Both thermoforming and injection molding are widely used plastic manufacturing methods. While both technologies require molds, they differ in many areas, including process, part design, cost, and volume. In general, thermoforming is best suited for large parts, including packaging and enclosures, while injection molding excels with small, end-use parts.
In this article, we compare thermoforming and injection molding so that you can make an informed decision on which technology will be best suited for your production.
The primary difference between thermoforming and injection molding is their processes, which influence each following category. The thermoforming process involves using a one-sided mold, heat, and a vacuum to soften a sheet of plastic and draw it over the mold. Once cooled, the plastic will retain the mold’s shape. As for injection molding, plastic pellets are fed through the machine past a heating element where they are melted. The molten plastic is then injected into a double-sided mold. The plastic cools in the mold and is then ejected.
Part Geometry & Size
While both processes use molds, these molds are not suited for the same parts or purposes. Thermoforming molds are only one-sided, meaning they create profiles rather than 3D objects like injection molding, which molds the part from both sides. As such, parts produced via thermoforming are most typically used as enclosures, housings, or packaging. Injection molding commonly produces end-use parts for a variety of industry applications. This is because side actions and pick-outs allow injection molding machines to make small, intricate pieces. Thermoforming cannot do this because small features would likely result in a lot of problematic webbing. As for part size, thermoforming is best used for parts larger than 4’ x 4’, and injection molding excels at producing parts smaller than that.
Tooling is required for both thermoforming and injection molding, meaning that molds will need to be cut from tool steel. These molds can take anywhere from 6-8 weeks to produce. For thermoforming, you can expect a mold to cost $2,000-10,000 minimum. One sheet of plastic material will cost $35-45. Your total cost spent on thermoforming material will depend on how many parts you can pull from one sheet as opposed to how much material is wasted. As for injection molding, tooling may be as low as $1,200. Your cost per part will greatly depend on the volume of your production, however, because injection molding follows the principle of economy of scale. The more you produce, the lower the unit cost will be. In some cases, you may be able to lower this unit cost to a few dollars or even $0.01.
Because thermoforming plastic sheets must be loaded into the machine and heated to glass transition temperature before each pull, this process takes a bit longer to complete than injection molding. As such, thermoforming lends itself to low to mid volume productions. On the other hand, an injection molding cycle may be completed in 2-5 seconds depending on whether there are side actions and pick-outs used, so it is best suited for high volumes. High volume injection molding runs are also cheaper to produce as mentioned in the previous section.
The material selection for both thermoforming and injection molding overlap in terms of the most common plastics. The materials that both processes share include ABS, Polyethylene, Polypropylene, Polystyrene, Acrylic, Polycarbonate, PVC, and PEI. As for raw material form, thermoforming material comes in sheets while injection molding material starts as pellets. It is important to note that while metal injection molding is offered by some companies, it is a much lengthier and more expensive process.
It is crucial to understand that thermoforming vacuum pressure pulls plastic taut over the mold to a uniform thickness. If your part’s integrity relies on a variation of thick and thin sections, injection molding will be better suited for your part’s production because the nature of the process and mold will allow for differing thicknesses within the part.
Both thermoforming and injection molding share similar post-processing steps. Parts manufactured via either technology will require trimming to remove excess plastic. This can be done manually or automatically. If completed automatically, thermoforming requires a die to trim, and injection molding removes the excess as the part is ejected from the mold. Other post-processing steps may include sanding, dyeing, painting, and more.
3D Printed Tooling
With a 6-8 week lead time to cut tooling, neither thermoforming nor injection molding is particularly fast. If your production can’t wait that long, there is the option to 3D print your tooling. These plastic molds can be ready to use in as little as 48 hours. However, these can be highly expensive and will only last 2,000 thermoforming cycles or 100 injection molding cycles before deformation occurs.
Manufacturing at 3 Space
Here at 3 Space, we offer a broad spectrum of manufacturing services, including thermoforming, injection molding, and 3D printed tooling. If you’re unsure which technology may be the best fit for your production, our expert engineers are happy to answer all your questions and make suggestions based on your part’s design and intended application. For further information, contact us today.