This white paper is based on a presentation that Steve Murrill, Chairman of the Board and former President of Profile Plastics, shared with the Society of Plastics Engineers (SPE) in 2016. Steve’s history of plastic pressure forming dates back to the 1980s but is as important in 2023 as it was just a few years ago. Today, heavy-gauge plastic pressure forming is a well-established, commercially viable manufacturing method for producing parts with tight tolerances, attractive finishes, and lower tooling costs than injection molding can provide.
The Origins of Plastic Pressure Forming
During the early 1980s, discussions began among a group of vacuum formers who were part of the Thermoforming Institute of the Society of Plastics Engineers (SPE), a leading trade association. The owners of eight vacuum-forming companies started talking about ways to improve the appearance of thermoformed parts so that plastic thermoforming could compete with injection molding for low volumes of large parts. It’s jobs like these where the economics of thermoforming are the most attractive.
John Grundy, the founder of Profile Plastics and its owner during this time, was an important part of these discussions. A talented engineer, John involved himself in all aspects of plastic pressure forming, which, like vacuum forming, is a type of thermoforming. John’s contributions to the plastics industry became so great that the SPE Thermoforming Division named him Thermoformer of the Year in 1993 and gave him its the Lifetime Achievement Award in 2001.
Plastic Pressure Forming Drivers:
Molded-In Features and Lower Mold Costs
Back in the early 1980s, John Grundy and his SPE vacuum-forming colleagues began discussing the idea of thermoforming with a negative mold to enhance the appearance of molded-in features, especially undercuts, which are difficult to achieve with positive parts. Each of the eight vacuum formers developed small experimental prototype molds to demonstrate some of the possibilities. They soon learned that applying a little pressure to the back side of the sheet, away from the mold, improved molded-in details. Moreover, greater pressure produced parts that looked like they’d been injection molded.
During this time, Profile Plastics prototyped a box-like part using a sheet-fed pressure former, a machine that Profile had purchased as a used vacuum former. Designed and built by Brown Machine Group (BMG), this 5-ft. by 10-ft. unit was a special-purpose machine that formed and then die-cut parts at the forming station. Die cutting required the machine to have two platens and locking bayonets, which hydraulically squeezed the platens together. The die-cutting feature had little marketplace value, however, and the original owner sold the machine to Profile.
Fortunately, this machine arrived on our shop floor at the same time that John Grundy and the other vacuum formers were searching for answers. Profile’s experimental part then made the rounds among thermoformers and industrial designers. Industry publications learned of this development and were eager to know more. After some initial successes, news about pressure-forming’s possibilities began spreading throughout the plastics industry.
Among thermoformers, tooling sources were exchanged and insights were traded, including about pressure-forming applications. As designers learned more, they were eager to try pressure forming because the mold costs were significantly less than injection molds. As the marketers of this era explained, pressure-form molds cost 25% of comparable injection molds. The lead times for tools were also a lot less since pressure-form molds took only six weeks while injection molds took 16 weeks.
Early Plastic Pressure Forming Projects and Troubles with Trimming
The first projects for pressure forming were expensive business machines and medical diagnostic equipment. The volumes were low, sometimes as little as 200 per year or 500 total parts over a product’s lifetime. The piece part costs were easily five times greater than injection molding; however, pressure-form molds cost five times less than injection molding tooling. Soon, advocates of plastic pressure forming described it as the perfect process for highly cosmetic, low-volume applications.
There was a post-processing problem, however. Pressure-formed parts require trimming but widespread acceptance of CNC machining for thermoformed parts was still nearly a decade away. At the time, Profile had one of the first five-axis CNC trimming machines. It was built by Thermwood Corporation in 1979 and then, as now, Thermwood was innovative. Although the machine-builder’s vision was clear, the equipment was difficult to program, unreliable, and expensive. Consequently, all of Profile’s early pressure forming projects required hand trimming.
Because hand trimming is slow and imprecise, Profile could not produce 100 to 200 sets of parts quickly. Some projects required four or five different fixtures just to hand trim a single part. This increased the risk of problems, and rejection rates soon exceeded 10%. In addition to part defects and material yields, thermoformers were concerned about parts mating and matching. Most pressure-forming projects involved multiple parts, and hand-trimmed tolerances were typically ± 0.030-in.
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, and family molds were discouraged because the challenges of hand trimming risked leaving an assembly short of parts. If that happened, filling a complete order with an equal number of each part would require additional pressure forming with the family tool – an added expense.
Still, advocates promoted plastic pressure forming as a bridge to injection molding. Customers could use it to get products to market faster and then use more expensive and slower-to-produce injection molds afterward. Yet these high expectations led to intense pressure 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 weren’t any rules books that explained how to handle these unknowns, but part designs needed to be completed quickly – and with high development costs because of the speed-to-market that pressure forming was supposed to provide. After a while, pressure formers noticed that its early adopters were entrepreneurs with varying levels of success.
Plastic Pressure Forming’s Early Adopters
Entrepreneurs were the early adopters of plastic pressure forming. Often, however, their concepts didn’t meet established market needs. Sometimes, their dreams died after an initial run of parts. Yet some were successful. That was good news for the entrepreneur but bad news for the pressure formers. With pressure forming’s high piece part costs, successful projects moved to injection molding quickly.
The thermoforming industry had paid a heavy price for rapid development only to realize that it had created a short life cycle for pressure-formed parts. That’s when we stopped talking about pressure forming as a “transition process” and started looking for applications where we could do it right the first time, and where there was the possibility of a long product life. Producing 500 units for a project seemed like a stretch goal, but it’s one we exceeded many times over.
Three Plastic Pressure Forming Success Stories
At Profile, there were three significant examples that more than paid for our development costs and provided us with increased confidence in the possibilities of pressure forming.
The first was a project with Eastman Kodak, which wanted to introduce a new electronic color printer quickly. On the same day injection molds were started, the company asked us to produce two pressure-formed covers. Injection-molded parts would eventually replace our pressure-formed ones, but the two types of parts looked alike. In the six months before their injection molded parts arrived, Eastman Kodak bought 5,000 sets of pressure-formed parts from us.
The second success was with a company called Life Fitness, which was developing its Club Exercise bike. This was a four-part application where, with some trepidation, we broke our “no family tools” rule. Over a seven-year period, however, we produced over 70,000 bike shrouds. Life Fitness chose injection molding for its next-generation model, but the project was profitable and paved the way for our third early success.
To this day, this third example may be the most successful pressure-formed part ever produced. It’s probably no surprise that the application was developed by John Grundy himself, back in 1986. The project, an A/C Plenum, was designed to replace a metal box and to improve the appearance of a very expensive and close-tolerance A/C unit that was used to control room temperature in a computer room. In 1987, this part became the winner of the first SPE Thermoforming Institute parts competition.
The Importance of Part Design in Plastic Pressure Forming
The A/C Plenum that John Grundy developed demonstrates the importance of part design to thermoforming success. The capstone part measures 2-ft. x 4-ft. and is made from a sheet of pre-colored Royalite R59 FR ABS that is .25-in. thick. The finished product is an assembly of 2 pressure-formed parts, 9 vacuum-formed parts, and a hardware kit. Everything is packaged in the customer’s box and shipped.
This part design has not required changes and is still an active and profitable application. By 2016, Profile had produced over 50,000 units with a total revenue in excess of $12MM. The fact that one of our first pressure-forming applications continues to be a model shows why part design matters. Based on successful applications like this, we have refined the list of requirements that we use when evaluating potential new opportunities.
The Best Applications for Plastic Pressure Forming
The best products for pressure forming usually have a computer onboard and two or more mating parts; however, each additional part increases the degree of difficulty exponentially. High-end applications require more parts, but this makes the economics of pressure forming more attractive. Larger parts are also a good choice, but consumer products with pressure-formed parts usually aren’t economically viable. Still, a pre-colored sheet with an acid-etched mold texture provides more value than parts painting.
Pressure-forming requires frequent part measurements against the customer’s print or 3D model. This is time-consuming, and the part tolerances on customer drawings are typically too tight. Pressure-formed parts also require considerable trimming. This usually takes twice as long as forming, and the CNC trim fixtures must hold parts consistently. In addition, most pressure-formed parts require back-side milling to adjust the fit. Across multiple parts, the de-mold temperature affects the consistency of the fit.
To control the fit, most part designs require negative molds with matching plugs and undercuts. All of the undercuts must be automated and connected to the process controller. In other words, there can’t be any loose pieces. Machined aluminum molds are better, and it’s best to integrate as many design features into the mold as possible. To eliminate the backfilling of thin corners, the part design usually requires modifications.
For uniform texture and gloss, and to minimize warp, pressure formers need to hold a live pressure of 50 to 90 pounds per square inch (PSI) for 1 to 3 minutes. This requires locking platens and, in the case of vacuum formers, new equipment. New ideas such as pressure-assisted vacuum forming typically take at least 10 years to achieve market acceptance, but strong competition helps to build a market – and pressure forming has become a widely accepted plastics manufacturing process.
Pressure Forming Costs vs. Vacuum Forming Costs
Why does pressure forming cost more than vacuum forming? In part, it’s because pressure forming machines are significantly more expensive than vacuum forming equipment. The molds that are used in pressure forming are also more expensive, and molded-in features such as louvers, logos, and openings add value along with costs. For many pressure-forming applications, such as medical equipment, expensive flame-retardant sheets may be required.
There are also differences in forming and trimming to consider. To control color and warp, pressure forming cycles are 30% to 40% slower than vacuum forming cycles. Pressure forming’s CNC trimming cycles are also significantly slower because more trimming is required. The points of attachment, or blocks, on a part’s backside add costs and trim time. Design for manufacturing (DFM), design for assembly (DFM), and internal process controls increase overhead requirements for engineering and quality resources.
Plastic Pressure Forming Costs and Part-to-Part Consistency
The key to pressure forming’s success involves lowering costs and improving quality. Today, the most inefficient step in the process is the CNC programming and trimming of parts. Yet this is a key step because it frees the part from the sheet and controls the part’s fit. From the customer’s perspective, trimming is where there’s the greatest variability. From the thermoformer’s perspective, variations in part shape or size during pressure forming also become problems during trimming.
When parts in a forming run differ dimensionally, the results of trimming differ. It’s also a problem if an untrimmed part doesn’t fit the trim fixture like it did when CNC trimming was programmed. Unless the thermoformer modifies the program, the fit won’t be the same. Yet trimming is also affected by differences in part shrinkage due to variations between sheets, resins, regrinds, or molding conditions. This is why pressure forming requires a degree of processing consistency that is beyond vacuum forming norms.
Logically, pressure formers want to know if they are producing identical parts within a run and across runs without stopping to measure each and every part. Even with a rigorous program like a capability study, how many part measurements are enough? And do we know what’s changed or what needs to change in order to produce identical parts in every run? If this level of control and consistency is truly required, injection molding is the process of choice. Yet there’s a place for plastic parts that are consistent enough.
Pressure Forming vs. Injection Molding
Pressure forming’s value proposition is the “consistent enough” production of plastic parts. Injection molding can provide greater part-to-part consistency, but with significantly greater costs. Pressure formers who forget this reality risk spending too much time and money on process changes and part inspections. Because of statistical process control (SPC), however, injection molding may dictate part acceptance. Yet an SPC-based comparison between these two plastics manufacturing processes overlooks a key difference.
Injection molding is a closed-loop process with a feedback loop that signals variation. Process controls enable the injection molder to adjust the process automatically, or to stop it entirely until the root cause of the variation is identified and corrected. By contrast, pressure forming is an open-loop process. Pressure formers assume that all of the inputs are the same as the last run, and that any variations are within a small, predictable range. That works where fit, function, and appearance are the quality standards.
What happens if there are major variations during pressure forming, and these variations go undetected? Inevitably, the result is a significant number of parts that are unusable and must be scrapped. That’s costly for the thermoformer and it raises a critical question. Is it possible to hold tight trim tolerances on pressure-formed parts with a normal variation in the properties of purchased sheets and a normal variation in forming conditions? If so, will the parts fit the trim fixtures as expected?
Plastic Pressure Forming Tolerances and Part Measurements
The main problem that pressure formers face is an inability to hold tight tolerances without constantly measuring parts and adjusting trimming programs. It’s possible to overcome this challenge, but at a considerable cost. For starters, it takes time to measure each part dimension, confirm what’s correct, and determine what’s not. There are also measurement errors and part-to-part differences that result in inconsistent trimming. In turn, the result of this trimming may appear to be a measurement error.
Finding the root cause of part-to-part differences requires a significant amount of time from talented people. In the end, splitting the dimensional difference between parts might be what informs the next run. Other adjustments may then be required, and those changes may cause other problems to occur. For pressure formers, there are at least possible solutions with the third option representing the best choice.
The first solution is to expand or loosen part tolerances so that normal processing variations fall within range. This allows pressure formers to present the process as-is but might not meet customer requirements. The second solution is to identify and use metrology techniques to rapidly measure parts during both forming and trimming and to quickly identify process variations. This provides a way to find outliers but it also means tolerating more part rejects and, consequently, higher project costs.
The third (and best) option is to tighten control of the forming process, the CNC holding fixtures, and measuring techniques to keep process variables within a tighter window. With more timely and more frequent data, pressure formers can identify process variations faster. In turn, this reduces systemic variation through human intervention and supports the production of close-tolerance parts. It also supports greater part-to-part consistency based on the closeness of parts to the drawing or model part.
Achieving this third solution requires improvements to measurement techniques so that monitoring and checking are fast and simple. Otherwise, pressure forming requires a degree of diligence that is manageable but inefficient. Pressure formers also need automated information from sensors that are connected to process controllers. Monitors that provide real-time alerts of process variations as they occur are also critical.
Plastic Pressure Forming Improvements:
Three Ways to Improve Forming Consistency
There are three ways for pressure formers to improve the consistency of the forming process.
- More consistent sheeting
- Better information about zoned sheet heating
- Monitoring and recording process variables that are currently uncontrolled
First, pressure formers need more consistent sheets; however, we depend upon the extruders who make the sheets for product information. Fortunately, most extruders are highly sophisticated and want sheet-to-sheet consistency as much as we do. Still, it’s important for us to communicate with extruders and obtain as much data as possible. Until we can correlate sheet variations to forming variables, we won’t be able to specify our sheet requirements effectively.
Second, pressure formers need a deeper knowledge of the sheet heating process. To get the perfect part, we spent a lot of time on the forming setup and zoning the oven. Then we save this recipe. Yet the sheet, burners, and room temperature all change. We adjust the oven during the next run, but we don’t know the real-time output of oven heat in the sheet. In other words, we need to know the heat in the sheet rather than just the heat the oven is imparting.
Today, pressure formers can use an infrared (IR) line scanner to capture information about a heated sheet as it leaves the oven. This is extremely useful and helps to improve the forming of problem parts. Thermal imaging has improved, however, and it’s also decreased in price. With IR charged coupled device (CCD) cameras, it’s possible to monitor the entire sheet in real-time while it’s in the oven. In turn, this enables a pressure former to monitor and verify a zoned oven’s consistency from run to run.
By saving and using this thermal image as a reference point, pressure formers can achieve greater run-to-run repeatability than by relying on burner setup information alone. Incorporating IR CCD technology into ovens can also improve part-to-part consistency, but there’s another potential benefit as well. By better understanding what’s happening inside the sheet, pressure formers can account for the effects of using sheets from different vendors, or sheets where there are different regrind variables.
Third and finally, our industry can improve pressure-forming consistency by recording and monitoring process variables that remain uncontrolled. This can be accomplished by:
- Adding inputs from the mold temperature controller and the mold
- An IR temperature device in the forming station
- Real-time output of the quality of the vacuum on the mold
- An IR temperature device on the part in the cooling station
All data must be recorded and monitored until the process is stabilized. What matters is that the numbers are the same from run-to-run since these variables will affect part sizes. If we want consistently formed parts, we need to gather information that is outside of the process controller.
Plastic Pressure Forming Improvements:
Five Ways to Improve Trimming Consistency
There are five ways for pressure formers to improve the consistency of the trimming process.
- The forming process
- Trim holding fixtures
- Trim programs
- Part measurement techniques
- Reducing the impact of CNC crashes
As explained previously, improving the forming process requires more consistent sheeting, better information about zoned sheet heating, and monitoring and recording process variables that are currently uncontrolled. Without more consistent forming, trimming issues will arise. Yet there are al
so improvements to make during trimming, starting with the trim holding fixtures. These fixtures can be produced by making a mold of a master part from the first forming run. They can also be pre-cut from the original part’s solid model. Either way, it’s an expensive and time-consuming operation – and problems result if a fixture is too large or too small.
Conventional wisdom suggests using model part data to make the trim fixture; however, even a model part may be imperfect. However, by comparing the formed parts to the model data, both can be improved. Trim fixtures are better when the first formed parts are checked against the solid model. If there isn’t time to make these measurements, then the trim fixture needs to be adjustable to allow for the best fit from a computer-aided manufacturing (CAM) program. Unless the master formed part is significantly different from the original model data, a CAM trim program is better than an adjustment-friendly program.
With improvements in high-resolution laser scanning, which is now up to 350K to 500K points per second, pressure formers can scan to an accuracy inside our print tolerances. We can also define edges clearly enough to use this technique to quality control (QC) trimmed parts. This is a significant breakthrough, and it’s one our industry must focus on in order to develop an external feedback loop. There’s also an area for improvement that’s not process-driven but purely cost-driven: CNC crash protection.
Unfortunately, CNC equipment is prone to frequent and expensive crashes, software-related failures that result in machine downtime. Reducing the operator errors that cause crashes is critical but it’s enough. That’s why the industry needs a bullet-proof system that will stop the machine head instantly or cause it to “go limp” so quickly that the head isn’t damaged, the machine can be reset, and the trim path restarted. GF Machining Solutions, a Swiss company, has developed a three-axis metal machining center with this feature. Adding this technology to the CNC routers used for trimming would be a major breakthrough.
Pressure Forming’s Future
When Steve Murrill, Chairman of the Board and former President of Profile Plastics, presented a history of pressure forming to the Society of Plastics Engineers in (SPE) in 2016, the industry had already made considerable advances. Yet cost reductions, quality improvements, part-to-part consistency, and better measurement technologies remain key concerns. Today, the risk of failure is higher than ever. At the same time, companies keep getting bigger – and that includes pressure forming’s customers and suppliers. There are also workforce challenges that are more acute today than in 2016.
Continuing advancements in machine vision, in-line measurement, and the proliferation of sensor-based Industry 4.0 technologies now offer manufacturers in general and pressure formers in particular new ways to tackle old challenges. Sound product designs will continue to evolve as well. Take the A/C Plenum that John Grundy, a pressure-forming pioneer, designed years ago. The first generation was vacuum-formed, the second generation was pressure-formed, and the third generation remains a marketplace success.
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