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MCHE 3920 Final > Injection Molding > Flashcards

Injection Molding Flashcards

1
Q

Which manufacturing method is described below?
High Form Freedom
High Production Rate
Great Functional Integration
Wide Range of material properties
Can be made in different Colors/Transparency, texture and with exotic plastics that are cool to the touch, have particular smells and shield electromagnetic fields

A

Injection Molding

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2
Q

Given the market for injection molded parts, it is not
uncommon for molds for seemingly similar parts to vary in
cost by a factor of ______________

A

5 or more

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3
Q

Exact cost of injection mold will depend on _______________ as well as _________and ___________

A

desired quality, part size, part complexity

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4
Q

Thermoplastics, (or plastics) are _______ which are large molecules comprised of many repeated subunits called __________

A

Polymers, monomers

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5
Q

In plastics, Monomers are typically _______________ which are compounds consisting of_________ and ________

A

Hydrocarbons, hydrogen and carbon

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6
Q

Molecular chains that tangle together
are held by ________________forces which are relatively ________ forces between molecules

A

Van der Waals, weak

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7
Q

Van der Waal forces cause plastic stiffness to typically be ____________

A

Low

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8
Q

The common plastic ABS has a young’s modulus of _______ GPa

A

1-2

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9
Q

To tear or rupture plastics, it is necessary
to break stronger _________ bonds

A

Covalent

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10
Q

What is the base strength and common strain to failure for plastic?

A

40 MPa, 50%

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11
Q

The building blocks of plastics are called
____________ and can be simple chains of _______ with two hydrogen atoms attached or can be complex chains with hydrogen, chlorine and side groups of other hydrocarbons

A

monomers, carbon

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12
Q

Monomers ________ need to consist of only carbon

A

Do not

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13
Q

Polyamides (nylons) have __________ in their monomer chains

A

nitrogen

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14
Q

Chain chemistry will have a strong effect on
__________________

A

material properties

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15
Q

Additives can greatly change material properties such as:
o Pigments for ___________
o Flame retardants
o Fillers
o Glass fibers for _____________
o Plasticizers for _______________

A

Color, strength and stiffness, faster production

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16
Q

What are the two main categories of plastics

A

Amorphous plastics & Semi-crystalline

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17
Q

_________________have a randomly ordered molecular
structure and do not have a set melting point. Instead, they soften slowly with increasing temperature

A

Amorphous plastics

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18
Q

_______________ plastics have a highly ordered molecular
structure and sharp melting points

A

Semi-crystalline

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19
Q

When amorphous plastic is heated up slowly, it experiences a
dramatic loss of stiffness at the ______________________

A

glass transition temperature

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20
Q

At the glass transition temperature, molecules have the freedom to _________ along each other.

A

Slide

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21
Q

At the softening temperature, the material is ___________ and
the concept of stiffness no longer applies

A

Melted

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22
Q

In ______________ plastics, the glass transition temperature is less pronounced and loses ___________ at the melting temp, beyond which it can be injection molded.

A

Semi-crystalline, strength

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23
Q

Melted plastics have a relatively ________ viscosity with typical ranges of ___________ Pa.s

A

high, 10-100

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24
Q

What are the viscosities of molasses and water?

A

5 & 0.001 Pa.s

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25
Q

High viscosity and narrow mold sections result in a need to force material into the mold under ______ pressures

A

High

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26
Q

Viscosity _________ slightly with increasing temperature, but
increasing temperature too much can lead to ______________

A

decreases, material degradation

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27
Q

Most plastics are injection molded at temperatures around _____°C

A

100

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28
Q

Consider a flat straight channel where length and width are
much greater than its thickness, d.
A _____________ will develop over the thickness with ______ velocity at the walls and _________ velocity in the middle

A

Velocity profile, zero, maximum

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29
Q

The velocity gradient, will be _________ at the walls
and _______ in the middle. This gradient is called the ___________ (units s-1)

A

Maximum, zero, strain-rate

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30
Q

Real polymers behave in a very ______________way. Specifically, their viscosity drops with _________ shear strain-rate called “_______________” which can cause the Newtonian pressure drop formula to be very inaccurate.

A

Non-Newtonian, increasing, shear thinning

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31
Q

The longer the polymer chain (i.e., the larger the molecular
weight), the ___________ viscosity a material will have at a
given shear strain-rate

A

higher

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32
Q

Injection molders use a metric called the _____________________
to estimate the rate at which a material can be processed

A

Melt Flow Index (MFI)

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33
Q

For low shear-strain rates, MFI is essentially the
inverse of viscosity so ______ viscosity = _____ MFI = __________ processing

A

low, high, faster

34
Q

MFI typically varies from _____ (g/min) to _______ (g/min)

A

3-30

35
Q

As in casting, the two halves of the die are pressed together
along the ____________

A

Parting Plane

36
Q

The ____________ clamping force is the pressure difference
times the _____________of the cavity into the parting plane

A

minimum, projected area

37
Q

In practice, a somewhat _________ force is used to reduce
the amount of flash formation

A

higher

38
Q

In injection molding you want to at least estimate the clamping force because the higher the clamping force, the _________ the injection molding machine will need to be

A

Larger

39
Q

As in high pressure die casting, the cavity is filled through a
number of narrow openings called __________

A

gates

40
Q

Because gates are ________ they lead to high shear strain-rate,
causing the viscosity to be _________, which is advantageous

A

narrow, lower

41
Q

When the flow through a gate shoots straight into the
cavity it’s called “___________”

A

Jetting

42
Q

Jetting produces ____________in the product and ______
material and can be prevented by aiming the flow against the wall of the cavity

A

flow patterns, brittle

43
Q

_____________ is often the limiting factor in injection molding, so
its usually good practice to inject at _______ points to minimize
flow length, L

A

Pressure, multiple

44
Q

In practice, want L/d to be below _____ for thicknesses
around ______ mm and below _______ for thickness
above ___ mm

A

100, .5, 400, 2

45
Q

Once product is sufficiently cooled, it is ejected from the mold
with __________

A

ejection pins

46
Q

Ejection Pins are _________ with cavity and pop up when mold opens which leaves visible marks on product.

A

Flush

47
Q

The best area to place ejection pin is on an ________________, if possible

A

interior surface

48
Q

As the mold fills, it is important that the flow
front expands _____________ which forces air out of the cavity and
prevents _____________

A

Gradually (Think: pouring pancake mix onto a pan), air bubbles

49
Q

Typical injection times are on the order of a ___________

A

second

50
Q

For smooth filling, want to place gates so melt flows from ________ to _________

A

thick, thin

51
Q

If the product contains a hole, the melt will
flow around the hole and merge beyond it which creates a “__________”.

A

knit line

52
Q

Cooling accounts for approximately ___________ of the total cycle
time

A

50-70%

53
Q

Tp ,Td , and Te are the _______________ temperature,
__________________temperature and the ___________
temperature

A

Melt Processing, mold wall processing, ejection

54
Q

Bottom line: The ________ the product the easier injection will be, but the longer the cooling time

A

Thicker

55
Q

Even though injection molding temperatures are _________
compared to metals, the thermal expansion of plastics is
________ to ____________ times that of metals so product shrinkage during cooling is a concern

A

low, 5, 10

56
Q

Shrinkage is handled in two primary ways:
1. Apply __________ pressure to squeeze in more material
during injection.
2. Make mold ______________. This is commonly done but requires complex simulations since calculating shrinkage of a product can be very ____________ for plastics

A

high, bigger, complex

57
Q

Injection molds are typically made out of _________, but
____________ molds are sometimes used as well

A

tool steel, aluminium

58
Q

The mold cavity often receives some sort of _______________ and molds are typically designed for about ___________ “shots”

A

surface treatment, 1 milion

59
Q

Molds are classified in three main groups:
o __________ molds
o __________ molds
o ____________ molds

A

Two-plate, Three-plate, Hot runner

60
Q

_______________ molds are simple, but injection system stays
connected to the product and can only be removed after cooling

A

Two-plate

61
Q

______________ molds can separate from product before cooling

A

Three-plate

62
Q

In ____________________ molds, the injection system remains hot, so there are no separation concerns

A

Hot runner

63
Q

A key design consideration for injection molded parts is the use of _________ angles to promote ejection, which are typically around ________ degrees

A

draft, 3

64
Q

The parting plane will be visible as a surface defect, so it is important to pay attention to it’s _________________

A

placement

65
Q

For high volume production, single cavities are too slow so it is common to use ____________molds which can have up to ___________ cavities in a single mold

A

Multi-cavity, 64

66
Q

_________ allow the mold to move in directions other than the
parting plane helps with ejection when it is not possible to get proper draft angles

A

Slides

67
Q

___________ distribute force of ejector pin across a
larger area to prevent damaging the part

A

Stripper rings

68
Q

Mold venting: The displaced air during injection is released
through _______________

A

small vent holes

69
Q

Mold cooling: molds typically have straight channels for
__________ cooling

A

water

70
Q

What method of injection molding is described below?
o Molten plastic is injected into a cavity (or cavities),
cooled, and ejected.
o About 80% of all injection molded products are made
using this method
o Invented in the 1920s but production took off in
1950s
o Machines are classified by tonnage (i.e., the clamping force
they can produce
o Machines range from 25 tons to 2500 tons of
clamping force

A

Compact (Standard) Injection Molding

71
Q

About 80% of all injection molded products are made using this method

A

Compact (Standard) Injection Molding

72
Q

Compact (Standard) Injection Molding was Invented in the ________ but production took off in________. The Machines are classified by ________ (i.e., the clamping force they can produce ranges from ________ tons to ________ tons

A

1920, 1950s, tonnage, 25, 2,500

73
Q

In _______________ Molding, special parts are placed ________ the cavity prior to
molding and the during injection the melt flows around these parts. This allows for the integration of other materials into the product.

A

Insert, inside

74
Q

If the inserted part is larger than the plastic part, it is called an
“________”

A

Outsert

75
Q

In ___________ Molding, a gas (often ________) is injected into the melt after the mold is partially filled creating ____________ in the part.

A

Gas, Nitrogen, Internal cavities

76
Q

In Gas Molding it’s Difficult to control placement of gas bubble, so method is only used for ________ shapes like handles and pipes

A

Simple

77
Q

__________________ allows for a thin decorative skin to be placed
inside the mold which then attaches to the melt

A

In-Mold Decoration (IMD)

78
Q

2k Injection Molding Allows for more than ________ shot of
different plastics into the same mold creating an integrated product with different combinations of ________ and/or colors.

A

one, materials

79
Q

In 2k Injection molding, the different materials can be joined
tightly, allowed to hinge, or be ___________ together. 2K (two constituents) is most common, but ________ products have been demonstrated

A

sandwiched, 7k

80
Q

2k Injection molding has ________ investment, but is a very versatile process

A

Higher

81
Q

Holes create “____” or “_____” knit lines and multi-point injection creates “_______” knit lines

A

warm, hot, cold

MCHE 3920 Final (10 decks)

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