Injection molding defects

I 3 Compounding and Processing of Plastic/Rubber Biends Table 3.7 Trouble shooting injection molding defects. 

Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. They occur when melt flow fronts collide in a mold cavity. Material characteristics can affect the knitting of the melt fracture. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of part strength. 

Zhang, J.G., Edirisinghe, M.J. and Evans, J.R.G., A Catalogue of Ceramic Injection Molding Defects and their Causes. Industrial Ceramics 9 72-82 (1989). 

What product defects would an injection molding operator observe if the following parameters were set incorrectly ... 

Several injection-molded polycarbonate (PC) disks used in a consumer product (i.e., a clock face) were analyzed, because the disks contained defects. The defects were analyzed initially and imaged using a stereoscopic microscope fitted with a digital camera. The defects present on the samples were easily identified. It was noticed that the defects were located on different regions of the molded part. This important observation eliminated a systematic problem or an imperfection at a specific site on the mold surface as the cause of the defect. 

A new GPPS resin was tested at a customer s injection-molding plant as an Improvement over an incumbent resin manufactured by a competitor. The new resin performed well in the process except that it created parts with a 5% rejection rate due to a splay defect. The competitive resin was reported to run well but with a lower... 

Many of the contamination defects that typically occur in extrusion processes can also occur in injection-molded parts. The most obvious defects are caused by foreign material contamination, resin degradation, and surface defects known as splay. The next sections provide case studies where contamination caused defects in injection-molded parts. 

Figure 11.27 Photographs for surface defects in an ABS injection-molded part a) surface photograph, and b) cross-sectional view showing a void just under the surface (photographs courtesy of James T. Seliskar of The Dow Chemical Company)...

Several case studies are presented in the next sections that show some common root causes of contamination in injection-molded parts. In these case studies, the problem is presented in a manner that the troubleshooter would encounter during a trial or information-gathering session. In each case study, the modifications required to fix the process are detailed along with supporting fundamental information. Two of the case studies used (ET) screws to eliminate the defects. ET screws and other high-performance screws will be discussed in Chapter 14. 

Figure 11.35 Photomicrograph of the silver-colored defect in a clear PS injection-molded packaging part. The flow direction was from the upper left to the lower right...

The modified screw was placed back into the injection-molding press and evaluated for performance. The barrel temperatures were maintained at 245, 255, 260, and 260 °C for the feed zone through the last barrel zone, respectively. This temperature setting was lower than that used for the original screw. The screw was rotated at a speed of 235 rpm, and the back pressure was set so that the pressure at the tip was 10 MPa. The 0.244 kg part and runner system was plasticated in 4.2 s for a specific rate of 0.89 kg/(h-rpm). All parts produced were completely free of the splay defect. The modifications were able to eliminate the bubbles and the unmelted material. 

Salamon, B.A., Koppi, K.A., and Little, J., Halo Surface Defects on Injection-Molded Parts, SPE ANTEC Tech. Papers, 44, 515 (1998)... 

The process provides fast molding cycles, unlimited shelf life for the sheet, large part capability, and design flexibility. The process also allows for scrap materials to be recycled. Trim waste from the molding operation and defective parts can be ground up and recycled into the basic sheet process in controlled amounts. Some of this waste has also been used as input for injection molding. 

Fig. 13.21 Schematic representation of the surface defects that appear periodically and alternate in location on opposite sides of a flat mold. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabilities in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos, and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

M. C. O. Chang, On the Study of Surface Defects in the Injection Molding of Rubber-modified Thermoplastics, SPE ANTEC Tech. Papers, 40, 360-367 (1994). 

Injection molding can be used with some thermosets in addition to thermoplastics, as long as the process can be controlled such that the crosslinking or curing takes place in the mold and not in the extruder barrel. It has been used effectively, for example, with thermosetting polyester resins. Scrap runners or defective parts must be discarded, however, as they cannot be remelted. 

Flow defects, especially as they affect the appearance of a product, play an important role in many processes. Defects can be identified and corrected.3 143 These flow analyses can be related to other processes and even to the rather complex flow of injection molding. 

In the 2P process (4), a thin photopolymerizable liquid layer is cured in situ between a smooth substrate (cut from a polymer sheet or molded) and a master and stripped off to effect pattern transfer. The liquid layer ensures accurate, defect-free replication. Examples of solvent-free formulations include multifunctional acrylates with adhesion, wetting, and release agents. Free radical and cationic polymerization processes have been described (9). At the present disk volume levels, this method is claimed to be cost competitive with injection molding. 

---