Thermoforming vs. Injection Molding: What's the Difference?

Jun. 24, 2024

Thermoforming vs. Injection Molding: What's the Difference?

Used in all types of industries, plastics provide versatility and strength across a wide range of applications, from automotive body parts to human body parts. Each application requires a unique manufacturing process that can mold the part based on specifications.

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Both thermoforming and injection molding&#;two of the most popular manufacturing processes for crafting plastic parts&#;offer unique advantages depending on the particular application. While thermoforming is commonly used for large-scale designs and shorter production runs, injection molding tends to be a better choice for small, intricate parts and large production runs.

What Is Thermoforming?

Thermoforming is the process of forming a heated plastic sheet to the surface of either a male or female mold. This is a single-sided plastic fabrication process, unlike injection molding; only one side of the plastic sheet is controlled by the mold or tool. Vacuum forming and pressure forming are both popular styles of thermoforming.

Depending on a project&#;s needs, thermoforming can offer several distinct advantages, including:

Lower tooling costs compared to injection molding
Quick product development and prototyping
Bright color and texture options
Extreme adaptability and simple adjustments

Thermoforming is ideal for smaller production quantities (250 to parts per year).

Lower Tooling Costs

Tooling for thermoforming is less expensive than injection molding. Molds for thermoforming are often made of inexpensive aluminum. In contrast, injection molds are typically composed of thicker aluminum, steel, or other heavier alloys to withstand higher pressures and enable continuous reuse over larger production runs. In addition, thermoforming uses only a single-sided tool rather than a double-sided injection mold. This results in virtually half the material usage for mold-making, which significantly reduces the up-front cost of thermoforming. However, the molds are less durable and cannot be used for large scale or repeat production.

The size of the component plays a significant role in thermoforming's cost-effectiveness compared with injection molding. The larger the component, the wider the gap between tooling costs. Thermoforming becomes increasingly cost-effective as the part increases in size.

Quick Product Development and Prototyping

Due to the speed with which thermoforming molds can be produced, thermoforming is much faster than injection molding when it comes to product development and prototype testing. Injection mold tooling is more time-consuming, as the molds are double-sided and composed of harder materials such as steel. Comparatively, thermoform molds are easier to design, fabricate, and modify, making them ideal for development and testing.

Versatile Texture and Bright Color Options

Thermoforming offers a variety of benefits for product design and branding. Bright colors can be incorporated into thermoformed plastics, facilitating vivid, durable coloring throughout the material. In addition, thermoform materials accept painting, silk screening, printing, stenciling, and coatings that provide unique designs, textures, and finishes to enhance the appearance and the longevity of the product.

Extreme Adaptability and Simple Adjustments

Since thermoforming uses a simple single-sided mold made from highly formable materials, thermoform designs can be modified quickly and with minimal cost. Injection molding, on the other hand, requires dual molds with heavier materials that are more time-consuming and expensive to tool.

Applications That Use Thermoformed Products

The lower costs, versatility, and adaptability of thermoforming make it ideal for many applications, including:

Automotive: Dashboards, seat components, interior panels, bumpers, and air ducts
Aerospace: Air ducts, seat components, interior panels, galley equipment, and window shades
Construction: Equipment housings, tool cases, interior, and exterior panels
Medical: Diagnostic and imaging equipment housings, bed and furniture components, assistive equipment, and wall and ceiling panels
Public transportation: Interior and exterior vehicle panels, seat components, dashboards, and light fixtures
Office equipment: Fax, printer, computer, and copier housings, electrical panels, wall and ceiling panels, and furniture

What Is Injection Molding?

Injection molding requires a great deal of upfront design and engineering to develop detailed tooling or molds. Crafted from stainless steel or aluminum, split-die molds are injected with molten liquid polymers at high temperatures under extreme pressure. The molds are then cooled to release complete plastic parts.

Plastic injection molding offers several distinct advantages of its own, including:

Detailed, highly engineered tooling with multi-cavity mold options
Precise, efficient processing for large volumes of small parts
Effective reduction of piece count
Efficient material use and low scrap rates

Plastic injection molding is ideal for large-volume orders and mass production in projects requiring thousands or even millions of the same part.

Detailed Tooling for Complex Parts

One of the primary benefits of injection molding is the ability to create extraordinarily complex components with an exceptional degree of detail. The high pressure used in the injection molding process allows the production of intricate components and unusual geometries, as material is forced firmly into even the smallest detailed cavities. Multi-cavity mold options allow the injection molding process to be further tailored to meet specific needs.

Extreme Precision and Efficiency

Injection molding involves the use of durable and reusable molds for repeated runs. Users may rely upon the mold to provide highly accurate, repeatable results for large production runs over many years. The process is particularly useful for extremely small, complex, and intricate components that are time-consuming or difficult to create using thermoforming, cutting, milling, and other fabrication methods.

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Plastic Injection Molding Cost Reduction

Although injection molding is more expensive than thermoforming, the mold design and manufacturing process may be modified in various ways to reduce overall production costs. Simplifying and streamlining the design can reduce some of the cost that goes into creating detailed molds. In addition, employing material reduction methods such as undercutting and coring, or simply modifying molds from a similar product may offer means to affordably meet the needs of a new project.

Efficient Material Use

Injection molding is a highly efficient process with extremely low scrap rates. The amount of material for each component is precisely measured to ensure that the mold is completely filled, ultimately generating little overflow or waste. An injection molded product can be molded to scale and requires little additional tooling after it is ejected from the mold.

Applications For Injection Molded Products

Applications that benefit from injection molding include:

Construction: Hand tools, fasteners, door and window locks, joiners, and handles, and other building accessories
Aerospace and automotive: Turbine blades and housings, lenses, panels, and gears
Food and beverage: Food-grade plastic for conveyor, filtering, and processing systems; and food and beverage packaging
Medical and pharmaceutical:Medical device components, diagnostic kits, X-ray components, and surgical prep kits
Point-of-purchase: Retail display equipment, such as dividers, hooks, and product stops

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Material Alternatives for Plastic Injection Molding

Plastic Alternative: PPO (Polyphenylene Oxide)
PPO Trade Names: Noryl, Tecanyl

PPO has high tensile and impact strength and is resistant to many chemicals including steam and hot water as well as mineral and organic acids but is sensitive to stress cracking. Despite its many attractive properties, PPO and its derivatives have found only limited commercial use.

The susceptibility of PPO to thermal oxidation in relation to its high glass transition temperature poses a significant problem for melt processing. For this reason, commercial resins are often blended with high-impact polystyrene (HIPS) with which it is fully compatible or with polyamide (PA). Some filled grades of PPO find uses in the automotive and electronic industry. Examples include pump parts, fan impellors, catalyst supports, plugs, insulators, and household articles. Commercial grades of (modified) polyphenylene ethers are available from Ensinger Ltd. (Tecanyl), and SABIC (Noryl).

Besides pure PPO, also blends of PPO and polystyrene and polyolefins (PP) are available that offer a wide range of flexibility, toughness, and flame retardant capabilities. The resins are available in injection-moldable, extrudable and foamable grades.

Plastic Alternative: PPS (Polyphenylene Sulfide)
PPS Trade Names: Fortron, Ryton, Torelina

The most widely used polyphenylthioether is poly(para-phenylene sulfide) (PPS), also called polyphenylene sulfide (PPS). It is a semi-crystalline engineering thermoplastic with a relatively high melting point of about 560 K. Because of its high melting point and its poor solubility, special processing methods have to be used to manufacture products from this resin. For example, PPS can be compression molded at temperatures between 550 and 650 K, at which PPS softens and undergoes crosslinking reactions to yield a totally insoluble plastic. PPS has outstanding heat and chemical resistance, good dimensional stability as well as high tensile and flexural strength due to the aromatic ring structure of the polymer backbone. Although the mechanical properties drop somewhat with increasing temperature, they level off at approximately 395 K, and moderately high mechanical properties can be expected up to 530 K. PPS is typically reinforced with glass fibers or mineral fillers. These grades have improved mechanical strength, are noticeably stiffer (higher modulus), and exhibit better strength retention at elevated temperature (higher heat deflection temperature, HDT). They also exhibit improved flame retardant capabilities and exceptional electrical and electronic properties, including outstanding dielectric strength, which is exceptionally stable over a wide range of temperatures and frequencies.

Poly(p-phenylene sulfide) is the most widely used polythioether. This semi-crystalline engineering thermoplastic has many attractive mechanical and electrical properties. For example, it offers high thermal stability and outstanding dielectric strength, which is exceptionally stable over a range of temperatures and frequencies. Because of these properties, it is extensively used for electrical and electronic parts such as plugs, connectors, relays, switches, and encapsulation of electronic parts. Other applications include mechanical parts in automobiles and precision engineering like air intake systems, pump parts, gaskets, valves, bushings, and bearings, particularly for service in corrosive environments.

Plastic Alternative: PPSU (Polyphenylensulfone)
PPSU Trade Names: Ultrason, Radel

PPSU is the highest-performing polysulfone. It is known for its high toughness, high flexural and tensile strength, excellent hydrolytic stability, and good resistance to chemicals and heat. Compared to the two other polyethersulfones, PSU and PES, it has superior mechanical properties, but it is also more expensive, and thus, less widely used. It also has the best chemical resistance of all polyethersulfones. For example, it is highly resistant to aqueous mineral acids, bases, and oxidizing agents and most solvents. However, aromatic solvents and oxygenated solvents, such as ketones and ethers, might cause some stress cracking. PPSU is often an excellent choice for components that are exposed to high temperatures and corrosive media because it has exceptional chemical resistance. Examples include pipe fittings, battery containers, medical device parts, and sterilizable products for health care and nursing. Polyphenylsulfone is also used in the automotive and aerospace industries for applications where superior thermal and mechanical properties relative to conventional resins are required. However, most (unfilled) grades are not suitable for outdoor uses because of poor weathering, ozone, and UV resistance.

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