Thermoforming

1. Thermoforming Edward Anderson David Miller Akhilesh Singhal

2. 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

3. 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

4. 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

5. 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

6. 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

7. History • Today • In addition to packaging, used for refrigerator liners, shower stalls, bathtubs, glove departments, automotive trunk liners and more

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. Vent Holes Image: Formech Int’l

16. 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

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

18. 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)

19. 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

20. 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

21. 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

22. 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)

23. 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

24. 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

25. 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

26. 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