Injection molding of polymeric material
Injection molding systems and methods useful for making microcellular foamed materials are provided as well as microcellular articles. Pressure drop rate and shear rate are important features in some embodiments, and the invention provides systems for controlling these parameters in an injection molding system. Another aspect involves an injection molding system including a nucleator that is upstream of a pressurized mold. Another aspect involves an extrusion system with the reciprocating screw for forming a single phase solution of non-nucleated blowing agent and polymeric material. Another aspect involves very thin walled microcellular material and very thin walled polymeric material. Another aspect provides a method for producing high weight reductions in very thin-walled parts with surfaces that have no noticeable differences from non-foamed parts.
1. A method of molding a polymeric foam article comprising:
conveying a polymeric material in a downstream direction in an extruder including a barrel and a screw, wherein the screw rotates and reciprocates within the barrel;
introducing a blowing agent into the polymeric material at a rate metered by the mass flow of the blowing agent and at a position that is a function of the axial position of the screw within the barrel to form a mixture of polymeric material and blowing agent; and
injecting the mixture of polymeric material and blowing agent into a molding chamber fluidly connected to the barrel.
2. A method as in claim 1 , wherein the amount of blowing agent in the mixture is between 0.1% and 2.5% based on the weight of the mixture of precursor material and blowing agent.
3. A method as in claim 1 , wherein the mass flow rate of the blowing agent is between 0.04 lbs/hour and 12 lbs/hour.
4. A method as in claim 1 , wherein the mass flow rate of the blowing agent can be controlled within +/−0.3%.
5. A method as in claim 1 , wherein the blowing agent comprises carbon dioxide.
6. A method as in claim 1 , wherein the blowing agent comprises nitrogen.
7. A method as in claim 1 , wherein the blowing agent is introduced into the polymeric material through a blowing agent port formed in a barrel of an extrusion apparatus.
8. A method as in claim 1 , further comprising forming a microcellular article having an average cell size of less than 100 microns in the molding chamber.
9. A method as in claim 1 , wherein the mixture is nucleated to form microcells by a pressure drop when injecting the mixture of polymeric material and blowing agent into a molding chamber fluidly connected to the barrel.