Transcript MFGT 142 Polymer Processing Chapter 14: Thermoforming Professor Joe Greene

MFGT 142
Polymer Processing
Chapter 14: Thermoforming
Professor Joe Greene
CSU, CHICO
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Thermoforming Process
• Overview
– Forming processes
• straight vacuum, pressure, plug assist
• reverse draw, free, drape,
• snap-back, matched die, mechanical
– Blister pack and skin pack
– Equipment
• machine, molds, plant considerations
– Product considerations
• materials, shapes, part design
– Operation and control
• critical parameters, trouble-shooting
• maintenance, and safety
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Thermoforming
• Advantages
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Low machine cost
Low temperature requirement
Low mold cost
Low pressure requirement
Large parts easily formed
Fast mold cycles
• Disadvantages
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High cost of raw materials (sheets)
High scrap
Limited part shapes
Only one side of part defined by mold
Inherent wall thickness variation
Internal stresses common
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Forming Process
• Straight Vacuum Forming
– Figure 13.1
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Forming Process
• Pressure Forming
– Positive air pressure (14.5 to 300 psi) on top of mold forms the part.
– Advantages include faster mold cycles, sheet formed with lower
temperatures due to higher forming pressure, greater dimensional
control and part definition.
– Figure 13.2
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Forming Process
• Plug-Assist Forming
– Top side is formed with core form. Vacuum applied to finish forming.
– Advantages include better wall thickness uniformity especially for cup
or box shapes, and reduces stretching or thinning of material during
forming.
– Materials of plug include wood, metal, thermoset polymers.
– Temperature of plug is a few degrees cooler than plastic to prevent
premature cooling. Plug is 10% - 20 % smaller than cavity.
– Figure 13.3
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Forming Process
• Reverse Draw Forming
– Used when very deep draws are needed.
– Material is heated to the sag point and then blown away from the mold
which thins the material in the center of the sheet.
– The size of the bubble is controlled with an electric eye.
– Plug assist procedures continue to form the part.
– Advantage is better thickness uniformity. Disadvantage is cycle time.
– Figure 13.4
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• Free Forming
Forming Process
– Forming is done without a mold.
– Part is expanded with air pressure as in reverse draw forming and then
cooled. Clamping ring designs can result in controlling shape to
hemisphere (circle ring) and elongated (tear drop ring).
– Example is canopies for racing vehicles.
– Advantage is achieving high clarity.
– Figure 13.5
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Forming Process
• Drape Forming
– Thermoforming over a male plug mold.
– Advantages include better part dimensional control on inside of
part, lower mold costs, ability to grain surface (tubs, showers,
counter tops, etc.) faster cycle times.
– Disadvantage is more scrap due to larger clamps and trim area.
– Figure 13.6
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Forming Process
• Snap-back Forming
– Variation of drape forming.
– Material is heated to the sag point and then drawn slightly into a
vacuum box below the part to thin the center of the part, male mold
pressed against the material to draw it further, vacuum is applied
through the male mold causing a snap-back and cool against mold.
– Advantage is well controlled part thickness, though longer cycle times
– Figure 13.7
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Forming Process
• Matched Die Forming
– 2 dies are used to form part with no vacuum or air pressure.
– Used with parts that do not have large draws.
– Advantages include excellent definition and dimensional contol on
both sides.
– Figure 13.8
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Forming Process
• Mechanical Forming
– Form the shape using a forming tool (not molds) to give shape.
– Figure 13.9
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• Blister Pack
Forming Process
– Thermoform a thin sheet of clear, stiff plastic using straight vacuum forming or
drape drawing. Many blisters are made simultaneously.
– Products to be packaged are placed in cavities while the sheet is still orientated
with the cavities down.
– Printed sheet of cardboard with adhesive on edges is laid over the cavities and
pressed together.
• Skin Pack
– Similar to blister packs in the use of cardboard, but the plastic is not stiff and no
cavity is formed.
– Rather, the plastic is pliable and the thermoforming is done so that the part is
encapsulated by the film against the cardboard by using sheets of cardboard that
are printed and then perforated with many very small holes.
– The cardboard would then have an adhesive applied to the upper (printed)
surface. The products are placed with one product in each pack.
– The heated plastic material is then laid over the sheet of cardboard and vacuum
is applied to draw the plastic sheet against the cardboard. (Draw forming)
• Skin pack has more wrinkles but is cheaper than Blister Pack
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Equipment
• Figure 13.10
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Molds
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Most common mold material is Aluminum, machined or cast.
Others include hard woods, cast epoxy, and water-cooled steel.
Draft angles from 2 degrees to 7 degrees.
Vent holes are back drilled and placed in all low parts of the mold to
provide for easy escape of trapped air.
• Undercuts
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Shapes and Part Design
• Draw Ratio
– Care must be taken that the depth is not excessive.
– Important parameter that is used to estimate the amount of wall
thickness variation that might occur.
– High draw ratios result in excessive thinning and thickness
variation.
– Draw ratio = depth of part / width of part.
– Draw ratio should be less than
• 2:1 for female molds
• 7:1 for male molds
• Area ratio approximates the amount of thinning of the part.
– Area Ratio = Area of Sheet before Forming / Area of part after forming
– Example area of sheet is 30 sq.in and part is 60 sq.in, the ratio is 1:2 and the
part thickenss will be 50% of sheet thickness.
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– Used to calculate blank size of sheet to make a part.
Operational and Control
• Critical parameters
– Speed of vacuum application
• quickly as possible
– Temperature of the mold
• normally at room temperature. If increase then cycle time and shrinkages
increases
– Size of the bubble for reverse draft forming
• Size of bubble not to exceed 50% to 75% of the shorter dimension of
clamped sheet.
• Clearance between bubble and female mold should be 2 to 4 inches.
– Plug size
• 70% to 85% of the mold cavity and the shape of the plug should mirror the
cavity
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Trouble-shooting guide
• Trouble-shooting table 13.2
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