ejection temp. for plastic part

experiment

The cooling cycle of an injection mold can determine the length of time it takes before you can eject a part successfully. There are many other variables discussed further down this response that you can also consider. Successfully is the key word. We will begin with the plastic material. The critical thing here after your material choice is the wall section thickness. Heat transfer between the plastic and mold surface causes the molded material closest to the mold to solidify first. This creates a hard skin which will resist deformation by the mold ejector pins or lifters during the ejection process. As soon as the skin is hard enough, theoretically you can eject the part. Real world experience shows that if the wall section is thick you will be limited by the heat transfer within the plastic. ( Post molding shrinkage, warpage and the necessity for cooling fixtures etc are all factors.) If you eject too soon the inside of the molded part may not be adequately set-up. Your product may warp or shrink in an uncontrolled manner. Different materials set-up at different rates. It is possible to find out what average cycle times are for most materials of a given wall section by contacting the material suppliers. Try www.matweb.com or www.ticona.com for basic manufacturer info. Plastics World or other Plastic magazines are good sources for manufacturers also. Phone the Manufacturer directly and request all help you need.

The next concern is the mold material. Heat transfer in Aluminium is ~ 8 times that of tool steel, with Beryllium Copper materials boast heat transfer rates 10 times that of steel. Again due to wear and cost and machining issues with Al and B-Copper Pre-Hard or treated Mold steel may be a better choice. Stainless claims, slower heat transfer initially compared to other mold steels, equalizes within months due to build-up inside cooling lines due to corrosion etc in non stainless steels. As the other responder says trial is the best way. Armed with the choices of mold material, plastic materials and wall section considerations you can make an educated guess as to what the mold cycle time will be. Even cooling is very important in the success of your mold. Using beryllium copper for cooling of thin cores is a common practice. Again it is a double edged sword in that if you do not adequately cool the base of the core you will fail as the core will get hotter, quicker than regular steel.

One last consideration, if your mold has many slides and lifters and delicate features you may have to sacrifice cycle time anyway to extend tool life. Cooling may not be as important an issue in driving your cycle time. To mold some plastic materials we need to provide heater cartridges or oil heating lines within the mold. Again even heating where oil is used to flatten out hot spots or equilibrate the mold temp may be a consideration.

So to recap. Plastic material choice. Plastic product wall section. Mold material choice and most importantly, even cooling within all sections and features of the mold add up to give your cycle time. All of this is general information for a general question. You can be very specific with your plastic and mold material choice and you will get more specific answers. I hope this is helpful.