Originally published September1997
Although medical thermoforming remains an art, it is becoming more of a science every day. Technology such as computerized modeling for thermoforming mold design has brought accuracy, reproducibility, and precision to a process that previously exhibited little of these qualities. Until recently, thermoforming depended on molds that were largely handmade or cast in metal from fabricated patterns. Though relatively imprecise, these molds had one great advantage: their surfaces could be refined to any shape that the mold designer needed. Molds were sanded, carved, blended, drafted, shaped, and contoured by hand to exactly fit the product and allow for the most efficient flow of plastic. The molds could be detailed, cast as a reverse, detailed from the opposite side, and cast back to the original--all to create the exact shape required. Manual milling machines were often used to give true, square edges, and to achieve a modicum of accuracy, but the final shape was determined by the application, not by the mold-making method.
Advances in technology allow current thermoformed medical trays to accommodate fairly severe draw ratios, multiple undercuts, and radical shapes. Photo: Prent Corp.
As computer-assisted design and manufacturing (CAD/CAM) began to make inroads into the thermoforming industry, many compromises were made in the name of efficiency and timesaving. Because most of the original CAD technology was two-dimensional, the first thing to be sacrificed was the sophisticated blending of surfaces that manual mold making had allowed. For example, end mills and cutters could only be driven efficiently in straight lines, giving relatively square and sharp corners, which compromised product fit. What suffered most was formability.
The best shape for most thermoforming is a hemisphere; the further away from this configuration one gets, the less the plastic material can be persuaded to take the shape of the mold. However, recent improvements in plastics, molds, and equipment have resulted in the production of mold designs today that couldn't have been made consistently even five years ago. Many of these improvements are largely due to advancements in computerized mold and process control that have enabled thermoformed package design to be less dependent on the method of mold fabrication and more responsive to formability concerns and product function.
MEDICAL PACKAGING APPLICATIONS
Plastic medical packaging can be classified as either sterile or nonsterile, and as either disposable or reusable. This article is concerned with what is by far the largest segment: sterile, disposable medical packaging. Thermoforms are also heavily used for nonsterile medical packaging, but because quality and function are normally not nearly as critical as they are for sterile packages, mold design and processing controls for nonsterile thermoforms can generally be quite simple.
There are two basic types of sterile, disposable packages: lidded and nonlidded. Whereas both types can entail sophisticated design and processing, lidded trays require by far the most attention. This is because lidded, sterile medical thermoforms must function as a sterile barrier in addition to protecting and organizing the components within during shipping, handling, and storage.
The preferred method of sealing thermoformed trays is with a peelable lidding stock. Although paper, foil, and other media can be used for making peelable lidding, the material of choice has long been a spun-bonded olefin (high-density polyethylene filament) coated with a heat-activated adhesive. This printable lid stock is microporous and breathable under pressure, which enables ethylene-oxide (EtO) gas to be used as a sterilant: forced through the membrane under pressure, the gas kills any bacteria inside the sealed package and is then removed under vacuum, leaving all items inside the package sterile until opened. Even after other sterilization methods that did not require the use of gas were perfected--for example, gamma or electron-beam irradiation--spun-bonded sheet remained the lidding media of choice because of its toughness, lack of particulates when peeled, and resistance to moisture.
Sterile barrier thermoformed packaging has progressed in response to industry demands. In the early years, packages were little more than simple open cavities, with dividing walls to organize components. Later, as equipment and mold design evolved to handle different varieties and thicknesses of plastic, more shapes became available. Current thermoformed packaging has progressed to the point of allowing fairly severe draw ratios, multiple undercuts, and radical shapes to be formed with great success. However, two basic requirements continue to be critical concerns for the medical package end-user: the need for consistent and substantial seal flanges, and the need for even wall distribution without thin spots.
The plastic seal surface generally referred to as the flange is located at the interface between the lidding stock and the plastic. This nominally flat surface --the remnant of the original plastic sheet used to form the tray--is probably the most vital element of the sterile barrier, since its flatness and consistency can be critical to the ability to seal the lidding to the tray. Proper design considerations must be followed in order to attain a consistent flange thickness.
The second major concern with thermoformed trays is sidewall integrity. This is especially important in EtO-sterilized trays that must withstand high pressure, heat, and moisture, as well as strong vacuum during the sterilization cycle. Any holes, tears, splits, gaps, or thin areas in the body of the tray will void the sterile barrier. Once again, careful design preparation is necessary to arrive at evenly thermoformed tray walls.
Achieving regularity in flange and sidewall design can be a major problem, because one of the hallmarks of the thermoforming process is its inconsistency. If 10 shots are formed, all 10 will be measurably different; the key is keeping the differences to a minimum. In the basic process, called vacuum forming, plastic sheet is heated past its deflection temperature, until it becomes semimolten and sags under gravity. The sheet's ultimate shape in this instance would be a hemisphere, and the further it sags or stretches, the thinner it becomes. This sheet is then positioned over a mold, and is pulled into or over the mold by vacuum. Wherever it comes in contact with the cooler mold, the plastic will not flow as readily; so that the top of the part is thicker, and the bottom thinner--sometimes so thin as to blow out and cause vacuum to be lost. Depending on the elasticity of the sheet, its starting thickness, and the design of the mold, the part produced can vary dramatically in strength and quality.
Often, vacuum forms are made using male molds, on which the sheet forms the bottom of the tray first, and is then pulled down to form the flange plane. This gives a stronger bottom to the tray, but tends to yield a much thinner flange and sidewalls. Irregularities can be overcome with increased sidewall draft and a thicker starting gauge, but these compensations make for a larger and more expensive part (see Figure 1).
Figure 1. Examples of vacuum-formed trays, highlighting areas of design concern.
Over the years, advances in technology led to the method of pressure forming, in which parts are formed within a sealed vessel--a hermetic shell containing both the tooling and the plastic (see Figure 2). In this process, the plastic sheet is prestretched through the application of positive air pressure on the side of the sheet away from the mold; the vacuum gate remains closed on the mold side of the vessel until prestretching is completed. The technique allows for a much more consistent wall thickness throughout the tray once the vacuum pulls the sheet against the mold to form its final shape (see Figure 3).
Figure 2. Pressure-forming cross section.
Plug-Assist Processing. Another processing improvement has been the use of plug assists in conjunction with pressure forming. These plugs are hobs shaped like the negative image of the mold; they permit further prestretching and preshaping of the plastic sheet on the pressure side of the vessel before vacuum is activated. This innovation has dramatically increased control over the thickness and quality of finished thermoformed parts, and has enabled processors to substantially reduce the starting thickness of the sheet needed to fashion a usable part.
Figure 3. In-line pressure-forming process.
As one can tell from the foregoing descriptions, thermoforming is not really a scientific method. The process itself incorporates numerous variables and tolerances, and can easily be affected by changes in material, equipment, temperature, pressure, and, especially, mold design. If steps are taken to minimize the variability of the first four items--that is, if material is rigorously inspected for quality and consistency, and equipment developed that can regularly operate within a narrow range of forming variations (probably via computerized controls)--then the path to thermoforming better and more consistent medical trays depends on effective part and mold design.
PART AND MOLD DESIGN
In thermoforming, part design and mold design are virtually inseparable. Unlike some other molding processes, thermoforming cannot achieve a different shape on one side of the part than on the other side: material is formed over or into a mold, and is not formed between two mold halves. Also, standard in-line or rotary thermoforming cannot predetermine material thicknesses in specific areas of the finished part, as can be accomplished, for instance, in injection molding. What thermoformers can do, however, is to target certain critical or hard-to-form areas and enhance material flow to these areas via improved mold and plug-assist design, spacing, and heat control.
The key to consistent, high-quality thermoforming is material flow. Whatever can be done from a design standpoint to enhance material flow will result in more-consistent, more-substantial, more-reproducible, and more-economical parts. The easier it is for the material to flow into a pressure-formed tray, the less variation in material thickness there will be. When plug assists are used, the better the material flows off of the plug assist, the more consistent the sidewall thickness will be. If material freezes on the plugs--either because of faulty design, incorrect temperature, or poor choice of plug material--most of the plastic will end up on the bottom and on the flange of the tray, leaving a thin band in between. If plug timing, spacing, or clearances are incorrect, similar gauge bands will occur. For high-speed, in-line pressure forming, it can be said that trays are designed not only to fit the product, but also so that an adequate plug can be shaped for the mold.
There are three basic ingredients for a good thermoformed tray: adequate sidewall draft, adequate radii, and reasonable draw ratios. Although trays can be manufactured that do not exhibit these three qualities, they cannot be made as efficiently, as economically, or as easily as those that do. Again, the key is material flow: the easier it is for the material to flow, the more it will do so; the less work the material has to do, the more easily it will do it. For example, a tray that has straight sidewalls, square corners, sharp edges, and deep cavities will be difficult to thermoform consistently. The same-sized tray, using the same gauge of plastic, will be substantially easier to form with moderately drafted walls, rounded corners and edges, and good draw ratios. A good rule of thumb is that, whenever possible, the plastic should be stretched over no more than three times its original surface area in order to avoid severe forming problems. Thus, for a 5-in.-sq tray--a starting surface area of 25 sq in.--the sum of all surfaces within the tray, including sides and bottom, should total less than 75 sq in.
Figure 4. Plug-assist pressure forming (Step 1).
For plug-assisted pressure forming, a well-designed female tray allows for a well-designed plug assist. Because the material in this process does not touch the tray mold until the last possible instant (when vacuum is pulled), the mold has little to do with how consistently the part forms (see Figure 4). However, since the shape of the mold dictates the shape of the plug to a large extent, plug clearance should be quite uniform throughout. A rounded, drafted form allows for a plug assist with more organic shapes; this, in turn, enables the plug to stretch and distribute the plastic material more evenly, so that the material flows off of the plug instead of freezing (see Figure 5). In this fashion, efficiently shaped plugs permit material to be distributed throughout the tray in a more specific manner. When the vacuum is pulled at the end of the process, very little additional stretching takes place. This stability makes for consistent sidewalls and strong corners, which are critical to sterile barrier thermoforms.
Figure 5. Plug-assist pressure forming (Step 2).
Properly applied, plug-assisted pressure forming can also dramatically improve flange thickness consistency. Because the part is pressure formed within a sealed vessel (the sheet is clamped off from the atmosphere) and because the plastic sheet itself divides the vessel into a top and a bottom pressure area, maximum control can be obtained over the sheet. When pressure is increased from the plug side of the tool, resistance is increased from the mold side--much like pushing on a filled balloon. The air inside the mold half of the tool is compressed, which pushes the plastic sheet away from the cold cavity. The more the plug is engaged, the more the pressure against it grows, stretching the plastic.
An additional benefit is that the plastic material at the flange plane is also kept away from the cold mold, allowing it to continue to stretch and flow until, finally, vacuum is pulled, and the sheet is forced against the cooled mold (see Figure 6). Through this technique, known as ballooning, the seal flanges of the tray are kept from freezing against the mold until the process is complete. This, in turn, allows additional plastic to be pulled from the scrap areas of the sheet (between parts or around the perimeter), lending more substance and consistency to the formed part.
Figure 6. Plug-assist pressure forming (Step 3).
CAD/CAM TECHNOLOGY FOR THERMOFORMING
All of the aforementioned design considerations relate directly to CAD/CAM mold making. The advantages of computerization can be significant: accuracy, reproducibility, and cost-efficiency. These are all desirable capabilities, but they are often gained at the expense of good thermoforming mold design.
What has often taken place is the following: a part is drawn up using two-dimensional CAD, and then transferred to two-dimensional CAM for programming. Because difficult blends, contours, and shapes are not practical from a time, equipment, or training standpoint, these design features are compromised or sacrificed in the name of mold-making efficiency. Cavities are milled in aluminum, plugs milled to match the cavities as closely as possible, and parts formed. Any thin spots in the parts are detailed by upgauging the plastic thickness, thus costing the customer money. Inefficiencies in the cycle time, scrap, yield, and mold spacing required to produce adequate parts are also paid for by the customer. Compromises in product fit and tray function are ultimately dealt with by the end-user.
In this common scenario, the thermoform mold design and mold-making process have been made more efficient by computer technology, but the tray design itself has been compromised. A more sophisticated approach involves a technology known as 2 12 D, which is somewhere between two- and three-dimensional CAD design. This method requires quite a bit more training and experience to use properly, and takes substantially more time and effort--which probably explains why it is not used very often in the thermoforming industry.
What 2 1/2 D permits is for more sophisticated surfaces and blends to be made in designated areas of the tray. For example, if the bulk of a particular tray forms easily and well, but there are corners or pockets in which the material thins excessively, these areas can be broken out of the CNC (computer numeric control) program and individually detailed. By adding more draft and radius, smoothing edges, or contouring sidewalls to fit the product properly, the designer can make the tray form and function better. However, the time involved in detailing these problem areas can be greater than that spent on the entire rest of the tray, which is why many thermoformers avoid this option.
Even fewer thermoformers make it to the next level: fully three-dimensional design and mold surfacing. Not only are equipment and training costs tremendously higher for this method, but most are not sophisticated enough as processors to take advantage of the technology. Fully 3-D CAD design allows for blended surfaces, constant corner radii, compound curves, and precise product fit--all of the things that were available 20 years ago when mold patterns were made by hand. In addition, all of the accuracy, reproducibility, and precision of computer technology can be applied to the mold-making process. When combined with 3-D CNC milling, molds offering excellent material flow, product fit, and design features can be manufactured. Furthermore, sophisticated plug assists can be designed to move material consistently and uniformly.
When a computerized mold making technology is compatible with a customer's technology, concurrent engineering becomes possible. If the customer is designing its product via CAD, wireframes of the product can be imported into the thermoformer's CAD system early in the development process. The mold can then be designed around this "virtual product," saving time. As the product changes, the mold can also be changed and updated, resulting in a very short development lead time once the product design is finalized.
Depending on the software being used, 3-D mold designs can be accomplished in a number of ways--for example, with wireframes, surfaces, or solid models. Once the basic product cavity has been determined, the key to success is to refine the shape so as to enhance material flow. With experience and forethought, a good thermoform designer should be able to contain the product securely while still allowing for adequate material flow, good draw ratios, and customer utility.
Good mold design does not necessarily equal good thermoform design, if tray function and utility are not adequate for the end-user. The computer or the CNC mill can only produce what it is asked to produce. CAD/CAM is only as good as the design itself, and will only give good results if the manufacturing process is under control. Sophistication in these areas is what the medical package buyer should look for when selecting a thermoformer.
Seachtling H, International Plastics Handbook for the Technologist, Engineer, and User, 3rd ed, Cincinnati, Hanser/Gardner, 1995.
Plastics Handbook, edited by the staff of Modern Plastics magazine, New York, McGraw-Hill, 1995.
Don Handrow is a design team leader at Prent Corp., the custom medical thermoformer located in Janesville, WI. He is responsible for coordinating thermoforming projects from initial concept to package completion, and has participated in numerous award-winning package designs.
James Kallenbach is senior designer and product development manager at Prent, where he directs the company's design team. An industrial designer with more than 20 years' experience in thermoforming, he has designed a wide variety of thermoforms for medical applications.