Advanced Structural Foam Molding Using a Continuous Polymer/Gas Melt Flow Stream Xiang Xu, Chul B. Park, John W. S. Lee, Xindong Zhu Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8 Received 8 April 2007; accepted 2 February 2008 DOI 10.1002/app.28248 Published online 20 May 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: This article introduces a patented advanced structural foam molding technology. By the implementa- tion of a means of continuing the polymer matrix melt flow stream, the processing conditions are made consist- ent, and the injected gas is thus uniformly dispersed throughout the polymer matrix. Compared with its prede- cessors targeted at similar applications, this new technol- ogy generates single-phase polymer/blowing agent solu- tions and is therefore capable of producing parts with smaller cell sizes, more uniform cell size distributions, higher void fractions, and similar geometric accuracy. Fine-celled parts (>10 6 cells/cm 3 ) with high void fractions (>30%) were successfully produced in this research with an advanced structural foam molding machine that was built in house. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 2855–2861, 2008 Key words: blowing agents; foams; injection molding; voids INTRODUCTION Structural foams are plastic foams manufactured with conventional preplasticating-type injection- molding machines; a physical blowing agent, a chemical blowing agent, or both are employed in the process to produce a cellular (foam) structure. The structural foam molding technology was invented and improved by Angell and coworkers. 1–3 Low-pressure, preplasticating-type structural foam molding machines are most commonly used to cre- ate structural foams because the required size of the molding system for producing large products is smaller than that of conventional injection molding on account of the lower cavity pressure. 4 Because the generated cells compensate for the shrinkage of injec- tion-molded parts during cooling, structural foams typically have outstanding geometric accuracy. The advantages of foam injection molding include the ab- sence of sink marks on the part surface, reduced weight, low back pressure, a faster production cycle time, and a high stiffness-to-weight ratio. 5–7 Because of this unique set of advantages, low-pressure, pre- plasticating-type structural foam molding technology has been widely used for manufacturing large prod- ucts requiring geometric accuracy. There are, however, a number of drawbacks to this technology. First, the cell number density (or cell density) of structural foams is typically very low (i.e., <10 3 cells/cm 3 ), the cell size is very large (i.e., >1 mm), and the cell size distribution is very non- uniform. Structural foams also have a low-quality surface, a very low void fraction, and poor mechani- cal properties (due to the large gas pockets). The processing conditions and the product quality are inconsistent as well. This article introduces an advanced structural foam molding technology, based on a preplasticat- ing-type system, that eliminates these drawbacks. This technology facilitates the uniform dispersion/ dissolution of gas in the polymer melt during the structural foam molding process, thereby minimizing the creation of large undissolved gas pockets. PROBLEM STATEMENT In today’s commonly used low-pressure structural foam molding technology using a low-pressure, pre- plasticating-type system (a so-called piggyback sys- tem), the amount of injected gas (typically N 2 ) is normally beyond the local or global solubility limit. Because of the overdosed gas content and the pres- sure fluctuations during each cycle, the injected gas cannot completely dissolve into the polymer. Furthermore, an inconsistent amount of gas is injected into the polymer melt because of the non- steady nature of the existing molding process. A shutoff valve is typically used between the plasticat- ing extrusion barrel and the plunger to prevent the reverse flow from the plunger to the extrusion bar- rel. However, this shutoff valve cannot completely decouple the functions of the extrusion barrel and the plunger. Because the valve needs to be shut off Correspondence to: C. B. Park (park@mie.utoronto.ca). Journal of Applied Polymer Science, Vol. 109, 2855–2861 (2008) V V C 2008 Wiley Periodicals, Inc.