Microcellular injection molding processing

Neat PLA, PLA/cellulose fiber composites (10-30 wt%), and PLA/carbon nanotube nanocomposites were successfully foamed in a microcellular injection molding process using nitrogen gas [3, 76]. All foamed samples exhibited a sandwich-type structure, with solid walls encapsulating a foamed... 

The crystallization behavior of PA6 and its nanocomposites undergoing a microcellular injection molding process was studied using TEM, XRD, SEM, and PLM the addition of polarized light microscopy was important to study crystal formation [492]. A synthesis approach to make PP nanocomposites by in situ polymerization was studied by OM, TEM, and... 

In this study, we investigate the effeets of POE content on the mechanical and thermal properties of PP and HDPE, using the microcellular injection molding process as the blending process. 

L. S. Tumg, Microcellular Injection Molding Process , SPE-ANTEC Tech. Papers p. 686-690. (2003). 

Figure 9.7. TEM Images of (a) Solid PLA-MWCNT Nanocomposite at 500 nm (Inset-a magnified view of agglomerated MWCNTs at 100 nm) (b) Microcellular PLA-MWCNT Nanocomposite at 500 nm (arrow indicates micro cell formed during microcellular injection-molding) (c) Solid PLA-MWCNT Nanocomposite at 100 nm and (d) Microcellular PLA-MWCNT Nanocomposite at 100 nm. Reprinted with permission from S. Pilla et al.. International Polymer Processing, XXII, p. 418,2007, 2007, Polymer Processing Society.

Table 2. Comparison of Injection-Molding Process for Various Products with and without Microcellular Structure ...

As with microcellular extrusion, the standard injection molding process parameters such as melt temperature and cooling temperatures also affect the cell sizes and the level of weight reduction achieved as well as the other typical characteristics, such as shrinkage. The fill rates and pressure levels that develop in the injection process have an additional major effect on microcellular molded parts as compared to standard molding in that they affect the cell structure. As described in the original research, the rate of pressure drop and subsequently the number of nucle-ation sites will determine how many cells are formed and how uniform they will be. 

Melt temperature and fill rates have been demonstrated to be very important process variables for microcellular injection molding. Recent work has studied the use of fast response thermocouples along with traditional pressure transducers to determine their effectiveness in providing practical process monitoring tools for the microcellular molding process [7]. Behind the ejector pin, pressure transducers and fast... 

Berry M, Kishbaugh L. Process monitoring and control for microcellular injection molding. SPE ANTEC 2010. 

Xu J., "Effect of Injection Molding Process Parameters on the Morphology and Quality of Microcellular Foams ", TH ITEC, SPE, 2770-2774 (2006). 

In response, microcellular injection molding was developed[3] and commercialized by Trexel Co. Ltd. as the Mucell process. The key insight of this process is the application of a supercritical fluid. The supercritical fluid is injected during the injection stage cycle, creating millions of micron-sized voids in otherwise solid thermoplastic polymer parts. 

In this study, we have investigated the efifeets of process parameters on the variation of the rim thickness (Fig. 3) by conventional and microcellular injection molding. The specification of the rim thickness is 0.02mm. If the rim thickness tolerance is out of specification, there is a running noise from the roller, and the textile roller should be replaced after a certain time. 

Although there are 4 cavities in one mold, only one cavity ( 1 in Fig. 5) was taken for the measurement in order to get consistent data. The rim thickness was the average of the five samples. The experiment was carried out by short shot first and showed that the runner system has unbalanced melt flow (Fig. 5). Some portions are filled whereas some are only partially filled. The injection molding process was done by the conventional method first, and then the microcellular process was introduced as the foaming molding method. Fig. 6 shows the rim thickness of the roller by conventional molding. The thickness variation is more than 0.10 mm, and points 4 5 have the least thickness. 

Journal of Injection Molding Technology 5,No.3, Sept.2001,p.l52-9 MICROCELLULAR FOAM PROCESSING IN RECIPROCATING-SCREW INJECTION MOLDING MACHINES Jingyi Xu Pierick D Trexel Inc. 

Thermoplastic polyurethanes are designed to be processable by standard plastic machinery such as extruders and injection molding machines. Thermoplastic polyurethanes have been used in biomedical applications. They also can be used in the microcellular form where the apparent density can be reduced. Some applications include tubing, handles, automotive parts, and shoe soles and heels. 

This book describes the chemistry, manufacture and use of the wide range of flexible polyurethane foams, from low-density open-cell upholstery foams to microcellular and reaction- injection-molded and reinforeed-reaction-injection-molded materials. The related effects of varying the raw-material chemistry and the production process and machinery on the properties and service performance of the final product are indicated. 

J. Xu and D. Pierick, Microcellular foam processing in reciprocating-screw injection molding machines, Journal of Injection Molding Technology, 5, 152-159 (2001). 

Microcellular foams can be produced by noncontinuous processes such as a batch process [2, 12, 15, 16, 31, 32, 34, 35], continuous processes such as extrusion and injection molding [24,33,36,37], orby asemicontinuousprocess [38]. Since the semicontinuous process is not extensively used in the scientific community or in the industry, it will not be described in this chapter. Readers are encouraged to refer Ref. 38 for detailed information on this process. To date, microcellular foams have been produced in amorphous polymers [12, 31, 32, 34], semicrystalline polymers [35], and in elastomers [16]. Recently, MCF structures have also been produced in plastics filled with inorganic nanoparticles (montmorillonite) [39-43], as well as organic cellulosic fiber filled plastic composites [12, 31, 32, 34]. 

Compared with the batch foaming process, relatively few publications feature continuous microcellular foaming of PLA in extrusion or injection molding equipment [3, 5, 73-79]. 

Typical processing methods - extrusion, extrusion coating, injection molding, microcellular foaming, spinning ... 

While the focus of this chapter is on injection molding, microcellular extrusion is also briefly discussed—primarily in the context of the historical development of tiie technology. Common traits are discussed as well as how the two processes differ, relative to the most important processing parameters that apply to each version of the microcellular process. 

There are some common traits of all foam injection molding, including structural foam molding and microcellular molding. The most important common trait may be that all of the processes are short-shot processes. With standard solid injection molding, the cavity is filled nearly completely with the screw movement under velocity control. At some point, just short of compete fill, the screw movement control is switched to pressure-based control. This is done because the material has already started to shrink and material must be forced into the cavity to compensate for this shrinkage. 

While virtually every thermoplastic material can be foamed, the specific characteristics that can be attained with microcellular processing (extrusion or injection molding) are very much dependent on the type of material, type of SCF, and the product design. 

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