This page contains a Flash digital edition of a book.
casting can limit this problem, as can the use of vertically gated ejector bosses outside the casting edge. However, these measures add weight and cost to the casting and/or shot. Sleeve ejectors can be used to apply force di- rectly around a problem core or casting feature where zero draft cores are employed. T e designer should

introduce die features that allow adjustments of die faces to remove fl ash, such as wear pads and overfl ow wells, which change fi ll patterns and remove trapped air/ fumes in the cavity. Vacuum technol- ogy can be used if casting performance requires minimal levels of porosity. T e use of robotics will have design impli- cations and may lower the casting-to- shot weight yield ratio. T e surface fi nish of any zinc alloy

die casting relies on die surface tem- perature, which is controlled with wa-

Zinc castings such as the heat sink cover shown must have adequate fi l- let radii and soft external edges to avoid cosmetic defects.

by the cavity fi ll time and gate

velocity, which can be correlated to the diecasting machine

performance and requires optimizing gate and runner size for effi cient operation and yield.

Die Casting Alloys Metallurgical changes are the

ter or oil pipes designed into the die. T e size and position of the pipes are determined at the die design stage and will be related to the weight/volume, number of cavities, cycle time and fi nish required. T is also is infl uenced

least likely to generate serious casting problems or surface defects in zinc die castings. However, changes in the aluminum and magnesium content of zinc alloys can aff ect some of the other factors that determine surface fi nish— die temperature, metal temperature at the end of the cavity fi ll, fi ll time, fi ll pattern and gate speed. Aluminum content below the

specified minimum will reduce the fluidity of the alloy, changing cavity conditions and increasing fill time. Magnesium is added to zinc to im- prove hardness and control the cor- rosion effects of certain impurities. Excessive magnesium has detrimen- tal effects on surface finish, reducing fluidity and impeding cavity fill con- ditions. Low magnesium zinc alloys have been introduced to overcome the problem. The alloys offer the designer a material that can be cast in thinner sections (down to 0.02 in. [0.5 mm]) while retaining the high surface definition associated with the more traditional zinc alloys. Iron is the most frequent con- taminant in the zinc diecasting process. Excessive iron contamina- tion depletes the aluminum content of zinc alloys, exacerbating the iron pickup and causing the formation of intermetallic particles. In severe cases, the intermetallic will appear after polishing as star-shaped im- pressions on the surface. The defect is often referred to as a hard spot or inclusion and is virtually impossible to remove by grinding or polishing. The phenomena also will appear after plating or painting, often de-


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60