Archive for Injection Molding vs Plastic Thermoforming
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When comparing a part manufactured with the heavy gauge plastic thermoforming process and the injection molding process, next to production volume, the largest factor that can impact the cost and even process feasibility is the size of the part.
Essentially, the larger the part is, the more expensive it becomes to produce with injection molding. Comparatively, part size has a very minimal cost effect on plastic thermoformed parts. The breakeven point on cost between the two manufacturing processes, with respect to annual production volume (Deciding Between Plastic Thermoforming and Injection Molding – The Choice is Not Always Obvious), increases as part size increases to approximately 5,000 parts or higher depending greatly on how large the part is.
Why Does Part Size Affect Cost and Manufacturing Process Selection?
The injection molding process requires a very large initial capital investment in the tooling and equipment needed to produce a part. This is because the nature of the process involves a very highly engineered 2-sided mold to create a part by feeding thermoplastic resin into a heated barrel with a rotating screw. The screw delivers the raw material forward collecting under pressure the amount required to fill the mold cavity and then injecting into the mold at high pressure and velocity. This action requires highly structured molds and equipment capable of withstanding very high clamping pressure.
As part size and dimensions increase, the complexity of design, engineering, and calibration required to construct, install, and process this 2-sided mold results in a significant increase in the cost of equipment, tooling and setup. The per-part production cost and lead time may also see an appreciable increase as the part size increases requiring much more robust molds and equipment. These increased capital expenditures will result in greater investment and overhead costs calculated in the piece price. Injection molding machines have a limited total mold size capability but can often accommodate multiple parts within the construction of a mold. Smaller part sizes equate to a higher number of parts manufactured per mold and machinery cycle. Larger part sizes decrease the number of parts that can be manufactured per mold and cycle.
Think of a muffin tray with 3-inch diameter muffin molds. Now take that same size tray but with 6 or even 10-inch diameter muffin molds and you imagine the impact on production and cost. In fact, most standard injection molding machines can only accommodate a maximum part size of 4’ x 4’. Larger machinery is available but is also drastically more expensive.
The heavy gauge plastic thermoforming process, on the other hand, involves considerably less pressure and most applications only require a single one-sided tool to produce a part. Additionally, only one part is formed per cycle in heavy gauge thermoforming applications. Consequently, the initial tooling investment is drastically reduced. While an increase in part size will still increase the tooling investment, the impact on cost is substantially less when compared to injection molding. Heavy gauge thermoforming equipment has oven zoning and variable sheet size capabilities which allow for a wide range of part sizes to be efficiently formed from the same equipment investment. The nature of the thermoforming process and flexible capacity capabilities makes scaling production for larger part sizes a relatively easy process. Since most heavy gauge thermoforming operations utilize cell-based manufacturing and CNC part trimming, a larger part can be produced with little impact, other than increased material, on per part cost, cycle time, and lead time. Thermoforming machinery can also manufacture part sizes as large as 10’ x 18’ providing a much larger part size capacity than injection molding.
Both injection molding and plastic thermoforming have widespread uses in a long list of industries. Each process has some unique features and benefits that are often advantageous for a specific application. In these instances, the choice to manufacture with plastic thermoforming or injection molding may be very obvious. This is most apparent in production volume. Low to mid volume tends to favor thermoforming, while high volume is usually more cost effective with injection molding.
However, a product’s needs and the capabilities of these two processes sometimes overlap. A part’s geometry may seem better suited for injection molding, but in a limited production run, but it may be drastically more cost effective to manufacture it with plastic thermoforming. This is just one example of an application where deciding between injection molding and plastic thermoforming may not be a clear choice. Selecting the right method in these situations requires a deeper appraisal of the features, benefits, and costs associated with each process.
The Clear Choice
As mentioned above, there are some instances when the type and specifications of an application drastically favor one or the other plastic manufacturing process when the choice is between injection molding or plastic thermoforming.
Injection molding offers the key benefit of cost effectiveness at the mass production scale. When an application requires the production of more than 3,000-5,000 Estimated Annual Usage (EAU) identical parts with uniform wall thicknesses, injection molding often is the clear choice. This can be attributed to a high upfront tooling investment that is gradually offset by a generally low per unit manufacturing cost. The volume range of 3,000 – 5,000 is due to a variation on part cost in respect to part size. Smaller parts are generally cheaper to manufacture than larger.
- Part production volumes > 3,000- 5,000
- Uniform part wall thickness required
Plastic thermoforming, on the other hand, has a substantially lower tooling investment and a slightly higher per unit manufacturing cost. This equates to a much lower total part cost at low to moderate part volumes. Plastic thermoforming becomes the clear choice when the volume of manufacturing is less than 3,000 – 5,000 parts per estimated annual usage. This process also has the capability to produce single parts with very large dimensions, whereas the injection molding process is limited to single part sizes of about 4 feet x 4 feet.
- Single part dimensions > 4’x4’
- Part production volumes < 3,000 – 5,000 EAU
Considerations When the Process Choice Is Not Clear
If your part or project doesn’t require a uniform wall thickness, large single part dimension, or has a volume requirement that is in the mid thousands, then you have landed in an area where the capabilities of plastic thermoforming and injection molding may overlap, and your process choice is not so obvious.
The good news is that you are now no longer handcuffed to a process that, while cost or size necessary, may not have the most comprehensive scope of benefits that would contribute the greatest to the success of your project.
Here are some points to consider for each process that can be taken advantage of or avoided now that you are free to choose a manufacturing method better suited to your project’s needs.
- Large single part capability (maximum dimensions approximately 10’ x 18’)
- Short lead time ( 6-12 weeks )
- Able to reproduce injection molded level detail
- Smaller investment in tooling
- Lower equipment capital investment leads to lower set up and machine time costs
- Can produce thinner wall parts than injection molding, resulting in weight savings
- Greater options for part surface finishing (textures, patterns, distortion printing, painting, etc.) that can be accomplished in the mold.
- Multi material structures for cosmetic and engineering structure options (e.g. Acrylic/ABS)
- Variable part wall thickness depending on depth of draw
- Improved cost effectiveness at lower to mid volumes (< 3,000-5,000)
- Lighter part weight compared to injection molding for most applications
- Less molded in stress than injection molding
- Twin sheet capability for hollow parts and added structure
- Longer lead time (22-24 weeks)
- Large investment in tooling
- Cost effective at high volumes ( > 3,000 – 5,000)
- Efficient material use
- High level of precise part detail
- Limited single part size capability (maximum dimensions approximately 4’ x 4’)
- Finished parts often require post processing painting or finishing
- Greater design freedom on single wall parts
Want More Information?
What you see above is just the tip of the iceberg when it comes to comparing these manufacturing processes. For more information and for assistance in choosing the right process for your project, please contact Productive Plastics and connect with our industry experts and engineers to see how we can put over 62 years of manufacturing experience to work contributing to your project’s success.