This set of Composite Materials Multiple Choice Questions & Answers (MCQs) focuses on “Compression Moulding”.
1. ______________ is used for transforming sheet-molding compounds (SMC) into finished products in matched molds.
a) Pultrusion
b) Vacuum bag molding
c) Filament winding
d) Compression molding
View Answer
Explanation: Compression molding is the molding process in which a polymer is preheated and placed in an open mold cavity. A top plug is used to close the mold and compressed so that the material spread all areas of the mold. Simplicity in process, cost-effectiveness, little waste of material etc., are some of the advantages of compression molding.
2. The principal advantage of compression molding is its ability to produce parts of complex geometry in a short period of time.
a) True
b) False
View Answer
Explanation: The main advantage of compression molding is its capability to produce complex geometric parts in short time periods. Ribs, holes, nonuniform thickness, flanges, bosses, and shoulders etc, can be incorporated with the compression-molding process. As a result, it helps in possibly eliminating many secondary finishing operations like forming, drilling and welding,
3. The compression molding process is suitable for the high-volume production of composite parts.
a) True
b) False
View Answer
Explanation: The compression-molding process is suitable for the high-volume production of composite parts since the whole molding process such as mold set up, placement of Sheet Molding Compound in the mold, and removal of the part from the mold, can be automated. Compression molding is used as the primary method for manufacturing many automotive components like bumpers, leaf springs and road wheels.
4. The compression-molding process begins with the placement SMC, usually, a stack of several rectangular plies called _________
a) charge
b) block
c) layers
d) sheet
View Answer
Explanation: The compression-molding operation starts with the placement of a previously cut and weighed amount of SMC, commonly a stack of several rectangular plies called charge, onto the preheated mold cavity bottom half. The ply dimensions are chosen so that it covers 60%–70% of the surface area of mold. The mold is suddenly closed after the placement of charge and the pressure on the charge is made to increase to a preset level by lowering the top half of the mold at a constant rate.
5. The SMC material in the mold starts to flow and fill the cavity with increasing pressure.
a) True
b) False
View Answer
Explanation: The flow of the material is essential to expel the air entrapped in the charge as well as in the mold. Depending on the fiber content, length of flow and part complexity, the molding pressure may vary from 200–5000 psi. High pressures are needed for molding parts that include deep ribs and bosses.
6. The mold is opened and the part is removed, with the aid of _________ after curing under pressure.
a) lever
b) handle
c) ejector pins
d) button
View Answer
Explanation: Ejector pins are important parts of the compression molding process as well as some other processes. The function of ejector pins is to exert a force to eject or push out the solidified part from the mold.
7. High molding temperatures in thick parts is essential.
a) True
b) False
View Answer
Explanation: High molding temperatures in thick parts should be avoided. It is because higher peak exotherm temperature may cause chemical degradation and burning in the resin.
8. What helps to reduce the peak exotherm temperature?
a) Decreasing the resin content
b) Increasing the resin content
c) Decreasing the filler content in SMC
d) Increasing the filler content in SMC
View Answer
Explanation: The fillers can play an important role in reducing the cure cycle since the increase in the filler content in SMC formulations decreases the peak exotherm temperature. Increased filler content replaces part of the resin and thereby decreases the total quantity of heat liberated. The time to achieve peak exotherm, which increases linearly with the thickness of the part, is also decreased with increasing filler content. So, it acts as a heat sink remaining within the material.
9. Which is not an efficient way to reduce the cure time in the mold?
a) To preheat the charge to pregel temperatures outside the mold
b) To finish curing with high mold-closing speeds inside the mold
c) To Increase the filler content
d) To Increase the resin content
View Answer
Explanation: On preheating the charge to pregel temperatures outside the mold, the temperature increases quickly and uniformly throughout the charge. The rapid mold- closing speed is desirable since it avoids the chance of premature gelation and generates the uniform flow pattern disregarding the charge thickness. Increasing the filler content also reduces the cure time as it reduces the peak exotherm temperature.
10. Preheating in compression molding is commonly accomplished by ___________
a) lighter
b) furnace
c) dielectric heater
d) hearth
View Answer
Explanation: The dielectric heaters increase the temperature quickly and uniformly throughout the charge volume on preheating. An alternating current generated by the dielectric heater is subjected to the charge which results in heating.
11. 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.
12. __________ 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.
13. 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.
14. _________ 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.
15. 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.
16. _________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.
17. 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.
18. __________ 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.
19. ___________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.
20. 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.
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