Injection Molding Flashcards by Robby Ratajczak

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

Injection Molding

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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 ______________

5 or more

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

desired quality, part size, part complexity

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

Polymers, monomers

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

Hydrocarbons, hydrogen and carbon

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

Van der Waals, weak

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Van der Waal forces cause plastic stiffness to typically be ____________

Low

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

1-2

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

Covalent

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

40 MPa, 50%

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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

monomers, carbon

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Monomers ________ need to consist of only carbon

Do not

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Polyamides (nylons) have __________ in their monomer chains

nitrogen

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Chain chemistry will have a strong effect on
__________________

material properties

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

Color, strength and stiffness, faster production

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What are the two main categories of plastics

Amorphous plastics & Semi-crystalline

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

Amorphous plastics

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

Semi-crystalline

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

glass transition temperature

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

Slide

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

Melted

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

Semi-crystalline, strength

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

high, 10-100

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What are the viscosities of molasses and water?

5 & 0.001 Pa.s

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

High

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

decreases, material degradation

Most plastics are injection molded at temperatures around _____°C

100

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

Velocity profile, zero, maximum

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

Maximum, zero, strain-rate

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.

Non-Newtonian, increasing, shear thinning

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

higher

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

Melt Flow Index (MFI)

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

low, high, faster

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

3-30

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

Parting Plane

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

minimum, projected area

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

higher

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

Larger

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

gates

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

narrow, lower

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

Jetting

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

flow patterns, brittle

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

Pressure, multiple

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

100, .5, 400, 2

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

ejection pins

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

Flush

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

interior surface

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

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

Typical injection times are on the order of a ___________

second

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

thick, thin

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

knit line

Cooling accounts for approximately ___________ of the total cycle time

50-70%

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

Melt Processing, mold wall processing, ejection

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

Thicker

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

low, 5, 10

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

high, bigger, complex

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

tool steel, aluminium

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

surface treatment, 1 milion

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

Two-plate, Three-plate, Hot runner

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

Two-plate

______________ molds can separate from product before cooling

Three-plate

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

Hot runner

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

draft, 3

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

placement

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

Multi-cavity, 64

_________ 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

Slides

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

Stripper rings

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

small vent holes

Mold cooling: molds typically have straight channels for __________ cooling

water

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

Compact (Standard) Injection Molding

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

Compact (Standard) Injection Molding

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

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

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.

Insert, inside

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

Outsert

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

Gas, Nitrogen, Internal cavities

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

Simple

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

In-Mold Decoration (IMD)

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.

one, materials

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

sandwiched, 7k

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

Higher

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

warm, hot, cold