Chapter 2: Molding, Extrusion, Calendering

Question 1

What is molding?

Answer 1

Molding is the process of converting plastic resins, powder, or pellets into useful products using some level of applied force.

Question 2

What are the main molding techniques?

Answer 2

The main techniques include compression molding, injection molding, transfer molding, rotational molding, and reaction injection molding.

Question 3

What is compression molding?

Answer 3

Compression molding involves placing plastic material in a mold cavity, which is then heated and subjected to pressure to form a product, typically used for thermoset compounds.

Question 4

What are the advantages of compression molding?

Answer 4

It generates less waste, is lower in cost, and can be automated or hand-operated. It also results in less product stress and mold erosion.

Question 5

What are the disadvantages of compression molding?

Answer 5

It is challenging to mold complex parts, fine details like inserts and injector pins can be damaged, and molding cycles may be long. Defective parts cannot be reprocessed.

Question 6

What are some products made with compression molding?

Answer 6

Examples include cookware knobs, electric plugs and sockets, automotive parts, tray (Mostly simple design)

Question 7

What is the basic process of compression molding?

Answer 7

The process includes cleaning and applying mold release, loading the preform into the cavity, closing the mold, applying heat and pressure, opening the mold to release the part, and finally cooling the part in a fixture.

Question 8

Why is venting necessary in compression molding?

Answer 8

Venting is required to release by-products like gas and water that may form during the molding process.

Question 9

What is crosslinking, and why is it important?

Answer 9

Crosslinking is a process involving heat and catalysts during molding, leading to chemical bonding that strengthens the polymer.

Question 10

What is transfer molding?

Answer 10

Transfer molding is similar to compression molding but uses a pre-heated plastic material loaded into a pot. A hydraulic plunger pushes the material into a mold cavity through a channel (sprue).

Question 11

What are the advantages of transfer molding?

Answer 11

It allows molding of complex parts with less mold erosion, less flash, and shorter mold and loading times compared to compression molding.

Question 12

What are the disadvantages of transfer molding?

Answer 12

It produces more waste from sprues and requires more costly equipment and molds.

Question 13

What is the process of transfer molding?

Answer 13

The uncured compound is pre-heated and placed in a transfer pot. A plunger pushes the material through sprues into the pre-heated mold cavity. Once cured or cooled, the mold is opened, and the part is released using ejector pins, followed by trimming excess material.

Question 14

What are the key differences between compression and transfer molding?

Answer 14

Transfer molding can produce more complex shapes, has less mold erosion, and shorter cycle times but generates more waste and is costlier.

Question 15

What are common problems in both compression and transfer molding?

Answer 15

Issues include cracks around inserts, blistering, short or porous molding, burned marks, mold sticking, warping, and orange peel surface.

Question 16

What are the four operations in the rotational moulding process?

Answer 16

• Place a pre-determined amount of polymer powder in the mould.
• Close, lock, and load the mould into the oven.
• Heat the mould while rotating it around two axes until the powder melts and adheres to the mould walls.
• Cool the mould by air, water, or both, and then remove the solidified product.

Question 17

What are the advantages of rotational moulding?

Answer 17

• Economically produced large products.
• Stress-free products.
• No polymer weld lines.
• Comparatively low mould costs.

Question 18

What are the disadvantages of rotational moulding?

Answer 18

• Long manufacturing times.
• Limited choice of moulding materials.
• Relatively high material costs.
• Difficulty in moulding specific geometrical features like ribs.

Question 19

What is the basic design feature of an injection moulding machine?

Answer 19

It consists of a reciprocating screw for heating and converting pellets, and the granules melt using heat from the barrel and frictional heat.

Question 20

Describe the injection moulding process.

Answer 20

• Granular put in to the screw and heated.
• Granules melt quickly due to external heat and frictional heat from screw.
• 3 zones screw: Feed (deep), transition(Depth narrowing, create friction), Metering(constant depth).
• The hot mass is injected into a closed mould cavity through non return valve.
• After cooling, the plastic part is removed.

Question 21

What are the advantages of injection moulding?

Answer 21

• High operations rates.
• Ability to use fillers and inserts.
  -Suitable for small or complex parts.
• Multiple materials can be injected into a mould.
  -Parts require little or no finishing.
• Thermoplastic scrap can be reused.

Question 22

What are some disadvantages of injection moulding?

Answer 22

High initial setup costs.
Requires a high number of parts for cost-effectiveness.
Process complexity.
Poor design can increase the scrap rate.

Question 23

What materials are commonly used in injection moulding?

Answer 23

Polypropylene (PP), Polycarbonate (PC), Acrylonitrile-Butadiene-Styrene (ABS), and Nylon 6 (N6).

Question 24

What is the extrusion process used for?

Answer 24

It produces continuous lengths of plastic with a constant cross-section, such as hosepipes and curtain tracks.

Question 25

What are the zones in a screw used in extrusion?

Answer 25

Feed zone: Greatest channel depth.
Transition zone: Decreasing channel depth.
Metering zone: Constant channel depth.

Question 26

What is filament extrusion?

Answer 26

A process where plastics are forced through small orifices in a die called a spinneret, forming single or multiple strands of plastic.

Question 27

What are common problems in injection moulding?

Answer 27

Bubbles. (Dry granule)
Short moldings. (Not enough material, make sure mold & material hv same T to not shrink)
Flashing. (Material too hot, due to high pressure)
Warped parts.
Sink marks.

Question 28

How does rotational moulding differ from injection moulding?

Answer 28

Rotational moulding is slower but suitable for large, hollow products with low stress.
Injection moulding is faster, capable of producing small and complex parts, but involves higher initial costs.

Question 29

What are the three types of spinning processes described?

Answer 29

Melt spinning, solvent spinning, and wet spinning.

Question 30

How does melt spinning work?

Answer 30

Plastics like PE, PS, and PVP are melted and forced out of a spinneret, then hit the air to solidify and undergo conditioning.

Question 31

What is solvent spinning?

Answer 31

Plastics are dissolved in solvents, forced through a spinneret into hot air, and the solvent is collected separately.

Question 32

Describe wet spinning.

Answer 32

Plastics are dissolved in solvents and extruded through a spinneret into a coagulating bath, forming a plastic gel that solidifies into filaments.

Question 33

What is the purpose of drawing in spinning?

Answer 33

It aligns molecular chains closer and parallel to the filament axis, increasing the strength of the filament.

Question 34

What is the basic process of blown-film extrusion?

Answer 34

Molten material is extruded through a die, shaped into a tube, expanded by blowing air to the desired thickness, cooled, and flattened with nip rollers.

Question 35

What factors affect the thickness of the finished film in blown-film extrusion?

Answer 35

• The volume and pressure of air determine the bubble size and film thickness.
• The pulling rate of the bubble determines the film’s thickness.

Question 36

How is extrusion coating performed?

Answer 36

A thin film of molten plastic is applied directly onto a substrate (e.g., paper, fabric, plastics) and pressed between rollers without adhesive.

Question 37

Why are substrates sometimes preheated and primed?

Answer 37

To promote adhesion and improve the bonding of the molten plastic to the substrate.

Question 38

What is the purpose of heating wires or cables before coating them with plastic?

Answer 38

To ensure better adhesion of the plastic coating.

Question 39

What is calendering?

Answer 39

A process where thermoplastic materials are squeezed to final thickness between heated rollers, forming sheets or films with specific finishes.

Question 40

What products use calendered films?

Answer 40

Textiles, rubber-like apparel, handbags, shoes, and luggage.

Question 41

What are the advantages of calendering?

Answer 41

Wide range of surface finishes and textures.
Can laminate fabric or other materials.
Produces continuous lengths of film.
Easy cleaning for colored or embossed films.

Question 42

What are the disadvantages of calendering?

Answer 42

Large and costly machines.
Limited product width and thickness.
Requires compensation for roller deflection.