Production Planning and Control in Batch Production

Job shops perform an array of jobs and each job is different. In job shop production predictions of demand for products is not possible. In a job shop different jobs are performed with a prescribed set of operations and the time taken by each operation. The equipment for job shop productions is divided for use in different departments. The requirement of each machine is different based on the operation to be performed for a particular job. The job shop is waiting line system in the respect that when a job is finished from one machine, it waits in line to enter the next machine because there are earlier jobs in line too. Vice versa, the machines also wait for jobs while running idly. So, a proper system of planning and control has to be in place for optimizing the job shop production.

Batch production

A variety of products are made but volume is not too large, to demand a separate line for each product. Here, items are processed in lots or batches and a new batch is undertaken for production only when the production on all items of a batch is complete. In fact, batch type production can be considered as an extension of job type system. The best example of batch type system is of chemical industry, where different medicines are manufactured in batches. Other example includes production of machines tools and printing press etc. this type of production generally adopted in medium size enterprises. It is a stage in between job production and mass production.

Production planning and control in batch production - Batch production is the manufacture of a number of identical articles either to meet a specific order or to satisfy the continuous demand. The decisions regarding jigs and fixtures are dependent on the quantities involved in the production batch.

Aspects of job shop production:

The proper sequencing and prioritizing are must for such production to avoid confusion and wastage.
The scheduling should be done based on the estimated time taken by each job.
The running time of a machine should be calculated while it is performing an operation

The performance of a facility is measured by following standards:

The average time taken by each job
The total processing time
The mean of the number of jobs completed before stipulated time and the jobs completed after the stipulated time
The mean waiting time of each job
The average of number of jobs to be done in a unit

The factors affecting a job shop unit are as follows:

Total number of jobs to be done
Total number of machines required
Manufacturing facilities
Evaluation and assessment of efficiency of men and machines

Batch production can be three types namely

A batch produced only once.⇒
A batch produced repeatedly at irregular intervals, when the need arises.⇒
A batch produced periodically at known intervals, to satisfy continuous demand.⇒

Planning problems in batch production

Two problems that may arise in batch production are:

Due to the size of the batch.⇒
Due to scheduling of production.⇒

If larger batch size: - more inventory carrying cost and low set up cost.

If smaller batch size: - low inventory carrying cost and more set up cost.

The solution to these problems depends on whether the production is governed by

External customer order: - if it is the case of external customer orders, the customer order size usually determines the batch sizes. The timing will also depend on the delivery dates specified by the customers.
Whether the plant is producing for internal consumption (i.e. a sub assembly used in the final product): - if it is for internal consumption, both batch size and the production scheduling problems are matters for internal management decisions.

The problems of optimal batch size have to take into account the set-up costs which are involved, before each production runs and the inventory carrying cost incurred, when the finished product is held in stock. The batch size determines the length of the production run and affects both the production schedule and batch size considerations of other products.

Points considered in PPC for batch production

Inventory carrying cost: - It consist of:

Cost of capital
Space cost
Material handling cost
Obsolescence, spoilage or deterioration cost.
Insurance cost
Cost of general administration

Set up cost (ordering cost): - It consist of Cost of time spent in setting up the equipment and organizing the labour for manufacturing batch (cost of idle time i.e. when material is not available on time).

Any cost associated with rearrangement or arrangement (i.e. pay of workers)
Cost due to rejects, scrap, rework generated during a set up.
Variable cost of administration paper work.

Planning and implementing economic batch size is the key focus.
Economic or optimal batch size: - it is such quantity at which total cost of carrying inventory and set up is minimum.

Determination of economic batch quantity
Let, A= annual demand; Q= manufacturing batch quantity; P= production rate; R= consumption rate; T= production cycle time; T*= small time within the production cycle (t), in which manufacturing is completed.
Cc= inventory carrying cost/ unit/ per period
Cp = set up cost/ batch
A × Cp × 2 / EBQ = Cc (P − R) / P (P − R)
Peak inventory level during any cycle = t* × A / Q
Set-up cost = Cp
Inventory cost = Cc (P − R) / 2P

Characteristics of PPC function in batch production

Before issuing manufacturing orders, need for new raw materials and tools, overloading and under loading of particular machines or work centers must be anticipated.
As products are diversified and several orders are handled simultaneously in different wok centers, scheduling and follow up becomes a different task.
Dispatching has to be done efficiently to avoid delays and bottlenecks in the production process.

Job shop productions: case 1 - First we will study the case where n number of jobs are performed by a single machine. In such a case prioritizing and scheduling is very important because the jobs are waiting in line to be completed. The objective is to improve the average idle time, waiting time, the work in process time. The shortest processing time (SPT) is aimed for in a job shop production.
Job shop production: case 2 - In such a scenario, n number of jobs are performed by two machines M1 and M2. The processing time on both the machines is calculated. The minimum time to complete a job is also stipulated. Then a sequence is worked out to line the jobs on both the machines in such a way that the target of maximum production in minimum time is achieved.

Prioritizing rules for job shops: The job which has the shortest processing time is selected. Manufacturers may also put the job which has the earliest delivery due date first in line. First come first serve rule may be applied to cut any slack or wastage. So, it depends on the manufacturer's priorities and the master schedule.
Examples of job shop productions: They include paint shops, machine tool factories and specialty restaurants. Given below is a brief outline of the paint manufacturing process, In this, paints are manufactured in a variety of colours and even patterns and the machines used are the same for all the jobs which are done.

First a color pigment is selected.
Then a brightener is added to it to bring out its texture.
Next, binder and plasticizer are added which will make it stick to the wall.
Lastly, additives and water are added to improve its consistency

Batch Production Case Study

If a design was to be manufactured by batch production several hundred would be made on a production line. Each person/machine on the production line would complete one part of the product and then it would be passed on to the next person/machine. Many items in your home will have been manufactured by batch production - can you name any?

Case Study in Bicycle Manufacture

Modern bicycle manufacture is component based. The company buys parts such as pedals from other manufacturers and puts the bicycle together on a production line. This is typical of a company manufacturing through batch production. Usually, a company will manufacture a certain number of a particular model of bicycle (two to three thousand at a time) and then move on to another model.

In each of the boxes of the flow chart shown below, write the key words representing each stage.

The tube for the bicycle frame is cut to size. One person will do this job and they cut enough tube for a batch of several hundred bikes in a week. The tubes are set up in a ‘jig’ which holds the frame together. The frame travels along the production line and the joints are preheated to save time. A gas torch is used to weld the frame together.
After welding the frames and the front forks need cleaning in the ‘Grit-Blaster’. This shoots tiny particles of sand, at high speed, at the frame and cleans away the ‘residue’ left behind by the welding process. All frames are checked to ensure that they are straight. Small adjustments can be made at this stage.
The frames are now ready for painting. This is done by using a fine spray which covers every part of the frame with paint. The frame then moves down the production line into a special oven which ‘bakes’ the paint giving it a tough finish. The wheels are assembled by hand and they are individually tested in a machine which automatically tensions each spoke to ensure that they are perfectly straight.
The frames are machined so that other parts such as the handle bars and the bottom bracket (pedals) can be attached. People operate the machine tools but they need some training before they can use the machines safely and efficiently. The bicycles are now ready for the shops where they will be viewed by customers and agents. Agents will buy ‘batches’ of bicycles for large stores.
Before sending the bicycles to the shops, agents view them and suggest changes for the next batch. This helps the manufacturer improve the design and production of bicycles.

Conclusion

In case of batch production, raw materials are converted into components or parts for stock but they are combined according to the customer orders. The products are manufactured usually in large batches. Every batch differs from others, but all units within a batch are identical. A number of heterogeneous finished products are manufactured with in a limited range of options. Therefore, PPC in intermittent manufacturing is a mixture of those used in process industry and job order production.

Sandeep Ghatuary

Finance & Accounting blogger simplifying complex topics.

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Fintaxman

Production Planning and Control in Batch Production | PPC Functions & Job Shop Aspects