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DDM 2 > Production Oriented Product Design > Flashcards

Production Oriented Product Design Flashcards

1
Q

What are the types polymers?

A

Thermoplastic

Thermoset

Elastomers

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2
Q

Generically the different cross-linking between the macromolecules determines the classification of polymers

A

Thermoplastics: No cross-linking

Thermosets: Intensive cross-linking

Elastomers: low-degree of cross-linking

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3
Q

Thermoplastics are…

A

soft and melt when heated and harden when cooled. Because of this behavior, these resins can be injection molded, extruded or formed via other molding techniques

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4
Q

The most important Thermoplastics and respective applications are:

A

Polyethylene (PE): bottles, cans, packaging materials

Acrylics: Plexiglas® lenses, window glazing

Fluorocarbons: Teflon® nonstick coatings, bearings, seals

Polyamides: Nylon®, Kevlar® fibers, engineering polymers

Polycarbonates: Lexan helmets, windows, wind-shields

Polyesters: Dacron, Mylar gears, connectors, rollers

Polyvinyl chloride (PVC): pipes, cable insulation, flooring

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5
Q

Unlike thermoplastics, thermosets…

A

harden after processing into an infusible solid and stay hard when heated again. So no repeated heating cycle is possible.

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6
Q

The most important thermosets and respective applications are:

A

Phenolics (Bakelite®): knobs, handles, cases

Unsaturated Polyesters: fiber-reinforced materials

Epoxies: fiber-reinforced materials

Silicones: waterproof and heat resistance materials

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7
Q

Thermosets form…

A

cross links, inter-connections between neighboring polymer molecules that limit chain movement. This network of polymer chains tends to degrade, rather than soften, when exposed to excessive heat.

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8
Q

Elastomers show…

A

extreme elastic extensibility under low mechanical stresses.

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9
Q

The most important Elastomers are:

A

Natural rubber (Latex): tires, shoes, seals

Silicones: seals, thermal insulation, electronics

Polyurethane: seals, gaskets, jogging shoes

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10
Q

The process of chemically bonding monomer building blocks to form large molecules, can occur by different reactions

A

Addition Polymerization

Condensation Polymerization

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11
Q

In addition polymerization…

A

a chain reaction adds new monomer units to the growing polymer molecule one at a time. Each new unit added creates an active site for the next attachment.

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12
Q

In condensation polymerization…

A

the reaction between monomer units or chain end groups releases a small molecule, often water.

Epoxy is a common example for condensation polymerization, where a resin and a hardener are combined

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13
Q

The C-C bond angle at atomic scale between one carbon atom and the next one is 109°, but there is random freedom at the scale above atomic bonding, how the molecules arrange. As a consequence…

A

macromolecule are not straight, but form rand three-dimensional messes (like a plate of spaghetti)

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14
Q

Crystallinity is…

A

the degree of order within the polymer’s overall macromolecular structure.When the polymer chains are folded over on each other in on ordered way, than a polymer is crystalline.

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15
Q

According to their crystallinity, thermoplastics can be further classified into two sub-segments:

A

Amorphous thermoplastics

Semi-crystalline thermoplastics

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16
Q

Amorphous polymers

A

Amorphous polymers, ones with little or no crystallinity, have random chain entanglements and lack a discrete melting point.

As they are exposed to heat, these polymers soften and become more fluid-like, allowing the polymer chains to slide past one another. As the polymer cools, chain movement diminishes, and the polymer’s viscosity increases.

Generally, the higher a polymer’s glass transition temperature, the better it will perform at elevated temperatures.

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17
Q

Semi-crystalline polymers

A

Semi-crystalline polymers have both amorphous and crystalline regions, they exhibit both a glass transition temperature, and a crystalline melt temperature.

Due to the molecular structure, some polymers crystallize quickly and reach high levels of crystallinity. Others require longer times in a hot mold to crystallize. The degree of crystallinity depends upon both the polymer and the processing technique.

Crystalline thermoplastics must be heated above the resin’s crystalline-melt temperature for extrusion and injection molding.

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18
Q

Why is the stress-strain curve for polymers not linear?

A

because of the zig zagging polymers having to slip past one another

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19
Q

A polymer’s mechanical behavior can be like a…

A

spring - energy- (impulsive) elastic chain deformation

spring and damper - entropic elastic deformation of a chain segment (mesh deformation)

damper - molecule creep

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20
Q

Why are polymers impregnated with fibers?

A

In order to increase the plastic’s strength and resilience.

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21
Q

What are the different types of polymers’ viscosities?

A

Thermoplastic - high viscosity

Thermoset - low viscosity

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22
Q

Which polymer type is better for fiber impregnation and why?

A

Thermoset because it has a lower viscosity allowing it to flow around the fibers easily

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23
Q

Compare the relationship between force and velocity for water and for polymers

A

The relationship is linear for water (important for water cutting) and for polymers the relationship is not linear (important for injection moulding)

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24
Q

Viscosity curves vs. time in the production process

A

Thermoset - Process time is determined by the reaction kinetics, time intensive (chemical cross-linking, not reversible)

Thermoplastics - Process time is determined by the heating / cooling capacity, fast (no cross-linking, reversibly plasticizable)

25
Different procedures for the fiber reinforcing process
Direct procedures - The procedural combination of fiber and matrix to a component takes place in-situ during the actual manufacturing process(hand lay-up, resin injection). Indirect procedures - The procedural combination of fiber and matrix takes place in advance via a semi-finished product stage (prepreg/autoclave, SMC/BMC, injection, granules).
26
Manufacturing technologies for fiber-plastic composites
Thermoset - prepreg/autoclave, SMC (sheet moulding compound)/BMC (bulk moulding compound), hand lay-up, resin injection, fiber spraying, pultrusion, winding, hot press Thermoplastic - extrusion, injection, diaphragm technique, cold press
27
Efficient component manufacturing criteria
Component design: geometry, complexity, breakthroughs, undercuts, inserts, stiffeners, fiber content, fiber orientation Quality: surface finish, dimensional accuracy, process safety, reproducibility, residual stresses Time: cycle time, process steps, preparation, post processing Cost: total cost, material cost, investment cost, plant cost, mould costs, salary costs, maintenance costs Ecology: ecological damage, power consumption, offcuts, recycling, waste, emissions
28
The most important process in the polymer industry
Injection moulding
29
What is the danger of adding too many monomers too quickly into a mixture?
The temperature will rise in the mixture due to the energy released from the covalent bonds forming between free monomers. Too much heat in the mixture can reach the destruction temp of the polymer. Called a runaway mixture.
30
Pros of injection moulding
Cyclic production process Highly complex polymer parts can be produced Part weights from 1 μg to 100 kg are possible 1.5 s to 15 min cycle time depending on part size
31
Injection molding versus extrusion
Injection moulding - cyclic process, high pressures, moulds are very costly, tight tolerances on parts Extrusion - continuous process, moderate pressures, dies/moulds are expensive, part dimensions may vary
32
Two main units of an injection moulding machine
Closing unit and plastification unit
33
Types of closing units and their advantages
Direct driven (force fit, hydraulic driven) - Easy to adapt mould height, easy to set up the closing force, very low wear Toggle (form fit, electric driven) - Fast opening and closing possible, energy is only for closing and opening necessary, high closing forces possible, only closing force in elongated position
34
Parts of an injection moulding machine
mould, backflow valve, screw, plastification cylinder with heating, machine nozzle, external water/oil mould temperature control
35
Injection moulding cycle
injection holding pressure dosing remove plastification open mould part ejection close mould apply plastification
36
Up to what pressure is used in injection moulding
2500 bar
37
Ideal pressure trend in the mould – cavity pressure
start injection melt reaches the pressure sensor cavity is filled (toggle point) volume changes during cooling, compensated by holding pressure polymer is freezed reaching atmosphere pressure – start of shrinkage
38
Benefit of needle valve nozzle
needs no movement of the plastification
39
The injection mold forms the molten plastic into the desired shape and:
provides the surface texture determines the dimensions of the finished molded article In facilitating mold-cavity filling and cooling the mold also influences the molding cycle and efficiency as well as the internal stress levels end-use performance of the molded part.
40
What does the success of any moulding job depend on?
The design and construction of the mould
41
What is a precision instrument and yet must be rugged enough to withstand hundreds of thousands of high-pressure molding cycles.
An injection mould
42
How common is a two plate mould and what does it consist of?
It is the most common mould configuration and has two mould halves that open along one parting line
43
The optimum runner design requires a balance between
Ease of filling Mould design feasibility Runner volume
44
Too wide of a runner can... and too thin of a runner can....
Lengthen cycle time needlessly and increase costs associated with regrind Cause excessive filling pressures and related processing problems
45
What is the the optimum runner cross-section and what are the pros?
Round cross-section runners minimize contact with the mold surface and generate the smallest percentage of frozen layer cross-sectional area. As runner designs deviate from round, they become less efficient.
46
How should runners for multi-cavity moulds be designed?
Runners for multi-cavity molds should be designed so that all parts finish filling at the same time. This reduces overpacking and/or flash formation in the cavities that fill first, leading to less shrinkage variation and fewer part-quality problems.
47
Why is the gate shape important when using fiber-impregnated plastics in injection moulding?
A properly shaped gate ensures that the fibers are mostly parallel throughout the mould. This is because the plastic starts hardening as soon as it touches the mould surface
48
Is the temperature of the polymer part different than the temperature set in the temp control system?
Yes, because of the different rates of heat transfer in the mould and polymer part
49
How is the temperature of the mould normally maintained?
With cooling fluid flowing through paths inside the mould
50
Why do some polymers need to be pre-dried? Name an example
When some polymers (like Nylon) form from monomers an H20 molecule is produced
51
What is the purpose of a gate in injection moulding?
Gates connect the runner to the part and provide a reduced thickness area for easier separation of the part from the runner system. }In designing the feed system, the first point to be considered is the wall thickness (t) of the molded part.
52
What can have a direct impact on part moldability, performance, appearance, and cost when it comes to injection moulding?
Gate position. The location of the gate determines the filling pattern and maximum material flow length.
53
For unreinforced plastics the gate would be positioned...
to balance filling and minimize flow length, typically near the center of the part or at strategic intervals for multi-gated parts. Often these best gate locations for filling are unacceptable for other reasons. For example, they might result in unsightly gate marks or weld lines in cosmetic areas, or increase mold construction costs.
54
Flow orientation in injection moulding affects what aspect?
Part shrinkage in the mold. Flow orientation has a large effect on fiber-filled plastics, which typically exhibit two or three times as much shrinkage in the cross-flow direction.
55
Why should the temperature of the mould be carefully controlled? What can differences in cooling rates do?
Optimizing mold temperature control promotes improved part quality and cost savings. Improper cooling can introduce elevated levels of thermal and shrinkage stresses resulting from cooling-rate variations throughout the part. Differences in cooling rate cause areas to shrink and solidify at different rates and by different amounts. In parts made of semi-crystalline resins such as Polyamide, POM or PBT, the cooling rate affects the degree of crystallization and shrinkage.
56
Mold temperature has influence on...
Shrinkage / Warpage / Post-shrinkage Surface Filling Mechanical properties Cycle time
57
Most engineering polymers require the moisture in the granules to be....
below a certain maximum level for processing in order to avoid hydrolytically degradation. Simple hot air driers of various types are not adequate for drying polyesters, for example, but dehumidified-air dryer systems are acceptable. Only these can provide the necessary constant and adequate drying, whatever the ambient climatic conditions may be. Apart from keeping the correct drying temperature, it is important to ensure that the dew point of the drying air remains lower than –20°C
58

DDM 2 (5 decks)

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