Thermoforming is a widely employed manufacturing process used to shape sheet thermoplastic materials to required profiles/shapes through the application of heat and pressure/vacuum. The various techniques within the umbrella term thermoforming include: vacuum forming, pressure forming, and combined vacuum/pressure forming. Each variant offers specific advantages based on the desired outcome and application requirements.
Thermoforming is employed in the manufacture of both simple and relatively complex single-sheet parts that follow a relatively low-stress process, by avoiding sharp transitions in the mold profile.
This article will discuss what thermoforming is, including its function, process, types, and uses.
Table of Contents
What Is Thermoforming?
Thermoforming is a simple process whereby a heated sheet of thermoplastic is stretched across a 3D-profile former and then forced into close conformity with it. It uses either pressure above, vacuum below, or both. The material is then cooled and released from the former, retaining the shape imposed upon it.
What Is the Function of Thermoforming?
Thermoforming is a low-equipment-cost method for producing 3D profiles from flat, thermoplastic materials. It allows the production of surprisingly complex net shapes with remarkable functionality.
Consider the example of the folding egg tray that is still widely employed in the retail packaging of eggs. This integrates a range of functions into a remarkably durable product with a 50+ year history of market success. The individual nests retain the egg in gentle support with a close fit. The product integrates shock-absorbing features that afford a high degree of damage protection simply by adding local collapse features and keeping the egg apart from the upper and lower package surfaces, to allow space for motion, so the impulse forces on the shell are reduced. The clam-shell design means one sheet of material makes both halves of the box. A pinch or partial piece at the hinge point allows local hinging, exploiting the crystalline resilience at the flex point for multiple closure operations without fracture. Snap features are integrated into the one-stage thermoforming process, so the box can be snapped closed until the consumer easily opens it.
How Do Thermoforming and Other Molding Types Differ?
Thermoforming is virtually alone in not utilizing a liquid thermoplastic stage. Injection molding uses a liquid stage. Extrusion forces a solid to near-liquid condition by shear. From this starting point, thermoforming can form complex 3D profiles. But in geometrical terms, the material is still essentially 2D (the original sheet distorted) rather than a reformed liquid. This means that the inter-molecular relationships of the origins sheet are distorted but essentially intact.
To learn more, see our full guide on Molding Types.
What Is the Process of Thermoforming?
A thermoplastic sheet is heated until it meets the required viscous, rubbery, semi-solid state, generally high up the glass-transition range. The necessary temperature for this is at the upper end of the glass-transition range, whereby interchain bonds that form the crystalline matrix are weakened and motile but not fully overcome.
The heated sheet is molded into a specific shape using two steps. First, the tool is raised on a movable table that rises to meet the underside of the sheet. This forces the heated polymer to drape over the tool, conforming to height but not overall shape. Then additional force is applied by vacuum below or pressure above, or a combination of these. In unusual cases, an upper tool component can be employed to assist in forming particular regions.
The formed part is cooled to return the material to the rigid state, retaining its formed shape as part of a now 3D sheet. Vacuum or pressure is retained during cooling, to prevent shape relaxation as the material returns to rigidity.
The completion of the molding process leaves a 3D profile (or multiples of the same profile) either as a male or female net shape relative to the sheet’s position. This incomplete part is removed from the thermoforming machine, either by hand or by automation. It is then passed to a trimming stage, in which excess material is cut away to achieve the final product. The trim can be manual or automated, cut or punched.
What Are the Two Thermoforming Processes?
Material gauge is one useful differentiator of thermoforming processes, with thinner gauge materials being primarily focused on single-use applications and thicker materials being more applied to situations requiring more endurance. Listed below are two custom plastic manufacturing processes:
1. Heavy-Gauge Thermoforming
Heavy-gauge thermoforming is a specialized classification of process, targeted at thicker polymer sheets, typically of 1.5 mm and greater. This type of material is employed when producing large, durable, and structurally robust plastic parts, often with complex geometries. However, localized internal curvatures are more limited as the material thickness increases. The depth of features can be quite large, however, as the material is forced to stretch and thin over higher tooling, local thinning occurs at pronounced features and stretch zones.
Heavy-gauge thermoforming is widely used in industries requiring sturdy and enduring-use components, such as: automotive panels, aerospace interior panels, and industrial applications such as: drip trays, multi-nest part trays, and architectural finishing pieces. It offers advantages in cost-effectiveness, quick production cycles, and the ability to create parts with intricate details and varying wall thicknesses without carrying high tooling costs.
An exception to the general durability of heavy-gauge thermoforming is in the molding of thicker foam materials such as takeaway food trays, which are often thermoformed in polypropylene, polyethylene, and polystyrene foams to make simple trays that are thick but of very low density and durability.
2. Thin-Gauge Thermoforming
Thin-gauge thermoforming is a process utilizing thin thermoplastic sheets, typically ranging from 0.008 to 0.080 inches (0.2 to 2.0 mm). This implies an overlap thickness range in which thin- and thick-gauge thermoforming overlap, leaving the definitions open to interpretation.
This type of material gauge is commonly used for producing lightweight, disposable products such as: packaging, blister packs, single-use trays/food containers, and some consumer goods of higher durability.
It offers great cost-effectiveness through rapid production cycles and the ability to create intricate and detailed parts from very low-cost materials and simple processing/tooling. This process is widely employed in industries in which high-volume production of thin-walled plastic components is beneficial and commercially necessary, but high costs cannot be borne.
How Long Does the Thermoforming Procedure Take?
The duration of the thermoforming procedure is highly variable and must be assessed on a per-design basis. It varies based on factors such as: material type, sheet thickness, and part complexity.
Generally, thin-gauge thermoforming processes for simple shapes may have shorter cycle times, often ranging from a few seconds to a minute or more per cycle. However, heavy-gauge thermoforming can have longer cycle times, spanning several minutes. High-volume and automated production systems may incorporate multiple molds and continuous processing for efficiency.
What Type of Mold Is Used in Thermoforming?
Molds used in thermoforming are very varied. A simple and slightly shaped piece of wood can act as a tool to form a basic tray or similar simple shape. More complex wooden and MDF (medium-density fiberboard) assemblies are often employed for more intricate profiles that are not expected to deliver long durability. CNC-cut tooling is widely used in higher volume and higher precision applications. This can be MDF, aluminum, and occasionally steel where long tool life is required.
In all cases, the tool is flat on one side and must have a tapered and narrowing profile, without significant axial enlargement at any point. Limited undercuts can be formed and bumped or stretched off the tooling so long as the design allows sufficient motility/elasticity in the region surrounding the feature, so the cooled plastic can come away from the mold without risk of fracture.
What Are the Materials Used in Thermoforming?
Thermoforming is limited to thermoplastic sheet materials, the process requiring softening by radiant heat or hot air as the first step. The most common materials applied are:
Polystyrene (PS) and high-impact polystyrene (HIPS).
ABS (acrylonitrile butadiene styrene).
Polycarbonate.
PET (polyethylene terephthalate) and PETG (polyethylene terephthalate glycol).
High-density polyethylene (HDPE).
How Much Does Thermoforming Cost?
Thermoforming is a low-technology process that can be implemented with higher technology enhancements such as process automation for: loading, unloading, handling/trimming, and potentially all stages.
Typically, the part cost for a thermoformed component is between 5 and 50 times higher than for the equivalent injection molded part, depending on whether the process is manual and labor-intensive or automated. This is a result of higher raw material costs and very high labor content potential.
However, the tooling costs can be a factor of 100 less for thermoforming, and economies of scale make low-volume injection molded parts particularly expensive because of this factor.
Overall, if volumes are low, a manual thermoforming process will always be lower cost than injection molding. Whenever volumes are higher, even with automation costs, thermoforming can be competitive with injection molding, as evidenced by the use of thermoforming in the automotive sector.
Is Thermoforming Cheaper Than Injection Molding?
Yes. It is considerably lower cost to establish production by thermoforming than by injection molding. The machines for basic thermoforming are 100 to 1000 times cheaper than for injection molding. The infrastructure requirements for thermoforming are minimal, whereas for injection molding they are considerable — down to high-strength floors to support heavy machinery.
Labor costs for the staff capability required in thermoforming are similar. However, the volume productivity of that labor is very high for injection molding. It’s also high for automated thermoforming, rendering this a competitive option.
Materials costs for injection molding are considerably lower than for thermoforming. Raw plastic granules can cost as little as $5 per kg, whereas even the lowest-cost HDPE sheet is likely to cost 5 to 10 times that price.
What Are the Different Types of Thermoforming?
Thermoforming falls into two process categories separated by the application of pressure above the sheet or vacuum below the sheet. In some limited cases, a hybrid of the two is used in the equipment, which some claim has improved results. The differentials are not great, however, so the higher equipment cost of hybrid pressure is often considered to be low value.
1. Pressure Forming
Pressure forming uses air pressure above the softened sheet, to force conformity to the tooling profile. Pressure-based processes are slightly lower cost to implement, as pressure sources are easier to make than vacuum sources.
A slight disadvantage is the somewhat increased potential for air entrapment within certain tooling features, although this can be countered by adding clear venting paths into the tooling.
2. Vacuum Forming
Vacuum forming applies a vacuum below the sheet to pull the material onto the tooling profile. This is essentially identical to pressure thermoforming, except the pressure above is atmospheric.
Vacuum equipment can be harder to regulate than a high-pressure source, but many consider this forming method to be superior. In particular, it allows better visual monitoring of the forming process as no upper enclosure is required, obstructing the operator’s view.
To learn more, see our full guide on the Vacuum Forming Process.
What Are the Uses of Thermoforming?
Thermoforming is a very widely used manufacturing process that allows relatively complex products to be made from essentially primitive tooling and a technically simple process.
It is used for forming 3D profiles in 2D materials that are thermoplastic:
For packaging containers, blister packs, and disposable utensils.
For interior automotive components and vehicle body panels and buffers.
For medical trays, equipment housings, covers/skins for prostheses, and dental molding products.
For interior paneling and seat components for aerospace.
For signage and display panels.
What Are the Industries That Use Thermoforming?
Most industries have the potential to make use of thermoforming, but the most common applications are in:
Food and product packaging
Automotive
Aerospace
Medical
Signage
How Is Thermoforming Used in the Manufacturing Industry?
Thermoformed components are either finished products or components to be integrated into finished products. Some examples are:
Egg trays are finished products, thermoformed, and used as-is.
Blister packs can be considered finished products ready to fill/close or they can be components to be integrated into packaging, such as blister windows built into card packages.
Medical and food trays are thermoformed and used directly, often with minimal finishing.
Automotive and aerospace panels and interior trim parts are built into vehicles.
Cosmetic parts for human prostheses are generally non-structural finishing components added as aesthetic finishers to the product.
Machine housings are components added as hygienic or aesthetic surfaces into equipment such as machine tools and medical instruments.
Signage parts are commonly made by thermoforming to give high aesthetic qualities to improve the overall impact of displays.
What Is the Quality of Thermoformed Plastic Sheets?
Thermoformed plastic components can be of exceptionally high quality, depending on the control of the process and the skill of the operators.
They depend heavily on three factors as listed below:
Good design that allows close and web/fold-free conformance to tooling.
Good surface finishes on the tooling, particularly for thin-sheet thermoforming in which there is a strong tendency for surface finish and unintended features to be reflected in the outer and generally cosmetic face of the formed component.
Quality and suitability of materials — choosing an excessive thickness of material causes loss of features and poor conformance; choosing a poorer standard of material will generally result in a lower quality product/component.
What Are the Advantages of Thermoforming?
Thermoforming has various significant advantages:
Cost of tooling is low compared with almost any other process.
Cost of equipment/setup is among the lowest of any plastic-forming process.
Materials options are considerable, both in polymer type and thickness.
Component quality can be very high when the tooling and process are well executed.
Component cost, when volumes are low, is overwhelmingly competitive with alternative processes.
Skill levels that are low to moderate can still produce usable results when the design is conducive to the process.
What Are the Disadvantages of Thermoforming?
Thermoforming does have some severe and restricting limitations and disadvantages:
Thickness variations can be hard to control as the material stretches over the tool profile.
Conformity to complex features can be hard to achieve repeatedly.
Most engineering materials and all exotic polymers are not available in sheet form or are not amenable to the process.
Designs must account for the trimmed edges, which are generally impossible to make highly cosmetic without considerable and moderately skilled labor.
The process is slow and costly when fully manual, or is considerably more expensive to set up when automated.
Volume productivity is low without heavy investment in automation, altering the cost-implications of the parts produced.
Quality can be operator-dependent, so the best outcomes do require some skill and judgment.
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