Molding - Manufacturing and Industrial Engineering Interview Questions

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Top Mechanical Interview questions and answers on Molding with explanation for Manufacturing and Industrial interview, competitive exams and entrance test.

1. What  is the  important characteristics of plastics?         

It can be molded into finished product by application of heat and pressure. 

2. What are the advantages of plastics over metals?
  • Low weight 
  • Corrosion resistance 
  • Insulation properties
  • Electrical properties
  • Cheaper 
  • Easy to handle 
  • Surface properties 
  • Reusable
 3. Explain the disadvantages of plastics? 
  • Low strength 
  • Low heat resistance
  • Poor mechanical properties
  • Difficulty to repair
4. How to classify the plastics?
  • Natural : milk of rubber tree, cellulose
  • Semi synthetic
  • Synthetic
5. Types of plastics based on chemical behavior?
  • Thermo plastics 
  • Thermosetting plastics
6. Difference between thermo set and thermo plastics?
 Thermoplastics:
  • Can be re softened and reused
  • No chemical changes during heating.
  • In granular form
  • Needle structure 
  • Hard but not brittle
Thermo set plastics:
  • Cannot be re softened and reused
  • Chemical changes during heating etc.
  • In powder form
  • Cross linked structure 
  • Hard and brittle 
7. Give Example of thermoplastics

ABS, PP, CA, PMMA, PS, PC, POM etc.

8. Perspex is the trade name of ---- ?

 CA

10. Perspex is the trade name of ---- ?

CA

11. Nylon is the trade name of ---- ?

 PA

12. Delrin is the trade name of ---- ?

POM

13. Teflone is the trade name of ……….?

PTFE

14. What  is MFI?

Flow capacity of different grades of thermoplastics are inversely proportional to molecular weight.

15. About ABS? 

Shrinkage 0.5%. It is heat resistant, provides good insulation and exhibit electrical properties. Examples are general purpose safety helmets, pipe fittings, television, radio etc.

16. About  CA ?

Shrinkage 0.3-0.8%. Its main properties are transparency and surface texture. Examples are Tool handle, goggles, TV screens.

17. About HDPE?

Shrinkage 3-4%. Main features of HDFE are high density, rigidity, strength, hardness and chemical resistance. Examples are milk bottle crates, fish boxes etc.

18. About LDPE?

Shrinkage 0.8-1.5%. It is crystalline and not available in transparent form. Examples are shopping baskets, waste baskets etc.

19. About PA?

 Shrinkage 0.7-1.5%. It is tough and high rigid. Examples are light duty gears, sprockets, bearings etc.

20.About PC?

Shrinkage 0.5-0.7%. It is known optical transparency, rigidity and toughness. Examples are goggles, lenses, safety helmets etc.

21.About PP?

Shrinkage 1-3%. It is flammable and degradable in sun light and stable. Examples are air cleaner, garden furniture etc.

22.About PS?

Shrinkage 0.2-0.8%. It is clear, brittle and it can be easily molded. Examples are toys, house hold appliances, electronic housings etc.

23.About PVC?

Shrinkage (FLEXI 1.5-3%,RIGID 0.2-0.4%). It is rigid and not very tough and very low cost. Example - Pipes fittings.

24. Examples for thermosetting materials ?

Alkyds, epoxies, pf, mf, urea, polyester

25. What  is fillers and additives?

Adding of small molecules to plastics to get some characteristics (color, flexibility etc.). Fillers are commonly used with thermosetting plastics. Additives are of two types,

  • Physical means such as plasticizer
  • Chemical means--- stabilizer.                      

26. What  are the mold release agents?

  1. External with mold surface e.g. Polyvinyl alcohol 
  2. Internal with resins eg. Silicon oil 

27. What  are the types of injection molding machines?

  • Plunger injection cylinder 
  • Two stage plunger injection cylinder 
  • Pre plasticizer two stage screw injection cylinder 
  • Reciprocating screw injection cylinder

28. What  are the divided sections of screw of injection molding machines ?

  • Feed zone 
  • Compression zone 
  • Melting zone  

29. What  do you meant by nozzle ?

Nozzle connected to the end of the barrel through which soften materials inserted in the mold. 

 30. What  are the types of nozzle ?

  • Reverse taper (melt valve)- for PA, ABS etc.
  • Removable tip
  • Standard of general purpose 

30. What are the types of nozzle ?

  • Reverse taper (melt valve) - for PA,ABS etc.
  • Removable tip
  • Standard of general purpose

31. Define drooling ?

Leakage of plastic material through the nozzle in between shots.
32. What is injection capacity or short capacity?

Maximum volume material injected by the screw during one cycle of operation.

33. What is plasticizing capacity ?

It is the amount of material that can be processed by the machine per hour. It is expressed in kg/h.

34. What is injection pressure?

It is the maximum pressure by which the material is injected through the nozzle. It is given in kg/cm2.

35. What is Injection rate or Injection velocity ?

It is the maximum rate at which the screw can inject or shoot materials from the barrel during one shot.

36. What do you meant by clamping force?

It is the maximum force that the clamping system can exert on the mold or it is the maximum force by which the mold halves can be closed together. It is given in ton or kilo Newton.

37. What is maximum daylight?

It is the maximum distance that the machine platen can be separated from each other and it can be obtained by adding the maximum mold thickness to the maximum opening stroke.

38. What is a mold ?

It is a custom build tool in which we converts plastic raw material into finished product.

39. What are the main elements of mold ?

The main parts of molds are core and cavity.

40. What are the general types of mold ?

Injection molds, compression molds, transfer molds.

41. What  are the types of injection mold ?

  • Two plate mold 
  • Three plate mold 
  • Hot runner mold 
  • Insulated runner mold 
  • Hot manifold mold 
  • Stacked mold 

42. What  is two plate mold ?

Mold which consists of core and cavity situated in plates. It is logical type tool where component require large gate. For simple type components there is only one daylight.

43. What  is 3 plate mold?

It consists feed plates with core and cavity.

44. What  is hot runner mold?

In this, runner kept hot to keep the molten metal into fluid state also called runner less mold. In this, runner contained in a plate of its own runner section of the mold is not opened during molding cycle.

45. Note down the advantages of runner mold ?

  • No molded side products 
  • No separating of gate
  • Cycle time can be reduced

46. What  is insulated runner mold ?

 It is a variation of hot runner mold in this type of molding. The outer surface of the material in the runner acts as a insulator.

47. What is hot manifold mold ?

This is a variation of the heated hot runner and not the runner plate. This is done using electric cartridge.

48. What do you meant by stacked mold? 

A stacked mold is a multiple two plate mold with mold placed one over the other. A stacked mold construction doubles the output from a single molding machine and requires the same clamping force.

49. Explain about injection molding?

 In this process, the plastic material is injected in to the mold through a sprue bush by means of a screw plunger. This process can be used for both thermosetting and thermoplastic materials.

50. What  is compression molding?

In this process, the plastic material is placed in the cavity and use a force for compressing the compound as the mold closes, these molds are generally used for thermosetting materials. 

51. What  is transfer mold?

In this process, the plastic materials is transferred from a transfer pot and then forced in to the cavity by means of plunger. This method is used for molding thermosetting materials only.

52. Write a short note about blow molding ?

Blow molding is very much like compression molding because the blow mold generally closes on a hollow shape that has been deposited in between the halves of the mold. Air or gas pressure is introduced at the center. This internal pressure causes a flow of the heated material in to intimate contact with the relatively cold mold sections. Here the plastic material solidifies and is subsequently ejected.

54. Explain vacuum molding?

The mold used for this process is similar that of the female half of the compression or blow mold auxiliary equipment on the machine heats the material and drags it over the cavity as indicated by the precise technique chosen.

55.Elaborate about rotational molding?

The machine provides the means of holding the mold and rotating it about two axis at the same time. This rotation causes the powdered material that was loaded in to the mold before it was clamped in to the machine to solidify and cling to the wall of the mold.

56. What  is impression ?

Gap between core and cavity is called impression. Impression provides the shape of the component.

57. What  do you meant  cavity?

Female portion of the mold and it gives external form.

58. What  is core ?

Male portion of the mold and it gives internal form.

59. What  is sprue bush?

Connecting member between register ring and runner.

60. What  is register ring ?

It is used to align the mold with machine platen.


61. What  is runner?

Connecting member between gate and sprue bush.
62. What  is gate?

Connecting member between impression and runner.

63. How can we calculate the cycle time of mold?
Cycle time Tc=m*3600/P
 Where ,Tc =minimum cycle time obtainable 
 M=mass of the shot 

P=plasticizing capacity of the machine with the particular polymer being molded (kg/h)

64. What  is parting surface?

The surface where top and bottom half closed together and prevents the loss of plastic material from the impression.

65. What  are the types of parting surface?

 Flat and non flat

66. Name the types of non flat parting surfaces?

 Straight, stepped and angular.

67. What  do you meant flash ?

 If the parting surfaces are not properly matched, the molding material from the impression will escape through the gap. This escaped melt is called the flash.

68. Why do we provide vent in mold?

Vents are provided in the mold to allow such air to escape freely. The vent is a shallow slot not more than 0.05 mm deep and 3mm wide. If the depth is more, the plastic material can pass through the slot and leave a flash mark.

69. What  are the types of runner ?

  • Fully round
  • Rectangular
  • Hexagonal
  • Trapezoidal
  • Modified trapezoidal 

70. Explain briefly about sprue gate ?

When the molding is directly fed from a sprue or secondary sprue, the feed section is term as sprue gate.The main disadvantage with this type of gate is that it leaves a large gate mark on the molding. This sprue gate is used only for single impression molds.

71. What  is rectangular gate?

This is the general purpose gate and it is a rectangular channel machined in on mold plate to connect the runner to the impression.

72. Write a short note about fan gate?

Fan gate can be considered as the other type of edge gate but it does not have a constant width and depth. The fan shape spreads the flow of the melt as it enters the impression and a more uniform filling is obtained with less flow marks and surface finish.

73. What  is tab gate?

This is particular gating technique for feeding solid block types moldings. This is mainly used to avoid undesirable jetting on the molded part.

74. What  is overlap gate ?

 It can be considered as a variation of basic rectangular edge gate and is used to feed certain type of molding. In this type of gate we have a better control over the flow rate. Main disadvantage is that it leaves big remnant mark after degiating.

75. What  is diaphragm gate ?

This gate is used for single impression tubular shaped moldings on two plate molds.

76. What  is ring gate?

The function of this gate is identical to that of a diaphragm gate. This type of gate is used for tabular type moldings when more than one impression is required in a simple two mold.

77. What  is film gate?

This is a long rectangular type edge gate and it is used for large thin walled components to assist in the production of warpage free products.

78. What  is pin gate?

Used for 3 plate mold and under feed mold. Small degating mark will be there.

79. What  is round edge gate?

This gate is formed by machining a matching semi circular channel in both mold plates between the runner and impression.

80. What is submerged gate?

Automated degating with less mark also called as sub surface gate or submarine gate.

81. What  is winkle gate?

This is a curved variation of the subsurface gate and for that reason it is sometimes called a “curved subsurface gate” or “curved tunnel gate”.

82.What  are the types of injection techniques ?

  1. Pin injection 
  2. Sleeve injection
  3. Stripper bar injection
  4. Blade injection
  5. Valve injection
  6. Air injection
  7. Stripper ring injection
  8. Stripper plate injection

83. What  is Pin injection ?

This is the most common and simple type of injection. The molding is ejected by the application of a force by ejector pins.

84. What  do you meant by sleeve injection ?

In this method, the molding is ejected by means of a hallow ejector pin which is called sleeve.

85. What  is stripper bar injection?

This method of injection is suitable for thin wall box type moldings because of the effective injection area obtainable are greater.

86. What  is blade injection ?

The main purpose of blade ejector is for the injection of very slender parts such as ribs and other projections which cannot be satisfactorily ejected by the standard type of ejector pin.

87. What  is valve injection ?

A valve ejector is basically a large diameter ejector pin. It is used for the injection of large components.

88. What  is air injection ?

This is the simplest to install. It is easy to connect to the fixed or moving half. The medium of actuation is compressed air.

89. Explain briefly stripper ring injection?

The stripper ring is a local stripper plate and it is used on molds with one or two impression only. The stripper ring is made to seat in the mold plate.

90. What  is stripper plate injection?

This injection technique is used mainly for the injection of circular box type moldings and for moldings with thin wall sections.


91. Name the methods used for actuation of stripper plate?

  1. Tie rod actuation
  2. Length bolt actuation
  3. Chain actuation
  4. Direct actuation
92. Define shrinkage ? 

When a hot plastic cools inside a mold, it contracts by an amount depending on the material being processed and the final product is smaller than the mold size is called shrinkage.

93. What  are the factors which affect the increase in shrinkage ?
  • Increase of material temperature.
  • Increase of mold temperature.
  • Increase in wall thickness.
  • Low injection pressure .
94. What  are the factors which effect the decrease in shrinkage ?
  • Low melt and mold temperature
  • High injection pressure 
  • Long injection time
  • Presence of filler materials and its content
95. What  is undercut molding?

A molding which has a recess or projection is termed as an undercut molding

96. What do you mean external undercut?

Any recess or projection on the outside surface of the component which prevents its removal from cavity is termed an external undercut

97. Name the methods of split actuation? 

  • Finger cam actuation
  • Dog leg cam actuation 
  • Cam track actuation
  • Spring actuation
  • Hydraulic actuation

98. How can we calculate finger cam length?

Finger cam length =(M/Sin Q) +(2c) l Sin 2Q

Where 

 M= split movement

 Q=angle of finger cam 

 L=working length 

 C =clearance 

99. Where we use side core and side cavity ?

A side core is a local core which is normally mounted at right angled to the mold axis for forming a hole or recess in the side face of a molding. The side cavity performs a similar function to the side core, in that it permits the molding of components which are not in line of draw. This element allows the components with a projection or projections on one or more of their side faces.

100. Where we use form pin straight action method ?

For the components which has internal undercut on one side of the wall.

101. Where we use form pin straight action method ?

We use form pin straight action method for the components which has internal undercut throughout the wall

102. What  is single daylight mold ?

When a basic mold is opened, there is only one space or “daylight “ between the two mold halves. This is called a single daylight mold.

103. Explain about double daylight stripper plate mold ?

A stripper plate mold consists of a fixed plate, a moving mold plate and a stripper plate. There are two day light when opened. This is called double daylight stripper plate mold.

104. What  is underfeeding mold?

An underfeeding mold is the one in which the feed system is arranged to feed into the underside of the component.

105. What  are the types of injection molding defects ?

Sinkmark, Streaks, Blisters/Bubbles, Weld line, Ejector marks, Cold weld, Jetting, Warpage

106.Define sink mark ?

Sink marks are the localized contractions or the depressions on the surface of the moldings.

107. What  is blister or bubbles ?

 Hollows created on or in the molded part.

108. What  is weld line ?

It is the witness marks of two or more melt flow fronts at their joining zone.

109.What  is jetting ?

It is the prominent in-homogeneous snake like strands on the surface of molding.

110.What is warpage ?

 Warpage is the deviation of the mold part from its required shape.

111.What  is bulk factor?

Bulk factor is defined as the ratio of the volume of the loose powder to the volume of the molded part.

112. What  are the types of compression molds?

  • Flash mold
  • Fully positive mold
  • Semi positive mold
  • Landed positive mold

113. Write a short note about performs ?

Performs are small pellets or tablets of molding material that are formed to shape in a special performs mold at room temperature. Curing won't takes place only densification.

114. Name the types of transfer molds ?

  • Pot transfer mold
  • Plunger transfer mold 

115. What  is die casting?

 Pressure die casting is the process in which the molten metal is forced under high pressure into a cavity in a metal disc with in fraction of a second and allowed to solidify.

116. What  are the types of die casting ?

  • Hot chamber process
  • Cold chamber process

117.Write the differences between hot chamber and cold chamber processes ?

Hot chamber processCold chamber process
The plunger and cylinder are submerged in the molten metal in the holding furnace.  Molten metal is ladled into the cold chamber. 
After shot, plunger is forced the molten metal through the nozzle. After ladling, the plunger advanced to force the metal in to the die.
Since both plunger & goose neck are submerged in the molten metal, it refills automatically when the plunger is withdrawn. Each time, we have to ladle the molten material in to the chamber.
For casting metals that melt at lower temperature such as zinc and lead. For casting metals that melts at high temperature such as aluminum, magnesium  and brass.

118. What  are the types of  die casting defects ?

  • Cold defects 
  • Hot defects
  • Miscellaneous defects

119. Example for cold defects ?

Lack of fill, cold shut, severe chill, chill, flow lines

120. Examples for hot defects ?

 Soldering, cracks, broken part, bent part, heat marks or shrinkage pits.