Molding defects

Molding defects can occur during the injection molding of plastic parts. Some common molding defects and possible causes are listed in Table A.6 (Belofsky 1995b). 

Blister Raised or layered zone on surface of the part Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater 

Bum marks Black or brown burnt areas on the part located at furthest Air entrapment in mold due to lack of venting 

Color streaks Localized change of color in plastic part Masterbatch was not mixed properly or the LID of the screw is not high enough 

Delamination Thin mica-hke layers formed in part wall Contamination of plastic resin 

Here we introduce some commonly seen molding defects in injection molded short glass fiber reinforced parts. Solutions for other defects not described in this chapter can be found in Rubin (1972).  

As a polymer melt is injected from a small size gate or runner into a large size cavity, the phenomenon of jetting or irregular flow can be observed.  

75 Microscopically the fiber exposure phenomenon in molded composite parts. 

16 Schematic of the formation of injection molded weldlines. (Reproduced from reference 79.) 

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Presser, m. presser compressor pressman. Press-erzeugnis, n. pressings, -fehler, m. molding defect, -filter, n. pressure filter, press filter, -fliissigkeit, /. pressure fluid, -form, /. press(ure) mold, compression mold, -gas, n. compressed gas. -glanz, m. gloss from... 

Special methods of incorporation drying is only necessary to prevent molding defects because the polymer has very high resistance to hydrolysis... 

The complexity of the injection molding process, and the interdependence of the many variables involved, means that any molding defect may have several different causes, of which more than one may be present at any given time. Table 10.14 shows a list of the defects that one may encounter in injection molding. 

A silver coating chemically precipitated in the laboratory helps render surface molding defects visible (mirror surface). 

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

Relative humidity (1820) (RH) n. The ratio, always expressed in percent, of the quantity of water vapor present in the ambient atmosphere to the quantity that would saturate it at the prevailing temperature. It is also the ratio of the partial pressure of water vapor present to the vapor pressure of water at the prevailing temperature. High relative humidity, which occurs in summer in many manufacturing-plant atmospheres, can cause condensation of water on chilled surfaces such as injection molds, with attendant mold defects. It can also cause blushing problems in the painting of plastics and metals. 

Key words injection molding, processes and variants, mechanical properties, fiber orientation, molding defects and corrections. 

Molding defects. Figure 1 indicates molding defects during injection molding... 

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. 

However, when low profile additives were applied to these new chemically thickened formulations rather than the BMC and preform unthickened systems, problems were observed. The reaction of the unsaturated polyesters carboxyl groups with the magnesium oxide changed the compatibility between the resin and low profile additive. This caused a separation of the components which resulted in an exudate on the surface of the SMC. Not only did this complicate the compound handling but caused molding defects such as scumming, sticking and porosity. 

The foaming reaction mixture must fill the entire cavity. All air, present in the cavity before the beginning of the filling stage, has to be displaced by the foam, so that the part will not show any molding defects, even in the area of vents. 

Figure 15-6-4. Molding defects molding ripples and poor material mixing.

The processing requirements for RIM are very stringent (a) the reactive streams must be effectively mixed by impingement (b) the system must not produce molding defects due to turbulence or advance polymerization (c) completion or near-completion of the polymerization must be achieved in an economically acceptable time and without excessive exotherms and (d) the reaction must be free of undesirable by-products. To date, very few polymerization systems have been used commercially with RIM. Polyurethanes account for over 90% of RIM materials produced. Other RIM systems include the anionic polymerization of caprolactam to produce nylon-6, and the ring opening polymerization of dicyclopen tadiene. Fillers are often incorporated into RIM formulations to lower raw material costs and to improve certain properties such as stiffness. 

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). 

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