Calculating Shrinkage Allowances

   In order to achieve the desired dimensions in our parts according to the tolerances we specified in a previous post, we had to account for the shrinkage of the injection molded parts after cooling. During injection molding, hot molten plastic fills the molds while under pressure and is held there for a period of time to cool. The pressure at which the mold core and cavity are pressed together as well as the cooling time are two parameters which can be manipulated to affect the degree of shrinkage during the production of the yo-yos.

   To compensate for the inevitable shrinkage of the yoyo parts during production, the yo-yo dimensions were scaled by the shrinkage factor percentage. To determine the shrinkage factor percentage, various yo-yos and the corresponding molds from groups of previous years with body and ring styles similar to our own group were measured. By taking the average critical dimensions of a few yoyo parts and the corresponding dimensions of the mold, the shrinkage factor could be determined. Subtracting the actual yoyo dimension from the mold dimension and dividing by the original mold dimension yields the percent shrinkage.

   The math for the body and cap of the yoyo to calculate the shrinkage factor is shown below. The results show that the body had a shrinkage percentage of 1.47% while the ring shrunk about 1.37%. However, to err on the side of the caution we approximated the shrinkage to be at 2% as the parts we used for comparison were not exact copies of our specific yoyo. Knowing this 2% shrinkage factor, we scaled the design of our yoyo body and cap accordingly in order to be within the correct tolerances and dimensions for our final product. 

By comparing the shrinkage factor of previous yoyo parts similar to our own, we were able to determine an approximate scale factor for our design to compensate for the shrinkage of our part during production.