Joining of Plastics HANSER. Handbook for Designers and Engineers 3rd Edition. Jordan Rotheiser

Transcription

1 Jordan Rotheiser Joining of Plastics Handbook for Designers and Engineers 3rd Edition Sample Chapter 4: Design for Disassembly and Recycling ISBNs HANSER Hanser Publishers, Munich Hanser Publications, Cincinnati

2 4 Design for Disassembly and Recycling 4.1 Introduction Each year municipalities in the United States dispose of approximately 320 billion pounds of solid waste. In 1984, 80% of that total went into landfills, 9% was incinerated, and 11% was recycled. Unfortunately, the availability of landfills is rapidly diminishing. Between 1980 and 1990, the number of landfills in the United States diminished by one-third. Despite vast improvements in landfill technology, no one is interested in having such facilities created nearby. Thus, the availability of landfills is certain to continue to decline, with consequent rises in the cost of disposing of solid waste, which will have to be transported ever greater distances for disposal. At some point, solid waste disposal will become so expensive that governing bodies will seek respite. While there is little debate over the desirability of recycling, members of the general public are inclined to consider the problem solved once they have separated their newspapers and some containers into recycling bins. The cost of garbage collection will certainly rise and there will, no doubt, be screams of agony as consumers discover that they must pay $100 to dispose of an old dishwasher. In Europe, governments have already turned to industry with product take back legislation. There they have discovered that the tomorrow of dealing with the problem is at hand. In the United States, the function, market, and cost priorities have elbowed disassembly and recycling to a secondary position. The evidence suggests that the issues of recycling and disassembly will soon rise to a higher level of priority. Indeed, in some areas, there is already pressure in this direction. When the recycling of automobiles was first proposed, someone figured out that to completely disassemble and sort the materials for one model year s worth of cars sold in the United States, several thousand containers would have to be spread over acres of land, and engineers would be needed to do the work! Frankly, this pessimistic estimate was not far wrong. For a product as complex as an automobile, the problem of disassembling and removing the component materials is quite formidable. Many different resins are used. Even parts made of the same resin will contain different additives and have different coatings. Nonetheless, the bulk of the volume of the product is readily salvageable. According to research from the Ford Motor Company, more than 94% of junked automobiles went to a dismantling center in 1994, and 75% of the content was recycled. An infrastructure exists for the recycling of automobile components, and steps are being taken to implement it. The Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) have developed marking systems for plastics (ASTM D ) similar to the system of the International Organization for Standardization (ISO). # Rotheiser 963ha004u.doc Schambach/Schröter

3 4.2 Design for Disassembly 59 Individually, the automobile companies have initiated programs on their own. Ford makes new splash shields from the polypropylene recycled from old batteries, taillamp housings from salvaged bumpers and grille opening reinforcement panels, and headliners and luggage rack side rails from recycled PET. Most business and household products are composed of a far more modest number of parts, and significant progress can be made toward achieving total or near-total recyclability. 4.2 Design for Disassembly Design for disassembly is not a new concept. Products with serviceable components have always been designed this way in fact, it was more widespread in the days before repairs became so expensive that it became cheaper for the consumer to dispose of most small products and replace them with new models than to have the original units repaired. In those days, it principally meant designing the housings with metal screws. If the housings were to be opened repeatedly, metal inserts were used. Self-tapping or thread-forming screws would suffice for housings not expected to be opened often. Frequently opened components, like battery doors, were snap-fit together for convenient access. These practices are still widespread and for the very same reasons. What is new is the adoption of these practices for parts that are not intended to be reopened. However, the term design for disassembly as used today usually refers to the disassembly associated with recycling. In this case, the objective is to free the part from mating components in an uncontaminated fashion so it can be isolated with other parts of the same material. Since there is no intent to reassemble, damage incurred during disassembly is of no concern. The biggest problems confronting the disassembly and recycling process are those of economics. Corporate good intentions notwithstanding, if the cost of the recycled material exceeds that of virgin resin offering the same properties, only legislative mandates will induce industry to move toward such recycling. Therefore, it is necessary to disassemble the product in the most efficient manner possible Reopenable Assemblies The most obvious contaminants are metal fasteners, since plastic parts containing them cannot be readily recycled. One technique is to partially eliminate such fasteners by substituting self-tapping or thread-forming screws for machine screws and metal inserts. Then there are only the screws to remove, and they are easier to get out than inserts. This approach has the disadvantage of limiting the number of times a product can be disassembled, however, because threads of these types are readily stripped by excessive torque. Once the product is out of the factory, it is impossible to control the application torque on these types of screws.

4 60 Design for Disassembly and Recycling Another approach is to eliminate metal fasteners altogether through the use of press and snap fits. Press fits have limitations because they cannot have a great deal of holding power if they are to be readily disassembled. The difference in holding power can vary greatly with just a few thousandths of an inch in interference. Thus, such fitments can be tricky to use. The reader is referred to Chapter 14, Press Fits/ Force Fits/Interference Fits/Shrink Fits, for more details. Snap fits are easier to control and can be designed with a through hole such as that illustrated in Fig. 4-1a for easy disassembly, or with a closed recess for a clean external appearance. In the latter case, a slot in the bottom, as illustrated in Fig. 4-1b, can provide access for a screwdriver blade, which will activate the snap finger. Figure 4-1 Snap fits: (a) through hole type and (b) tool activated There are some concerns regarding the use of snap fits. First, both the snap finger and the recess it fits will require tooling significantly more expensive than is needed for the same part without these details. Second, the maintenance cost is greater for these types of molds. Next, the mechanisms required to make these elements will require more space in the mold, which could cause either a reduction in the number of cavities that can be run in a given molding machine or call for a larger press, with a higher operating rate. These factors, plus the longer cycle associated with such mechanisms, could increase the processing cost. Finally, there is some material in the snaps themselves which adds to the material cost. Chapter 15, Snap Fits, presents a more extensive discussion of this topic. When metal inserts cannot be avoided, they can at least be designed in a fashion that promotes the most efficient handling for recycling. Molded-in inserts are the most difficult to remove and should be avoided. Inserts made of steel can be readily removed from the regrind with magnets, but they can damage the grinder during the regrinding operation prior to their removal. As illustrated in Fig. 4-2, an insert can also be removed by heating it with a hot probe until the plastic around it softens enough for it to be pulled out. This process is not recommended for plastics with high melting points, since its efficiency is highly dependent on the melting point and strength of the material. In most cases, faster methods can be found.

5 4.2 Design for Disassembly 61 Figure 4-2 Hot probe removal: (a) hot probe melting plastic and (b) insert removal Some inserts are readily removed with tools. A tool called the Easy Out is illustrated in Fig It is used to wrench inserts of various types from low to medium strength plastics. It can handle screws up to 3/8 in. diameter. Other tools used for this purpose are the T wrench and the power impact wrench. Inserts that can be removed in this way include coiled wire inserts, press-in inserts, double-threaded inserts, and any made of a material soft enough for this method to be effective. There are two types of expansion insert: those with a center disk, which forces the insert to expand (Fig. 4-4a), and those that use the screw to force expansion (Fig. 4-4b). The latter are less expensive, but sometimes fall out of their own accord when the screw is removed. They are quite easy to remove, however. The type of expansion insert shown in Fig. 4-4a is more difficult to remove. Inserts such as these, molded-in inserts and those that are emplaced with adhesives, are often used for thermosets. When they are in bosses, they can sometimes be broken out, since such bosses are often resin-rich (short of glass reinforcement) and therefore quite brittle. If this is not possible, they must be drilled out, a slow process that leaves metal chips that must be cleaned up to prevent contamination of the plastic to be recycled. Figure 4-3 Insert removal with Easy Out

6 62 Design for Disassembly and Recycling Figure 4-4 Expansion inserts: (a) disk expansion insert and (b) screw expansion insert and knockout notch The breakout approach can be used for thermoplastic inserts as well. In fact, notches such as illustrated in Fig. 4-4b can also be used for bosses, stakes, ultrasonic joints, or any other type of joining detail. This concept is similar to the knockouts used for optional holes in metal electrical boxes. Depending on the material used, it may be possible to knock out the welded area with a hammer in the same fashion. Unfortunately, notches also create weak points in the part which detract from its function. Another device used for this purpose is the hole saw (Fig. 4-5), which removes the boss and a circle of its surrounding material. Figure 4-5 Hole saw removal In some cases, the insert or screw can be placed in a through hole, such as that illustrated in Fig. 4-6, and pressed out from behind. There are other fasteners besides inserts to be concerned with. Self-tapping and drive screws, of course, can be unscrewed, although some are more economically pressed out if they are in through holes. Cap fasteners, such as illustrated in Fig. 4-7a, can be wrenched off with pliers. The push-on fastener shown in Fig. 4-7b can be pried up with a screw driver and twisted off with pliers or simply broken off with its stud. The spring-loaded version shown in Fig. 4-7c can be removed by pressing down the upper segment to relax the tooth grip on the stud. The self-threading nuts illustrated in Fig. 4-7d have a single tooth form that cuts a coarse thread when pressed on during assembly. They can be removed with pliers or a socket wrench, depending on the shape of their heads. The latter can also be removed with an electric or pneumatic screwdriver. These and push-on versions can also be twisted off with pliers.

7 4.2 Design for Disassembly 63 Figure 4-6 Press-out method Figure 4-7 Stud fastener removal: (a) plier twist-off, (b) push-on fasteners, (c) spring-loaded lock, and (d) self-threading Permanent Assemblies Permanent assemblies include all the welding methods plus stakes and adhesive joints. Stakes are the easiest to deal with because it is usually possible to knock their heads off with a chisel or to pry them off as illustrated in Fig. 4-8a. If there is adequate clearance, they can be knocked off from the back side with a hammer. If the stake is a flat-head type, where the surface is flush with the surface of the part, or if there is not enough space to get under the head of the stake, then a wedge, like that illustrated in Fig. 4-8b, can be driven between the two parts and they can be separated in that fashion.

8 64 Design for Disassembly and Recycling Figure 4-8 Removal of: (a) headed stakes and (b) flush stakes Snap fits can also be designed to be permanent rather than reopenable. If the snaps are too strong to be broken off, the area around the snaps must also be designed with a weak segment that would allow them to be broken out much like the knockouts described earlier and shown in perspective in Fig It is far more difficult to separate parts that have been welded together, although the level of difficulty depends on the location of the welds and their strength. Intermittent welds like those often created by ultrasonic energy directors or spot welds like the one shown in Fig. 4-9 are most vulnerable to a wedge at points between the welds. A pryopen slot for this purpose is also illustrated in that figure. Figure 4-9 Spot weld opening More robust welds may need to be cut away with a band or circular saw. That is probably the best approach to adhesive joints. Both they and induction/electromagnetic welds involve the use of contaminating third materials that substantially reduce the value of the recycled resin because it becomes a mixed material recycled material. In the case of the adhesive joint, it is the adhesive itself. In electromagnetic induction welding, there is an insert containing ferrite particles between the two parts. The latter joint is one of the easiest to disassemble because the joint can be remelted if the cost of the time and equipment can be justified. Solvent welds are also easier to deal with. Since solvent welds are most commonly used to join parts of the same material, it may not be necessary to disassemble them at all. The best approach is to establish whether solvent welds indeed were used before attempting to disassemble the parts. If the parts must be disassembled, they can sometimes be softened enough for disassembly by exposing them to the same solvents used to assemble them. If not, they should be treated the same way as other welded joints. 4.3 Design for Recycling The design of a product involves a wide variety of considerations, of which recycling is only one. The following discussion presumes that recycling is the primary concern,

9 4.3 Design for Recycling 65 all others being secondary. In this way, the designer can quickly rate the recyclability of the assembly method under evaluation, for reference along with the other considerations Simplification Part reduction was discussed in Chapter 3 as a means of cost reduction. However, it has disassembly importance as well, since it is clearly less expensive to disassemble fewer parts. Furthermore, the fewer parts there are, the less likely it is that the product will break down and require service. If it does fail, the repair will cost less because less time will be needed to effect it. Thus, a simpler design is not only cost-effective, it is environmentally responsible as well Assembly Method Selection From a recyclability standpoint, the author recommends the following assembly method selection hierarchies. This presumes that it is not possible to make the two parts of the same material such that no disassembly would be required Reopenable Methods 1. Press fit Easy to disassemble, often without heat or tools. However, it can require very tight tolerances and be too easy to disassemble. If the two materials have significantly different coefficients of linear thermal expansion (e.g., a plastic gear on a metal shaft), heat can be used to disassemble them. 2. Snap fit Often thought of as a permanent assembly method, it can be readily reopenable if so designed. Increases the cost of tooling, maintenance, and processing, however, it saves the cost of a fastener. 3. Self-tapping, thread forming, drive screw, or sheet metal nut Saves the cost of the opposite threaded fastener, its assembly, and its disassembly. However, it can be opened only a limited number of times, the drive screw least of all. 4. Machine screw with simple expansion insert This is the easiest of the threaded inserts to remove. Unfortunately, it will sometimes fall out of its own accord, once the screw is removed, and be lost. 5. Machine screw with helical wire, double-threaded insert, or onsert These are the easiest of the female fasteners to remove, but they either do not have the strength and durability of a full metal insert and their installation is time-consuming. 6. Machine screw with molded-in or postmolded insert It does not matter which way these inserts are emplaced; they are all difficult to remove. Molded-in inserts are usually more difficult to remove simply because they are often custom-made for a specialized requirement. As such, they can have odd shapes or be more flimsy than standard inserts. When fasteners are required, their number can be reduced through careful design as illustrated in the previous chapter.

10 66 Design for Disassembly and Recycling Permanent Methods By definition, the permanent assembly techniques are all difficult to disassemble. Therefore the hierarchy for these methods is much shorter. 1. Press or snap fit Both these methods can be designed to be permanent, although the press fit may require expansion of one part with heat to accomplish. However, they can also be designed to be readily disassembled with limited difficulty. 2. Staking In most cases, the head of the stake can be located such that a tool can be used to pry it off. Failing that, the parts can be designed to be readily pried apart. 3. Hot gas, hot die, solvent, ultrasonic, and vibration welding Products made by all these processes are difficult to disassemble, the level of difficulty varying with the structure and material. When possible, design the assemblies so that the weld can be broken out or the parts can be pried apart as illustrated in Fig Adhesives and induction/electromagnetic Products joined by these means are more difficult to disassemble than those above in that they are contaminated with foreign matter, which must also be removed Material Selection A great deal of disassembly can be avoided through material selection with recycling in mind. First, those materials must be identified that can meet the product s function and cost parameters. Once that has been accomplished, the recycling concerns can be considered in arriving at the final selection. All plastic materials can be recycled in some fashion. When selecting a material that is to be recycled, the designer must establish the level of recyclability of the resin that has otherwise met the function and cost criteria. The thermosets, for example, have very limited recyclability because they cannot be remelted and reused. Current recycling of thermosets calls for the material to be shredded, glass fibers to be separated out, and the remaining resin to be mixed with calcium carbonate and ground to a fine powder. The powder, referred to as a composite filler, is mixed with virgin resin at a level of around 7% by weight to form a new compound. Thermosets have also been chopped up and used as filler for a variety of applications. There are more options for the thermoplastics. To begin with, if they can be reground and recycled through the same process, the processor has the opportunity to recycle sprues, runners, cut-offs, and scrap parts. This is one aspect of recycling that has proven to have current economic benefits. Provided the returned parts are not contaminated and the same material is still in use, this also provides the manufacturer with a means for dealing with returned parts. The SPI has a Clean Sweep program to help companies identify internal scrap that can be reused, establish collection methods, and find a market for the reprocessed material. Under this program, Ticona reclaims and sells acetal, polyester, nylon 6/6, polyphenylene sulfide, and liquid crystal polymers. General Electric has a program for repurchasing scrap polycarbonate for resale, mixed with virgin. Many materials must be mixed with virgin to have reliable physical properties and be reused in this fashion.

11 4.3 Design for Recycling 67 In addition, there are commercial markets for some post-consumer recycled resins. Polyethylene terephthalate, the material used in soda bottles, is known to be recycled into sheeting, textile fibers, films, and bottles. High density polyethylene, the milk bottle material, is made into traffic barriers, detergent bottles, drainage pipes, lumber, pallets, flower pots, toys, and boat gears. Polypropylene is recycled for packaging, closure, automotive, and medical applications. The designer concerned with creating a product that can be recycled must select a resin that has an established market for the recycled material and an organized method of collecting, disassembling, and sorting the product for reuse. To interest the commercial market, a plastic must be available in a significant and steady supply. Note that all the polymers for which applications were listed in the preceding paragraph are commodity resins produced in large volume. If a resin is to be used for which such a market does not exist, it will be necessary to create a market. This is a less formidable task than it might seem if the resin is a high value engineering resin and the volume is adequate. Look first to the resin supplier, who may have a recycling program, or at least people on staff who know what is taking place in the market for their recycled resin. To be successful, a recycling program must be carefully planned. One cannot simply manufacture a product made of a material that could be recycled and presume that it will be recycled if one is to be regarded as serious about recycling Additives Plastics materials are often referred to as compounds instead of resins because, as used, they contain a great deal more than the basic resin. Additives are used to enhance the properties of the material and its processing. However, these additives can adulterate the material sufficiently to limit its value as a recycled material. The following is a partial list of such additives. Antioxidants Antistatic agents Colorants (dyes and pigments) Coupling agents Fillers/extenders (calcium carbonate, talc, kaolinite, alumina trihydrate, feldspar, silica) Flame retardants Foaming agents Heat stabilizers Impact modifiers Lubricants Organic peroxides Plasticizers Preservatives Property enhancers (mica, spheres) Reinforcements (carbon, glass, aramid)

12 68 Design for Disassembly and Recycling Stabilizers Ultraviolet inhibitors Although these additives add substantial benefit to the material, they limit its acceptance to the high value recycled material stream. Filled and reinforced compounds are being recycled; however, each specific material under consideration needs to be investigated with respect to its ability to be recycled. When the total cost picture is evaluated, it may be more desirable to go to a resin with higher properties that can be used in an unadulterated form Contaminants There are many ways for a product to become contaminated once it leaves the manufacturer and is, therefore, uncontrollable. Usually these are sources of contamination, dirt or grease, and the recycler deals with them through washing protocols. However, some contaminants are within the control of the manufacturer. Paint and metallic coatings represent the type of coating that can make it difficult to reclaim and recycle a material. While one cannot deny the necessity to provide EMI/RFI shielding, enhance the external appearance of the product, or protect it from ultraviolet light through the use of color or metallics, there are choices to be made. An integral color, such as a pigment, may be an acceptable alternative, depending on the intended use of the recycled resin. Color can be eliminated from internal parts completely, thereby making them more desirable from a recycling standpoint. Lubricants also contaminate the recycled material stream. Two types of external lubricant are in use. One is the variety used in the function of the product, such as that which is often found on bearings and the like. The other is the mold release used in the processing of the part. While it may be difficult to remove the former, the latter can be reduced or eliminated through the use of greater draft angles. These reduce the force necessary to strip the part off its core, thus eliminating the need for a mold release. As added ecological and economic benefits, increased draft angles can reduce the cycle time, which reduces energy usage as well as lowering the machine cost Material Reduction There are several ways to reduce the amount and number of materials to be recycled. By using materials sufficiently alike to be recycled together, the number of parts to be disassembled can be reduced. This can have an effect on the assembly aspects as well, since similar materials usually have melting points close together. This characteristic facilitates the assembly techniques that utilize heat for welding. Of the materials that can be solvent-welded, those that are chemically close to each other lend themselves most easily to the process. Although it is always advantageous to eliminate a disassembly operation, it is particularly desirable with these assembly methods because products so manufactured are more difficult to disassemble.

13 4.3 Design for Recycling 69 Sometimes the number of materials used can be reduced by combining parts. Variations in geometry, derived carefully by a designer mindful of molded-in stress limitations, can alter the strength and stiffness of a component such that the need for an additional part of another material can be eliminated. The stiffness of a given material can also be altered through the use of ribbing or foaming. Thus, one component could be made of solid resin by injection molding and the other might be structural foam molded out of the same material. When the number of materials can be no further diminished, it may be possible to reduce the amount of material used through a technique called thinwalling. While essentially a cost reduction technique, it also has recycling and ecological benefits because, in addition to reducing the amount of resin used, it can reduce the cycle time during processing. Two criteria must be met for effective thinwalling: strength and moldability. The wall thickness that meets the strength requirements of the part can be determined by manual methods, however it is best done with finite element analysis. A mold filling analysis program will help determine the minimum wall thickness that will fill. While new, high flow rate versions of many resins have been developed to accommodate this practice, there is significant danger that high molding temperatures and cavity packing pressures will result in distortion and high levels of molded-in stress. Figure 4-10 Plastics recycling code for use in packaging applications: Examples of container code system for plastic bottles. The stand alone bottle code is different from standard industry identification to avoid confusion with registered trademarks (From Rosato, Plastics Encyclopedia and Dictionary, p. 151.) Identification and Disassembly Instructions Regardless of which material and assembly technique is used, the designer must recognize that this will not be immediately obvious to the disassembler. A sticker

14 70 Design for Disassembly and Recycling bearing printed instructions should be affixed to the interior of the main housing. Directions on opening the main housing should be etched into the mold on the bottom part. Material identification should be in the mold for all the parts according to one of the internationally recognized systems. For packaging use (only), the Society of the Plastics Industry has produced the Plastics Recycling Code, illustrated by the example in Fig The balance of consumer and industrial products are identified for recycling under the SAE, ISO, and ASTM standards, which are regarded as quite similar. The ASTM standard is designated D and is available from: American Society for Testing and Materials (ASTM), 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA , (610) , fax (610) ,