Unless the length of the melt flow exceeds practical limits a single gate is the preferred option. Multiple gates always create weld lines where the flows from the separate gates meet.
A distinction can be made between center and edge gating of a part. Center gated parts show a radial flow of the melt. This type of gate is particularly good for symmetrical parts, such as cup shaped products or gears, because it will assure more uniform distribution of material, temperatures, and packing, and better orientation effects it gives very predictable results. On the other hand, linear flow and cross flow properties often differ. In flat parts, this can induce additional stress and results in warpage or uneven shrinkage.
Because of their simplicity and ease of manufacture, edge gates are the most commonly used. These work well for a wide variety of parts that are injection molded. Long narrow parts typically use edge gates at or near one end in order to reduce warpage. But it is very difficult to mold round parts using this type gate, as they tend to warp into an oval shape. While a single gate into the body of the part might incur a higher initial tool cost, lower scrap rates and higher part quality will quickly justify this expense.