Injection Molding Flashcards
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
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
Exact cost of injection mold will depend on _______________ as well as _________and ___________
desired quality, part size, part complexity
Thermoplastics, (or plastics) are _______ which are large molecules comprised of many repeated subunits called __________
Polymers, monomers
In plastics, Monomers are typically _______________ which are compounds consisting of_________ and ________
Hydrocarbons, hydrogen and carbon
Molecular chains that tangle together
are held by ________________forces which are relatively ________ forces between molecules
Van der Waals, weak
Van der Waal forces cause plastic stiffness to typically be ____________
Low
The common plastic ABS has a young’s modulus of _______ GPa
1-2
To tear or rupture plastics, it is necessary
to break stronger _________ bonds
Covalent
What is the base strength and common strain to failure for plastic?
40 MPa, 50%
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
Monomers ________ need to consist of only carbon
Do not
Polyamides (nylons) have __________ in their monomer chains
nitrogen
Chain chemistry will have a strong effect on
__________________
material properties
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
What are the two main categories of plastics
Amorphous plastics & Semi-crystalline
_________________have a randomly ordered molecular
structure and do not have a set melting point. Instead, they soften slowly with increasing temperature
Amorphous plastics
_______________ plastics have a highly ordered molecular
structure and sharp melting points
Semi-crystalline
When amorphous plastic is heated up slowly, it experiences a
dramatic loss of stiffness at the ______________________
glass transition temperature
At the glass transition temperature, molecules have the freedom to _________ along each other.
Slide
At the softening temperature, the material is ___________ and
the concept of stiffness no longer applies
Melted
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
Melted plastics have a relatively ________ viscosity with typical ranges of ___________ Pa.s
high, 10-100
What are the viscosities of molasses and water?
5 & 0.001 Pa.s
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
