Groover: Fundamentals of Modern Manufacturing, 5e
Case Study by Daniel Waldorf, California Polytechnic State University
Injection Molding Case Study: Awesome Amps and its Amplifier O-Frame
Awesome Amps, Inc., is a small start-up company that has designed a portable amplifier for digital music players and musical instruments. The product design includes an important component, the O-frame, that provides structure to the amplifier and compartmentalizes the various electrical circuits and subassemblies. The O-frame geometry is fairly complex, but it is small enough to fit in the palm of a hand. An initial production run is anticipated for 5,000 units, with possibly larger runs to be completed later. The engineer in charge needs to design a production process for the O-frame that is economical and environmentally responsible and that leads to a consistent, high-quality part.
Since the structural load on the part is modest and the required dimensional tolerances are fairly loose, a polymer is suggested for the O-frame material and a plastic molding process considered for production. Since high-density polyethylene (HDPE) is inexpensive and easily molded, it is chosen for the part.
Ultimately, injection molding is selected for the part. A horizontal machine with a reciprocating-screw injector is acquired and a two-plate mold is constructed for the part. The process is run according to the recommended machine settings found on the website of the HDPE supplier, with two exceptions. First, in order to eliminate weld lines on the part, a higher injection temperature is specified. And second, to correct a problem of incomplete parts, a higher injection pressure is applied.
Although the parts are successfully produced, it is eventually discovered that the parts from the initial run are considerably larger than designed. The outer diameter is specified to be 112.5 +/- .25 mm, but most parts measure between 112.9 and 113.0 mm. The engineer is troubled and perplexed by the problem.
WATCH THE THREE VIDEOS on Plastic Injection Molding
1. Which material type is more commonly injection molded, thermoplastics or thermosets?
2. What machine settings have important effects on the part properties in injection molding?
3. What two mechanisms provide heat to melt the polymer in the molding machine barrel?
4. Most industrial machines for injection molding are structured horizontally. What types of molded parts are typically produced on a vertical machine?
5. A three-plate mold for injection molding is more complex and expensive than a two-plate mold. What advantage of the three-plate system allows this extra investment to be justified?
GO TO THE TEXT: Chapters 8 and 13
6. Some types of plastics are readily recycled, if they can be repeatedly heated (melted) and cooled with little material degradation. Will the choice of HDPE for the O-frame allow for the part to be recyclable? See Section 8.2.
7. Why is the O-frame part appropriate for the process of injection molding? See Section 13.6.
8. How could the use of a three-plate mold possibly correct the problem of the weld lines on the part without needing to increase temperature? See Section 13.6.
9. Why did the parts come out too large from the initial run? See Section 13.6.
SOLVE
10. Use the shrinkage values in Table 13.1 to determine the correct size of the outer diameter in the mold cavity.
11. If the actual shrinkage rate is just one-half of the rate in the table, what would be the correct diameter for the mold cavity.
12. What can be done if the mold has already been made and purchased with the wrong diameter?