Vacuum forming and injection molding are two of the most popular methods used to process plastics.  This article provides an overview of each method and discusses the advantages and disadvantages of each.

What is Vacuum Forming?

Vacuum forming (also known as “thermoforming”) is a manufacturing process in which plastic sheet goods are preheated in automated vacuum forming equipment until soft and pliable (think fettuccini al dente).  Next, the preheated sheet makes contact with a mold or form and a vacuum source is turned on, which “sucks” the sheet to the mold.  Once cooled, the sheet takes the shape of the mold.  It is then trimmed with either a 5 axis CNC router or simply die cut.  If additional detail is required, positive air pressure is used to assist the forming process, a technique commonly known as “pressure forming.”

Vacuum forming molds can be either “male” or “female,” depending on the geometry of the parts and where the weakest areas require more material thickness.  Molds can be textured to impart different finishes using a female mold and positive air pressure.

What is Injection Molding?

Injection molding is a far more complex manufacturing technique than vacuum forming and requires a great deal of engineering.  The process of injection molding begins with either pellets or granules of polymer which are placed in a hopper and then dropped into a barrel.  A screw pushes the material into a heater where it is melted.  Next, the liquid polymer is injected into a steel or aluminum split-die through a “gate” being held in a “press” under extreme pressure.  After cooling, the die opens and pins eject the finished parts.

A great amount of engineering time and cost is required to fabricate the parts and dies used in injection molding, making vacuum forming the preferred process for many applications where time-to-market and low cost is critical.  Still, each method has its own distinct merits and also unique drawbacks.

Advantages of Vacuum Forming

• Ability to create large parts (up to 48” x 96”)
• Relatively fast prototyping and production time frames, sometimes as quickly as 4 weeks
• Lower start up costs — patterns and molds can be made inexpensively from MDF, high density foams and epoxy
• Ideal for repeat jobs — aluminum castings can be made which have virtually unlimited lifetimes
• Good price point on small and medium runs

Disadvantages of Vacuum Forming

• Intricacy of parts is limited — additional details can be added with pressure forming
• Some clear parts will exhibit “mark-off” (i.e., defects or dirt from mold will transfer to parts)
• Consistent wall thickness is not possible — very deep parts can be problematic
• Higher per-piece costs make vacuum forming non-competitive with other automated processes where quantities are larger
• Only one material can be formed at a time
• Finishing costs can be costly and labor intensive

Advantages of Injection Molding

• Allows for high production output rates
• Can use inserts within the mold and fillers for added strength
• Close tolerances on small intricate parts are possible
• More than one material can be used at the same time when utilizing co-injection molding
• Typically requires very little post-production work — ejected parts usually have a very finished look
• Very little waste – all scrap can be reground to be reused
• Full automation is possible
• Lower cost-per-part compared to vacuum forming

Disadvantages of Injection Molding

• Extremely high start-up costs
• Requires a great deal of engineering time
• Long time frames necessary to fabricate tooling, making time-to-market a major drawback

Conclusion

In summary, both processes have their advantages and disadvantages. In determining which method is best for your particular product, consider these issues:  production quantity, design and engineering requirements, time frame, start-up costs and overall budgetary concerns.  For additional information on vacuum forming and injection molding, please contact your E&T Plastics representative.