Method of injection molding a skin layer around a core resin

An injection molding method using a multilayer injection molding device for forming an injection molded product having a core layer resin surrounded by a skin layer resin, comprising:

• applying injection molding pressure at a first predetermined time to the skin layer resin in a skin layer injection unit to inject the skin layer resin into a cavity formed within a metal mold in the multilayer injection molding machine while blocking a core layer resin passage extending from a core injection unit to the cavity;
• applying injection molding pressure to the core layer resin in the core injection unit to inject the core layer resin together with the skin layer resin into the cavity after opening the core layer resin passage at a second predetermined time after the first predetermined time;
• terminating the injection of the core layer resin by removing injection pressure from the core layer resin while continuing to apply injection pressures to the skin layer resin and blocking the core layer resin passage at a third predetermined time after the second predetermined time;
• and
• opening the core layer resin passage at a fourth predetermined time following the third predetermined time while applying a holding pressure to the skin layer resin that is less than the skin layer resin injection pressure and of a level causing the skin layer resin to flow back to the core layer resin passage.