Injection Molding vs. Thermoforming: Differences and Comparisons
Learn more about the differences between injection molding and thermoforming.
Two of the many plastic product fabrication methods that are available are injection molding and thermoforming. Injection molding is the manufacturing process by which molten plastic is injected under pressure into a mold to create parts. Thermoforming is the manufacturing process by which a sheet of heated plastic is forced and stretched (via vacuum or pressure) onto the surface of a mold to create parts. The three main differences between the two are in the process, the size of parts produced, and the lead time. Injection molding can mold highly-detailed parts between the halves of a mold in one process, whereas in thermoforming, the same halves of a similar part must be molded and joined together separately.
Injection molding is better for small parts and thermoforming is better for larger parts. This is because part sizes are restricted by the tooling and equipment needed to form parts in injection molding. Production lead times and parts made per cycle between the two processes also differ significantly. Injection molding is better for large-lot production since multiple products can be made in the same cycle while thermoforming is better for shorter, smaller runs. One major advantage of injection molding is that highly detailed, small parts can be produced quickly and efficiently. For thermoforming, one major advantage is that its upfront cost and lead time for tooling are much lower than for injection molding. In this article, we dive deeper into the similarities and differences between injection molding vs. thermoforming.
Injection Molding Definition and Comparison to Thermoforming
Injection molding is a plastic manufacturing method involving an injection molding machine (IMM). In injection molding, plastic pellets are fed into the injection barrel and fill the “flights” or areas between the threads of the injection screw. The rotation of the screw and heater bands placed on the barrel melt the pellets. The rotation also allows the material to flow from the feed zone into the transition or compression zone. Here, the molten material is compressed as tight as possible to evacuate all air. As the material is compressed and melted, a “melt pool” is developed and travels to the metering zone of the screw as it continues to rotate. When molten material enters the metering zone, almost all the volume between the flights of the screw and barrel walls are filled with molten material. At this point, an appropriate amount of material is shot into the mold. The IMM holds pressure on the mold until products have been properly formed and cooled, at which point the completed part is ejected. This entire process is repeated continuously until all parts are made.
Injection molding was first developed in 1872 by brothers Isaiah and John Hyatt. Their first injection machine was used to mold hair combs, buttons, and other small items. Unlike thermoforming, injection molding is the most cost-effective and efficient way to make plastic parts. For more information, see our guide on what is injection molding.
Figure 1 below shows an injection molding machine:
An injection molding machine.
Image Credit: Shutterstock.com/Baloncici
What Are the Advantages of Injection Molding Compared to Thermoforming?
Listed below are some advantages of injection molding compared to thermoforming:
- Ideal for high-volume production since injection molds can have several cavities
- Great for producing small and/or intricate parts.
- Adaptable to a high level of automation for high-volume production.
What Are the Disadvantages of Injection Molding Compared to Thermoforming?
Listed below are the disadvantages of injection molding compared to thermoforming:
- High tooling and equipment costs can be a barrier to entry for some companies wishing to use injection molding.
- Long lead times for mold building and repair due to the complexities of the shapes typically being molded.
Thermoforming Definition and Comparison to Injection Molding
Generally, thermoforming is the process where heated thermoplastic sheets are pressed into a single-sided mold to make products. Thermoformed products can be achieved by exerting pressure onto the sheet or by forcing all air out of the mold. This approach is called vacuum forming. After a part has been formed, it is trimmed and finished based on customer specifications. Thermoforming is ideal for simple geometries and parts with large tolerances, like packing inserts, but is not suitable for small, detailed parts or high-volume production.
The concept of thermoforming originated back in 1855 when cellulose was developed. John Hyatt, one of the people credited with inventing injection molding, experimented with cellulose as raw material, intending to create a harder material. Through his experiments, he combined nitrocellulose, camphor, and alcohol and began to form the material sheets and rod. He often left the final fabrication of products such as toys, bottles, and other items to other companies. Today, a wide variety of products are made by thermoforming - from toys to entire bathtubs and vehicle dashboards. Compared to injection molding, thermoforming’s tooling costs are relatively cheap, and large parts can be made quickly for low-volume orders.
What Are the Advantages of Thermoforming Compared to Injection Molding?
Listed below are the advantages of thermoforming compared to injection molding:
- Great for low-volume production runs or one-offs, since only one part can be made at a time
- Faster prototyping and product development, since it is easier to make thermoforming tooling than injection molding tooling
- Low tooling costs, since thermoforming tools, are typically made from aluminum
- Easy to make repairs and modifications to tooling, since thermoforming tooling is relatively simple
What Are the Disadvantages of Thermoforming Compared to Injection Molding?
Listed below are the disadvantages of thermoforming compared to injection molding:
- Unsuitable for high-volume production, since production output is low with thermoforming
- Labor-intensive, since some thermoformed parts must be post-processed to meet customer specifications.
- Wasteful, because correctly thermoformed parts may require several attempts before they attain the desired shape.
| Attribute | Injection Molding | Thermoforming |
|---|---|---|
Attribute High-volume production | Injection Molding Yes | Thermoforming No |
Attribute Make small, detailed parts | Injection Molding Yes | Thermoforming No |
Attribute Make large parts, but with big tolerances | Injection Molding No | Thermoforming Yes |
Attribute Has low tooling costs | Injection Molding No | Thermoforming Yes |
Attribute Has repeatable processes | Injection Molding Yes | Thermoforming Yes |
Attribute Wide range of available materials | Injection Molding Yes | Thermoforming No |
Deciding whether injection molding vs. thermoforming is right for a project comes down to the specific requirements. If high-volume production is required or the application is a small, complex part, injection molding is better. If larger parts at low-volume production are needed, thermoforming is better.
Deciding whether injection molding vs. thermoforming is right for a project comes down to the specific requirements. If high-volume production is required or the application is a small, complex part, injection molding is better. If larger parts at low-volume production are needed, thermoforming is better.
Injection Molding vs. Thermoforming: Lead Cost Comparison
Tooling costs for thermoforming can be significantly less than for injection molding. Because pressures used in thermoforming are not as high as injection molding, most thermoforming tooling is made from aluminum. Compare that to the injection molding process, which uses expensive tool steels such as P20 or H13. Fabricating tooling for injection molding can take a long time and may be much more labor-intensive than thermoforming molds due to the complexity of the parts being molded.
Injection Molding vs. Thermoforming: Speed Comparison
Both injection molding and thermoforming are excellent at producing parts at a rapid pace. The main difference is that with injection molding, molds can sometimes have multiple cavities fed by the same supply of molten material, leading to much higher output. The development of final product design and molds is faster with thermoforming because the tooling can be easily modified.
Injection Molding vs. Thermoforming: Volume Comparison
Injection molding can support much higher production volumes than thermoforming. This is because injection molds can have multiple cavities. Some molds can have over 100 cavities. In the thermoforming process, on the other hand, just a single part is made during every molding cycle. As a result, injection molding is much better for large or frequent production orders.
Injection Molding vs. Thermoforming: Materials Comparison
Injection molding and thermoforming can generally use the same plastic materials, such as polyethylene, polycarbonate, and PVC. Because injection molding uses plastic pellets and thermoforming uses plastic sheets, the materials used in thermoforming generally have more malleable and ductile characteristics.
What Are the Mutual Alternatives to Injection Molding and Thermoforming?
Mutual alternatives to both injection molding and thermoforming include:
- Blow molding
- Extrusion molding
Blow molding and extrusion molding are both forms of plastic manufacturing that utilize many of the same materials used in injection molding and thermoforming. Both produce plastic products in large volumes and are highly repeatable processes. Blow molding is mostly used for making containers and bottles, while extrusion molding is used for items with fixed cross-sectional shapes over long lengths such as weather seals, pipes, and door frames.
What Are the Similarities Between Injection Molding and Thermoforming?
Similarities between injection molding and thermoforming include:
- Both use thermoplastic materials to make parts.
- Both are repeatable processes that are excellent at producing high-quality parts.
What Are the Other Comparisons for Injection Molding Besides Thermoforming?
Listed below is another alternative to injection molding:
- Injection Molding vs. 3D Printing: 3D printing is a fabrication process that can create parts with complex geometry with ease, as injection molding does. Some complex-geometry parts cannot be made with injection molding due to undercuts and other unmoldable features. 3D printing avoids this problem and can print any part, no matter the complexity, with both ease and speed.
What Are the Other Comparisons for Thermoforming Besides Injection Molding?
Listed below is another alternative to thermoforming:
- Thermoforming vs. Blow Molding: While thermoforming can only be used to make one half of an assembly at a time, blow molding can make both halves of a hollow container in one cycle. Items such as bottles, containers, and other items can easily be made with blow molding.
Summary
This article presented the comparison of injection molding and thermoforming, explained what they are, and discussed the different attributes of each. To learn more about injection molding and thermoforming, contact a Xometry representative.
Xometry provides a wide range of manufacturing capabilities, including injection molding and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.
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