Jordan Rotheiser - Hanser Publications · Handbook for Designers and Engineers 3rd Edition Jordan...

Joining of PlasticsHandbook for Designers and Engineers

3rd Edition

Jordan Rotheiser

ISBNs978-1-56990-445-9

1-56990-445-6

HANSERHanser Publishers, Munich • Hanser Publications, Cincinnati

Sample Chapter 4:Design for Disassembly and Recycling

4 Design for Disassembly and Recycling

4.1 Introduction

Each year municipalities in the United States dispose of approximately 320 billionpounds of solid waste. In 1984, 80% of that total went into landfills, 9% wasincinerated, and 11% was recycled. Unfortunately, the availability of landfills is rapidlydiminishing. Between 1980 and 1990, the number of landfills in the United Statesdiminished by one-third. Despite vast improvements in landfill technology, no one isinterested in having such facilities created nearby. Thus, the availability of landfills iscertain to continue to decline, with consequent rises in the cost of disposing of solidwaste, which will have to be transported ever greater distances for disposal.

At some point, solid waste disposal will become so expensive that governing bodieswill 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 theyhave separated their newspapers and some containers into recycling bins. The cost ofgarbage collection will certainly rise and there will, no doubt, be screams of agony asconsumers discover that they must pay $100 to dispose of an old dishwasher. InEurope, governments have already turned to industry with product “take back”legislation. There they have discovered that the “tomorrow” of dealing with theproblem is at hand.

In the United States, the function, market, and cost priorities have elboweddisassembly and recycling to a secondary position. The evidence suggests that theissues 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 tocompletely disassemble and sort the materials for one model year’s worth of cars soldin the United States, several thousand containers would have to be spread over acres ofland, and engineers would be needed to do the work! Frankly, this pessimistic estimatewas not far wrong. For a product as complex as an automobile, the problem ofdisassembling and removing the component materials is quite formidable. Manydifferent resins are used. Even parts made of the same resin will contain differentadditives and have different coatings.

Nonetheless, the bulk of the volume of the product is readily salvageable. Accordingto research from the Ford Motor Company, more than 94% of junked automobileswent to a dismantling center in 1994, and 75% of the content was recycled.

An infrastructure exists for the recycling of automobile components, and steps arebeing taken to implement it. The Society of Automotive Engineers (SAE) and theAmerican Society for Testing and Materials (ASTM) have developed marking systemsfor plastics (ASTM D1972-94) similar to the system of the International Organizationfor Standardization (ISO).

# 2091812 Rotheiser 963ha004u.doc Schambach/Schröter

4.2 Design for Disassembly 59

Individually, the automobile companies have initiated programs on their own. Fordmakes new splash shields from the polypropylene recycled from old batteries, taillamphousings from salvaged bumpers and grille opening reinforcement panels, andheadliners and luggage rack side rails from recycled PET.

Most business and household products are composed of a far more modest numberof parts, and significant progress can be made toward achieving total or near-totalrecyclability.

4.2 Design for Disassembly

Design for disassembly is not a new concept. Products with serviceable componentshave always been designed this way – in fact, it was more widespread in the daysbefore repairs became so expensive that it became cheaper for the consumer to disposeof most small products and replace them with new models than to have the originalunits repaired.

In those days, it principally meant designing the housings with metal screws. If thehousings were to be opened repeatedly, metal inserts were used. Self-tapping orthread-forming screws would suffice for housings not expected to be opened often.Frequently opened components, like battery doors, were snap-fit together forconvenient access.

These practices are still widespread and for the very same reasons. What is new isthe 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 disassemblyassociated with recycling. In this case, the objective is to free the part from matingcomponents in an uncontaminated fashion so it can be isolated with other parts of thesame material. Since there is no intent to reassemble, damage incurred duringdisassembly is of no concern.

The biggest problems confronting the disassembly and recycling process are thoseof economics. Corporate good intentions notwithstanding, if the cost of the recycledmaterial exceeds that of virgin resin offering the same properties, only legislativemandates will induce industry to move toward such recycling. Therefore, it isnecessary to disassemble the product in the most efficient manner possible.

4.2.1 Reopenable Assemblies

The most obvious contaminants are metal fasteners, since plastic parts containing themcannot be readily recycled. One technique is to partially eliminate such fasteners bysubstituting self-tapping or thread-forming screws for machine screws and metalinserts. Then there are only the screws to remove, and they are easier to get out thaninserts. This approach has the disadvantage of limiting the number of times a productcan be disassembled, however, because threads of these types are readily stripped byexcessive torque. Once the product is out of the factory, it is impossible to control theapplication torque on these types of screws.


Another approach is to eliminate metal fasteners altogether through the use of pressand snap fits. Press fits have limitations because they cannot have a great deal ofholding power if they are to be readily disassembled. The difference in holding powercan vary greatly with just a few thousandths of an inch in interference. Thus, suchfitments can be tricky to use. The reader is referred to Chapter 14, “Press Fits/ ForceFits/Interference Fits/Shrink Fits,” for more details.

Snap fits are easier to control and can be designed with a through hole such as thatillustrated in Fig. 4-1a for easy disassembly, or with a closed recess for a cleanexternal 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.

There are some concerns regarding the use of snap fits. First, both the snap finger andthe recess it fits will require tooling significantly more expensive than is needed forthe same part without these details. Second, the maintenance cost is greater for thesetypes of molds. Next, the mechanisms required to make these elements will requiremore space in the mold, which could cause either a reduction in the number of cavitiesthat can be run in a given molding machine or call for a larger press, with a higheroperating rate. These factors, plus the longer cycle associated with such mechanisms,could increase the processing cost. Finally, there is some material in the snapsthemselves which adds to the material cost. Chapter 15, “Snap Fits,” presents a moreextensive discussion of this topic.

When metal inserts cannot be avoided, they can at least be designed in a fashionthat promotes the most efficient handling for recycling. Molded-in inserts are the mostdifficult to remove and should be avoided. Inserts made of steel can be readilyremoved from the regrind with magnets, but they can damage the grinder during theregrinding operation prior to their removal.

As illustrated in Fig. 4-2, an insert can also be removed by heating it with a hotprobe until the plastic around it softens enough for it to be pulled out. This process isnot recommended for plastics with high melting points, since its efficiency is highlydependent on the melting point and strength of the material. In most cases, fastermethods can be found.

Figure 4-1 Snap fits: (a) throughhole type and (b) tool activated


Some inserts are readily removed with tools. A tool called the Easy Out isillustrated in Fig. 4-3. It is used to wrench inserts of various types from low to mediumstrength plastics. It can handle screws up to 3/8 in. diameter. Other tools used for thispurpose are the “T” wrench and the power impact wrench. Inserts that can be removedin this way include coiled wire inserts, press-in inserts, double-threaded inserts, andany 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 forcesthe 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 accordwhen 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. Insertssuch as these, molded-in inserts and those that are emplaced with adhesives, are oftenused for thermosets. When they are in bosses, they can sometimes be broken out, sincesuch bosses are often “resin-rich” (short of glass reinforcement) and therefore quitebrittle. If this is not possible, they must be drilled out, a slow process that leaves metalchips that must be cleaned up to prevent contamination of the plastic to be recycled.

Figure 4-2 Hot probe removal: (a) hotprobe melting plastic and (b) insertremoval

Figure 4-3 Insert removal with Easy Out


Figure 4-4 Expansion inserts: (a) disk expansion insert and (b) screw expansion insert andknockout notch

The breakout approach can be used for thermoplastic inserts as well. In fact, notchessuch as illustrated in Fig. 4-4b can also be used for bosses, stakes, ultrasonic joints, orany other type of joining detail. This concept is similar to the “knockouts” used foroptional holes in metal electrical boxes. Depending on the material used, it may bepossible 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 itsfunction. Another device used for this purpose is the hole saw (Fig. 4-5), whichremoves the boss and a circle of its surrounding material.

In some cases, the insert or screw can be placed in a through hole, such as thatillustrated in Fig. 4-6, and pressed out from behind.

There are other fasteners besides inserts to be concerned with. Self-tapping anddrive screws, of course, can be unscrewed, although some are more economicallypressed 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 priedup 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 theupper segment to relax the tooth grip on the stud. The self-threading nuts illustrated inFig. 4-7d have a single tooth form that cuts a coarse thread when pressed on duringassembly. They can be removed with pliers or a socket wrench, depending on theshape of their heads. The latter can also be removed with an electric or pneumaticscrewdriver. These and push-on versions can also be twisted off with pliers.

Figure 4-5 Hole saw removal


4.2.2 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 headsoff with a chisel or to pry them off as illustrated in Fig. 4-8a. If there is adequateclearance, 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, likethat illustrated in Fig. 4-8b, can be driven between the two parts and they can beseparated in that fashion.

Figure 4-6 Press-out method

Figure 4-7 Stud fastener removal:(a) plier twist-off, (b) push-onfasteners, (c) spring-loaded lock,and (d) self-threading


Snap fits can also be designed to be permanent rather than reopenable. If the snaps aretoo strong to be broken off, the area around the snaps must also be designed with aweak segment that would allow them to be broken out much like the “knockouts”described earlier and shown in perspective in Fig. 4-9.

It is far more difficult to separate parts that have been welded together, although thelevel of difficulty depends on the location of the welds and their strength. Intermittentwelds like those often created by ultrasonic energy directors or spot welds like the oneshown in Fig. 4-9 are most vulnerable to a wedge at points between the welds. A pry-open slot for this purpose is also illustrated in that figure.

More robust welds may need to be cut away with a band or circular saw. That isprobably the best approach to adhesive joints. Both they and induction/electro-magnetic welds involve the use of contaminating third materials that substantiallyreduce the value of the recycled resin because it becomes a “mixed material recycledmaterial.” In the case of the adhesive joint, it is the adhesive itself. In electromagneticinduction 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 ifthe cost of the time and equipment can be justified.

Solvent welds are also easier to deal with. Since solvent welds are most commonlyused to join parts of the same material, it may not be necessary to disassemble them atall. The best approach is to establish whether solvent welds indeed were used beforeattempting to disassemble the parts. If the parts must be disassembled, they cansometimes be softened enough for disassembly by exposing them to the same solventsused to assemble them. If not, they should be treated the same way as other weldedjoints.

4.3 Design for Recycling

The design of a product involves a wide variety of considerations, of which recyclingis only one. The following discussion presumes that recycling is the primary concern,

Figure 4-8 Removal of: (a)headed stakes and (b) flush stakes

Figure 4-9 Spot weld opening

4.3 Design for Recycling 65

all others being secondary. In this way, the designer can quickly rate the recyclabilityof the assembly method under evaluation, for reference along with the other consi-derations.

4.3.1 Simplification

Part reduction was discussed in Chapter 3 as a means of cost reduction. However, ithas disassembly importance as well, since it is clearly less expensive to disassemblefewer parts. Furthermore, the fewer parts there are, the less likely it is that the productwill break down and require service. If it does fail, the repair will cost less becauseless time will be needed to effect it. Thus, a simpler design is not only cost-effective, itis environmentally responsible as well.

4.3.2 Assembly Method Selection

From a recyclability standpoint, the author recommends the following assemblymethod selection hierarchies. This presumes that it is not possible to make the twoparts of the same material such that no disassembly would be required.

4.3.2.1 Reopenable Methods

1. Press fit Easy to disassemble, often without heat or tools. However, it can requirevery tight tolerances and be too easy to disassemble. If the two materials havesignificantly different coefficients of linear thermal expansion (e.g., a plastic gearon a metal shaft), heat can be used to disassemble them.

2. Snap fit Often thought of as a permanent assembly method, it can be readilyreopenable if so designed. Increases the cost of tooling, maintenance, and proces-sing, however, it saves the cost of a fastener.

3. Self-tapping, thread forming, drive screw, or sheet metal nut Saves the cost of theopposite threaded fastener, its assembly, and its disassembly. However, it can beopened 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 threadedinserts to remove. Unfortunately, it will sometimes fall out of its own accord, oncethe screw is removed, and be lost.

5. Machine screw with helical wire, double-threaded insert, or onsert These are theeasiest of the female fasteners to remove, but they either do not have the strengthand 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 waythese inserts are emplaced; they are all difficult to remove. Molded-in inserts areusually more difficult to remove simply because they are often custom-made for aspecialized requirement. As such, they can have odd shapes or be more flimsy thanstandard inserts. When fasteners are required, their number can be reduced throughcareful design as illustrated in the previous chapter.


4.3.2.2 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 beused 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 allthese processes are difficult to disassemble, the level of difficulty varying with thestructure and material. When possible, design the assemblies so that the weld canbe broken out or the parts can be pried apart as illustrated in Fig. 4-9.

4. Adhesives and induction/electromagnetic Products joined by these means are moredifficult to disassemble than those above in that they are contaminated with foreignmatter, which must also be removed.

4.3.3 Material Selection

A great deal of disassembly can be avoided through material selection with recyclingin mind. First, those materials must be identified that can meet the product’s functionand cost parameters. Once that has been accomplished, the recycling concerns can beconsidered in arriving at the final selection.

All plastic materials can be recycled in some fashion. When selecting a material thatis to be recycled, the designer must establish the level of recyclability of the resin thathas otherwise met the function and cost criteria. The thermosets, for example, havevery limited recyclability because they cannot be remelted and reused. Currentrecycling of thermosets calls for the material to be shredded, glass fibers to beseparated out, and the remaining resin to be mixed with calcium carbonate and groundto a fine powder. The powder, referred to as a “composite filler,” is mixed with virginresin at a level of around 7% by weight to form a new compound. Thermosets havealso 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 bereground and recycled through the same process, the processor has the opportunity torecycle sprues, runners, cut-offs, and scrap parts. This is one aspect of recycling thathas proven to have current economic benefits. Provided the returned parts are notcontaminated and the same material is still in use, this also provides the manufacturerwith a means for dealing with returned parts.

The SPI has a “Clean Sweep” program to help companies identify internal scrapthat can be reused, establish collection methods, and find a market for the reprocessedmaterial. Under this program, Ticona reclaims and sells acetal, polyester, nylon 6/6,polyphenylene sulfide, and liquid crystal polymers. General Electric has a program forrepurchasing scrap polycarbonate for resale, mixed with virgin. Many materials mustbe mixed with virgin to have reliable physical properties and be reused in this fashion.


In addition, there are commercial markets for some post-consumer recycled resins.Polyethylene terephthalate, the material used in soda bottles, is known to be recycledinto sheeting, textile fibers, films, and bottles. High density polyethylene, the milkbottle 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 aresin that has an established market for the recycled material and an organized methodof collecting, disassembling, and sorting the product for reuse. To interest thecommercial market, a plastic must be available in a significant and steady supply.Note that all the polymers for which applications were listed in the precedingparagraph 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 tocreate a market. This is a less formidable task than it might seem if the resin is a highvalue 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 istaking place in the market for their recycled resin. To be successful, a recyclingprogram must be carefully planned. One cannot simply manufacture a product made ofa material that could be recycled and presume that it will be recycled if one is to beregarded as serious about recycling.

4.3.4 Additives

Plastics materials are often referred to as “compounds” instead of resins because, asused, they contain a great deal more than the basic resin. Additives are used toenhance the properties of the material and its processing. However, these additives canadulterate the material sufficiently to limit its value as a recycled material. Thefollowing 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)


• Stabilizers• Ultraviolet inhibitors

Although these additives add substantial benefit to the material, they limit its accept-ance to the high value recycled material stream. Filled and reinforced compounds arebeing recycled; however, each specific material under consideration needs to beinvestigated with respect to its ability to be recycled. When the total cost picture isevaluated, it may be more desirable to go to a resin with higher properties that can beused in an unadulterated form.

4.3.5 Contaminants

There are many ways for a product to become contaminated once it leaves the manu-facturer 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 toreclaim and recycle a material. While one cannot deny the necessity to provideEMI/RFI shielding, enhance the external appearance of the product, or protect it fromultraviolet light through the use of color or metallics, there are choices to be made. Anintegral color, such as a pigment, may be an acceptable alternative, depending on theintended use of the recycled resin. Color can be eliminated from internal partscompletely, thereby making them more desirable from a recycling standpoint.

Lubricants also contaminate the recycled material stream. Two types of externallubricant are in use. One is the variety used in the function of the product, such as thatwhich is often found on bearings and the like. The other is the mold release used in theprocessing of the part. While it may be difficult to remove the former, the latter can bereduced or eliminated through the use of greater draft angles. These reduce the forcenecessary to strip the part off its core, thus eliminating the need for a mold release. Asadded ecological and economic benefits, increased draft angles can reduce the cycletime, which reduces energy usage as well as lowering the machine cost.

4.3.6 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 bedisassembled can be reduced. This can have an effect on the assembly aspects as well,since similar materials usually have melting points close together. This characteristicfacilitates the assembly techniques that utilize heat for welding. Of the materials thatcan be solvent-welded, those that are chemically close to each other lend themselvesmost easily to the process. Although it is always advantageous to eliminate adisassembly operation, it is particularly desirable with these assembly methodsbecause products so manufactured are more difficult to disassemble.


Sometimes the number of materials used can be reduced by combining parts.Variations in geometry, derived carefully by a designer mindful of molded-in stresslimitations, can alter the strength and stiffness of a component such that the need foran additional part of another material can be eliminated. The stiffness of a givenmaterial can also be altered through the use of ribbing or foaming. Thus, onecomponent could be made of solid resin by injection molding and the other might bestructural foam molded out of the same material.

When the number of materials can be no further diminished, it may be possible toreduce the amount of material used through a technique called “thinwalling.” Whileessentially a cost reduction technique, it also has recycling and ecological benefitsbecause, in addition to reducing the amount of resin used, it can reduce the cycle timeduring processing. Two criteria must be met for effective thinwalling: strength andmoldability. The wall thickness that meets the strength requirements of the part can bedetermined by manual methods, however it is best done with finite element analysis. Amold filling analysis program will help determine the minimum wall thickness that willfill. While new, high flow rate versions of many resins have been developed toaccommodate this practice, there is significant danger that high molding temperaturesand cavity packing pressures will result in distortion and high levels of molded-in stress.

4.3.7 Identification and Disassembly Instructions

Regardless of which material and assembly technique is used, the designer mustrecognize that this will not be immediately obvious to the disassembler. A sticker

Figure 4-10 Plastics recycling code for use inpackaging applications: Examples of containercode system for plastic bottles. The stand alonebottle code is different from standard industryidentification to avoid confusion with registeredtrademarks (From Rosato, Plastics Encyclopediaand Dictionary, p. 151.)


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 bottompart. Material identification should be in the mold for all the parts according to one ofthe internationally recognized systems.

For packaging use (only), the Society of the Plastics Industry has produced thePlastics Recycling Code, illustrated by the example in Fig. 4-10.

The balance of consumer and industrial products are identified for recycling underthe SAE, ISO, and ASTM standards, which are regarded as quite similar. The ASTMstandard is designated D1972-94 and is available from:

American Society for Testing and Materials (ASTM),100 Barr Harbor Drive,P.O. Box C700,West Conshohocken, PA 19428-2959,(610) 832-9585, fax (610) 832-9555,www.astm.org

Jordan Rotheiser - Hanser Publications · Handbook for Designers and Engineers 3rd Edition Jordan...

Documents

Transcript of Jordan Rotheiser - Hanser Publications · Handbook for Designers and Engineers 3rd Edition Jordan...