Thermoforming
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Thermoforming Tech
Academy > Extrusion
Process
The purpose of sheet extrusion is to transform thermoplastic resins,
usually in pellet form, into roll or sheet stock through a combination
of heat and pressure. The sheet may then be processed into shapes through
thermoforming or pressure forming.
There are three primary techniques used to manufacture thermoplastic
sheet. These are:
-
Extrusion
through a flat die onto casting rolls.
-
Extrusion through
an annular die onto a sizing mandrel. The pipe-like cross section
that is extruded will be slit in one or more places and then flattened
and handled as sheet.
- Resins and additives
will be plasticated between large rolls and then sized through a
series of additional rolls into a flat sheet. This process is known
as calendering.
Each
of these methods has advantages and disadvantages depending on factors
such as type
of polymer being processed, thickness and width of sheet, and surface
quality desired.
Single Layer Flat Sheet extrusion is the most common technique used
in extruding plastic sheet for the thermoforming industry. The classic
machinery components for this process can be described as follows:
Resin is fed into an extruder where it is plasticated into a melt.
The extruder, consisting of a heated barrel with an internal rotating
screw, pumps the melted resin into a flat sheet die which sizes the sheet
(thickness and width).
The sheet exits the die in a semi-viscous state and travels through
a series of rolls to cool. These rolls also determine final sheet size,
thickness, and width.
The
flat sheet may then be wound onto continuous rolls, or "pre-sheared" into
discrete lengths.
Coextrusion
is a process that allows the combination of different materials and
colors in a single sheet. This is done to achieve special properties
which are specific to a certain polymer, or for aesthetic effects with
color, or for economic reasons where an inexpensive material "sub-strata" is
combined with a more expensive material "cap".
Auxiliary Equipment
The proper auxiliary equipment is required to achieve the desired results
in the sheet extrusion process:
Blenders
Most feed to the extrusion hopper is made up of mixtures of resins, colors,
and other additives. In order to provide a uniform mix in the sheet
a blender is required before the hopper. This might be a batch ribbon
blender, a conical blender, or in some cases, an automatic metering
device which feeds multiple streams of material to the hopper in precise
dosages.
Screen Changers
These are commonly used after the material exits the extruder barrel
to filter the plastic melt and remove the occasional foreign particles
and break up conglomerates of color. Changers are usually automatic
so the process need not be shut down to change dirty screens for fresh
ones.
Melt Pump
This device is used where a variety of different resin types are extruded.
Each polymer would require a special screw for optimum throughput.
The melt pump eliminates the need for fine tuning temperatures and
reduces gauge variation in machine direction. In addition, as screws
wear, their performance is affected. The melt pump overcomes these
deficiencies and ensures positive uniform flow to the die.
Static Mixers
Used to improve distributive mixing in single-screw extruders, static
mixers are well suited for color mixing and melt temperature homogenization
of a broad range of thermoplastics. Additional benefits include achieving
uniform product density, eliminating melt lines in the machine direction,
and reducing the time needed to change colors. Add-on mixing units
provide compounders a low cost means to increase extruder productivity,
achieve higher yield from raw materials, and improve physical characteristics
of the product.
Texture Rolls
When the melt leaves the die and enters the roll nip it is soft enough
to take the finish of the rolls it is in contact with. Thus, textured
or grain rolls will impart a mirror image of their finish to the sheet.
This can give both functional and aesthetic qualities to the sheet.
Though haircell, matte, and sandblast are
among the most common textures, there are literally hundreds available
on a custom basis.
Grinders
One of the many advantages of thermoplastic sheet products is that they
can be recycled and processed directly back into the sheet. The equipment
that is used to prepare the plastic material for reprocessing is typically
called a grinder or granulator. Since thermoplastics can be recovered
and reused so easily, grinding equipment has become vital to the plastics
industry.
Dryers
Many resins are hygroscopic (they absorb moisture). Thus, they must be
dried before being fed to the extruder. The most common dryer uses
hot dehumidified air which passes through and around the resin particles
in a tall cylindrical hopper.
Feedstock
Generally direct extrusion or roll fed thermoforming is limited to sheet
under 0.100 thick. Cut sheet stock is usually cut in line at the time
of extrusion to the desired dimensions for a particular forming application
and molded in thicknesses from 0.030 to 0.450 inches depending on finished
part design requirements.
Sheet stock can also include coextruded sheet, laminated sheet and foam
core sheet. Coextruded sheet with a weatherable cap is used for outdoor
applications requiring protection from UV exposure. Decorative and protective
laminated films are used for such applications as luggage, office furniture
and for applications needing abrasion resistance.
The following sheet properties have a significant influence on thermoformability
and formed part quality;
-
Dimensions
(length, width, thickness, and flatness)
-
Surface
type and color
-
Orientation
-
Contamination
-
Toughness
-
Moisture
content
Sheet Gauge
Uniformity is recommended to be +/- 1.0% or less both across the sheet
and down the sheet in the extruded direction. For more critical commercial
thermoforming applications gauge uniformity of 0.05% is required. This
is often necessary when sheet gauge is less than 0.100 inches. It is
difficult to obtain this close tolerance during extrusion, but the benefits
in these cases are significant. Proper sheet gauge control as described
above can translate to higher part output rates, less part-to-part thickness
variations and less scrap.
Orientation (internal strain)
During extrusion, a thermoplastic resin can be stretched to cause
the polymer molecules to line up more in the direction of stretch
than in
other directions. This is usually in the direction of extrusion and
is described as uniaxial orientation. The amount of orientation can
be determined
by placing samples of the material in an oven at 325 degrees F for
30 minutes, between two thin sheets of aluminum dusted with talc.
The amount
of shrinkage represents the amount of orientation. Shrinkage of 10
to 15% in the extrusion direction is normal, though less would be
more desirable.
Shrinkage in the cross machine direction is usually lower,
normally 5% or less. A large amount of orientation will cause differential
drawing during forming. Resistance to draw is greater in the oriented
direction than in unoriented direction. for sheet thicker than 0.175" high
orientation (>15%) can cause sheet to pull free of the clamps during
the heating process. For sheet less than 0.175" thick, high orientation
(>25%) can cause the same phenomenon.
Toughness
The impact resistance of a sheet is its ability to resist cracking when
struck with an object. The toughness can influence the performance of
the thermoformed part during assembly, shipment, or in its end use. Impact
resistance is most commonly determined using a falling dart impact test
(see Material Testing).
Moisture Content & Contamination
A frequent cause for rejecting thermoformed parts is moisture or contamination
in the sheet stock. Hygroscopic materials absorb moisture into the sheet
as well as on the surface, and this can cause surface defects during
forming. For hygroscopic materials such as ABS, Styrene, and Polycarbonate,
it can be helpful to wrap the extruded sheet in polyethylene film to
minimize moisture absorption. This will also protect sheet from surface
contamination and damage during storage and shipping. Thermoforming output
rate can also be improved (and sheet moisture addressed) by preheating
the sheet prior to forming.
Extruded sheet sometimes develops a static electrical charge. This condition
causes the sheet to attract dust and foreign particles from the surrounding
environment. Thermoformed parts made from dusty or dirty sheet will exhibit
surface defects. A polyethylene cover (mask) should be used during any
prolonged shutdown periods such as holidays and weekends. In many cases,
contamination or moisture problems effect only the top and/or bottom
sheets of a stack.
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