Thermoforming characteristics

Rigid PVC is currently the most widely used forming film because of its ideal thermoforming characteristics. A typical thickness before thermoforming is 250 pm (10 mil). PVC does not provide a good barrier for moisture-sensitive products. When better barrier properties are required in a thermoformable blister, PVC is laminated or coated with other materials. Because of environmental issues, other materials such as PP compete for blister usage with PVC. In the medical device industry it has been completely replaced by materials such as PET. 

Plastic memory is a phenomenon of TPs that has been stretched while hot beyond its heat distortion point to return to its original processed or molded form. Different plastics have varying degrees of this characteristic and degree of return is basically dependent on temperature (Chapter 4, Thermoforming, Memory). 

This is an extension of the cold forming process. It uses thermoformed B-stage thermoset reinforced plastic (RP) skin to improve surface and other characteristics to a cold molded thermoplastic. The mold is closed and the fast, room temperature curing RP plastic system hardens. The finished product has the smooth TP-formed sheet. 

PE crosslinked foams offer higher stability and mechanical strength, better insulation characteristics, and improved energy-absorption properties. Most of these foams can be thermoformed, embossed, printed, laminated, or punched, using conventional equipment. 

NatureWorks PLA can be extruded, cast or biaxially oriented, and thermoformed using conventional processing equipment. The company claims that NatureWorks PLA performs like traditional petroleum-based plastics, and in some cases offers better performance characteristics, including gloss, clarity, strength, and flavour and aroma barrier. 

The resulting poly(oxy-1,4-phenylene sulfonyl-1,4-phenylene) (PES), CAS 25667-42-9, is a valuable polymer used to make various household appliances, medical devices, automotive parts, etc. The polymer can be shaped by thermoforming and is used in various composite materials. It has good fire safety characteristics and meets the requirements for direct food contact. Other polyethersulfones with structures similar to PES also have important practical applications and are used for the manufacturing of different appliances, cookware, automotive and aerospace components, etc. 

Thermoforming consists of forming a preheated plastic sheet or film into a desired shape against a mold surface. Forming can take place pneumatically or mechanically. In the first case, a differential air pressure is used, either vacuum on the mold side, compressed air on the other, or both. In the second case, the preheated sheet of material is pressed into the mold using either a preform, a plug, or by a combination of both. Viitually all thermoplastics can be thermoformed. However, because of limitations in the material characteristics, some can only be used for simple shapes with small draw ratios. 

Polyetherimide (Ultem 1000, GEC) is a high performance engineering thermoplastic with high heat distortion temperature (> 200°C), high mechanical strength and inherent flame-retardancy characteristics. Recently blends of polyetherimide with polycarbonate have been commercially offered as thermoformable sheets and as molding compounds (Table 15.28). The primary reason for... 

PS is one of the most versatile packaging resins. The amorphous grade, crystal PS, is used to make bottles for pills, tablets, and capsules. High impact PS (HIPS) is commonly used as thermoformed containers for dairy products. PS foam has good shock absorbing and heat insulation characteristics. Applications in food packaging include egg cartons and meat trays. Varieties of styrene-based copolymers have also been developed to exhibit special combinations of properties. Typical PS properties are shown in Table 4.10. 

Medical packaging has been another big user of plastics. As converting techniques improved, so that accurate molding of small vials could be accomplished at low cost, and as new polymers became available with the necessary characteristics, plastics have been substituted for glass in many applications. As medical procedures became more complex, more disposable kits were introduced, designed to have complete sets of equipment for specific procedures. These kits require special packaging to keep the parts organized and easily usable. Here thermoformed... 

Poly(ether imidejs (PEI)s are a particular class of PI which combine the high-temperature characteristics of PI but still have sufficient melt processability to be easily formed by conventional molding techniques such as compression molding, gas assist molding, profile extrusion, thermoforming and injection molding." ... 

The carbon fiber RP also show excellent electromagnetic interference (EMI) and radio frequency interference (RFI) shielding characteristics, without use of additives. Fligh performance injection molding compounds can give attenuation values of 60-70 dB RPs for stamping, laminates and thermoforming can reach 90-100 dB and above. 

---