Metin Atmaca 030080007 7th week definitions - part 2

3. Preform Molding (Manufacturing Process):

There is no previous definition.

New Definition:

There are several good reasons to preform the reinforcement before loading the mold. One is to speed up the process and to free the production mold from everything except loading, injection, in-mold cure, and demolding. Another is to improve the quality and reduce part-to part variations. For fast cycle times the ideal is to make the preform so stiff that it becomes self-locating in the production mold. In other cases when the mechanical properties are of paramount importance one often wants to minimize the amount of preform binder since the mechanical properties can be adversely affected by the binder. In general, a good perform is required to be inexpensive to make and it must be stiff enough to be stacked and handled before injection. The fibers must stay in the direction in which they have been placed during preforming both during handling and injection. To achieve all these goals it is common to apply some form of preforming agent. Both thermoplastic and thermosetting powders are commonly used for this purpose.

The preforming methods can be roughly classified into five basic types:

1. Cut and paste

2. Spray-up of chopped fiber on perforated models

3. Thermoforming

4. Weft knitting

5. Braiding

(Dave, R. S., Loos, A. C., Processing of Composites, p. 361)

4. Mechanical Overload (Mechanical Failure)

Previous Definition:

As the name implies, This type of fracture results when load applied exceeds the available strength of a component. This seems straight forward enough, but there are some subtleties. For example, "What is the available strength of a component?". Actually, it is a function of two things. It is derived from both the inherent strength of the material and how much of that material is available to carry the load. If the material strength is compromised (through alteration of its heat treatment, for example) then the strength of that component is not as intended. Similarly, if the amount of load bearing material is deminished (through corrosion or cracking) then the component is similarly weakened.

(Broker J.P., Hill P.F., Bicycle Accidents: Biomechanical, Engineering, and Legal Aspects, pg.242, Kayra Ermutlu)

New Definition (Better):

The strength of a material is usually measured in some kind of laboratory test on a standard test piece. By far the most commonly quoted measurement is the tensile strength and such values are widely used to make comparisons of different materials. However, the way the force is applied and the geometry of the test piece itself affect the result and there are other mechanical properties that are equally important as absolute tensile strength in determining the ability of a component to withstand externally applied loads, for example, toughness (which is a measure of the ability to absorb energy as a fracture develops) and ductility or malleability (the capacity to be deformed before fracture occurs). In many materials events begin to happen that would destroy the integrity of an engineered component long before the tensile breaking force is reached. Among the most difficult types of material to use successfully and safely in highly stressed situations are those that behave in a brittle manner despite having very high tensile strength.

Other strength data commonly quoted are determined by means of compression tests and shear tests where the units are the same as for tensile tests, i.e., force per unit area, but others are expressed in different units and must always be related to the geometry of the test piece and the way the test was conducted. Highly sophisticated tests that measure properties such as fracture toughness are of vital significance in material specifications and design parameters for structural products used in aerospace, pressure vessels, etc.

Different types of materials exhibit different behaviors depending on the way their constituent atoms and molecules are bonded and how these stack together to form crystal structures. The art of materials science is to control these arrangements by mechanical processing and heat treatment in order to make a product with the required properties for a particular application. The engineering dimension comes in when the product has to be designed to resist all the foreseeable loadings likely to be encountered under service conditions and to ensure that a given component will safely withstand the stresses and strains (and abuse) likely to be experienced during its entire service lifetime, regardless of whether these are static loadings or loadings of a cyclic nature likely to be applied many millions of times.

(Peter Rhys Lewis, Colin Gagg, Ken Reynolds, Forensic Materials Engineering: Case Studies, p.36)

5. Process Manufacture (Manufacturing/Organization Method)

There is no previous definition.

New Definition:

Typically process manufacturing involves a combination of physical parameters. These could be a combination of temperature, pressure, density, flow rate, moisture level, and chemical concentration that are set at the machine to process the material. If these settings are suboptimal, then the process operates sub-optimally in terms of throughput, quality, and efficiency. These types of optimization problems are ideally solved using the Six Sigma methodology and tool set.

Process manufacturing is fundamentally different from discrete manufacturing in the way material flows. Material flows in a continuous stream in process manufacturing, while parts move in discrete batches in discrete manufacturing. 

Since there has been so much work done in developing these methodologies in discrete manufacturing and very little in process, it might seem logical to apply them “as is” to process manufacturing industries.

However, this approach is like trying to fit square pegs into round holes.

The better approach is to adapt these techniques within a process improvement framework that identifies the various forms of waste in the process manufacturing value stream, and manages the wastes with the appropriate concepts and tools.

Lean manufacturing defines seven types of waste that make a production system inefficient and costly:
1. Over-production: Producing too much, too soon.
2. Inventory: Extra production required to buffer process variability.
3. Transportation: Movement of materials without adding value.
4. Waiting: Increasing production cycle time without adding value.
5. Movement: Movement of operators without adding value.
6. Defects: Product that does not conform to customer specifications.
7. Over-processing: Processing a material more than is necessary to meet customer specifications.

The first three types of wastes above relate to a lack of material flow. By the very nature of process manufacturing, material flows in a continuous stream from one process to the next, without periods of stopping and waiting in between (the possible exceptions being some batch processing in the chemical and steel industries). The Lean ideal of flow occurs by default. As a result, over-production, inventory, and transport are either non-issues or only minor issues in process manufacturing.

Movement waste is also less relevant to process manufacturing because operators typically monitor automated equipment.  Their movement usually does not have an adverse impact on the ability of the equipment to continue processing the material.

However the three types of waste -- waiting, defects and over-processing -- do exist in process manufacturing and are fertile ground for the application of Lean and Six Sigma methodologies.

§  Product changeovers, which in process manufacturing can sometimes take 18 hours or more, are an example of waiting waste.

§  Defects are the result of production of material that does not meet the specifications of the downstream internal/external customer.

§  Over-processing occurs when the material is processed to a greater extent than is required by the downstream customer.

(Herb Lichtenberg, Square Peg In A Round Hole: Applying Lean Principals In Process Industries)