Thermoforming - Polymer Processing - Lecture Slides, Slides of Software Engineering

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MFGT 142

Polymer Processing

Chapter 14: Thermoforming

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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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Forming Process

• Straight Vacuum Forming

• Figure 13.

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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.

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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.

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Forming Process

• Free Forming

• 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.

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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.

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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.

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Forming Process

• Blister Pack

• 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 PackDocsity.com

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Equipment

• Figure 13.

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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.
• Used to calculate blank size of sheet to make a part. Docsity.com

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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