Case Study - Statistical Process Control Background: On a daily basis, Mobley Concrete Company loads and average of 40 to 50 loads of concrete into mixer trucks that deliver the material to the job site. Prior to loading each truck, the front-end loader adds the raw ingredients of the concrete mix into the plant bins and bulk hauling trucks load cement powder into the silos. Upon the command of the batch man, the proper amounts of raw materials are weighed up into the scales on the plant. Once the batch computer system is satisfied that all of the raw materials are in the scales, the material is dropped into the truck where it begins mixing. Once the material has dropped into the truck completely, the driver is notified to pull out from under the plant and the driver proceeds to the wash stand. The particular plant we are discussing has two wash stands so that two trucks may be washing at the same time. The driver exits the truck and goes into the batch room to collect their delivery ticket. The driver is responsible for washing down the mixer truck both for the sake of prolonging the life of the mechanical components and for the image of the company. During the wash-down process, the driver also has the responsibility to check the wetness of the concrete to make sure it matches the consistency desired by the specification on their delivery ticket. Once the load is the proper consistency and the truck has been washed down, they can then travel to the job and deliver the concrete to the customer.

Problem: Upon observation on a particular day, it appeared that the two wash stands were insufficient for the plant as trucks seemed to be stacking up at the plant prior to leaving and slowing the delivery to the customers.

Data: The Operations Manager, having observed this problem, decided to randomly choose four trucks operating from the plant on one day and time their flow through the plant from the time they began pulling into position, to the time they left plant. A spread sheet has been provided to show the statistical data collected from that day.

Case study: Statistical Process Control

• Based on the data, what part of the process can you determine may be either a bottleneck or not in statistical control? Explain and support your answer.
• What are some possible recommendations to help the process become more streamlined and consistent?

Background:

Cement powder is a primary ingredient in ready mixed concrete. Cement, when mixed with the correct proportions of water, sand, and gravel in a fashion similar to mixing a cake, will eventually dry and harden into concrete. The cement powder is produced by a supplier and stored in silos for us to pick up when needed. We send our powder hauling trucks to pick up the material based on demand as we have limited space for the material at our plants. The cement supplier has their own schedule that does not always match up with the concrete production schedule. This presents a supply/demand issue on some occasions that must be handled appropriately to successfully service the customers.

Problem:

The cement supplier that supplies the cement for Mobley Concrete Company has decided to close on Friday for the upcoming holiday weekend. Today is Wednesday and we can foresee an issue arising for Friday as our concrete customers are not stopping work for the holidays. We need to make sure we have enough cement to service our customers.

Data:

• Silo Capacity: 100 tons
• Current cement in inventory: 78 tons
• Number of powder hauler trucks: 2
• Powder truck capacity: 25 tons
• Cement used per cubic yard of concrete: 500 pounds
• Anticipated customer demand on Thursday: 320 cubic yards
• Thursday business complete by 6:30 PM
• Cement terminal closes at 10:00 PM
• Anticipated customer demand on Friday: 580 cubic yards
• Powder haulers need 3 hours travel time to include loading at the plant
• Powder haulers need 45 minutes to unload at the concrete plant

Case Study: Demand/Supply Chain

• Elaborate if there is a problem you can with the cement inventory versus the demand.
• Choose a solution and explain why you believe it is the best solution.