MULTISHOT TOOLING
When it comes to the injection mold, its design and construction are determined by the relationship and shape of the two materials used to create the part. The multishot mold style will determine the specific kind of multishot press and any ancillary equipment required to mold the part.
In a single-shot mold, resin is injected into a cavity, and after the appropriate amount of cooling time, the two halves of the mold split open and the part is ejected. In multishot molding, two or more resins are injected into the mold to create a multi-material part. For each subsequent shot, space must be created in the mold to accommodate the additional resin. Put simply, steel must move after the first shot to make new space for the next resin.
There are three principal ways of creating space between first and second shots in a multishot mold:
1. Common core (using a rotary platen or turntable).
2. Core and cavity steel change (using a built in rotary indexing plate or robotic transfer).
3. Core toggle.
(Note that there are instances where a combination of the above can be used to create specific part geometry features.)
Common core: In a common-core multishot mold, the two materials are injected simultaneously. This is achieved by having two different sets of cavity steel on the same mold face (often top and bottom, but right side/left side can be used as well), with one half for the first shot of material and the other for the second. The core side of the mold is mounted on a rotary platen or turntable, which is fastened to the moveable platen of the press.
Using our example of a one-cavity, two-shot mold, that means we’ll have two core-and-cavity stacks—while both cores are identical, there is a difference in the cavity steel.
Once injection and cooling of the first resin is complete and has produced a part in the first-shot half of the mold, the mold opens and the part remains on the core. The rotary platen rotates the core side of the injection mold 180° to align the parts with the opposite half of the cavity side of the mold, ready for the second shot. The mold closes and the second shot of resin is injected. After cooling, the mold opens and the completed two-material part is ejected.
Core and cavity steel change: This technique enables molders to add second-shot features to both the cavity and the core side of the part—something not possible with the rotary-platen method.
This method uses a rotating indexing plate built into the mold to lift first-shot parts from the first-shot core steel after injection. The parts are not ejected, but are transported via the rotating indexing plate to the second-shot core position. The part design must have features that allow the stripper plate to hold, move, and accurately place the first shot onto second-shot core steel. The part component produced by the first shot can be nested into the second shot, allowing material to be injected in, around, and through the first shot.
Core toggle/core pull: Indexing plates use simultaneous first and second injection just like the rotary-platen method. Note that in some cases, an end-of-arm fixture and robot can be used to manage the lift-and-replacement activity, depending on part design/geometry.
This method is unique in that it does not use any type of rotation. Instead, actions to move steel are designed and built into the mold itself. Rather than opening after the first shot, the mold generally stays closed and internal actions move elements of the core and/or the cavity steel, creating new geometry space between shots. It seems no matter which element, core or cavity, is moved, you hear this method called “core pull” or “core toggle.”
Injection is sequential, so cycle time is the total of the two shots plus additional cooling for first shot—which can be over twice as long as the two previous methods, where cycle time is determined by the resin with the longest cooling time. However, in the core toggle method, all cavity and core stacks are identical and create a finished multi-material product on every cycle.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
