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Thermoforming takes on more engineering applications.

Thermoforming Takes on More Engineering Applications

Recent advances in materials, process machinery, and computer-aided engineering are spurring the use of thermoforming for many applications that previously could only be accomplished through injection molding, blowmolding, or other thermoplastic processes.

Manufacturers have traditionally turned to injection molding to create parts with great detail, complexity, and definition, while they have used blowmolding techniques to produce double-walled or hollow-cavity structures too large or with wall thicknesses too thin to be injection molded.

Yet while these are the two most widely used processes in the plastics manufacturing industry for producing three-dimensional parts, they have inherent characteristics preventing them from seeing even wider use: injection molding tooling costs are relatively high and production lead times are long; blowmolding cannot be easily used to mold parts using more than one material, nor can it incorporate support ribs or other inserts within the hollow part.

Exciting new developments in sheet processing allow thermoformers to produce parts with near-injection-molded quality at production levels too low for injection molding to be economically feasible, and also produce many types of hollow-walled parts far more economically than any other process. Moreover, in many cases this can be accomplished through simple and inexpensive modifications to existing processing machinery.

The Thermoforming


Thermoforming--the heating, stretching, and molding of a thermoplastic sheet onto a male or into a female mold--traditionally has been a process used to manufacture low volumes of relatively simple parts with no real detail, such as dunnage trays, skylights, and truck bed liners. One common area of application--packaging--is an exception in that it supports very high-volume production via automated roll-fed equipment.

In general, a sheet of thermoplastic material is clamped around its perimeter and placed in an oven. It remains there until the material reaches its forming temperature, which is generally measured by the degree of sag or drape the sheet exhibits. At this point, the sheet and clamp are removed from the oven and the part is formed.

The simplest and most commonly used forming method is vacuum forming. When the sheet is pulled into a female or over a male die, it is sealed against a vacuum box. In sequence, a vacuum valve is opened and air is evacuated from between the pliable sheet and the mold, pulling the sheet against the mold surface (see Fig. 1). However, since vacuum forming is limited to a theoretical maximum forming pressure of 14.7 psi, parts have been denoted by large corner radii and minimal detail. While this rounded, softer look was generally acceptable in the past (and still is with some parts and applications), consumers and end-users are demanding sleeker designs with sharper, crisper corners and more attention to detail.

Pressure Forming

The advent of pressure forming offered thermoformers the ability to rival injection molding detail for the first time, with tight radii, stiffening ribs, surface texturing, and embossed or debossed logos. In pressure forming, compressed air at up to 150 psi is introduced through a pressure box that closes on the mold (most frequently a female mold) and seals it along the edges of the heated material. The very rapid forming speeds possible with these increased pressures, coupled with hotter dies, improve the thinning characteristics of the formed material by minimizing cooling of those portions of the hot sheet that initially make contact with the die. This improves the material's ability to be stretched toward the corners (see Fig. 2).

Twin-Sheet Forming

Thermoforming has further evolved into twin-sheet forming, where two simultaneous vacuum- or pressure-forming operations are used to produce hollow parts. In essence, two female molds are used in conjunction with pressure and vacuum to form two sheets of material at the same time. The sheets can be of the same or dissimilar materials.

The force on the upper and lower platens supporting the mold halves is used to "pinch" the two sheets, producing a welded integral bond joining them, and ultimately forming a hollow part similar to one that has been blowmolded. Equipment manufacturers and sheet suppliers have made notable advances in modifying equipment so that all this can be done on a single machine (see Fig. 3).

Materials Innovation

Great strides in materials technology have been a prime reason why thermoforming is becoming an attractive alternative for high-performance, sophisticated, and relatively low- to medium-volume applications.

Hot-melt strength--the ability of a sheet to support itself during transfer from the oven to the forming station--is the preeminent characteristic of an ideal plastic thermoforming material. This high-heat property allows the material to remain soft enough to be stretched into a larger shape without separating during the forming or stretching processes.

Materials that have been widely accepted for thermoforming include acrylic, ABS, polycarbonate, polyvinyl chloride, and PPO/PPE.

Many thermoplastics manufacturers have developed engineering thermoplastic sheet materials that are well suited to thermoforming. They offer high-performance properties that exceed those of many commodity-type materials. Although GE sheet materials must be dried before forming to eliminate the possibility of moisture bubbling, they can be used in the same types of molds, ovens, and other thermoforming equipment required to process other sheet materials. These products can be thermoformed into complex shapes with sharp detail on conventional forming equipment designed with top and bottom ovens and rapid sheet transport systems to shuttle the heated sheet from the oven to the forming station.

Comparison With


For many low-volume applications, twin-sheet forming may be more cost-effective than blowmolding. Initial capital equipment and tooling costs are much lower than those for blowmolding, and the lead time for both production and prototype tooling is typically much shorter. In addition, the initial start-up costs for blowmolding include the expense of purge and start-up materials, whereas the first twin-sheet formed part is typically usable in production.

Twin-sheet forming also allows relatively easy changes in material thickness. All that is necessary is simply to increase or decrease the thickness of the starting gage. On the other hand, such changes in blowmolding entail some very sophisticated and time-consuming parison modifications that affect part-to-part consistency and wall-thickness uniformity.

For additional flexibility, a thermoformer can twin-sheet form a part, for example, with a thin sheet on one sid and a thicker sheet on the other. A hollow-cavity part can also be formed using different materials on either side of the mold. This is not easily possible in blow-molding because a parison can be extruded from only one material.

Because of this capability, material properties never before available, such as clarity and opacity on opposite side, or UV resistance on one side and chemical resistance on the other, can be incorporated into one part design. For example, an aircraft seat back tray could be twin-sheet thermoformed with a high-heat material on one side and an impact-resistant material on the other.

In addition, twin-sheet forming can eliminate the need for secondary attachments. The sheets can be heated and brought together around a metal structure to form a hollow part with attachment bosses, stiffening ribs, or other secondary structures inside.

An example is an air flow resonator, which is a hollow structure with a series of baffles inside. With twin-sheet forming, one need only place the baffles between the two formed sheets prior to closing the molds. Before the advent of twin-sheet forming, the part had to be taken out of the machine, carried to another station, and trimmed out. Following the insertion of baffles, the part was blued and then cured.

Comparison With Injection Molding

At volumes of 5000 to 10,000 parts or less, thermoforming can economically produce parts with near-injection-molded quality. Although initial material costs are higher than for injection molding (because one must start with a sheet that has already been extruded, cast, or calendered), overall costs are less. The cost for tooling for a thermoformed part can be roughly 10% of the cost of the tooling required for an equivalent injection molded part.

Injection molding tooling requires costly, close-tolerance machining of matched-metal dies, stripping plates, part ejectors, and runner systems. Thermoforming tooling, on the other hand, is typically a single-side casting, which reproduces only one side of a part contour. A female cavity would be cast for parts requiring surface detail on the outside of the part, while a male mold would produce those parts that require interior detail. Thus, thermoforming molds are less complicated and must simpler to build. They can be produced in as few as six weeks, unlike injection molding tools, the production of which requires months.

Unlike injection molded parts, thermoformed parts can be removed from a mold without the use of ejection pins. Formed parts remain rigidly held in the clamp frame throughout the forming cycle. After the forming process has been completed, the part is automatically stripped from the mold.

Historically, injection molded parts are always "near net shape," needing little or no secondary processing. However, thermoforming requires that parts be trimmed and any holes, louvers or grilles routed or machined into the final part as a secondary operation. However, the introduction of computer-controlled robotic trimmers has reduced the cost and time associated with these operations and has made trimming of complex shapes possible.

Advances in the thermoforming process now permit the designer to consolidate parts and improve assembly and subassembly production. For example, undercuts, when combined with properly designed trimmed areas, can facilitate snap-fit assemblies, and such formed-in details as countersinks and counterbores can reduce or eliminate secondary processes. Like insert molding for injection molding, mounting or stiffening hardware can be insert-formed to simplify later joining processes.

Comparison With Hand Layup

In the aircraft industry, thermoforming can serve as an economic alternative to some of the hand-layup construction that occurs.

A good example is ducting. Currently, aircraft ducts are made using a free-cast rotomolded plaster-of-paris mold that is hand-wrapped in a cloth impregnated with thermoset resin. The wrapped mold is put into a vacuum bag and sealed, and subsequently heat-cured. Once curing is complete, a mallet is taken to the part to remove the plaster mold. The banging, however, usually creates microcracks, which must be sealed with a slurry of plastic on the interior. The entire process can take up to 45 days. With the mold destroyed, a new one has to be made every time a duct is to be built--a costly, time-consuming, tedious, and rather primitive procedure.

Studies have shown that twin-sheet thermorforming can be substituted for this process, which now takes between six and eight man-hours, reducing the work time to five minutes. The result will be a part that provides equal performance and greater part-to-part consistency at a reduced cost.

Machine Design Developments

The wider range of thermoformable sheet from suppliers has galvanized machinery manufacturers to mount major development efforts in machine design. There is increased availability of sophisticated new sheet-fed equipment that is capable of withstanding higher thermoforming pressures and providing increased levels of efficiency and precision, particularly in the large parts area.

Computerization is becoming more common on thermoforming equipment. One machine manufacturer has installed a computer-control system that coordinates the entire operation, including sheet feeding, heating cycles, indexing, and all mold movements. The system can be programmed to run four different jobs simultaneously and change conditions for each forming cycle.

Another manufacturer's development of a gear-drive rotation system for its rotary sheet-fed equipment has resulted in the large-scale reduction of moving parts. The aim is to provide more positive and accurate indexing and avoid inertia overdrive so that more precise large parts, such as truck bed liners, hot tubs, and signs, can be produced from heavy sheet.

In order to thermoform high-melt-flow materials and thin sheets effectively, one machine builder has devised an air-support design whereby a hermetic chamber under the clamp frame is pressurized to provide a supportive cushion of air for the heated sheet suspended above it. The result is a sheet that can be formed at production speed without problems.

Enhancing process efficiency of sheet formers is an issue of continuous improvement to machine manufacturers. One builder recently developed a double-end machine with two forming stations that share the same oven. Consequently, downtime is eliminated while a part is being formed, and machine versatility is maximized by enabling two different parts to be formed at the same time.

Heating Advances

Traditionally, plastic sheets were heated by the single-sided method, with double-sided heating chosen occasionally. However, since the types of heater used--nichrome wire, calrod, and black-body heating units--had limited functionality, achieving a uniform heating pattern was very difficult.

One frequently used method of controlling heat and attaining uniformity has been to manually change the input of heat in certain areas by applying screens or shading devices between the heaters and the material. However, this could only be peformed on large zoning areas, making finite adjustment impossible.

Today, new heating developments include ceramic heaters, quartz heaters and smaller modular black-body units, as well as infrared sensors that help a thermoformer to determine the actual surface temperature of the sheet. All are characterized by increased versatility and finite adjustment capabilities. Proper forming temperature can be ensured on every cycle by measuring actual sheet temperature rather than by using input timers to handle heater control.

A benefit of finite adjustment is selective heating. Now, instead of the sheet's having to be uniformly heated, the shape of the part and its complexity determine the amount of heat to be applied in certain areas. Therefore, it is possible to control the thickness of a material as it is formed into the mold: a higher heat can thin the sheet in a specific area, while cooling off another area can increase thickness.

One manufacturer has turned to computerized heating control on its new industrial forming machine. The benefits are simplified initial heater settings and automatic programming of forming-area temperatures via a single keyboard and cursor locations on a VDT, rather than through manual adjustment of individual potentiometers for each heat zone. Automatic mold-changing systems, as well as automatic setup of forming conditions and machine variables when changing jobs, increase overall productivity.

A newly introduced rotary thermo/pressure forming machine designed for the automotive industry features precise heat control and energy-efficient ovens. Programmable controls allow profiled sheet heating patterns, complete repeatability, and heating cycle control accuracy. Energy efficiency is attained by having both the upper and lower ovens insulated and fitted with the customer's choice of heater type.

Under development are manufacturing processes using selectively heated tools and modified (existing) shuttle equipment that enable the processor to heat two sheets simultaneously in a single clamp frame (twin-sheet forming). This will be accomplished by superheating and regulating the inflation air with flow-control meter.

The objective is to create, by altering an existing piece of equipment, a process by which any processor can utilize high-performance materials.

Pinpointing Material Flow

Because thermoforming essentially entails taking a two-dimensional sheet and stretching and forming it into three-dimensional part, nonuniform material distribution occurs in different areas of the wall where thickness varies.

Predicting those thicknesses has been left to the mercy of such geometric calculations as draw and stretch ratios, which can estimate the relative thickness of the sheet based on the specific forming process used.

Draw ratio is defined as the ratio of the greatest part depth or height to the narrowest part width (see Fig. 4). Draw ratio can approximate stretching and thinning of the thermoplastic sheet as it is formed into its final configuration. A material's limiting draw ratio is a function of the material's properties and the specific thermoforming process. While a maximum draw ratio of 1 is generally preferred, higher ratios can be achieved through the use of plug assists, billow forming, snap-back forming, or other process techniques. Designing the tooling to form more than one part simultaneously can modify the effective draw ratio. For instance, a deep narrow part with a draw ratio of 3 could be designed so that three parts could be formed at one time. The result would be the same formed depth, an increase in width by 3, and a draw ratio of 1.

Stretch ratio is calculated by dividing the area of the usable blank prior to forming by the net area of the formed part. Based on the conservation of mass, the resulting average thickness of the formed part will be the original thickness multiplied by the strech ratio. However, a part designer should be aware that since this is only an average thickness, there will be portions of the formed part that will be much thinner than this theoretical average.

Conservation of mass. The conservation of mass approach has two primary disadvantages. The first is that it does not incorporate any material model to calculate thickness distributions, but merely conserves mass of the polymer stretches into the mold. The second is that this technique cannot be easily extended to more complex, three-dimensional problems.

It is assumed that the strectching and thinning of the sheet as it is formed into the mold will stop once the heated sheet makes contact with the cooler mold or plug assist. Therefore, by studying mold geometry, plug assist, the amount of drape or sag of the heated sheet, and the forming process, one can estimate which portions of the sheet will cool first, and, as a result, will be thicker or thinner than the estimated average thickness.

Sheet Thinning Analysis

for Thermoforming (STAT)

While it is standard industry practice to specify a minimum wall thickness or indicate the starting gage of the sheet material, material flow must be predicted much more accurately when aerospace, automotive, and other precision components are at issue.

With this in mind, GE Plastics developed a process simulation computer program for designing thermoformed parts. This program, called Sheet Thinning Analysis for Thermoforming (STAT), helps to reduce design development cycles on application by minimizing time-consuming and costly prototype test procedures. STAT, which is based on GE Plastics' Polymer Inflation Thinning Analysis (PITA) program for blowmolding, uses finite element analysis techniques that enable a designer to predict material distribution in a thermoformed part.

The STAT simulation tool shows the feasibility of a design and either confirms or invalidates engineering assumptions made during the design phase. Designers then have the flexibility to consider several different design concepts and processing methods before creating an actual prototype. This not only reduces the time traditionally spent on prototype testing, but often results in a design or processing method that would not have been developed simply by trial and error. It also minimizes material and processing costs while at the same time maximizing part performance.

STAT can be used to simulate the optimum processing and design of such thermoformed parts as automobile bumpers and ground-effect packages, packaging containers, medical parts, computer and business equipment, and aircraft interiors and components.

Information for the STAT program is generated when engineers construct a three-dimensional model for the software using either engineering drawings or hand-fabricated prototypes. Once the model is generated on STAT, engineers can then simulate numerous variations of processing techniques, including male and female molds and plug assist forming. The design is subsequently transferred to a standard structural analysis program in order to determine part performance.

Part of the STAT development process involved vacuum forming three different GE thermoplastics--XENOY sheet (polycarbonate-polybutylene terephthalate), LEXAN sheet (polycarbonate), and ULTEM sheet (poletherimide)--into a rectangular cavity with an undercut along one side (see Fig. 5). Finite element analyses were performed to simulate the thinning behavior of each polymer as it stretched into the mold cavity to produce final parts with the dimensions shown in Fig. 6.

The predicted wall thickness distribution was in excellent agreement with the experimentally derived data. The program is now being expanded with design additions in the female cavity and plug assist areas that will permit a processor to increase productivity and become more cost competitive.
COPYRIGHT 1990 Society of Plastics Engineers, Inc.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:Mulcahy, Charles M.; Berns, Evan M.
Publication:Plastics Engineering
Date:Jan 1, 1990
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