Imagine arriving at work to find everything has changed: your computer keyboard now consists of clunky metal keys, and when you go to the water cooler you find an awkward wooden keg instead of the smooth translucent container you’re accustomed to. This isn’t a nightmare: this is simply an illustration of how plastic seamlessly folds into our daily lives, sometimes in ways we notice and appreciate, sometimes in ways we don’t. The creation of plastic products we have come to depend on is often contingent upon thermoforming, a process wherein a plastic sheet is heated, stretched, and cooled into a final product.
Thermoforming: How the Process Works
Thermoforming, like most plastic formation processes, uses heat to manipulate the plastic. Once heated, the plastic sheet is transported through the thermoforming machine and a mating mold and pressure box enclose the sheet. A vacuum removes air as the plastic is stretched onto or into a mold (aluminum molds are common, but epoxy and wood are also used,) and pressurized air helps the plastic assume the mold’s shape. If the mold is aluminum, water-cooling or a chiller system is often used to expedite the formation process. A reverse vacuum helps eject the product from the mold before trimming begins.
Trimming removes the newly created plastic component from the sheet on the machine. With inline formers, steel rule or forged dies can trim the product while it’s still on the machine—this saves time and enables rapid packaging and shipment. In high-volume production runs, however, matched-metal punch-and-die trimming makes use of a separate machine. The thermoforming machine can automatically feed the plastic into the trimming press upon completion, which makes for a smooth transition from on noe step to another. Since a trimming press is more expensive than a steel rule or forge die, it mayt be worthwhile for smaller production runs.
Types of Thermoforming: Thin and Thick Gauge
The thermoforming process can be divided into two general categories, thin-gauge and thick-gage.
With sheets that are less than 1.5 mm thick, a sheet extruder or rolls feed the sheet directly into the thermoforming machine. Products made using thin-gauge thermoforming often comprise parts of rigid and semi-rigid disposable packaging.
- Thick-gauge
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When sheets are greater than 3mm thick they must be manually fed into the machine. Products made through thick-gauge thermoforming are found in a wide range of industries—thick-gauge formed plastics are often part of structural components on airplanes, cars, showers, and even electrical devices. Thick-gauge products sometimes need additional hand-work and hole-drilling, depending on the specific intended use of the part.
Although the thermoforming process may seem relatively straightforward, there are a few tips that may help the process go more smoothly. The Plastics Distributer and Fabricator—Your Industry Magazine offers the following top-ten tips to increase thermoforming efficiency.
Because thermoforming has been around for a while, so has the equipment. Making sure your equipment is in good working condition and routinely checking out new technology to see what else is out there, are good ways to maintain efficiency.
Having a flawed or old mold can really damage a product. On the other hand, a mold that is up-to-date and has a fast cooling rate can make the whole process faster and more productive.
Examining the parts of the cycle that are slowing the process down and improving a specific step can increase the overall cycle time.
Using smaller and thinner sheets reduces the amount of scrap created, and also reduces material costs.
Older machines need more attention. If possible, avoid using no-zone ovens. Ovens with zones eliminate the need for physical screening or blocking the sheet from heat transfer.
To avoid overheating the sheet, an infrared pyrometer can be used to monitor temperature and material sag.
PLC controls may increase efficiency, but it’s important to thoroughly investigate all aspects of a control system to determine which one is right for a particular operation.
Managing the cooling process is just as important as managing the heating process. Adding fans and finding out which molds can be used with an in-machine cooling system can help the machinist gain better control over cooling time.
Instead of slowing down the thermoforming process to match the trimming process cycle time, it is often possible to increase trimming speed.
A faulty vacuum can result in insufficient pressure and recovery time. Determining the correct amount of pressure needed at various stages of the process and continually checking the vacuum are key in efficient thermoforming.
