Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mold. Cast or machined aluminum is the most common mold material, although epoxy, wood and structural foam tooling are sometime used for low volume production. The sheet is held against the mold surface unit until cooled. The formed part is then trimmed from the sheet. The trimmed material is usually reground, mixed with virgin plastic, and reprocessed into usable sheet [1]. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.
Process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft and flowable. Then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.
Thermoforming process
The thermoforming process involves the following steps:
- Extrusion of sheet
- Place the sheet on a mould
- Draw the sheet into the shape of the mould by heat and negative force (vacuum).
Raw Materials
Most thermoplastics are usable. Must be in sheet form.
Tooling
Generally, machined aluminum is used, although poured composites and even wood can be used for short runs.
Cost
Tooling costs are generally low and piece prices are strictly dependent upon the speed of the machinery.
Examples of Application
Covers, displays, blister packaging, trays, drinking cups & food packaging.
Some of the possible advantages of thermoforming over injection molding include
- Large surface-area parts formed on inexpensive molds andmachines, due to low pressure and temperature requirements
- Easy formation of very thin-walled parts that are difficult to make by other techniques.
- Very high rates of production of thin-walled parts atrelatively low capital investment.
Advantages
- Extremely adaptive to customer design needs
- Rapid prototyping development
- Material and process is optimized for cost effectiveness
- High-speed production allows for just-in-time shipments
- Flexible tooling design offers a competitive advantage
- On-the-fly product enhancements with low additional costs
- Visually pleasing appearance
- Weight savings for consumer and manufacturer
- Wider design scope
- Lower tooling costs
- No anticorrosion spray necessary
- Paintable and colored plastic availability
- Fully integrated process with limitless flexibility for small to large product designs
Disadvantages
- High initial equipment investment
- High startup and running costs possible
- Part must be designed for effective molding
- Accurate cost prediction for molding job is difficult
Choosing The proper forming parametres
Optimum forming conditions depend on part and molddesign, part draw ratio, host polymer, sheet thicknessand thermoforming method.
Sheet storage
To maintain the properties of Stat-Rite sheet, Noveonwraps and seals the rolls in heavy polyethylene with adesiccant to help prevent damage and moisture pick-upduring shipping or storage. Handling guidelines follow:
- Rolls should be used within 6 months after receipt.
- Do not remove the wrapping until you are ready to use the roll.
- Rolls should be opened only in a controlled humidity andtemperature environment.
- Rolls should be used as soon as possible after removal ofprotective wrapping
- Stat-Rite sheet should be stored under controlledtemperature of 60°F to 80°F and low humidity conditions.
- Sheets stored for any length of time should be pre-driedbefore forming.
Forming temperature
Sheet temperature should be determined with aninfrared pyrometer. For best results, the infrared deviceshould be mounted through the oven wall. Unlike theirhost polymers, Stat-Rite alloys show little sheet sag attheir optimum forming temperature. Heat transfer inthermoforming depends on heater radiation, air convec-tion, and conduction through the plastic. With infraredtemperature measurement and heating cycle time control,sheet temperature can be controlled to within +/- 10°F or +/- 5°C.
For thin-gauge thermoforming, where the sheet thicknessis typically less than 0.060" (1.5 mm) conduction throughthe sheet is usually less important than radiation and convection to the sheet surface. Sheets thicker than about0.010" (0.25 mm) should usually be heated on both sides.
When determining the best forming temperatureremember:
- Electrical properties will be affected by excessive heating.
- Maximum thermoforming temperature for Stat-Rite is 374°F(190°C) for acrylic, 338°F (170°C) for PETG (see Table 1).
- Forming at lower sheet temperatures yields the best hotstrength, minimum spot thinning, and shorter forming andcooling cycle times.
- Forming at higher sheet temperatures yields lower internalstress levels, better mold surface replication, deeper draws,longer cooling times, higher formed part shrinkage, morenonuniform part wall thickness, and vacuum hole nipples.
- Plug assist forming often produces improved parts.
- The heater temperatures should be selected to meet thedesired heating time and overall cycle time.
Heating Time
The time required to heat a sheet to its proper formingtemperature depends on sheet thickness, surface finish,material color, heater temperature, and the type ofheaters used. Generally, excessive sheet heating leadsto color shift, discoloration, surface blistering, delamina-tion, and loss of both physical and electrical properties.For thin-gauge thermoforming, the time to form andcool the sheet against the mold surface must equalthe time to heat the sheet to the forming temperature. Cooling begins the instant the sheet is transferred fromthe oven to the forming station. It is recommended thatthe average sheet temperature drop not exceed 10°F(5°C) during this transfer. Thus, transfer time should beonly a few seconds
Heating source
SInfrared heating elements are the most commonly usedheating source. Ceramic and quartz tube heaters arereplacing older metal rod heaters, since they are farmore energy efficient and more easily controlled. Theintensity of the heating source, usually given in Watt/in2or kW/m2, is usually controlled by the power and thefraction of time the heater is on. Ovens should bedesigned to provide even heat distribution over theentire sheet surface. In certain instances, screens orheat shields can be used to shadow local areas on thesheet to aid in improving wall thickness distribution
Cooling time
The formed part should be cooled to a temperaturebelow its distortion or set temperature. Cooling timesdepend on mold temperature, mold material heat transferproperties, coolant type, part wall thickness, part design,sheet temperature and ambient temperature.
Vacuum
A good vacuum system with the capacity to quicklyevacuate the volume of the mold is essential. A goodrule of thumb is that the volume of the vacuum tankshould be at least four times the volume of the moldcavity. And the vacuum developed by the vacuum pumpshould be 28.5 inches of mercury or 35 Torr.
Mold design
Machined or cast aluminum molds are recommended forcommercial Stat-Rite thermoforming. Water coolingchannels are recommended for mold temperature uniformity and cooling cycle control. Highly polishedmolds are not needed or recommended for vacuum forming. Matte part surface is achieved by grit blastingor chemical etching. In certain instances, polyfluorocarbon-impregnated aluminum surfaces are used to allow thesheet to locally slide during forming. This can yield apart with more uniform wall thickness.Syntactic foam and epoxy are recommended for plugs.For deep draw parts, plugs should be bull-nosed andpolyfluorocarbon-coated, to provide local slip and moreuniform wall thickness.
Thermoforming tool design guidelines
Modern developments in tooling, thermoforming machines and techniques, together with improved thermoformable polymers, have made thermoforming one of the most rapidly growing polymer processing areas.
As with all processes, there are processing limitations. The following are guidelines to assist you with designingyour product for optimum strength, appearance and performance.
- Minimum draft angle should be 2° to 5° on male molds ormale portions of female molds and 1/2° to 1° on female molds. For textured mold surfaces, the draft angle should beincreased 1o per 0.2 thousands of an inch or 5 µm of texture.
- To minimize nipple height, the diameter of any vacuum hole should not exceed the local sheet thickness. For very thin sheets, alternate means of air evacuation, such as slot vents or porous plugs, should be considered to avoid nipple formation. If the rate of air evacuation is too low, the sheet will not fully form against the mold. This indicates that there are an insufficient number of vacuum holes.
- Undercuts should be avoided. If undercuts are necessary, they should be discontinuous around the periphery and should be shallow. If deep or continuous undercuts are required, breakaway portions of the mold will be needed to affect part removal without scuffing.
- Molds must be oversized to allow for polymer shrinkage. On male molds and male portions of female molds, 0.3% to0.5% shrinkage allowance is recommended. On female molds, 0.5% to 0.8% shrinkage allowance is recommended. The polymer grade, coefficient of thermal expansion, part geometry, mold temperature, initial sheet temperature, initial sheet thickness, and forming cycle all affect polymer shrinkage.• Radii on ribs and fillets should not be less than the localsheet thickness. The radii should be as much as four times the local wall thickness in areas where high loading is encountered or good stiffness is required.
- The draw ratio is given as the surface area of the formed part divided by the surface area of the sheet used to form the part. The average thickness reduction is the reciprocal of the a real draw ratio. Often, the depth-to-width ratio, viz,H:D, is used for axisymmetric parts but it is not accurate for rectangular parts since it ignores the effects of the length dimension. • In vacuum or drape forming, the depth of draw is usually limited to the narrowest width of the part, viz, H:D <1.greater>
- The best part-to-part dimensional tolerance is achieved by forming against a heated mold. However, cooling cycletimes increase with increased mold temperature.
