Much focus from various fields is being placed on the hydrogen and utilizing the technique. Nowadays, the applied researches of hydrogen energy have been trying as one of an alternative energy source for fossil fuel. And, since we could not think lightly of consumed energy by machine tools and industrial machinery, Eco-friendly manufacturing is going to be achieved by hydrogen energy. In this study, to prevent the weldline that is a representative defect in the field of plastic injection molding, heat & cool technology using hydrogen energy, LmNi4.90Mn0.10 hydride, is applied to the mold. This method can achieve heat & cool technology by using only hydrogen energy. So, since electricity and warm/cool water that are for heating/cooling the mold are not necessary, this method should thus be at an engineering advantage in terms of eco-friendly manufacturing. After manufacturing the hydrogen-used-prototype-mold that is dedicated to preventing the weldline, the comparison between the experimental data and numerical simulation was carried out. As result, using this approach, the mold surface temperature difference from initial temperature was rapidly increasing to 40 K, in 18 seconds from the beginning of hydrogen-absorbing and the maximum temperature difference ΔT in this experiment has become 48 K.