Lecture 7: Plastic

Question 1

What is the goal of most processes for plastics?

Answer 1

Most processes aim to achieve net-shape or near-net-shape production, completing the part in one process to minimize costs.

Question 2

Define polymer.

Answer 2

A polymer is an extremely large molecule composed of a repetitive pattern of an unspecified number of monomers.

Question 3

Why are additives added to polymers?

Answer 3

Additives enhance various properties of plastics, such as flexibility, color, resistance to UV radiation, and flame retardancy.

Question 4

What are composites?

Answer 4

Composites are materials made by combining chemically distinct and insoluble phases with a recognizable interface.

Question 5

Name three types of fibers commonly used in composites.

Answer 5

Glass fiber, Aramids (e.g., Twaron, Kevlar), and Carbon fiber are commonly used fibers in composite materials.

Question 6

describe Extrusion

Answer 6

shaping viscous plastics by pushing
them through mold/die
a. Can only be done with thermoplastics and elastomers
i. Not with thermosetting plastics
because we need viscous material, so
we need to heat it up, which is
impossible with thermosetting plastics

Question 7

What is injection molding?

Answer 7

Injection molding is a process of filling a mold with high pressure (70-200 MPa) to allow viscous material to fill every part of the mold.

Question 8

What are 4 advantages of injection molding?

Answer 8

It is used in mass production, offers high accuracy, excellent surface finish, and allows for the creation of complex products in one go.

Question 9

What is the primary disadvantage of injection molding?

Answer 9

The complexity and cost of the dies required, as defects in the die can be replicated on the product surface, necessitating high-quality, expensive dies.

Question 10

What types of materials are commonly used in injection molding?

Answer 10

Thermoplastics and elastomers are heated until viscous and then injected into the mold. Thermosetting plastics undergo a chemical reaction inside the mold, making them irreversible.

Question 11

What is rotational molding?

Answer 11

Rotational molding involves pouring material into a mold and rotating it to coat the walls, allowing for the creation of large product geometries and hollow products.

Question 12

What are some 3 drawbacks of rotational molding?

Answer 12

It results in considerable variance in wall thickness, lower quality surface finish, and is a slower process suited for small batches.

Question 13

what are some advantages of rotational molding and material used

Answer 13

1.Large product geometries possible
2.Dies are fairly cheap
3.We can make hollow products ➔ they are fairly light
We have to heat up the material so it becomes viscous
and sticks to the wall ➔ we use thermoplastics, not
thermosetting plastics

Question 14

Describe the process of extrusion blow-molding.

Answer 14

In extrusion blow-molding, a tube of plastic is extruded, and a blow pin blows air into the tube to expand it and assume the shape of the mold.

Question 15

What are the characteristics of extrusion blow-molding materials?

Answer 15

The material is heated to become viscous, allowing it to deform and expand to fill the mold. This process is suitable for thermoplastics due to their ability to be reheated.
the bigger the diameter of the mold, the thinner the walls will be
* example – water bottles have thin walls except for their necks and bottoms which are pretty sturdy
c. fairly cheap process ➔ mass production

Question 16

What is injection blow-molding?

Answer 16

Injection blow-molding involves creating a small shape using an injection molding machine, then placing it into a larger mold and expanding it until it touches the die.

Question 17

What advantage does injection blow-molding offer in terms of wall thickness?

Answer 17

It allows for the creation of different wall thicknesses, resulting in less variation between the wall thickness and bottoms and necks of bottles.

Question 18

How does the precision of injection blow-molding compare to other methods?

Answer 18

The process is more precise but also more expensive compared to other molding methods.

Question 19

What is calendaring?

Answer 19

Calendaring is the process of pressing material together with rolls to create foil, film, or sheet.

Question 20

What is thermoforming?

Answer 20

Thermoforming involves heating a plastic sheet, pushing the die up to create a vacuum, and then trimming the sheet to create a usable product.

Question 21

What types of materials are used as the base material in thermoforming?

Answer 21

Thermoplastics in the form of foil or sheet are commonly used as the base material for thermoforming.

Question 22

How is heating achieved in thermoforming?

Answer 22

Heating can be done using an oven or infrared heat sources to soften the plastic sheet for forming.

Question 23

What are some characteristics of molds used in thermoforming?

Answer 23

Molds used in thermoforming are relatively cheap, require low forces, and allow for varying wall thicknesses in the final product.