The article deals with a problem related to a product produced by injection moulding technology focusing on the importance to design the product geometry in the right way. The mould structure is often characterized by uneven wall thickness throughout the entire volume. Thicker walls and a greater accumulation of material can cause the overflow and other distortions. The accumulated material solidifies unequally, so internal tension is developing. Its influence may result in various deformations and surface defects. There is described the application of specific solution for the mould wall thickness correction in real practice. The relief pins method is concerned that was used at the product with commercial name Box Series S-BOX 106 made of polyamide PA 66. After proposed modification of the form, the mould quality has improved and production efficiency has increased. Software Cadmold 3D-f was used for simulation of cooling and injection pressure.