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TRIZ Improves Pem-nut Production

| On 01, Dec 2008

By Jaewook Lee and Byoungkon Roh

Project Summary

Pem-nuts are used to attach circuits to a chassis-base, but they are too expensive for regular use because of the numerous processes required to producethem using a traditional machining process. The authors used TRIZ (Theory of Inventive Problem Solving)tools and the application of the 40 inventive principles to design a mold-form to improve production and reduce the cost of pem-nuts.Figure 1 shows the circuit components used in the pem-nut production process (PDP).

Figure 1: Circuit Components Used in PDP

Standoff nuts are used for printed circuit boards (PCB) and other assemblies. They are manufactured in various shapes and formed in a variety of ways based on how they will be used. Most standoff nuts are built using a machining style and are called pem-nuts after the developers – the Penn Engineering and Manufacturing Company (PEM). Pem-nuts are made from free-cutting steel that improves machining ability. Automatic lathes form the pem-nuts through a machining process and the cutting tools are changed as necessary to get the proper shape.

Figure 2: Standoff Nuts Built Using a Machining Process

Figure 3: Standoff Nuts – Cross Section and Actual Object

Purpose

There are several forming methods – the use of an injection mold, stamping and forging. This project’sinventors did not include the injection mold method on the target techniques list, because the injections affect the electric conductivity of the components. The stamping and forging methods, however, maintain the key properties of the raw material. Stamping is the best forming method for productivity.

The goal was to reduce costs and improve the productivity of nut manufacturing by implementing a stamping method of forming the nuts instead of machining standoff nuts, so they applied TRIZ to stimulate idea generation for solutionson thebest forming method for shaping parts for this project.

Problem Statement

System Analysis and Structure

During the system analysis stage the researchers referred to the system as the “nut injection process.” The nut injection process includes adding nuts to the main base chassis to fix the PDP circuit to the panel and requires the use of nuts and the chassis. The system structure is shown in Figure 4. The function of each component is as follows:

  • Guide: Allows for the smooth assembly of the PCB
  • Screw: Firmly holds the PCB
  • Body: Determines the height and maintains the hardness of the nut
  • Groove: Under-cut to strengthen the coherence between the nuts and the base chassis
  • Base chassis: Keeps the PCB assembly strong
  • Head serration: Prevents the nut from turning and enforces the injection strength

Figure 4: System Structure

Current Injection Structure

The under-cut is important functionally. The base chassis moves to this groove, and the nut and the base chassis are mechanically joined. Figure5 represents the “before nut injection.”

Figure 5: The Before Nut Injection

The “after nut injection” is shown in Figure 6. Because the base chassis is deformed in volume due to its plasticity, it rushes into the head of the nut at point A. The nut is fixed firmly against both the upward and downward forces to separate it from the base chassis.

Figure 6: The After Nut Injection

Problem Definition

When using the current stamping method, it is impossible to form the groove. But, it is also necessary because the groove is key to the injection function. Therefore, the authors formulated a contradiction – the groove must not exist for stamping but rather for strength.

Contradiction Definitions

Ideal Final Result

The ideal final result (IFR) was a reliable nut structure that would be superior to the existing manufactured nuts based on the injection effect and press forming. The system must make grooves for the machining style nuts and maintain the current shape of the nuts in the manufacturing process.

Function Diagram

The function diagram made it evident that the injection groove met the desired function – holding the nut and base chassis simultaneously.

Figure 7: Component Name

Figure 8: Function Diagram

Contradiction Definition

  • Physical contradiction: The head groove must exist and must not exist.
  • Technical contradiction 1: When there is a head groove the strength after injection is high, but the stamping processing is impossible.
  • Technical contradiction 2: When there is no head groove the stamping processing is possible, but the strength after injection is low.

Figure 9: Contradiction Definition

Generate Solution

Generate Ideas from the Physical Contradiction

Separation principles were applied to determine the physical contradictions. In this case “separation in time” could be done – the groove does not exist at the press forming stage, but does exist at the nut injection stage. Separation in time is shown in Figures10 and 11. Before the injection there is nogroove, but afterthe nut is injected the energy generated forms a nut-groove.

Figure 10: Before Injection

Figure 11: After Injection

Generate Ideas from the Technical Contradiction

The second technical contradiction was taken from the 40 inventive principles. The researchers generated ideas to produce the nuts by forming them without grooves and improving the effects of the injection.

  • Feature to improve: 12,shape
  • Feature to worsen: 14,strength

The researchers also used the contradiction matrix to select four inventive principles:30,flexible membranes and thin films; 14,spherical shapes; 10,preliminary action; and40,composite materials.

Figure 12: Contradiction Matrixand Technical Contradiction

Among the four recommended principles, number 10 (preliminary action) provided feasible ideas. Instead of grooves, small bores could be made prior to injection. If the small bores worked like grooves, the contradiction could be overcome.

Final Solution

The final solution was derived using the inventive principles (10, preliminary action) and the separation principles (time). The following stagesare based on the separation principles and 40 inventive principles.

  1. In advance, the bores are formed in the nut head.
  2. Manufacture nuts without grooves using the stamping method.
  3. The bores in the nut head gently form grooves.

The injection process provides the natural pressure to change the grooves.

Figure 13: Injection Process Improvement

Summary

By utilizing TRIZ tools and principles of innovation these inventors developed and implemented an idea that increased productivity and reduced costs by more than 20 percent. The basic idea for improvement was derived from the separation principles and the 40 inventive principles of TRIZ. This project served as another example of how TRIZ offers unique and creative ideas for problem solving.

Figure 14: Process Before and After Use of TRIZ