This set of Composite Materials Multiple Choice Questions & Answers (MCQs) focuses on “Compression Moulding – Set 2”.
1. As the temperature of SMC charge increases in the mold, the resin viscosity also increases.
a) True
b) False
View Answer
Explanation: With the increase in temperature of the SMC charge the network structure formed as a result of thickening reaction with magnesium oxide(MgO) breaks which leads to the increase in resin viscosity. The resin viscosity is required to be increased since it avoids the premature gelation within the mold which can result in voids and interlaminar cracks in the composite casting.
2. __________ is the result of small internal voids or surface pits caused by the entrapment of air or other gases during compression molding.
a) Blowholes
b) Cold shuts
c) Misrun
d) Porosity
View Answer
Explanation: Porosity caused during the compression molding results in poor appearance and surface finish of the casting. It can also affect the performance of the part.
3. Which is not a reason for the introduction of air into SMC?
a) Inefficient wetting, at the fiber–resin interface
b) Compaction between carrier films in the SMC
c) The mechanical blending of liquid resin in the resin plate
d) Preheating of SMC
View Answer
Explanation: Air gets entrapped in SMC at various stages of the compression molding process like the mechanical blending of liquid resin, compaction between the carrier films in the SMC etc. However, the air volumes entrapped in the SMC can be expelled by providing proper venting system in the mold.
4. _________ are interlaminar cracks formed at the end of molding due to excessive gas pressure in the interior region of the molded part.
a) Protrusion
b) Microporosity
c) Blisters
d) Gas holes
View Answer
Explanation: Blisters are formed as a result of the high internal gas pressure developed from unreacted styrene monomer during molding in undercured parts and from the entrapped air in between the charge layers. It can also appear because of some post-molding process such as high-temperature baking in an oven resulting from the expansion of the air entrapped in the charge. The blister normally appears as a dome-shaped bulging near the surface.
5. Which of the following practice won’t help in reducing blisters?
a) Minimize the entrapped air
b) Using coupling agents
c) Reducing contamination between layers
d) Increasing the molding temperature
View Answer
Explanation: Blisters can be controlled by increasing the radial flow of entrapped air by stacking more charges in a small area instead of stacking fewer charges over a large area. Another way is to increase the interlaminar shear strength which can be achieved by the usage of coupling agents, decreasing the mold temperature, ensuring proper cure and reducing the layer space contamination.
6. _________are linear domains of aligned fiber orientation and are formed at the joining of two divided flow fronts.
a) Knit lines
b) Stream lines
c) White lines
d) Fiber lines
View Answer
Explanation: Fibers tend to align themselves along the knit because of the multiple charge plies used in the compression molding. It will result in the reduction in the strength of the part in the perpendicular direction. By the proper placement of charge plies, the development of knit lines can be reduced.
7. Non-uniform cure is critical in thick-section moldings.
a) True
b) False
View Answer
Explanation: The non-uniform cure develop a gradient of properties in the direction of thickness. By the result of non-uniform curing, the interlaminar shear strength gets reduced due to undercuring of the interior part since the curing process is initiated from the surface and progressively moves to the interior part.
8. __________ are small surface depressions normally observed above the ribs in compression-molded SMC parts.
a) Gas porosity
b) Sink marks
c) Blowholes
d) Cold shuts
View Answer
Explanation: Sink marks are formed as a result of the differential cooling rates. It can be reduced by changing the geometry of the part, reducing packing pressure by optimized packing etc.
9. ___________is the process in which top mold is retracted by a small amount and the liquid paint is injected over the top surface.
a) Mold coating
b) Anti-mold coating
c) Nano mold coating
d) In-mold coating
View Answer
Explanation: In-mold coating is the process of covering or shielding the surface defects with less viscous paint or coating. It is done just after the cure cycle is completed.
10. Warpage is critical in thin-section moldings.
a) True
b) False
View Answer
Explanation: Warpage is caused due to differential cooling rates between sections of different fiber orientations and thickness. The strength of the molded part is reduced due to high residual stresses resulting from unbalanced heat dissipation.
Sanfoundry Global Education & Learning Series – Composite Materials.
To practice all areas of Composite Materials, here is complete set of 1000+ Multiple Choice Questions and Answers.
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