Modern Plastics Handbook 2011 Part 10 pot

cost differential can cover quite a volume before injection molding becomes more competitive. However, as product designers create structural foam designs with detail equivalent to injection-molded parts or require gas counterpressure molds, that advantage has diminished. When the cost of a structural foam mold is nearly that of an injection mold, the process will be selected for its unique attributes. Such features as weight reduction, filling of hollows, and the ability to make larger parts (due to the low pressure) become the principal selection criteria. For some parts, there might be one more stop on the pecking order before going to injection molding, that is, gas-assisted injection molding. This process provides a rigid wall with a hollow channel within its thick sections and a solid wall elsewhere. Molding pressures can be as low as 15% of injection molding pressures with the resultant increase in the size of the part which can be molded in a given molding machine. Gas-assisted injection molding is of particular value in the elimination of assembly operations used to fabricate box structures. There have been successful applications in the elimination of parts in automobile door frames where the gas has been used to create the box structures within the frame. For most parts, the ultimate in production volume economies can be reached with injection molding. At a size of about 5 ft 2 of surface area, however, injection molding drops out of the competition because molding machines which can handle that size are very scarce and are usually reserved for proprietary applications. In that case, structural foam molding may be the most efficient process. Unfortunately, structural foam molding can handle a much more limited palette of materials than injection molding can. The materials that are readily molded with structural foam are ABS, ionomers, polycarbonate, polyethylene, and polypropylene. Most of the remainder can only be handled with difficulty and some, like styrene and acrylic, cannot be processed at all. The only high-strength and temperature material in the readily processed group is polycarbonate. For cost or chemical compatibility reasons, polycarbonate may not be acceptable. That means that the next step up in volume economies for a part which is too large for injection molding and which requires higher- temperature resistance than the structural foam materials can inex- pensively offer may be a thermoset process like compression molding. Machining. With the ready availability of most machining equipment and little or no tooling required, machining is the ideal process for very low-volume applications, provided the shape is one which can be readily machined. Piece part cost will be highest with machining as each piece must be processed individually. Machining also is the process which permits the closest tolerances for extremely critical applications 8.30 Chapter Eight 0267146_Ch08_Harper 2/24/00 4:45 PM Page 30 where the cost can be justified. The high cost is due to the fact that the work must be done slowly to avoid thermal expansion or melting which would result in imprecision. It should be noted that some plastics are too flexible to be readily positioned for machining or may deflect away from the cutting tool when pressure is applied. Machining is also used in combination with other processes. Some processes require that gates be removed or parts trimmed by mechanical means before they are ready to be used. Machining may be used to create details the process cannot create. An example of this type of application would be holes in thermoformed parts. Finally, machining may be used because the product’s production volume is too low to war- rant the additional tooling cost necessary to mold the detail into the part. An example of this type of application would be a hole in an injection-molded part parallel to the parting line that would require an expensive side action. The additional amortization cost could exceed the cost of drilling the hole for low volumes. Many plastic prototypes are machined and fabricated. However, this process is only useful to resolve fitment, appearance, and ergonomic issues. That is because the strength, internal stresses, and environ- mental behavior characteristics of the part made in this fashion will be significantly different from the properties of the same part made by the ultimate production process. Furthermore, many plastic parts are in the strength and stiffness range where the characteristics of the assembly are different than those of the individual parts. Therefore, testing should be performed on the completed assembly made with production parts. Even tests performed on parts made by the proto- typing method for a given process cannot be completely reliable because the processing conditions would be different and plastics are process-sensitive. Thermoforming and pressure thermoforming. Thermoforming has the repu- tation of being the process to use for applications requiring large parts with thin walls that have low tooling budgets. Tooling for thermoforming is relatively inexpensive because this is an “open-mold” process, meaning that a mold with only one side is used. Thus, it is the first step up in tooling cost from simple machining. Product piece parts are typically less costly for thermoformed parts than for machined parts. However, this is a three-stage process because there are two additional stages besides the thermoforming stage. First, resin must be extruded into sheet before it can be thermoformed and, after thermoforming, the excess sheet (offal) must be trimmed and any holes or openings cut in the parts. These factors can result in fairly high piece part costs. Therefore, as the volume of a given product grows, it may reach a point where the additional cost of injection molding tooling is more than offset by the reduction in piece part cost. Design of Plastic Products 8.31 0267146_Ch08_Harper 2/24/00 4:45 PM Page 31 Thermoforming is a stretching process which permits thinner wall thicknesses than machining, for which walls must withstand machining forces and heat, or the molding processes that require wall thickness thick enough to permit melt flow. For products that do not require thicker walls for strength, this can result in lower piece part prices. There are actually several thermoforming process variations, and they can be considered according to the thickness of the sheet used (“thin gauge” or “heavy gauge” thermoforming), the manner in which it is supplied (roll or sheet), or the contour of die used (male or female). Thin-gauge (thickness less than 0.060 in) thermoforming uses material supplied in a roll. It is the high-volume variety of this process and is generally associated with packaging. With the exception of dispos- ables, such as cups and plates, it is not generally used for product manufacturing. Heavy-gauge (thickness greater than 0.060 in) material is supplied in sheet form. However, it should be noted that gauges less than 0.060 in are sometimes used in sheet thermoforming. This is the variety of thermoforming that product designers are normally concerned with. It can produce relatively flat parts with rounded corners in its simplest form, known as “vacuum forming.” However, its “pressure forming” ver- sion can produce detail that can rival injection molding. Heavy-gauge thermoforming tends to lose its competitive edge over other processes when the part size falls below 1 ft 2 . Conversely, it has a very large part capability. This allows the combination of several parts into one in many cases, thus eliminating some assembly operations completely. Pressure thermoforming uses air pressure, often with the addition of a plug assist, to force the material deeper into the mold cavity. With pressure, the fine detail and surface finishes associated with injection molding can be achieved. However, injection molding usually requires much higher annual volume than thermoforming to be economically feasible. Thus, this development increases the capabilities of the product designer by extending the range of products which can use such detail to those with lower annual volumes. Injection molding becomes a more serious contender for the application as its volume increases. Then the piece part cost differential can be applied to the substantially greater cost of the injection molds. The cost of the tooling for injection molding rises substantially with increasing size, and the payoff volume, the point at which the additional tooling cost is offset by piece part savings, goes up accordingly. Thermoforming can, however, make substantially larger parts than can injection molding. Structural foam molding, gas counterpressure structural foam molding, and co-injection molding. The high cost of tooling is the factor which governs access 8.32 Chapter Eight 0267146_Ch08_Harper 2/24/00 4:45 PM Page 32 to the injection molding process. This cost can be reduced through the use of related processes which require less molding pressure, such as structural foam molding, gas counterpressure structural foam molding, and co-injection-molding. Lower pressure allows for a less substantial, and therefore less costly, mold. The part produced by structural foam molding is not solid like the part produced by thermoforming or injection molding. Within the outer skin, there is a cellular structure with the cell size increasing toward the center. The part must have a wall thickness of 0.187 in in order for any significant amount of foaming to take place. Thus, the structural foam part may actually require more resin than the equivalent part made by either thermoforming or injection molding if the wall thickness must be increased to accommodate the foaming process. In addition, the molding cycle for structural foam molding is much longer than that of injection molding due to the time required for gas expansion. For these reasons, the piece part cost will be greater for a part produced by structural foam molding than for a similar part made by injection molding. The reduced molding pressure does permit larger parts to be molded in a given molding machine, at least to the lim- its of the machine platens. Parts made by the structural foam molding process have a charac- teristic swirled surface. Gas counterpressure structural foam molding and co-injection molding are variations of the process which can produce a solid, nonswirled surface. Depending on the application, the additional mold cost for the gas counterpressure feature reduces the tooling savings over the injection molding alternative. Coinjection molding permits a solid material to be used for the outer skin and a foamed material for the inner structure which can also be a less expensive material. This process requires sophisticated equipment. These three processes use closed molds and low pressures. The closed molds are more costly than the open molds used in thermoforming because there are two halves instead of one. However, the low pressure keeps the tool cost significantly lower than traditional injection molding. Unfortunately, it is that high pressure associated with injection molding that permits its fast cycles. Thus, piece part prices are higher for these methods than they are for injection. As in thermoforming, these processes are most competitive with injection molding for large parts. The larger the part, the greater the mold cost advantage over injection molding. Unless, of course, one designs parts of such intricacy that this advantage is negated. Piece part costs are a different matter. As a broad statement, parts from these processes will be less costly than those from thermoforming, but more costly than those from injection molding. However, there are other reasons for using them besides simple piece part cost. Design of Plastic Products 8.33 0267146_Ch08_Harper 2/24/00 4:45 PM Page 33 One purpose could be to change the “feel” or “heft” of a product such as a steering wheel or a vacuum cleaner handle. That permits the handle to be made in one piece instead of the older method which consist- ed of two injection-molded halves usually assembled with screws and nuts. These foam molding processes eliminate the need for assembly and the cost of the fasteners. This results in a lower product cost even though the injection molding process has a faster molding cycle. Injection molding. While the extrusion process is the highest-volume process, injection molding is used for the greatest number of product design applications, largely because it provides the lowest piece part cost for volume applications. Injection molding can accommodate a range of applications from huge parts which require a cycle of several minutes to high-volume bottle caps that have been molded at the rate of 288 parts every 4 s. Injection molding gets its name from the fact that plastic is injected into a mold at very high pressure. That gives the process high output capability plus the ability to produce fine detail and tight tolerances. However, the high pressure at which the plastic is injected into the mold requires sturdy, robust steel molds that are inherently expensive for that reason alone. Added to that is the fact that the process is used for the most precise, demanding piece part designs which also require expensive core and cavity details. Finally, the low piece part cost provided by the injection molding process is often obtained through the use of multiple cavities. Hence, the cost of each cavity is multiplied by the number of cavities in the mold. Size is probably the major limitation to injection molding. As the parts grow larger, the cost of tooling becomes prohibitive for many applications and the number of molding machines available that are large enough to make them diminishes significantly. Many of the very largest injection molding machines have been manufactured for spe- cial applications, are owned by proprietary manufacturers, and are not available for custom molding. Gas-assisted injection molding significantly increases the size of the part which can be molded in a given size molding machine because of its much lower pressure. Thermoset shapes 1. Machining 2. Casting 3. Lay-up or spray-up 4. Cold-press molding 5. Resin transfer molding 6. Reaction injection molding 8.34 Chapter Eight 0267146_Ch08_Harper 2/24/00 4:45 PM Page 34 7. Compression molding, bulk molding compound (BMC), sheet molding compound (SMC), low-pressure molding compound (LPMC), transfer molding, and thermoset injection molding Since many of the thermoset processes are best suited to large parts, two pecking orders exist in this area as well. Up to 1 ft 2 , the next step after machining and casting would, depending on part con- figuration, be either compression molding, transfer molding, or injection molding. All of these are processes which require expensive tooling, however, the alternatives do not lend themselves to small parts very well. For large parts, the pecking order would include the full gamut of thermoset processes with the exception of transfer or injection molding. Machining. As with thermoplastics, thermosets can also be machined. In fact, most of the thermosets are somewhat easier to machine than the thermoplastics in that the melting problem is less of a factor. While local- ized heat at the machining surface can still create difficulties, the tem- peratures are much higher and charring or burning is the likely result. A considerable amount of machining is done with thermosets because, with the exception of a couple of methods, the mechanical removal of molding flash is necessary in thermoset processes. Drilling holes and cutting openings is also commonplace in some processes because it is difficult, bordering on impossible, to mold them in for many designs. Casting. Casting is a low-pressure closed-mold process; however, mold costs are kept low because it is often possible to cast the mold directly from the model. It is difficult to create fine detail to close tolerances with this process. Casting cycles are long and, consequently, piece part prices are high. Casting is often used to enclose an object, usually an electrical com- ponent, in order to protect it. Casting applications also include furni- ture and decorative objects where fine detail is required or simulation of wood is desired in relatively low volume. Lay-up and spray-up. The largest of parts (mine sweeper hulls) can be made by lay-up and spray-up. However, the ability to create fine detail is limited and close tolerances are not possible. Machining is necessary to create holes and trim the parts. The construction is laminated with polyester with glass reinforcement, and it is the reinforcement application method which defines the name of the process. Open molds are used; therefore, one side of each part is rough and unfinished. Mold costs are low; however, they only last for a small number of parts. Thus many molds would be required for high volumes, although the pattern needs to be made only once. Since these are very slow processes, piece Design of Plastic Products 8.35 0267146_Ch08_Harper 2/24/00 4:45 PM Page 35 part costs are high. However, robotics have been used to reduce labor costs. Cold molding or cold-press molding. This process is a step up from the open- mold processes previously discussed in that it is a closed-mold process. Therefore, the parts are finished on both sides. However, the process does not quite produce the surface quality required by the transporta- tion industry. Hence, cold-press–molded parts are more likely to be used for interior parts. To a limited degree, this process can provide a part with some inside structure such as ribs, etc. However, it has poor toler- ance control and, therefore, somewhat limited application. Cold-press molds are plastic, which makes them comparatively inexpensive. Relatively long cycle times and postmolding machining result in parts which are more expensive than compression-molded parts, but less costly than parts produced by the lay-up or spray-up processes. Resin transfer molding (RTM). Resin transfer molding can produce parts of higher finish, greater complexity, and wall thickness consistency than cold-press molding; however, the part cost can also be higher. Therefore, the processes are sometimes used together with external parts made by the resin transfer molding process using cold- press–molded parts for internal supports. Closed plastic molds are used which are sometimes plated for longer life and better finish. These molds are considerably less expensive than compression molds, thus considerable volume is required before the additional cost of tooling can be offset by the lower piece part cost of compression molding. Parts as large as truck hoods, small boat hulls, and car body halves have been produced using this process. Reaction injection molding (RIM). RIM is a low-pressure process using closed molds. However, it has a much shorter cycle than resin transfer molding, which results in lower piece part costs. Unfortunately, very few materials are available. Thermosetting polyurethane is the prin- ciple material available, with epoxy, nylon, and polyester also available but to a limited extent. Reinforced reaction injection molding (RRIM) is available with chopped glass used as the reinforcement. This process is most widely used in the automotive field, although there have been other applications where large parts or limited volumes are required. Compression molding, BMC, SMC, LPMC, transfer molding, and thermoset injection molding. Compression molding is the primary thermoset process; the other processes in this group, with the exception of thermoset injection molding, are derived from it. The term compression molding also includes BMC, SMC, and LPMC which actually describe the molding 8.36 Chapter Eight 0267146_Ch08_Harper 2/24/00 4:45 PM Page 36 compound used. BMC and SMC refer to the type of reinforcement and the manner in which the resin is prepared for molding. Compression molding, transfer molding (not to be confused with resin transfer molding), and thermoset injection molding compete for short fiber–reinforced or –unreinforced applications with fine detail and close tolerances. Molds are expensive, but piece part costs are low. For a product which can be manufactured by all three processes, compression molding will have the lowest tooling cost and highest piece part cost. Injection molding will have the lowest production cost with a higher tooling cost, and transfer molding is somewhere in between them (the use of preheated resin allows transfer molding cycles to approach injection molding cycles). That is a gross generalization because part design details will likely favor one process or the other in most cases. In BMC, long strands of reinforcement ( 1 ր 4 to 1 ր 2 in) are placed in the material along with other additives. A ball, slab, or log of this mixture is formed and placed in the mold. This method of reinforcement is less expensive than SMC, however, it is not as strong. SMC fibers are spread into a resin paste to form a sheet. Reinforcement fibers can range from the very smallest to those of indefinite length, although they usually do not exceed 3 in. However, SMC is generally known as a long-fiber process; it is used for higher strength applications such as truck tractor hoods and fenders. LPMC material is prepared like SMC but is formulated to permit molding at a lower pressure. The reduction in pressure results in less expensive molds, often constructed of aluminum. The additional cost of tooling requires a high production level for the piece part savings to justify a change of process from cold-press molding or resin transfer molding to one of the compression processes. However, the use of LPMC can make compression molding competitive at relatively low levels. Through their large-part capability, the compression molding processes permit the combination of parts with the resultant assembly savings. They are also processes which offer economies of high volume. Thus, they are the processes of choice for large-part, high-volume applications. Hollow part processes. Hollow products can be made either by assem- bling parts made by most of the processes (even extrusions can be capped) or by one of the hollow part processes. Which is most cost effective varies considerably according to the application and to the state of the art of the techniques being applied. The savings associated with molding the part in one piece may be offset by the use of Design of Plastic Products 8.37 0267146_Ch08_Harper 2/24/00 4:45 PM Page 37 robotics in automating the assembly process coupled with a more cost- effective molding process. For thermosets, there really is only one hollow part process—fila- ment winding. That process is very limited in shape and structure such that, except for pressure applications, most thermoset hollow products are assembled. There are three hollow part processes for thermoplastics and they do have a pecking order of sorts. When proceeding with the development of thermoplastic hollow shapes which do not lend themselves well to assembly, the following sequence should be used, starting with the lowest volume requirements: 1. Rotational molding or twin-sheet thermoforming (application dependent) 2. Blow molding Processes for hollow shapes have a much simpler pecking order. The ideal shape for rotational molding is a sphere. The ideal shape for twin- sheet thermoforming is a flat panel. The selection of which process to begin with would be based on the shape of the particular part to be manufactured, although it should be noted that rotational molding can make parts with sections as thin as 1 in. Additional factors would be the size of the part (rotational can go larger) and the selected material, since each of these processes favors different materials. Depending on the size and shape of the part, blow molding may become competitive as the volume grows. However, there are size lim- its to which this step can be taken as the other two processes can make larger parts than blow molding. Rotational molding and blow molding make similar hollow parts. In some cases, such as automotive ducts, the ends are removed from a hollow shape to create the final part. Large containers can be molded so they are integral with their covers which are cut off to form the two parts. Other shapes can also be molded as one hollow piece and cut apart to make multiple parts (usually two). Structural components are usually molded as double-walled parts which can be filled with foam for greater strength and rigidity. Rotational molding, blow molding, and twin-sheet thermoforming can also make large, double-walled parts which are relatively flat (such as pallets). Twin-sheet thermoforming is best suited to the flat- test of such parts. Rotational molding. The rotational molding process, sometimes referred to as rotomolding, can produce parts ranging in size from small balls to enormous containers. The principal material used in rotational molding is polyethylene, however, some nylon parts are also made. For simple 8.38 Chapter Eight 0267146_Ch08_Harper 2/24/00 4:45 PM Page 38 [...]... 14-ft boat hull 11⁄2 ϫ 41⁄2 ϫ 14 ft 42 in deep 10- ft high ϫ 821⁄2 ft deep 4 ft ϫ 4ft 6 in ϫ 7 ft Continuous roadway Limited by size of stock available 15 ϫ 100 in 10 ϫ 10 ft Casting SOURCE: Largest commercial 9 ⁄2 ft long ϫ 1 ft deep ϫ 4 in thick, 28 in deep ϫ 44 in long Limited only by physical ability to handle molds and moldments 2 ϫ 5 ϫ 5 ft 10 ϫ 10 ϫ 11⁄2 ft 4 ϫ 5 ϫ 8 ft 12 ϫ 12 in 13 ft deep... packing of the part Improper gate location or inadequate number of gates are other ways to cause nonuniform cooling and part distortion 8.2.2 Shrinkage and the use of draft in plastic parts The shrink rate is the amount the part will reduce in size as it cools from processing temperature to room temperature It is described in terms as the amount of shrinkage per inch of part size Thus, a piece part 1 in... piece part to compensate for this expansion When the part shrinks onto a core, it grips the core very tightly and cannot be removed without considerable force That force will tend to distort the part; therefore, the cycle must be extended long enough for the part to be removed from the mold without damage The force can be reduced by placing a slight angle on the walls of the part perpendicular to the parting... mold to be machined with great care Thus, a mold with a broken parting line is considerably more expensive than one with a straight parting line 0267146_Ch08_Harper 2/24/00 4:45 PM Page 45 Design of Plastic Products Hole in part Shutoff Undercut 8.45 Heating/cooling line Hole formed in part Cavity Removable side action Runner Core pin Parting line Ejector pin (knock out) Edgegate Core (a) (b) Figure... Fundamentals for Plastic Parts The basic engineering formulas for structural design can be applied to plastic part design, within the limitations of the data available for the material properties, as previously discussed This information is widely available and is not particular to plastic part design Therefore, it will not be covered in this chapter It is, however, found in Machinery’s Handbook, 24th Edition,... that side For there to be no temperature differential between those two parts of the core, the corner must receive more coolant than the side This could require a very expensive cooling system which the budget cannot support Figure 8 .10 Voids, warpage, and sink in plastic parts (Source: Jordan I Rotheiser, Joining of Plastics Handbook for Designers and Engineers, Hanser Publishers, MunichHanser/Gardner... offset the low output Rotational molding is well suited to large parts and, for the largest of parts, it has no real competition in thermoplastic processes Twin-sheet thermoforming is generally used to produce large, flat, double-walled parts Nearly all of the thermoplastics can, at least theoretically, be thermoformed However, the bulk of parts made by this process are usually polystyrene or ABS The... in Fig 8.9b Where the two (or more) parts of the mold meet is called the parting line To prevent plastic from leaking from the mold, the two halves must match perfectly When the parting line is flat and on one plane, it is relatively easy to align the two halves However, some designs require the mold to have a contoured parting line This is referred to as a broken parting line and requires the halves... depth of the hole is the insert length plus 0.030 in 8.2.8 Design for multiple part assemblies Thus far, the design discussion has centered around the design of individual piece parts However, most products require multiple part assemblies, often consisting of parts made of different materials The first step is to ensure that the parts fit together properly—not merely at room temperature, but at the temperature... per ejector which can also result in distorted parts or an extended molding cycle The shape of the part will also dictate gate location Ideally, the gates should be located where they permit the cavity to be filled uniformly If one part of the cavity fills before the rest, it will begin to cool immediately, resulting in differential cooling through the part This phenomenon can also result in premature . roadway Machining No limit 10 ft wide or 15 in deep Limited by size of stock available Pultrusion 1 ⁄16 in deep 12 ϫ 12 in 15 ϫ 100 in Reaction injection 4 ϫ 12 in 3 ϫ 4 ϫ 10 ft 10 ϫ 10 ft molding Resin. of the particular part to be manufactured, although it should be noted that rotational molding can make parts with sections as thin as 1 in. Additional factors would be the size of the part (rotational. form the two parts. Other shapes can also be molded as one hollow piece and cut apart to make multiple parts (usually two). Structural components are usually molded as double-walled parts which

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