I received this e-mail from a molder overseas…
We are molding PS 750 (NL) tubes in an eight cavity mold. When we use a higher mold temperature, part ejection is good, but the parts display sinks near the slides. With a lower temperature, we can mold parts which look good, but are difficult to eject. We believe we have enough draft on the part and extensive polishing has been done to assist part ejection, yet the parts is hard to eject. If we spray mold release, the part will release from the mold, but this is not a good long term solution. Do you believe the fault is in the material?
Note: Nathan also provided many photos and specifications for both the tool and parts which we will avoid displaying these to protect his customers.
The three biggest factors that hinder part ejection and hold the core to the part are: the force holding the part to the core, the vaccuum forces holding the part to the core, and the coefficient of friction between the core and the part.
First, the hoop stresses due to shrinkage causes the part to contact the core even after it begins moving off the core. The warmer the part is during the time of ejection, the less this stress will hinder part ejection.
Second, highly polished surface of the core can create such a good seal with the core, it can help to maintain the vacuum seal during part ejection. Applying a very slight texture or surface treatment to the core surface can often reduce this seal and help gas get under the part, thus reducing these vacuum forces. Many molders will also use porous steel and air poppet valves to help facilitate this removal process.
Third, the coefficient of friction for most tool steels can be improved significantly through the use of dry lubricating surface treatments such as dicronite. This is one of quickest ways to overcome friction and improve part ejection.
One great benefit to surface treatments such as dicronite, you can see the coating on the core, so any wear or damage to that coating can be easily tracked and monitored.