Design of Production Systems

Plant location means selecting the geographical site where the factory will be built. The decision is long term and costly to reverse, because land, buildings and machines are fixed investments.

Importance of plant location

• It decides the transport cost of raw materials and finished goods.
• It affects availability and cost of labour, power and water.
• It influences the production cost and so the selling price and competitiveness.
• It affects expansion possibility and relations with the community.
• A wrong choice cannot easily be corrected and may cause long-term loss.

Factors affecting plant location

1. Nearness to raw materials: important when raw material is bulky or perishable or loses weight in processing (sugar mill, cement plant).
2. Nearness to market: important when the product is fragile, perishable or heavy (bakery, soft drinks).
3. Transport facilities: road, rail, air, waterways; cost and reliability.
4. Availability of labour: skilled and unskilled labour, wage level, labour relations.
5. Power and fuel: cheap and reliable electricity, water supply (e.g. hydropower for electro-intensive industry).
6. Water and waste disposal: supply of water and facility for effluent treatment.
7. Climate and land: soil bearing capacity, drainage, flood risk, cost of land, room for expansion.
8. Government policy: taxes, subsidies, industrial estates, environmental rules, zoning.
9. Capital and financial facilities: banks, insurance, availability of finance.
10. Community and living conditions: housing, schools, hospitals, security, safety from hazards.
11. Competitors and supporting industries: nearby suppliers, repair and service facilities.

Selection method

Because many factors conflict, the final decision is made using methods such as the weighted factor rating, break-even (cost) analysis or the centre-of-gravity method.

Method

In the weighted factor rating method, each factor is given a weight (sum = 1.00) and each site is scored. The weighted score is $\text{weight} \times \text{score}$, and the site with the largest total is selected. The weights add to $0.25+0.20+0.20+0.15+0.12+0.08 = 1.00$, so the totals are directly comparable.

Calculation

Decision

Site C has the highest weighted score (7.21), then A (6.92) and B (6.71). Site C is selected. The margin over A is small (0.29), so the result should be checked for sensitivity, e.g. by changing the weight of the raw-material factor, because the weights are subjective.

Answer: Site C, weighted score 7.21 (A = 6.92, B = 6.71).

Method

Total annual cost $TC = FC + v\,Q$. Two sites cost the same at the volume where their lines cross; the cheapest site changes at these break-even volumes.

(a) Break-even points

X and Y:

$$\begin{aligned} 600000 + 40Q &= 900000 + 30Q \\ 10Q &= 300000 \Rightarrow Q = 30000 \text{ units} \end{aligned}$$

Y and Z:

$$\begin{aligned} 900000 + 30Q &= 1500000 + 20Q \\ 10Q &= 600000 \Rightarrow Q = 60000 \text{ units} \end{aligned}$$

X and Z cross at $Q = 900000/20 = 45000$, but at that volume Y is cheaper than both (Rs 2,250,000 against Rs 2,400,000), so this crossing is not used.

(b) Output of 50,000 units

• X: $600000 + 40(50000) =$ Rs 2,600,000
• Y: $900000 + 30(50000) =$ Rs 2,400,000
• Z: $1500000 + 20(50000) =$ Rs 2,500,000

Y is cheapest, which agrees with the range 30,000 to 60,000.

Answer: Break-evens at 30,000 and 60,000 units; choose site Y (cost Rs 2,400,000 per year) for 50,000 units.

Principle

The centre of gravity (load-distance) method finds the point that minimises total weighted distance to the markets, treating the shipment volumes as weights:

$$\bar{x} = \frac{\sum x_i W_i}{\sum W_i}, \qquad \bar{y} = \frac{\sum y_i W_i}{\sum W_i}$$

Calculation

$$\bar{x} = \frac{69000}{1400} = 49.29\ \text{km}, \qquad \bar{y} = \frac{83000}{1400} = 59.29\ \text{km}$$

The centre is at about (49.3, 59.3) on the grid, pulled towards M3 because it has the largest shipment.

Limitations

• Straight-line distances are assumed; real roads, rivers and hills are ignored.
• The calculated point may be unavailable or very costly land, so a nearby suitable site is chosen.
• Only transport cost is considered, not labour, land or taxes.
• Cost per tonne-km is assumed equal for all routes and constant with volume.

Answer: Centre of gravity = (49.3 km, 59.3 km).

The factory building should suit the process, allow smooth material flow, provide light and ventilation, and permit future expansion. The common types are:

1. Single-storey building

All operations are on one floor.

• Suitable for heavy machines, heavy products and continuous flow production (steel, automobile assembly).
• Advantages: easy material handling, strong floors, easy layout change and supervision, lower foundation and maintenance cost per area.
• Disadvantage: needs more land.

2. Multi-storey building

Operations are on several floors, often with gravity flow from top to bottom.

• Suitable where land is costly and the product is light (electronics, garments, flour mills).
• Advantages: less land, compact, good for processes using gravity.
• Disadvantages: costly structure, lifts needed, vibration and limited floor loading, difficult layout change.

3. Saw-tooth (north-light) roof building

A single-storey building with a roof shaped like saw teeth; the glazed vertical face is oriented to give diffused daylight without glare.

• Suitable for textile mills, machine shops needing uniform natural light.

4. Monitor or clerestory roof

A raised central portion with windows gives light and ventilation along the length of the shop.

• Suitable for foundries and forge shops needing ventilation for heat and fumes.

5. Pre-engineered (steel frame) building

Factory-made frame members are bolted on site; quick to erect, cheaper, with long spans without columns. Suitable for warehouses and medium industries.

Selection factors

Nature of product and process, plant layout, cost of land, expected expansion, building codes and the need for natural light and ventilation.

Plant layout is the arrangement of machines, equipment, storage, service areas and workers in a plant so that the production process runs smoothly at minimum cost. It is the physical arrangement of facilities to give the best use of space, labour and machines.

Objectives

• Minimum material handling and transport distance.
• Smooth, continuous flow with no back-tracking.
• Effective use of floor space and equipment.
• Safe, comfortable working conditions.
• Flexibility for changes in product or volume.
• Easy supervision and communication, and lower work-in-process.

Product (line) layout

Machines are arranged in the sequence of operations of one product, as in an assembly line or bottling plant.

Process (functional) layout

Similar machines or functions are grouped together (lathe section, milling section, grinding section) and jobs move between sections, as in a job shop.

Examples

• Product layout: car assembly, soft-drink bottling, sugar plant.
• Process layout: machine shop, hospital, repair workshop.

(a) Fixed position layout

In this layout the product stays at one place because it is too large or heavy to move, and the workers, machines and materials are brought to it.

• Examples: ship building, aircraft assembly, bridge, dam and building construction, large turbines.
• Advantages: no movement of the heavy product, flexible for changes in design, work can be assigned to skilled teams, suitable for one-off projects.
• Disadvantages: material and equipment movement is high, space around the product may be crowded, equipment utilisation is low, and skilled labour and good scheduling are needed.

(b) Group technology (cellular) layout

Group technology (GT) groups parts with similar shape, size or processing needs into part families. Machines needed for one family are placed together in a cell, which works like a small product line inside a job shop.

  Process layout           Cellular layout
  [L][L] [M][M]            +-------+  +-------+
  [G][G] [D][D]            |L M G  |  |L D M  |
  (jobs travel far)        |cell 1 |  |cell 2 |
                           +-------+  +-------+

• Steps: classify parts (coding), form families, group machines, arrange cells.
• Advantages: shorter travel and lead time, less work-in-process, simpler scheduling, better quality, team responsibility.
• Disadvantages: machines may be under-used, cell reorganisation when product mix changes, cost of coding and rearrangement.

GT layout is a compromise between the flexibility of process layout and the efficiency of product layout, and is suitable for medium-volume, medium-variety production.

A flow pattern is the path followed by materials as they move from receiving, through the operations, to the shipping of the finished product. A good pattern avoids back-tracking and cross-flow, keeps distances short and allows expansion.

Common flow patterns

1. Straight line      2. L-shape       3. U-shape
 In -> -> -> -> Out    In->->->        In ->->->->  v
                            |                         v
                            v->-> Out  Out <-<-<-<-  v

4. Circular (O)       5. S/Serpentine   6. Odd angle
   +---->----+         In ->->->->|       (follows
   ^         v              |<-<-<-       process
   +----<----+              ->->-> Out     needs)

1. Straight line: Simple and used for short processes with few operations and enough length. Easy to control but may need a long building.
2. L-shape: Used when building shape or other facilities do not allow a straight line.
3. U-shape: Receiving and shipping are at the same side, which allows common use of docks, supervision and material handling equipment. Common in JIT cells.
4. Circular: Input and output at the same point; useful when the operator or fixture returns to start (e.g. pallets).
5. S-shape (serpentine): Long process lines are folded to fit in a compact area.
6. Odd-angle: Used for short runs where handling is mainly by conveyors, or where space is irregular.

Factors for selection

Building shape and size, number and sequence of operations, product size and volume, material handling system, location of receiving and shipping, and possible future expansion.

(a) Cycle time

$$C = \frac{\text{available time}}{\text{required output}} = \frac{480}{60} = 8\ \text{min}$$

(b) Minimum number of stations

Total task time $= 4+3+5+4+3+6+2+3 = 30$ min.

$$N_{min} = \frac{\sum t}{C} = \frac{30}{8} = 3.75 \Rightarrow 4\ \text{stations}$$

(c) Ranked positional weight (RPW)

RPW of a task = its own time plus the times of all tasks that follow it.

Order of rank: A, C, B, D, E, F, G, H. Assign to each station the highest-ranked available task that fits in the remaining cycle time.

Explanation: after A (4 min) only 4 min remain in the station. C (RPW 19) needs 5 min, which does not fit, so B (3 min) is taken. Task D cannot join station 2 because C+E already use 8 min, and F cannot start until D is finished, so D takes a station alone. F (6) and G (2) fill station 4 exactly.

(d) Efficiency and balance delay

$$\text{Efficiency} = \frac{\sum t}{N\,C} \times 100 = \frac{30}{5 \times 8} \times 100 = 75\%$$ $$\text{Balance delay} = 100 - 75 = 25\%$$

The four-station minimum is not reachable because of the precedence limits, so five stations are needed.

Answer: Cycle time 8 min; minimum stations 4 (theoretical); 5 stations used; efficiency 75%; balance delay 25%.

Method

The load-distance score is $\sum (\text{loads}_{ij} \times d_{ij})$, where $d_{ij}$ is the distance between departments along the row. The layout with the smaller score has less material handling. In a row, distance $= 10 \times$ (difference in slot numbers).

Present layout P-R-S-Q (slots 1, 2, 3, 4)

Slots: P = 1, R = 2, S = 3, Q = 4.

Proposed layout P-Q-R-S (slots 1, 2, 3, 4)

Slots: P = 1, Q = 2, R = 3, S = 4.

Comparison

Saving $= 2650 - 1850 = 800$ load-m per day, i.e. $800/2650 = 30.2\%$ reduction. The proposed layout puts the high-flow pairs P-Q, Q-R and R-S next to each other. It is therefore better, though rearranging costs must be compared with the saving.

Answer: Present = 2,650 load-m/day; proposed = 1,850 load-m/day; proposed layout reduces handling by about 30%.

Material handling is the art and science of moving, storing, protecting and controlling materials during manufacturing, storage and distribution, so that the right material reaches the right place at the right time at minimum cost. It adds no value to the product but adds cost, so it should be reduced to the minimum.

Objectives

• Reduce handling cost and production cost.
• Reduce delays, damage and loss of materials.
• Improve safety, working conditions and use of space.
• Increase productivity and keep production flowing smoothly.

Principles

1. Planning: plan all handling activities in advance.
2. Flow: keep a smooth flow along a straight path; avoid back-tracking.
3. Simplification: remove, combine or simplify moves; handle only when necessary.
4. Gravity: use gravity (chutes, slides) wherever possible.
5. Unit load: move materials in unit loads (pallets, containers) rather than singly.
6. Mechanisation: use machines where volume justifies.
7. Space utilisation: use vertical space and keep aisles clear.
8. Safety, standardisation and flexibility: use standard, flexible equipment safely.
9. Maintenance and obsolescence: plan upkeep and replacement.

Classification of equipment

Equipment is selected according to the material, quantity, distance, path and cost.

Material handling equipment is chosen by whether the movement is continuous or intermittent, the path is fixed or variable, and the load is light or heavy.

Notes

• Conveyors suit product layouts where a large volume moves between fixed points; a belt conveyor moves bulk material such as sand or grain, and a roller conveyor moves boxes.
• Cranes lift loads vertically and move them across a shop without using floor space; used for dies, engines and castings.
• Forklifts pick up palletised loads and travel anywhere in the plant, so they suit process layouts and storage.