Pressure Forming: past, present, and future

Today, heavy-gauge pressure forming is a well-established, commercially viable plastics manufacturing method. This sheet-fed process delivers parts with tight tolerances and attractive finishes, with lower tooling costs than injection molding can provide.

But how did we get here, and what will it be like in the future? As industry evolves, it’s good practice to review this question regularly. Steve Murrill, Chairman of the Board and former President of Profile Plastics, first considered it with the Society of Plastics Engineers (SPE) in 2016. This article is based on his original presentation.

The origins of pressure forming

In the early 1980s, vacuum formers in the SPE’s Thermoforming Institute wanted to improve the appearance of thermoformed parts. The owners of eight companies discussed ways they could compete with injection molding for large parts with high levels of aesthetic finish in lower volumes.

Profile’s founder John Grundy was an important part of these conversations. A talented engineer, John involved himself in all aspects of pressure forming, the new type of thermoforming they developed. John’s contributions to the industry were recognized by the SPE Thermoforming Division in 1993, when they named him Thermoformer of the Year.

How features and mold costs drove development of pressure forming

Grundy and his colleagues played with the idea of using negative molds. Vacuum forming in the 80s used positive molds. Switching to negative tools could enhance the appearance of molded-in features and allow for undercuts, which are difficult to achieve otherwise.

These pioneering thermoformers experimented with small prototype molds to demonstrate the possibilities. They discovered that applying pressure to the back side of the sheet forced it against the mold, producing fine molded-in details and textures. The new process created parts that looked like they’d been injection molded.

Profile constructed a pressure box model to fit the unique vacuum forming machine that we acquired used about the same time. Built by Brown Machine (now BMG), this 5 ft x 10 ft machine included the capability of die cutting parts at the forming station.

Die cutting required the machine to have two platens and locking bayonets which hydraulically squeezed the platens together. While the feature had little marketplace value, it was perfect for Profile’s testing. Essentially, the box part converted this machine into a sheet-fed pressure former.

Our experimental component made the rounds among the companies involved, then it went out to more thermoformers and industrial designers. Industry publications learned of this development and wanted to know more. News about the possibilities of pressure forming began to spread throughout the plastics industry.

Thermoformers shared tooling sources and insights, including potential applications for pressure forming. Designers who learned about pressure forming were eager to try it because of the significantly lower mold costs compared to injection molds.

Pressure forming molds were marketed at the time as costing 25% of equivalent injection molds. Lead times for tools were also shorter, only six weeks versus 16 weeks or more.

Early pressure forming projects and troubles with trimming

The first projects for pressure forming included expensive business machines and medical diagnostic equipment. Advocates of the process described it as perfect for highly cosmetic, low-volume applications.

Volumes for those early parts were sometimes as low as 200 per year or even just 500 total over a product’s lifetime. While piece part costs were easily five times greater than injection molding, the tooling cost five times less.

Post-processing became a critical issue. Pressure formed parts require trimming.  Hand trimming is slow and imprecise, so producing up to 200 sets of parts quickly wasn’t possible. Some projects required multiple fixtures just to hand trim a single part, increasing the risk of problems.

Rejection rates soon exceeded 10%. In addition, thermoformers were concerned about parts mating and matching. Most pressure forming projects involved multiple parts, and hand-trimmed tolerances were typically ± 0.030-in or more.

Profile had one of the first five-axis CNC trimming machines, built by Thermwood Corporation in 1979. Widespread acceptance of CNC machining in thermoforming was still nearly a decade away.

Although the machine-builder’s vision was clear, that first CNC trimmer was difficult to program, unreliable, and expensive. Consequently, our early pressure forming projects still required hand trimming for the first few years.

Faster, cheaper molds vs. high development costs

Back then, hiding trimmed edges through innovative part design was paramount. Fewer mating parts per application were preferred. The challenges of hand trimming risked leaving an assembly short of parts. If that happened, filling an order with an equal number of each part meant additional material and processing.

Still, advocates promoted pressure forming as a bridge to injection molding. It could get products to market faster, giving customers time to have more expensive and slower-to-make injection molds prepared for mainstream production.

Yet these high expectations put intense pressure on thermoforming companies to develop the rest of the pressure forming process on the fly. Specifically, thermoformers need to address how to attach pressure formed parts to each other and to machines.

There were no rule books that explained how to handle these unknowns, but part designs needed to be completed quickly. Development costs at the time were high because of the speed-to-market that pressure forming was supposed to provide.

After a while, pressure formers noticed that the early adopters of the process were entrepreneurs who saw varying levels of success.

Three pressure forming success stories

At Profile, three particular projects more than paid for our development costs and gave us increased confidence in what pressure forming could achieve.

Our first example may be the most successful pressure-formed part ever produced. John Grundy developed a plastic air conditioner plenum in 1986 to replace a metal box for a customer, improving the appearance of the conditioned space. It was for the emerging “computer room” market, now called data centers. In 1987, this part became the winner of the first SPE Thermoforming Institute parts competition.

The second success was developing the Life Fitness Club Exercise bike. With some trepidation, we broke our “no family tools” rule for this four-part application. We produced over 70,000 bike shrouds over seven years. For their second generation, the customer chose injection molding. This profitable project paved the way for our other early success.

The third was a project for Eastman Kodak. They wanted to introduce a new electronic color printer quickly. While injection molds were being made, the company asked us to produce two pressure formed covers. The parts made by the two processes looked alike. In the six months before their injection molded parts arrived, Kodak bought 5,000 sets of pressure-formed parts from us.

The importance of part design in pressure forming

The A/C Plenum that John Grundy developed demonstrates the importance of why part design matters to thermoforming success. This design has not required changes and is still an active and profitable application today after many years.

The 2ft x 4ft capstone part is made of pre-colored ABS. The finished product is an assembly of two pressure formed parts, nine vacuum formed parts, and a hardware kit. We package everything in the customer’s box and ship it.

By 2016, we produced over 50,000 units. We refined our list of requirements for evaluating potential opportunities based on successful applications like this.

The best applications for pressure forming

The best products for pressure forming usually have electronics onboard and two or more mating parts. While adding parts increases the degree of difficulty exponentially, high-end applications that require more parts can make the economics of pressure forming more attractive. Larger parts are also a good choice.

Pressure formed parts for consumer products usually aren’t economically viable. Still, using a pre-colored sheet with an acid-etched mold texture provides more value than painting.

Also remember that pressure formed parts require considerable trimming. This usually takes twice as long as the forming, and CNC trim fixtures must hold parts consistently. In addition, some pressure-formed parts require back-side milling to adjust the fit.

Frequently measuring pressure formed parts against the customer’s print or 3D model is a must. While this consumes time, and the part tolerances on customer drawings are typically too tight, it’s critical for refining the process to maintain the highest quality.

Cost: Pressure forming costs vs vacuum forming

Why does pressure forming cost more than vacuum forming? In part, it’s because the machines are significantly more expensive. Cycle times are also 30-40% slower to control color and warping.

Tooling is more expensive because of built-in cooling, pneumatic controls, and surface finishes. While molded-in features such as louvers, logos, and openings add real value, they increase cost.

Also consider the differences in forming and trimming. CNC cycles are slower because more complex parts require more trimming. Points of attachment, or blocks, on a part’s backside add costs and trim time.

There are other factors as well. For applications like medical equipment, expensive flame-retardant plastic sheets may be required. Design for manufacturing, design for assembly, and internal process controls increase engineering and quality resource requirements, but the value is visible in higher part quality and appearance.

The future of pressure forming

The thermoforming industry has made considerable advances since Steve Murrill originally presented this history in 2016. Even today, improving quality and part-to-part consistency, better measurement technologies, and cost reduction remain key concerns.

Industry 4.0 technologies provide pressure formers new ways to tackle old challenges. Continuing advancements in sensors and machine vision for in-line measurement and even AI facilitate quality and control costs by improving machine control and throughput.

At the same time, companies keep getting bigger. Not just pressure formers, customers and suppliers face issues with growth and market changes. And today’s workforce challenges are more acute than ever. Every manufacturer feels that pain. Automation and training have become critical to address these issues.

Conclusion

The development of sheet-fed, heavy-gauge pressure forming offers OEMs cost-effective plastic manufacturing, especially for large parts in low to mid volumes. Technology changes since the early 1980s make the process more capable and consistent.

That said, sound product design continues to play a central role in making pressure forming useful for OEM customers. Even as design technology changes, the core concepts remain. The A/C plenum that pioneer John Grundy designed years ago and is still in production today proves that.