How does thermoforming differ from injection molding in terms of cost, speed, and application suitability?
In the world of manufacturing plastic products, two processes dominate the discussion, namely injection molding and thermoforming. Both are long-established processes that have been applied across multiple industries, including packaging, medical devices, automotive parts, and consumer goods. While these two techniques can be fairly similar, they are not interchangeable. Utilizing one technique instead of the other will likely lead to significantly increased tooling costs; longer lead times for getting product to market, and/or final product(s) that do not conform to your definition of quality. This guide will provide you with a straightforward comparison of both thermomolding and injection molding so that you can choose the method that best suits your needs when designing your next product.
The Basics: How Each Process Works
What is Thermoforming?
A thermoplastic sheet is made pliable by heating it to a specified temperature before forming it into the required shape. The mated thermoplastic sheet can then be cooled while held in the formed position. After being formed, the part is cut from the sheet; any excess material is referred to as “regrind” and can sometimes be recycled.
There are two basic types of thermoforming: vacuum forming (this method uses suction to pull an otherwise pliable thermoplastic tightly to the mold) and pressure forming (this method uses compressed air to apply pressure and force the pliable thermoplastic against the mold, resulting in better definition and detail than vacuum forming).
Thermoforming is most commonly used in producing large, shallow parts (e.g., trays, packaging, automotive panels, or medical device housings).
Injection Molding
The process of injection-moulding is melting plastic pellets and injecting the molten material (plastic) under high pressure into a closed metal mold. Once the plastic cools and hardens, the finished part is ejected from the mold with usually little or no secondary processing needed.
The process of creating injection-moulded parts allows for the production of very complex, highly toleranced parts with consistently uniform wall thicknesses and the ability to incorporate features such as threaded holes, snaps, bosses and other items directly in to the part and finished part, all while often eliminating or vastly reducing the amount of additional processing needed for the part (surface finish) before use.
You can find injection-moulded parts in a wide variety of products and industries, including, but not limited to, bottle caps, dashboards, electronic enclosures, syringes for the medical industry, and hundreds of other items that are used every day.
Cost Comparison: Upfront vs. Long-Term
The molds used to create each part are the most significant difference in cost between the two processes.
Tooling Costs
Thermoforming molds are usually made from aluminum or composite materials, which makes them much cheaper to produce. A simple thermoforming tool can range from $1,000 to $50,000, depending on its size and complexity, so there is a clear advantage for startups and prototypes, as well as for products that may not have a large enough demand to justify the cost of other processes.
Injection molds, on the other hand, are made from hardened steel and designed to withstand extreme pressures. A single injection mold could cost anywhere from a minimum of $10,000 for a simple single cavity to well over $500,000 for a complex multi-cavity mold. As such, lead times for tooling can take several weeks to several months.
Per Unit Costs
At high volumes, the costs of production per unit using injection molding can be reduced dramatically after the tooling costs for creating the mold have been accounted for. For production runs of 100,000 or greater, the amortization of the tooling cost becomes irrelevant, and unit economics remain competitive due to the high efficiency and speed of the process.
Thermoforming typically has higher per-unit costs at larger volumes as a result of trimming and waste associated with material, slower production cycle time, and more labor required to finish each unit.
Point of Break Even
As a rule of thumb, thermoforming can frequently be lower than the total cost of production for production volumes under 10,000 to 25,000 units; however, for production volumes in excess of that threshold, especially for smaller components, injection molding will generally be a lower-priced manufacturing process over the entire life of the product.
Speed: Cycle Times, Lead Times & Time-to-Market
Cycle Times
The cycle time in injection moulding is faster since many parts can be produced in one single shot, depending on the type of part and material (from 5 seconds to as much as 2 minutes). In high-speed multi-cavity moulds, dozens of parts can be created every minute.
While cycle times are longer in the case of thermoforming (anywhere from 20 seconds to many minutes for a part, depending upon how much trimming has to be done), there are also additional trimming times that add to this overall time for production. When dealing with large quantities, this slower production rate can have a significant impact on throughput.
Tooling Lead Time
Thermoforming has a big-time lead with regard to tooling lead times. Tooling is simpler to make and uses softer materials, allowing it to typically be completed in weeks, not months. This leads to much faster time-to-market, which is essential when products have very short launch windows or the speed of prototype is critical for the development team.
Injection-molded parts will have longer lead times for tooling because of the precision involved in making complex tooling with side actions, lifters, and multi-cavity layouts. These tools can take 3-6 months from the design phase through machining, sampling, and validation phases before flow-testing.
Thermoforming allows you to have a physical component in your hand weeks earlier than injection-molded products, which will assist with speed of iteration, demoing to investors, and validating the market before committing to costly production tooling.
Application Suitability: When to Use Which
Choose thermoforming when:
- You want a part size that is large, shallow, or wide – ex, packaging trays, refrigerator liners, vehicle panels, and point of sale display units
- Your production volume is low to medium (less than approximately 25, 000 units per year)
- You are in the prototype or early commercialization stage and need to control your initial costs
- Your design may still be changing – thermoforming tooling is inexpensive to change out/create new tooling
- Your surface finish requirement is moderate, with no need for complex integrated features
Choose injection molding when:
- You have large quantities – 10k-100k (i.e., high volume)
- Your part geometry is complex (internal passages/holes; threaded inserts; living hinges; force fit/snap-together; undercuts)
- Dimensional accuracy of your part must be exact (i.e., medical devices, electronics, precision assemblies), so no exceptions.
- Your part must have uniform wall thickness (throughout the part).
- Your ultimate cost per part must be as low (long term) as possible.
Side-by-Side Comparison
Factor | Thermoforming | Injection Molding |
Tooling Cost | Low ($1K–$50K) | High ($10K–$500K+) |
Production Speed | Moderate (seconds–minutes/cycle) | Fast (5 sec–2 min/cycle) |
Part Complexity | Low-to-medium | Very high |
Material Waste | Higher (trimming waste) | Minimal |
Best Volume | Low-to-medium runs | High-volume runs |
Wall Thickness Control | Variable, harder to control | Precise and uniform |
Tooling Lead Time | Weeks | Months |
Material Considerations
Thermoforming and Injection Moulding are both considered to be able to manufacture parts from a large variety of thermoplastic materials; however, they do have their preferred materials.
Thermoforming processes are typically limited to using ABS, PETG, and polycarbonate, as well as other thermoplastics in the form of sheets. The only thing that restricts what a thermoformer can thermoform is by using thermoplastics, which are materials that can be softened with heat and hardened when cooled.
Injection Moulding has an even broader range of thermoplastics as it also includes other types of thermoplastics such as Nylon (PA), POM (Acetal), PEEK, and glass-filled composites, which would be extremely challenging, if not impossible, to thermoform due to their flow characteristics and/or temperature limitations. If high-performance engineering plastics are required in your application, then Injection Moulding will usually offer you a much wider selection of applicable materials than using Thermoforming.
Design Flexibility & Part Complexity
This is likely where the two methods differ most.
The process of thermoforming involves heating a flat piece of plastic and stretching it over a shape. This results in parts that are essentially derived from that flat shape. You cannot make very deep draws or parts that have highly vertical walls or very complex internal geometries with this method. As the material is stretched, the wall thickness will vary significantly across the part; thus, you need to keep that in mind during the design process.
Injection molding is a type of manufacturing characterized by the use of pressurized molten plastic filling a closed mold cavity. This allows for unlimited geometric complexity in the production of a design. Wall thickness can be uniform throughout the part, sharp corners can be created, very complex internal features will be possible, and two-shot molding or overmolding will allow for multiple materials to be used in one shot. Therefore, the design freedom is far greater than with the thermoforming process.
Here is a simple rule to help you determine which of these processes may be best suited for your particular application. If your part has the appearance of having been formed from a flat sheet or from a simple shape, then the thermoforming process will likely work well. Whereas if your part features are on more than one plane, contain any type of internal channeling, or must mate with precision, then the injection molding process will probably be the most appropriate method of manufacture.
Conclusion
While both processes of thermoforming and injection molding are excellent methods of manufacturing, they cater to two very different types of projects. With its speed to market and lower capital investment, as well as greater size and simplicity of geometric shapes, thermoforming is typically the best manufacturing method for projects that are in their early stages or for lower volumes of production. On the other hand, injection molding is generally the best manufacturing method for high-volume production when precision and complexity are required, and a greater investment is to be made in tooling.
Ultimately, the decision between the two processes comes down to four questions: How many units will you need? How complex is your part? What is your budget for tooling? How quickly do you need to be in the market?
Once you have answered these questions honestly, the appropriate manufacturing process will become evident. It is also important to realize that there is nothing wrong with beginning your project with thermoforming and then transitioning into injection molding; in fact, this is often the best route to take as you work through the manufacturing process. The determining factor is using the right process for the point in time that your project is currently at, and not using one process simply because you have historically done so.