Thermoforming

Chapter 1: What is Thermoforming?

Thermoforming is a plastic manufacturing process that uses pressure or the force of a vacuum to stretch thermoplastic material over a mold to create a three-dimensional shape, part, configuration, or other form of plastic product. Cups, containers, lids, trays, and clamshells are formed by thermoforming using thin sheets of thermoplastic, while thicker sheets of thermoplastic are used to produce car doors and dash panels, refrigerator liners, and plastic pallets.

The two processes used for thermoforming are vacuum forming and pressure forming, which are used to stretch the heated thermoplastic over the surface of the mold. Although the two processes are similar, they have unique properties that make them applicable to fit the needs of a project’s design and must be chosen in accordance with a project’s requirements.

The forming phase of thermoforming happens in a mold cavity when the plastic sheet is drawn by air or vacuum pressure. The mold cavity contains the shape of a single part. The mold tool, sometimes referred to as "tooling", is a collection of mold cavities.

The steps of thermoforming are simple and straightforward, which makes it suitable for high-volume manufacturing of molded products due to its fast turnaround times. Thermoplastic sheets, are continuously fed into the heating chamber and formed into the desired shape. For the thermoforming of larger parts, the thicker thermoplastic sheets are fed individually. In some operations, an extrusion machine is placed upstream of the thermoforming machine. Certain set-ups are designed to produce multiple parts with each stroke of the press using molds with several cavities.

Chapter 2: Thick (Heavy) Gauge and Thin Gauge Thermoforming

The part thickness determines the gauge of thermoplastic that is used for the manufacturing process. The different thicknesses require machinery and techniques applicable to fit the material’s thickness. A wide variety of materials are used in the thermoforming process and require that designers are aware of the characteristics of the various materials in order to provide the highest quality, timing, performance, and reliability of the end product.

• Thick Gauge or Heavy-Gauge Therforming

  Thick or heavy-gauge thermoforming produces parts from 0.060" – 0.500" (1.5 - 12.7 mm) thickness. Cut sheets of thermoplastics are the starting material, which is heated in an oven. Heavy gauge thermoplastics are used to create thicker and more durable parts that have permanent end-applications. The products produced using heavy gauge thermoplastic are lighter and have superior impact resistance.

  The higher gauge of thick thermoplastics makes it possible to produce complex and intricate parts that have smoother shapes with exceptionally attractive appearance. Additionally, the thermoplastic can be colored to match the requirements of a product or application. Some of the positive properties of thick gauge thermoplastics include ultraviolet (UV) protection, flame retardance, electrical conductivity, and solvent resistance. As with thin grade thermoplastics, thick grade thermoplastics can be manufactured from FDA-approved materials.

  

  
  

• Thin Gauge & Thermoforming

  Thin gauge thermoforming produces products with thicknesses of less than 0.060" (1.5 mm). Thermoplastics are roll-fed or come from an upstream extrusion process. Thin gauge thermoforming, produces thin products; which are intended for disposal or recycling but are an important part of everyday life. Cosmetic packaging, candy trays, clamshells, and display packaging are some examples of thin gauge thermoforming. Production of thin gauge thermoplastics is quick with high volume runs and is customizable.

  FDA Thermoforming Grade is a thin polypropylene (PP) that has been approved for food packaging due to its resistance to chemicals. It is 60% the density of PVC film. PP meets the safety standards that specify that the material, when it degrades, will not be a threat to human health.

  

  
  

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Chapter 3: Thermoforming Process

The thermoforming process takes a sheet of thermoplastic, carefully heats it until it is sufficiently pliable, places it over a forming mold that forms it into a three-dimensional shape, and completes the process by trimming and finishing it into the desired shape of the product. It is a simple process that is quick, efficient, time-saving, and highly productive.

Regardless of the simplicity of thermoforming, each step of the process has to be completed with precision and accuracy in order to produce quality parts and products. Any errors can lead to deformed, damaged, and useless sheets of plastic.

Heating Plastic Sheets:

Plastic sheets to be molded, which has length and width greater than the finished product, is clamped into a holding device and transported into a heating equipment to raise it to the forming temperature. The sheet is heated by contact heating using panel and rods (conduction), by exposing them to circulating hot air or using infrared heaters.

The type of heating system is chosen depending on the material and the amount of necessary heat. The heating process is critical to the forming process since it creates the necessary pliability and flexibility.

Forming temperature vary depending on the type of thermoplastic being used, the application for the finished part, and the forming technique. This is one of the most important operating parameters in thermoforming to meet certain quality standards. Take note that the true forming temperature of a sheet is its core temperature, not its surface temperature. Hence, it is important to calculate heat transfer across the sheet.

When it comes to the variation of temperature across the sheet, the 10-10-5 rule must be met. The first 10 applies to the 10 locations on the sheets, which includes both sides of the sheet, each of its four corners, and the middle of the four sides. The next 10 refers to the allowable variance of 10°F (-12.2 ° C) in the 10 locations. Five refers to the temperature on both sides of the sheet in each of the 10 locations with an allowable variance of 5°F (-15°C). The 10-10-5 rule applies to heating, forming, and cooling to achieve optimal thermoforming.

Forming Plastic Sheets in Mold Cavities:

Heated plastic sheets are removed from the heating equipment and transported to a temperature-controlled and pre-heated mold tool. At this stage, the plastic sheet takes the shape of the mold cavity, which contains the desired form of the finished product. This stage gives the product its three-dimensional characteristics (length, width, and height).

The mold tool may be a positive or a negative tool, depending on its form:

• Positive Tool

  Positive Tool, or "male mold" is convex-shaped - the heated plastic sheet is positioned above the convex tool. The "humped surface", or the convex surface, will now give the plastic sheet its final shape. The exterior surface of a positive mold tool will give the shape of the inner surface of the part.

  

  
  

• Negative Tool

  Negative Tool, or "female mold", on the other hand, is concave-shaped - the interior surface contour of a negative mold tool will give the shape of the outer surface of the part.

  

  
  

After forming, the plastic containing the new shape solidifies by cooling using air circulation or liquid cooling systems. The tool material used significantly affects the cooling cycle, thus also affecting the quality of the parts.

Additional shaping steps are involved, with thick gauge thermoforming, such as drilling, cutting, or finishing.

Trimming Formed Sheets:

The sheet containing the formed parts goes through a trim station or five-axis CNC router, where a die, abrasive wheel, or circular saw cuts the parts to separate them from the sheet web. The trimmings are recycled and reprocessed to form other parts.

Chapter 4: Leading Thermoforming Machines

There are many machines available for thermoforming in both the United States and Canada. These machines are important in today's society because thermoforming allows for cost-effective and efficient manufacturing of various products and packaging, contributing to industries such as food, medical, automotive, and consumer goods. We provide information on many of the leading thermoforming machines below.

Brown Machine's C-Series Thermoformer

Brown Machine is a prominent manufacturer of thermoforming equipment. The C-Series Thermoformer is popular for its efficient and reliable performance. It offers precise control over heating, cooling, and material handling, making it suitable for producing a wide range of products, from thin-gauge packaging to heavy-gauge industrial components.

SencorpWhite's 2500 Thermoformer

SencorpWhite is known for its high-quality thermoforming solutions. The 2500 Thermoformer stands out for its ease of use, rapid tool changeovers, and advanced process control. It is commonly used for manufacturing medical trays, blister packs, and other precision-formed products.

GN Thermoforming Equipment's GN800 Thermoformer

GN Thermoforming Equipment is a reputable manufacturer of thermoforming machines. The GN800 Thermoformer is popular for its versatility, capable of handling a wide range of materials, including PET, PS, PP, and PLA. It offers excellent energy efficiency and precise control, making it suitable for producing items like food containers, electronic components, and consumer goods.

WM Thermoforming Machines' FT and FC Series

WM Thermoforming Machines is a leading European manufacturer with a strong presence in North America. The FT and FC Series are well-regarded for their robust construction, reliable performance, and high throughput capabilities. These machines are commonly used for producing items like cups, trays, and packaging materials.

Multivac's R-Series Thermoformers

Multivac, a global leader in packaging solutions, offers a range of R-Series Thermoformers. These machines are popular for their automation features, user-friendly interface, and high level of hygiene. They are commonly used in the food industry for producing vacuum packs, MAP (modified atmosphere packaging) products, and other food-related applications.

Please note that the popularity and availability of specific models may have changed since this posting. As a result, it is recommended that you check with the manufacturers or local suppliers to get the most up-to-date information on thermoforming machines.

Chapter 5: Types of Molds

The mold cavity to be used in the forming step is carefully designed by the manufacturer to create the required profile of the finished product, depending on the customer needs or end-user application. Initial stages in the development of a mold tool involve detailed drawings in the CAD software and CNC program to realize the desired patterns. Some of the materials used to create the mold tool are the following:

• Tooling with Wood:

  An inexpensive type of tooling material, wood can be shaped fairly easily; hence, the manufacturer can readily make complex designs or make changes to the details of the part. However, it poses many disadvantages: uneven and lengthy cooling (since wood is an insulator), moisture which can cause voids and potential transfer of wood grains to the part. Tooling made from wood is commonly used to produce prototypes and patterns for a production mold.

• Tooling with Fiberglass:

  As with tooling from wood, fiberglass is an economical, permanent mold tool if a manufacturer only produces lower volume parts. However, the cooling cycle is two to three times longer than a temperature-controlled mold.

• Tooling Aluminum:

  Aluminum provides excellent temperature control, which leads to shorter cycle times and excellent quality parts:

  • Cast Aluminum

    Cast Aluminum tools are derived from a machined pattern.

  • Fabricated Aluminum

    Fabricated Aluminum tools are made from a single or multiple aluminum blocks that are honed and cut to produce the mold. The fabricated tool is costly but more accurate in terms of dimensions, and the manufacturer can make more complex designs.

  

  
  

Chapter 6: Thermoforming Methods

The following are description of two common forming methods, vacuum and pressure forming.

Vacuum Forming:

With vacuum forming a vacuum is generated underneath the sheet to draw the plastic sheet against the mold cavity until it takes its desired shape. Vacuum forming is the simplest of all thermoforming methods. However, part thickness distribution is difficult to control. Vacuum pressure should be uniform and sufficient throughout the mold.

Pressure Forming:

Similar to the vacuum forming method, air pressure is utilized together with the vacuum applied under the cavity to push the plastic sheet. The added air pressure creates greater detail (e.g. textured surfaces, undercuts, and sharp corners) on the finished product; that is not easily created by vacuum forming, making this method suitable for products with complex designs.

Matched Mold Forming:

Matched mold thermoforming is where the heated thermoplastic sheet is shaped by a male and female mold, which can be made of metal, plaster, wood, or epoxy resin. When the halves of the mold are closed, they distort the sheet of thermoplastic to take the shape of the halves of the mold. As the mold closes, excess air is removed to form a tight seal by the application of a vacuum. The walls from matched mold forming are more uniform and adhere closely to design tolerances. The process allows for exceptional dimensional control and offers the ability to create intricate and complex shapes.

Twin Sheet Forming:

Twin sheet forming involves two plastic sheets simultaneously heated and formed using two mold tools for each half of the parts. The mold tools are then precisely pressed together on the edges to connect the two halves. This method is used in producing double-walled, three-dimensional parts and hollow tubes such as air ducts, pipes, and tanks.

Chapter 7: Materials Used in Thermoforming

Thermoplastics are the raw material of the thermoforming process. Thermoplastics are a broad class of polymers that can be heated to a certain elevated temperature and re-casted reversibly, without altering their chemical properties and associated phase change. It can survive multiple cycles of heating and cooling. Given this nature, thermoplastics can be reprocessed, and are recyclable materials. Only thermoplastics can be thermoformed. Thermosetting and elastomeric plastics, in contrast, cannot be reshaped once the polymeric chains have been cross-linked.

A forming temperature is any point located above the glass transition temperature and below its melting temperature. When the temperature of a thermoplastic is increased gradually, the intermolecular forces in the polymeric chains are also weakened gradually, until it reaches the glass transition temperature. Above the glass transition temperature, the once rigid and brittle solid is turned into a soft and pliable rubber-like material.

Thermoplastics are grouped into either amorphous or semi-crystalline structures.

Amorphous Thermoplastics:

These materials have a random molecular structure and have a wide range of softening temperatures. Some advantages of amorphous thermoplastics they have good dimensional stability, higher impact resistance, bond well with adhesives, and are easier to thermoform than semi-crystalline thermoplastics. However, they have poor fatigue resistance and are prone to stress cracking. Some of the amorphous thermoplastics are polycarbonate, acrylic and high-impact polystyrene.

Semi-crystalline Thermoplastics:

These exhibit an organized lattice at a temperature lower than its melting point. This type is known for its excellent wear and bearing resistance, making it ideal for structural applications and durable plastic parts. This type is also known for its better chemical resistance and insulation properties. Some disadvantages of this type: they are difficult to thermoform and/or bond with other formed parts, and they only have average impact resistance. Examples of semi-crystalline thermoplastics are polyethylene, polypropylene, and nylon.

There are many thermoplastics suitable for thermoforming. The table below presents the most notable:

Chapter 8: Problems and Quality Issues Encountered During Thermoforming

The key to successful thermoforming is proper tool management and design. In order to prevent contamination-related defects, all materials and tooling should be kept and uniform temperature and should be free from moisture and plastic buildup.

Parameters that need to be optimized and controlled in every thermoforming process are the following:

• Forming temperature
• Mold tool temperature
• Vacuum and/or air pressure
• Liquid or air coolant flow rate and temperature

Note:

1. When adjusting the forming temperature, ensure that it is still between the forming and the melting temperatures.
2. Part thickness distribution may be improved through pre-forming of the plastic sheet before it is drawn to the mold cavity.

Chapter 9: Pros and Cons of Thermoforming

The goal of thermoforming is to take a warm sheet of plastic and place it in or on a mold such that it takes on the desired form. Even though the process is simple and efficient, it produces highly durable and resilient products that are easily disposable and recyclable or long-lasting. From its beginning as an answer for aircraft design, thermoforming has rapidly grown to be a cultural phenomenon that provides convenience and superior quality.

Benefits of Thermoforming

Low Cost:Large parts are normally used in larger assemblies and products. Although they can be produced using other forming methods, thermoforming is capable of producing large parts at half the cost and in less time than any other plastic production method. From car door panels and instrument panels to tail lights and consoles, thermoforming can complete the job quicker, easier, and at less expense.

Durability:A key to modern production is the ability of products to last and endure the harsh and rugged treatment they receive. One of the main factors in customer satisfaction is how long a product will last and is a main marketing point. Heavy gauge thermoforming produces large, th

Tooling Costs:Thermoform molds are easily engineered using 3D printing or computer aided design (CAD). They are made from silicone, fiberglass, or other materials and do not require grinding, machining, or other forms of tooling. The creation of a metal mold is expensive, time consuming, and labor intensive. It requires highly experienced professionals with the proper skills.

Thermoforming molds are produced and placed in production on the same day. The materials are far less expensive than the steel and iron required for other molds but produce the same kind of high quality products.

Development:Thermoforming uses tools made from wood or epoxy. The tools for thermoforming can be used to create an assortment of finished parts that represent the initial design. Prototypes are formed from the same materials as those used for the final product, allowing for the identification of design flaws or issues before approving production tooling.

Design:Thermoforming has very few limitations in regard to designs regardless of the intricacies, details, or size of a part's design. This aspect of thermoforming is one of the main reasons for its popularity, especially in automobile design where the weight of components is a major concern.

Disadvantages

• Individual part costs can be higher than injection molding.
• Molded-in components, such as screws, fasteners, and clips, cannot be included.
• With any geometry, the front side will be the same as the back side.
• Part thickness can be an issue and may not be even across all surfaces of a part.
• All forms of thermoforming produce a great deal of waste, which can be recycled.

Conclusion:

• Thermoforming is the process of heating thermoplastics to their forming temperature and drawing them over a mold cavity wherein, they take on three-dimensional characteristics. Thermoforming has a wide range of applications.
• Thermoforming is grouped into two categories depending on the sheet thickness of the formed part: thick gauge thermoforming (0.060" – 0.500") - (1.5 - 12.7 mm) and thin gauge thermoforming (less than 0.060").
• The first step in the thermoforming process is heating of a plastic sheet to its forming temperature.
• Forming stages give the three-dimensional characteristics (length, width, and height) to the once flat sheet. Forming methods may be vacuum forming, pressure forming, mechanical mold forming, and twin sheet forming. Afterwards, formed parts are trimmed from the sheet web.
• Thermoplastics in the form of sheets are the starting material of the thermoforming process. These plastics can survive cycles of heating and cooling, allowing them to be recycled. Thermoplastics may be amorphous or semi-crystalline.
• The forming temperature is the temperature above the glass transition and below the melting temperature.
• The mold may be a positive or negative tool, and its material significantly affects the heat transfer across the sheet.
• The parameters to be optimized and controlled are forming temperature, mold tool temperature, vacuum and/or air pressure, and liquid and/or air coolant temperature.
• Proper tool management and temperature control is the key to successful thermoforming.

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