Design of Plastic Products
The times we live in are often known as the “Computer Age.” It could also be referred to as the “Plastics Age,” as the production of plastics has exceeded that of steel (by volume) since 1979. In fact, the volume of plastics produced has more than doubled in the last 20 years. Nonetheless, most students who graduate from the major engineering universities are generally unprepared to design in plastics. Thus, it is left to the individual engineer to learn plastics engineering on his or her own, often by trial and error.
Unfortunately, this type of education can come at great cost—both to the company and the career of the individual. First, second, and even third and fourth efforts can be disastrous. That is because plas- tics design is more complicated, and more time consuming, than designing with metals. There are several reasons for this and they center around the processes which are used to manufacture plastic parts, the tooling used for those processes, and the nature of the plas- tics themselves.
Unlike metals, the properties of most plastics vary considerably with- in normal operating temperatures. A particular acrylonitrile butadiene styrene (ABS) whose tensile strength is 5500 lb/in2 at room temperature can drop to 2800 lb/in2 at 125°F. Other properties are also affected. For example, brittleness increases as the temperature drops, etc.
What does this mean to the design engineer? Basically, it means there will be more work to do. Various pertinent properties will need to be examined at both extremes of the service range. Furthermore, the design parameters must be explored more fully. It cannot be assumed that the product will survive the temperatures endured in cleaning, shipping, or storage unscathed.
Other exposures can cause problems with plastic components as well. Ultraviolet light causes or catalyzes chemical degradation in many resins. Plastics are vulnerable to attack from many chemicals, particularly in heavy concentrations. Some are even affected by water and there is one, polyvinyl acetate, that actually dissolves in water (think of soap packets).
Engineers will want to calculate the various performance values in the traditional fashion. Unfortunately, these computations cannot be regarded as valid. It is not that the laws of physics are different for plas- tics, it is simply that the data employed is not reliable for calculations.
That is not to suggest any skullduggery on the part of the test engi- neers; it is simply that the standard test sample and conditions are narrowly defined and likely to be significantly different from those to be endured by any specific product. The values obtained for most plas- tics will vary according to the process, gating, wall thickness, rate of loading, etc. It should be noted that there is some latitude within the test procedures themselves which can affect results. Most plastics engineers use the data sheets principally for the purposes of compari- son in material selection.
It is important that the design engineer be cognizant of this when employing the data in an empirical fashion. Generous safety factors are customarily used. When greater precision is required, the proposed material may be independently tested under conditions more appro- priate to the actual application. When the anticipated market is suffi- cient, the resin manufacturer may be inclined to perform these tests at the company’s expense. Otherwise, it will be the obligation of the prod- uct manufacturer to bear the cost.
The general stiffness range of most plastics, combined with the gen- eral effort to use the thinnest possible wall thickness, means the geom- etry has a pronounced effect. Other than through comparison to similar constructions, the stiffness of the actual part is difficult to pre- dict in a precise fashion. Although the traditional equations will pro- duce approximate results, stiffness remains a question until the first part is molded. (Finite element analysis results are vulnerable to the many variables involved.) Fortunately, there are so many compound variations available within a given resin, it is usually possible to adjust stiffness within a reasonable range. This has been the saving grace of many an engineer. Also, many plastics engineers withhold the placement of ribbing until the first parts are tested.
Even if the material maintained its properties throughout the prod- uct’s temperature range and the data was perfectly reliable, the prod- uct’s performance could still vary. That is because the plastics processes are subject to tooling quality and process parameter variations.
Nonetheless, the fact that plastic parts can be successfully designed is attested to by the wide variety of products in the marketplace. It is clearly, however, more work to design in plastic and it is virtually impossible to perfectly predict the initial results. That is the reason prototypes are frequently made.
It is tempting to test a fabricated sample before constructing tooling. However, it should be noted that the final part is likely to produce sub- stantially different test results than one fabricated by some other process. Furthermore, sophisticated plastics engineers will often delib- erately underdesign a product, adding a little material at a time in a prototype mold to determine the thinnest acceptable wall. This approach also permits the testing of the complete assembly whereby the walls of the mating parts reinforce each other to produce a stronger overall structure.
8.1.2 The holistic design approach
There are four principle elements to a successful plastic product: mate- rial selection, part design, tooling, and processing. Typically, product designers are effective part designers but have limited background in the other disciplines. This leads to products which are more expensive than necessary and difficult to manufacture—which also increases the cost. Many companies have solved this problem through the use of multidisciplinary design teams. However, team members report that such teams can be dysfunctional, often due to the fact that team mem- bers’ schedules are difficult to synchronize or a lack of availability of required skills.
Ideally, the part designer would know enough about these other dis- ciplines to be able to design with them in mind. That utopian situation could create the ultimate in efficient product design—the holistic design approach.
Bottle cap example. An example of this type of thinking would be the case of a closure for a bottle containing cosmetics. In order to protect the contents, the closure must seal the opening. Typically, that seal is creat- ed with a seal ring or liner which is clamped down with force provided by screw threads, thus creating what is generally known as a bottle cap.
Process selection is simplified because the process of choice for most bottle cap applications is injection molding. Compression and transfer molding are possibilities, however, they are slower than injection molding and are rarely used for thermoplastic materials. Therefore, they would only be considered if the material of choice turned out to be a thermoset.
In the case of a cosmetic bottle cap, the most extreme temperatures it is likely to encounter will be in transit or washing prior to applica- tion. This limits the range to that which is readily accommodated by most thermoplastics without deformation. However, thermal expan- sion will need to be considered as the cap cannot loosen enough to break the seal at elevated temperatures nor contract enough to crack at low temperatures. Furthermore, it must not fail due to stress relax- ation over time nor impart an odor of its own to the contents. For a cos- metic application, there may also be an appearance requirement of a high-gloss surface. Most importantly, it must withstand the chemical attack of the contents. While resin manufacturers typically perform limited tests (more on this topic will be discussed later) for resistance to various chemical compounds, they cannot do this when the compo- sition of the exposure is a secret such as with a cosmetic. The manu- facturer is expected to conduct such tests privately.
There is another element of material selection for a bottle cap which involves the tool building and processing disciplines. It derives from the fact that the decision must be made as to whether the threads are to be stripped off the core, turned off the core or the core is to be col- lapsed to permit ejection. The tool for the former will be far less expen- sive and the mold will operate at a much faster rate of speed. However, the material must be one which is flexible enough to strip off the mold, yet rigid enough to perform its other functions.
The problem is created by the fact that the formation of a thread cre- ates plastic at a point inside the largest diameter of the hardened core as shown in Fig. 8.1a. As the force of ejection pushes on the base of the cap to remove it from the core, it must be flexible enough to expand off the core as illustrated in Fig. 8.1b. The part becomes more rigid as it cools in the mold. Even an essentially rigid material might be success- fully ejected from the mold if this function is performed while the part is still soft. However, the part must also be rigid enough to withstand the force of ejection without enduring permanent distortion.
The point at which the cap is cool enough to eject, yet warm enough to strip off the core, will vary according to the means of ejection employed. Ejector pins provide very localized forces at the base of the cap. An ejector plate creates an ejection force which is distributed uni- formly across the base of the cap. Therefore the cap can be ejected in a softer condition with the use of a stripper plate. That results in a cycle
Figure 8.1 Stripping an undercut off a core: (a) as molded; (b) part ejection.
reduction on the order of 30%, however, the stripper plate adds a sig- nificant increment of cost to the tooling.
The amount of force required to eject the part can also be attained through the use of interrupted threads on the bottle cap. By breaking the continuity of the thread, the amount of material which must be stretched to permit removal of the cap from the mold is significantly reduced.
The determining factor in how deep a thread can be stripped is its strain rate. For the sake of this bottle cap example, it will be presumed that it must be made of acrylic for appearance reasons because this polymer can provide a very high level of gloss. Acrylic is an amorphous thermoplastic with a very low rate of strain. In this case, too low to permit the part to be stripped off the mold. Therefore, the cap would need to be turned off the core of the mold.
In order to turn the part off the core, an unscrewing mechanism must be employed. There are several ways to go about this, however, all of them incur significant additional cost. Furthermore, the space required for the mechanism limits the number of cavities which can be placed within a mold base. If the platens of the molding machine have sufficient space, a larger mold base can be used. However, if the mold was already sized to the limits of the platen, the number of cavities will need to be reduced or a larger molding machine will be required. Either way, fewer bottle caps will be produced with each molding cycle and the machine cost for each cap will be increased, thus reducing the efficiency of the production. The machine cost will also be increased by the longer cycle necessitated by the time required for the unscrewing
mechanism to function. Thus, a cap produced in an unscrewing mold will always have a greater machine cost increment than one which is stripped off a core—all other elements being equal.
There is another method for producing internal threads which are too rigid to be stripped off a mold. That involves a core mechanism which collapses. Such “collapsing cores” are patented and there is an added cost for this mechanism. Molds utilizing these cores cycle near- ly as fast as stripper plate molds and the mechanisms require a mod- erate amount of additional space. However, these molds are reported to have higher maintenance costs than the other types of molds and are generally thought of as a solution for applications with lower pro- duction quantities.
Note that, at this point, the discussion has involved material selec- tion, processing, and tool design with scarcely a mention of part design. That is the whole point of the holistic design concept—that a higher level is achieved when all of these elements are considered simultaneously. The details of the part design are dependent on the decisions reached as a whole. Clearly the thread design will be depen- dent on the determination of whether the cap is to be turned or stripped off the core or whether a collapsing core is to be used instead. When appearance is of greater importance than molding efficiency, esthetic requirements may be the determining factor.