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Thermoforming. Edward Anderson David Miller Akhilesh Singhal . What is Thermoforming?. Thermoforming is the process involving heating a plastic sheet and forming it into a cavity or over a tool using vacuum, air pressure, and mechanical means Types: Thin thermoforming Thick thermoforming

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thermoforming

Thermoforming

Edward Anderson

David Miller

Akhilesh Singhal

what is thermoforming
What is Thermoforming?
  • Thermoforming is the process involving heating a plastic sheet and forming it into a cavity or over a tool using vacuum, air pressure, and mechanical means
  • Types:
    • Thin thermoforming
    • Thick thermoforming
  • Methods:
    • Vacuum forming
    • Pressure forming
history
History
  • 18th Century
    • Tortoiseshells and hooves thermoformed into combs and other shapes
  • World War II
    • Development of Thermoplastics
    • Acrylic aircraft cockpit enclosures, canopies and windshields
  • 1950’s
    • High volume production and products of thermoplastics.
    • Sheet manufacturing most productive
history1
History
  • 1960’s-
    • Era of Industrial growth
    • Development of blister packaging

and food packaging divisions

    • Major market share
    • Sign manufacturers introduce vacuum forming in their production Images: Wikipedia
history2
History
  • 1970’s
    • Extremely high volume output
    • Need for high speed machines
    • Scrap handling and reduction methods introduced
    • Gain of confidence in thermoplastics making
    • Research and exploration of new machines and products
history3
History
  • 1980’s
    • Revolution in technology. Machinery produces pellets and self-handles scrap
    • One employee per machine
    • Flexibility in size of containers use by the same machine
    • Improvement in quality and cost reduction
history4
History
  • Today
    • In addition to packaging, used for refrigerator liners, shower stalls, bathtubs, glove departments, automotive trunk liners and more
thermoforming basics
Thermoforming – Basics

Polymer sheet (cut or roll) is clamped and then heated (radiative heating) [Step 2/3]

Softened sheet is lowered over a mold (or mold brought up to sheet) [Step 4]

Images: Formech Int’l

thermoforming basics1
Thermoforming – Basics

Air trapped between the mold and plastic sheet is quickly drawn out through vent holes. The sheet quickly cools. [Step 5]

Vent holes can be used to push air back under the cooled, molded part to help release it from the mold. [Step 6/7]

Images: Formech Int’l

the mold
The Mold

Two major types: Male and Female

Male molds have the convex (inner) side against the mold; the concave side is against the mold face in female molds.

Mold type should bechosen so that theimportant side contacts the mold

Off side is harderto control shape

Male Mold:

Female Mold:

Images: Formech Int’l

the mold1
The Mold

Molds can also come in single or multiple impressions:

Multiple-impression molds increase the risk of webbing, especially in male molds.

Webbing is extra plastic aroundor between molded form(s).

Can reduce webbing by usinga female mold, a reducing frame,pre-stretching (bubble), a plug assist, slower vacuuming, or a thicker plastic sheet

Webbing

Images: Formech Int’l

the mold plug assist
The Mold – Plug Assist

A plug assist is an additional, removable molding component used to help stretch the plastic (consistent thickness) for deep draws or to minimize webbing.

Plug is usually lowthermal conductivity,polymer should coolon the mold, not on the plug:

Wood, Resin

Image: Formech Int’l

the mold plug assist1
The Mold – Plug Assist

Plugs allow for deeper draws without excessive thinning. Rule of thumb:

Plastic in cavity thins drastically when deeper than 75% of cavity diameter (see below).

Plugs pre-stretch the plastic, allowing for deeper drawing than with vacuum alone.

Image: Formech Int’l

the mold venting
The Mold - Venting

Venting is accomplished using a vacuum pump to draw air from under the plastic through small vent holes.

Vent holes should be less than half the sheet thickness in diameter at the surface, although sub-surface the holes may be much larger. They should also be minimized in number and placed at the points where the plastic would last contact the mold (see next slide).

Image: Formech Int’l

slide15

Vent Holes

Image: Formech Int’l

the mold other considerations
The Mold – Other Considerations

Undercut molds are possible, so long as the geometry of the finish part allows it to be removed. Otherwise requires more expensive split tooling (see below).

Mold temperature needs tobe right to prevent thinning or chill lines.

Heater temperature to prevent singeing.

Positive-Pressure Thermoforming instead of Vacuuming.

Image: Formech Int’l

thermoforming youtube
Thermoforming YouTube
  • Society of Manufacturing Engineers:
    • http://www.youtube.com/watch?v=U60mdDW5Ulc
  • Miller Plastics Inc.:
    • http://www.youtube.com/watch?v=WL-h0HAcdFA
details of thermoforming
Details of Thermoforming

The maximum depth to width ratio is between 0.5 and 2

Is generally a batch process not a continuous one

Is economical for large and small batch sizes

Gives products with excellent mechanical properties

Must use Thermoplastic sheet not pellets

This is more expensive, since the sheets have already been made (typically by someone else)

details of thermoforming1
Details of Thermoforming

Can be used for a large range of sizes which can range from plastic cups to boats

The product must be trimmed after forming and the scrap can not be directly recycled

The mold tends to mark the sheet during processing

Has much lower tooling costs then injection molding

details of thermoforming2
Details of Thermoforming

The side of the sheet that is away from the mold can not be controlled although its behavior can be predicted

The side of the sheet that is formed against the mold can be controlled with close tolerances

Any required tolerances for the side that is away from the mold requires further finishing techniques

applications of thermoforming
Applications of Thermoforming

Thermoforming can only be used for thermoplastics not for thermosets or elastomers because of the crosslinking they posses

Commonly used in the food packing industry but can also be used for other products such as bathtubs

micro thermoforming
Micro Thermoforming

Thermoforming can be accomplished on the micro scale

This provides an alternative to injection molding which was previously the only processing technique which could create technical micro parts such as polymer microchips (which are used by the life sciences)

micro thermoforming1
Micro Thermoforming

The process consists of one half of the mold containing micro-cavities, and the other half having holes for vacuum and gas pressure

A 20 to 50 micron thick sheet of polymer is placed between the two halves is sealed, heated and then pressure is introduced just like with normal thermoforming

micro thermoforming2
Micro Thermoforming

A process much like twin sheet forming can be used to create closed container parts

Pre and post processing are possible with techniques such as ion bombardment and uv-based surface modification

Thermoformed parts have a smooth inner surface that is difficult for other techniques to create

limits and future work
Limits and Future work

Current limitations include manual operations that result in a 10 minute cycle time

Development efforts

Automating pressure build up and demolding systems

Advanced heating systems to preheat sheet and allow for constant mold temperature

Objective is to reduce cycle time and increase reproducibility

references
References

Granta Design Limited. CES EduPack 2008.

Associated Thermoforming inc. “Thermoforming plastic parts FAQ: benefits, forming kinds and more.” 15 Feb. 2009 http://www.ati-forms.com/faqs.html

AllBusiness. “Micro-thermoforming makes its debut.” 15 Feb. 2009 http://www.allbusiness.com/manufacturing/chemical-manufacturing-resin-synthetic/3955884-1.html

Formech International Ltd., “A Vacuum Forming Guide.” http://support.knowlton.ohio-state.edu/files/FormechVacuumGuide.pdf

Florian, John. “Practical Thermoforming: Principles”. New York: CRC, 1996

Osswald, Tim A.”Polymer Processing Fundamentals”.Munich: Hanser, 1998