Parts with flash boil down to one or two issues. First, Flash is when material seeps beyond the boundary of the part. There are three causes. Either the tool is damaged, the part is being over packed or the molding machine is too small to hold the mold halves closed while under pressure..
Tool damage requires an experienced tool engineer to make a repair. Sometimes this can cause extensive delays in delivery.
Over packing a part is generally not enough to cause it to flash. However, when extra pressure is trying to force more plastic into the cavity, even a .003 gap in the shut off can leak.
Wrong sizing of the molding machine can result in the mold blowing open while under pressure. Be sure that the correct tonnage is calculated. As a general rule of thumb, 2 to 5 ton of clamp tonnage per square inch of projected part area. For example, if you have a part that is 3 inches x 5 (depth is not calculated in area) that is 15 square inches. So that is between 30 and 75 tons of clamping pressure needed.