Voids molded parts

Sufficient time must be allowed to cool the molded part under pressure to 90°C to prevent vacuum voids and distortion. 

If the resin were insufficiently dried, the moisture present could cause voids within the molded part. This happens when the water evaporates and leaves open spaces. One effect of voids can be a lower bulk density. Bulk density was measured by the displacement method in accordance with ASTM D792 [7], and was found to be substantially equivalent for the two samples, as shown in Table 12. 

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)...

Through a compression and decompression action the density of the mold contents is increased, and the more intimate contact with the mold walls improves the heat reduction. The surface of the molded part is substantially improved and voids as well as sink marks disappear. The improved dissipation of the heat reduces the cycle time up to 15% depending on the polymer. 

Voids in Part Injection Pressure too Low Excessive Injection Rate Short Injection Time Low Melt Temperature Low Mold Temperature Cooling Rate too Low Gate Size too Small Incorrect Pad Size... 

The total absence of volatile generation during cure serves to explain the reproducible and facile preparation of void-free cmolded parts to thermal shock. 

KINEL molding powders and KERIMID 601 leuninating resins dre based on a thermosetting polyimide, which cures without evolution of volatiles. KINEL compounds are transformed by compression, transfer, or extrusion molding into void-free parts, whose thermomechanical properties exceed those attainable from conventional thermoplastics or thermosets. 

Although this process is inexpensive and easy to perform, the molded parts are not of the highest quality and may not completely have the properties expected. The laminates produced by this method contain more resin than those produced by other methods, and more voids may be present. Little control can be exercised over the uniformity of the wall thickness. Typical cure time for polyesters is about 10 h at room temperature. Epoxies are used in special applications. They can be cured in as little as 3 h at 82°C. Hand layup is very labor intensive, but it is an excellent molding process when large sizes and a few units are needed. 

X-ray inspection Radiography (X-ray) inspection can be used to detect voids or discontinuities in molded parts, adhesive bonds, iaminates, composites and panel constructions. This method is more expensive and requires more skilled experience than ultrasonic methods. The product must contain some metal powder or other suitable fdler to create enough contrast to make the defects visible. This method is applicable to honeycomb sandwich structures as well as metal and nomnetal joints. 

Figure 2 Voids in molded part for the fan gate and end gated molds...

Usually a process simulation can be used to design the injection and vent locations with the input parameters of geometry of the mold and the permeability of the fabric. The vents are usually placed at the locations where the resin arrives last so voids can be prevented. This will produce void-free parts if these conditions are replicated from one part to the next. However, material placement and variability will change the permeability of the fabric in certain locations from one part to the next altering the resin flow pattern which will not ensure that the resin will arrive at the vents for all the parts. Hence it is important for the designer to anticipate disturbances in the flow due to material placement and variability and develop optimization and control approaches to address them. On the next page we identify various micro and macro issues that may cause variation in the flow pattern. 

Typical specimens that can be polished and observed optically include composites of polymers and hard fillers, tough polymers, molded parts and extrudates. The distribution of the fillers, such as glass or graphite fibers or nuneral particles, can be observed by reflected light. The shape of the fiber cross section, the presence of voids and the overall structure of the material can be observed by this method. Figures 4.3 and... 

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