As depicted on the “Mold Base Machining Dimension” pages of the D-M-E Collapsible Core Design and Assembly Guide, the cross-section represents a typical collapsible core mold design. The Collapsible Core is mounted in the ejector retainer plate; the center pin is mounted in the bottom clamp plate. An extended stripper plate has been incorporated for proper ejection and cylinders are shown installed for the last stage of the required 2-stage ejection. The use of guided ejection is vital to the design.
The molding sequence is as follows: The injection molding machine platens open and the mold parts at the main parting line. When sufficient mold opening space is achieved, the ejector plate assembly is moved forward by the mechanical or hydraulic knockouts of the press. The ejector plate assembly with the collapsing core moves forward with the necessary stroke required to move the core of the center pin to collapse the core. (This is the first stage of ejection.)
If the segments of the core fail to collapse for any reason during this stage, the positive collapse sleeve will come into play and ensure the start of the collapse. Also, the stripper plate and ejector plate assembly move together because the return pins are located directly under the stripper plate. This simultaneous movement continues until the ejector plate assembly is fully forward. At this point, a limit switch is actuated. This, in turn, actuates the cylinders to take over and continue to move the stripper plate with the stripper insert, moving the part away from the collapsed core. (This is the second stage.)
When automatic part stripping is required, a means must be provided for carrying the molded part off the collapsed core at the completion of the ejector stroke. This is commonly achieved by providing a ring projection (.010 x.010 min.) on the face of the stripper insert. Shock dislodges the part from this ring and permits it to drop out of the mold at the end of the stripper stroke. The part must not drag over the core. When removing the part manually, the stripper ring and cylinders are not required.
Note that the stripper plate actuation MUST be sequenced so that the cylinders have returned the stripper plate before the ejector plates are returned. This applies to all parts whether through molded or not. The stripper plate must always be returned to its original position before re-expanding the core. This will avoid interference of the stripper insert with the core and possible core damage.
When the top of the collapsible core closes against the cavity, all cores must be at the same overall length. Whenever this condition exists, an early ejector return must be employed. The early ejector return will ensure that the core is expanded before the mold closes at the parting line.