Reach for finished product Grasp finished product Remove finished product form work table

194

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musculoskeletal disorders, all possible ergonomic interventions were brain-stormed. It was realized that behavioural management and change of work-schedule would not be very much effective in the shopfloor scenario under study as the key ergonomic stressors were associated with the anthropometric mismatch between workers and workplace work accessories resulting in uncomfortable working posture. Therefore, the most feasible and implementable solution to ensure dimensional compatibility between worker and workstation hardware could be achieved by introducing easy, low cost and locally manufacturable work accessories design. Before conceptualization of any specific hardware based design solution, it was envisaged that the height adjustable work stand and the retrieving tool could be potential design solution towards addressing the issues of awkward working postures. Organizational context should be considered for making workplace interventions (McVicar et al., 2013). Therefore, after discussions with production supervisors/managers and operators, different models of these two hardware were conceptualized using morphological chart (Norris, 1963) and thereafter concepts were screened and finalized using Pugh Chart (Pugh, 1990). The CAD models of the final concepts are shown in Fig. 6. The list of final accessories is provided below.

Table 9 Operation chart for work activities observed in the proposed workstation. Left hand

Symbol

–

Symbol

Right hand

O

– – – – – – Reach for finished product Grasp finished product – –

– – – – – – o O – –

O O O O O O – O O O

Transport finished product to work stand Position finished product on work table Grasp finished product

O

–

Reach for de-flashing hand tool Grasp de-flashing hand tool Reach for retrieval tool Grasp retrieval tool Transport retrieval tool Position retrieval tool Retrieve finished product – Transport retrieval tool Place retrieval tool Release grasp on retrieval tool –

O

O

Reach for finished product

O

O

– – – –

– – – –

O O O O

– – Invert and position finished product position finished product on work stand Grasp finished product

– – O

O O O

O

O

O

O

– – – – – – – – – – Remove finished product form work stand

– – – – – – – – – – O

O O O O O O O O O O O

Position cutting tool on finished product De-flash finished product Grasp cut runner Reach for dropping runner Release cut runner into collector Reach for finished product Grasp finished product Invert and position finished product position finished product work stand Position cutting tool on finished product De-flash finished product Reach for product label 1 Remove product label 1 Reach for finished product Stick product label 1 Reach for product label 2 Remove product label 2 Reach for finished product Stick product label 2 Grasp finished product Remove finished product form work stand

(a) height adjustable work stand for positioning plastic furniture for deflashing (b) squared shaped collector (for collecting de-flashed material) (c) height adjustable stand for keeping the product labels (d) cylindrical collector for cut-runner (e) retrieval hand-tool (f) stand for keeping retrieval hand tool and de-flashing tool Collector for runner was designed to easily fix collecting bag. Collector for gathering de-flashed material was designed to hold more amount of de-flashed material. Dimensions of collector (for collecting de-flashed material) was designed to be 1000 mm × 1000 mm × 250 mm with one side open for allowing de-flashed materials to fall into it. The open end of the collector for dropping the runner (removed from the freshly molded chair) should be 600 mm from ground level considering height of gunny bags (commonly used) and also for facilitating 5th p, 50th p and 95th p males to drop the runners without significantly bending their thoracic and lumbar regions. The top surface of the work stand (height adjustable) from the surface of the ground should be 631 mm for 5th p, 809 mm for 50th p and 832 mm for 95th p while height of adjustable product label stand from ground level should be 995 mm for 5th p, 1046 mm for 50th p and 973 mm for 95th p. The height of the work stand may be suitable adjusted by the individual workers (of different body dimensions). Such adjustments in height will help different percentile workers to work in good, less risky work postures using same workstation fixtures. Other design dimensions (diameter, support at the base, round base at the top provided to position the chair for de-flashing operation) of the stand may be decided contextually, considering the strength and stability

physical demand and effort as observed in the subjective workload assessments. Hence it was primarily deemed essential to design a concept workstation (evaluated using virtual ergonomics technology) in order to minimize awkward work postures with body segments positioned within comfortable range of motion. Such an approach may also help to reduce the work pace and temporal demand of the workers concerned. Following initial identification of existence of awkward/risky working postures and consequent prevalence of symptoms of

Table 10 Injection molding machine cycle times. Factory

A B C

Inferential statisticsa

Descriptive statistics Existing workstation

Proposed workstation

Mean

SD

Mean

SD

54.8 51.6 52.1

0.7 0.6 0.6

54.9 51.6 52.2

0.7 0.5 0.5

Inferential statisticsb

Existing Vs. Proposed workstation

Factory

Existing workstation

Proposed workstation

NS NS NS

A vs. B A vs. C B vs. C

* * *

* * *

‘NS’ – Not Significant (p > 0.05); SD – Standard Deviation; ‘*’ - Significant difference (p < 0.05). a Wilcoxon signed rank test. b Mann-Whitney test. 195

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Table 11 Observed operator work cycle times. Factory

Operator

Existing work station

A B C

a b c d e f

Inferential statisticsa

Descriptive statistics Proposed workstation

Mean

SD

Mean

SD

46.6 42.9 37.5 35.9 31.6 32.9

3.2 2.1 3.1 1.9 2.3 3.2

44.4 41.7 34.5 34.4 30.9 31.4

2.4 1.8 1.5 1.3 1.6 2.2

Inferential statisticsb

Existing Vs. Proposed workstation

Operator

Existing workstation

Proposed workstation

NS NS NS NS NS *

a vs. b

*

NS

c vs. d

*

NS

e vs. f

*

NS

‘NS’ – No Significant difference (p > 0.05); ‘*’ - Significant difference (p < 0.05). a Wilcoxon signed rank test. b Mann - Whitney U test.

Fig. 6. Final proposed model of workstation fixture.

requirements on account of the various models and varied quality considerations resulting in dissimilar overall weight of the chairs being manufactured by different companies. Length of the retrieval hand tool is 1100 mm and it helped 5th p, 50th p and 95th p DHM to retrieve the finished product (from under the injection molding machine) without bending the lumbar region significantly. De-flashing hand tool holder may be provided at a height of 900 mm on the tool stand and other dimensions of tool stand may be suitably appropriated accordingly. The concept workstation model enabled 5th p, 50th p and 95th p digital human models to perform work activities with body segments within comfortable range of motion as visualized by presence of green color in body segments. Further, all working postures were categorized as negligible risk and also under low risk category (Table 7). Spinal load analysis using DHM software indicated that compressive and shear forces (for L4-L5 segment) generated for working postures were within safe limits for 5th p, 50th p, and 95th p digital human models for all work postures. Physical mock-up of the final proposed model was constructed for trials with different factory workers (Fig. 7) for elucidating feedback as well as for conducting work study. The proposed workstation model was well received by workers and appreciated by production supervisors/managers. Needs of the users must be considered in design (Blasco et al., 2016) and therefore thrust must be on user centered designs. Since the proposed workstation fixture design consisted of individual modules, the entire setup can be positioned as per desire of the worker and also based on their working hand (right/left). Work study techniques were used to visualize benefits of design implementation in terms of work activities, operator cycle times before and after design modifications. Work activities in existing and proposed workstation were charted using an operation chart (Tables 8 and 9). Operation chart may vary based on hand orientation (right/left) and tool holding preferences of individual workers. Number of work activities was marginally less in the proposed workstation when compared with the existing workstation. Workers will definitely be

Fig. 7. Trial by different workers.

benefitted by minor reduction in work activities in a typical work cycle because of the large number of repetition of work cycles every day. Injection molding cycle time significantly varies (Table 10) from factory to factory (in spite of similar products manufactured), depending on the type of polymers (virgin/recycled) used, quality of finished product etc. Injection molding cycle time determines the time available for workers towards processing the finished product (deflashing activities, sticking product label). Significant difference in observed operator work cycle times were observed between two (skilled and experienced) operators within factory (Table 11) while processing similar finished products even though no significant differences in injection molded machine cycle times were observed (Table 10) within factory. Reduction in observed operator work cycle times was noticed 196

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when operators used modified/re-designed workstation models, but reduction in observed operator cycle times were not significant (Table 11). However, since a large number of products are manufactured every day, workers/operators are definitely expected to be immensely benefitted due to reduced work cycle time. Further in the modified/re-designed workstation setup, the observed operator work cycle times between operators within individual factories were not significantly different from each other (Table 11). No significant difference in injection molding cycle times (Table 10) was noticed (during timing of observed operator work cycle times) before and after workstation design modifications in the respective factories.

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