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the Technology Interface/Spring 2008 Klein show the dramatic shrinkage of HDPE during the molding process. There are many variables that affect part shrinkage including part thickness, processing temperature range and part cooling time. Common mold shrinkage (the difference in dimensions of the mold and the molded part) for HDPE is 3%. [10] Per the Shrinkage Table below, the actual mold shrinkage was about 2% in length and less than 1% in part width. The aluminum also exhibited shrinkage as it was smaller than the plaster pattern. The aluminum showed 0.6% shrinkage in length and over 2.3% in width. The common pattern shrink allowance is 1.3% for pattern makers. [11] It must be noted that the tooling plaster showed no measurable shrinkage which makes it an ideal pattern making material. The total shrinkage from wood pattern to HDPE part was 2.5% in length and 3% in width. This “real” data will be useful when designing future products for rotational molding. IX CONCLUSION This project was an excellent method to tie together many skills gained in different classes into a unique, and fun, learning activity. The student had to evaluate current products from a design perspective, and then apply their insight to their own design using Solid Edge CAD software. Once the design was determined the model building process had to be determined including the processes and materials to use. Since wood turning is not part of the curriculum, the student had to study this process and make some practice parts before attempting the model build. Laminating techniques were studied that would allow the model to be precisely split in the center to produce two halves. A new material and process for making the foundry pattern had to be studied. Although tooling plaster had been used for centuries, it is typically not studied in our materials courses. Students do have an opportunity to perform some dry sand aluminum casting but had never used a fragile, thin pattern. The casting process was actually the most time consuming. The student first had to design and build a flask suitable for this large product. Because of its size, the mold was very heavy and would often collapse from its own weight. Four Page 11PDF Image | 12 mil Reinforced Polyethylene
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