Tài liệu The Plastic Product P1 docx

11 2 The Plastic Product Plastics have evolved to be a very useful material. Today, plastics are used in almost every area, from small bottle caps, disposable cutlery, and packages for dairy products, to large containers, such as laundry baskets and garbage pails. Plastics have transitioned from a “cheap” substitute for metal and glass to the material of choice providing almost unlimited design freedom, unique properties, and significant cost savings. Figure 2.1 shows various industrial containers and house wares that create durable products in cycles from 10–30 seconds. Figure 2.2 shows various thin-walled containers are typically used in the dairy industry and are molded with wall sections typically less than 0.7 mm with cycles of 20 shots per minute. Figure 2.3 shows a collection of PET bottles for water, soft drinks, etc. and some of the preforms used for blowing these bottles. Today, more than 500,000 tonnes annually of plastic are converted into bottles. Cycle times for molding these parts have been reduced from 35 to 8 s in the last 20 years. In addition, cavitations have increased from 8 to 144 cavities, resulting in significantly lower product costs. Figure 2.4 shows a sampling of “stadium cups” with printed or in-mold labelled decorations. Figure 2.6 shows samples of small, thin-walled technical products made from engineering plastics such as ABS, Acrylic, and PC. Figure 2.1 Molded products of various sizes (Courtesy: Husky) Figure 2.2 Various thin-walled containers (Courtesy: Husky) Figure 2.3 PET bottles for water, soft drinks, etc. and some of their preforms (Courtesy: Husky) Figure 2.4 Stadium cups Figure 2.5 Small and large technical (engineering) products, heavy-walled jars for cosmetics, and tubular containers with integral, hinged lids (Courtesy: Husky) 1281han02.pmd 28.11.2005, 10:4811 12 2 The Plastic Product 2.1 The Product Design The following contains suggestions for the product design and how it may impact the mold design and the productivity of the mold A new mold is usually required  For a new product  After the redesign of an existing product  To increase the productivity and the output of the production facilities already in place. This usually provides a good opportunity to reevaluate and improve the product, and to reduce manufacturing costs, particu- larly through the reduction of the plastic mass of the product. The mass of the plastic accounts for a significant portion of the cost of every product. Reducing wall thickness and reduction of unnecessarily heavy cross sections will not only reduce the cost of plastic material for the product, but will also result in – sometimes significantly – faster molding cycles. The result is that more of the products can be made per hour at lower cost than was possible with the preceding design. In such a case, important considerations are  The output of the plasticizing unit and the dry cycle of the machine manufacturing the product before the planned changes  If there was special handling equipment (product removal, stacking, printing, etc.) with the old mold, will it be able to handle the greater output, or will it need improvements as well The above will be discussed in more detail later in this book. Figure 2.6 Small, thin-walled technical products made from engineering plastics 1281han02.pmd 28.11.2005, 10:4812 13 2.2 Product Drawings 2.2 Product Drawings Occasionally, only samples or CAD models of a new product are available. This may be of some advantage to better visualize the product, but it is absolutely necessary, to minimize risk for all parties involved in the final decision, to have a complete detail drawing of the product, showing all features, tolerances, and specifications. This is also the moment when the designer has the greatest opportunity to decide on the most suitable design for the mold, and/or to make suggestions on how the product design might be modified to improve the productivity, to simplify the mold design, and to reduce mold costs. This is also the time to consider any ancillary equipment required for this production. An opportunity graph (Fig. 2.7) shows symbolically the value of planning a project. At the outset of the project, the opportunity to make improvements, revisions, and selections is highest to affect the final outcome of the project, while the costs are lowest. After concept analysis, once the elements of the project have been agreed upon and as engineering of the mold progresses, the opportunity to make conceptual changes or improvements diminishes, and any costs associated with it will increase. By the time the project reaches completion and gets into testing and production, the opportunity to make changes is low, and any costs could be very high. To confirm that the part drawing is acceptable to all parties it should always be signed off in writing as acceptable. Appendix 12 provides some general advice for the designer on how to critique a part drawing. 2.2.1 Product Shape: How Can the Product Best Be Molded? Here, even an experienced, conscientious designer may want to consult with another (knowledgeable) colleague, and/or with anyone else who is familiar with the type of product for which the mold is to be built, and discuss problems of making and of operating such a mold, to get their input regarding the proposed product design. In the following, some of the most important areas to be contemplated are discussed. 2.2.2 Parting Line (P/L) Is There an Obvious Location for the (Main) Parting Line? In many products, the location of the parting plane (parting line, P/L) is obvious. It is along the largest cross-sectional dimension of the product, at right angles to the motion of the opening and closing of the mold, and should preferably be in one plane. This is the least expensive, and fortunately, the most frequent case. However, there are many cases where the P/L cannot be It is critical that complete product drawings are available for the mold designer before any mold design is started Opportunity Opportunity Costs Costs Time Period of evaluation of product, opportunity for changes is high, changes are easy to obtain, and low in cost. During engineering, opportunity for revisions is still fairly high. Changes are still relatively inexpensiv e During manufactoring, there is little opportunity to make revisions. Changes can be quite costly. Mold tests and production: Figure 2.7 Opportunity graph The old proverb “a stitch in time saves nine” applies here too: Spend more time at the beginning of the project, to save much time later on 1281han02.pmd 28.11.2005, 10:4813 14 2 The Plastic Product located there, and requires special consideration. A few examples are listed below:  Simple parting lines (Fig. 2.8)  Sometimes, the P/L must be offset because of the shape of the product (Fig. 2.9).  It may be of advantage to place the P/L at a level, which is not at the largest cross section, to force the product to stay on the side from where it will be ejected, as can be the case with flat products. This would not affect the mold cost; however, flat products often cause trouble at ejection, because they do not always stay reliably with the side from where they are ejected. Additional mold features, such as sucker pins, or grooving in the side of the product (“pull rings”) may be required to hold the product on the ejection side to make sure that the mold can operate automatically, without interruptions (Fig. 2.10).  The P/L is curved. This is sometimes unavoidable because the product shape will not permit a straight P/L; for example in some toys, but occasionally also in technical products. A typical example is the P/L for plastic forks or spoons. In all these cases, the matching of the P/L is difficult and expensive. It may need special, costly grinding equipment or expen- sive fitting by hand (“bluing”) (Fig. 2.11). Figure 2.9 Example of simple mug handle, using offset P/L Figure 2.11 Typical mold profile for cutlery Figure 2.10 Typical flat piece with undercut below parting line Figure 2.8 Examples of straight, simple parting lines (top: at the opening; bottom: at the largest diameter) 1281han02.pmd 28.11.2005, 10:4814 15 2.2.3 Side Cores Is There a Need for Side Cores, Splits, or for Other Methods to Release Severe Undercuts or Threads? Any of these features will add considerable cost to the mold (and to the cost of the product), not only because of the added complexity of the stack but also because each stack requires much more space than a simple stack without side cores. For the same number of cavities, a much larger mold and therefore often also a larger machine size may be required just to accommodate the mold in the available platen area, even though the clamping forces required would be little more than for the mold without side cores or splits. Such side cores, splits, etc will lengthen the cycle time and reduce productivity compared to molds that do not have such features. Could a Redesign of the Product Avoid the Need for Side Cores? In some cases, round holes or “odd shape” openings generated by using side cores or split cavities could be redesigned without sacrificing the usefulness of the product, and possibly without significantly changing the appearance, by creating such holes or openings in the side walls (or even in ribs inside the product) with a design method where core and cavity meet on a “shutoff”. This may require the use of special inserts in either or both of cavity and core, which may necessitate a change in the shape (or in the draft angle) of the side wall of the product, or require an opening in the bottom of it. In many cases, this could be acceptable for the end use of the product and allow a much simpler, less costly mold [1]. By just giving a bit more thought to the product design before planning and designing a mold, and by understanding the application for which the product is used, a little redesign can often result in spectacular savings in mold and product costs. Selecting Other than the Conventional Parting Line Occasionally, the choice of the obvious placing of the parting line would require a side core, while by slanting the P/L, the product could be molded with a simple up-and-down mold. An example is a simple louver (Fig. 2.13), but the principle applies to any similar case. The cost of a mold with a “slanted” P/L is somewhat higher than that of a mold with an ordinary P/L, but much lower than a mold with a side core. Investigate Shape of Threads and Undercuts Often, a design specifies threads or undercuts, on the inside of the product (Fig. 2.14). Is the specified shape of thread or undercut designed with molding in mind? Many such threads or undercuts could be molded without un- screwing, or the need for collapsible cores, by changing the shape of the undercut so that the product can be stripped off the core, i.e., the undercuts can easily slip out of the grooves that created them when pushed by ejectors or a stripper. Figure 2.13 Example of louver; top: needs side core; bottom: tilted – it becomes an “up-and down” mold Figure 2.14 Typical bottle cap with tamper-proof ring and stripped thread for simpler ejection (no unscrewing mold required). This product is outside-gated, using a hot runner hot tip gate 2.2 Product Drawings Figure 2.12 4-cavity handle mold with 3 side actions per cavity (Courtesy: Topgrade Molds) 1281han02.pmd 28.11.2005, 10:4815 16 2 The Plastic Product Figure 2.15 shows the difficulties of a typical unscrewing mold. The core must rotate out of the cap before it can be ejected. This makes core cooling more difficult and results in 30% longer cycle times than a stationary core. Unscrewing molds are much more complicated than “bump-off” (stripped) closure molds. Figure 2.16 shows a schematic of a much simpler mold, where the thread (and the cap) can be stripped. Here, core cooling can be very efficient. The cycle time for a typical (28 mm) bottle cap made from HDPE MFI 19, weighing less than 3 g, molded in a 24-cavity mold running in a 90 t (1,000 kN) machine is in the order of 4.0 s, equaling a productivity of 21,600 caps per hour. Figure 2.17 exemplifies of how a small change in the angle of the flank of the thread can allow a thread to be stripped from the core, rather than requiring an unscrewing mold. Small changes like this can have a major impact on product cost because mold cycle, cost, and maintenance will be significantly improved with a stripped product. Need for Two-Stage Ejection or Moving Cavity This applies to a shape or design feature of a product consisting of  Deep ribs on the cavity side, as is often the case with containers with “false” bottoms. Such ribs could also be specified on technical enclosures, etc., as illustrated in Fig. 2.20. The depth of the rib F and the ratio of the thickness of the rib t, as well as the draft angles of the rib are critical considerations, or  Deep ribs (often circular) on the core side; typically, the underside of an over-cap, as illustrated in Fig. 2.21 (even without the thickening at the end of the rib as shown in this illustration). In both cases, if the ratio of F/t > 2, or if there is any thickening at the end of the rib (as in Fig. 2.21), either a “two-stage ejection” or a “moving cavity” are necessary, which will increase the mold cost by about 15–20%. In both cases, it is important to provide especially good venting at the end of the ribs to ensure proper filling. Failure to use these methods will make it very difficult Figure 2.18 72-cavity unscrewing mold (Courtesy: Stackteck) Rachets Rotating core Stationary ratchet ring Figure 2.15 Schematic of difficulties of a typical unscrewing mold. Stripper ring Core Figure 2.16 Mold where thread can be stripped Types of closures Top of thread almost flat, less than 15°. If stripped will be greatly deformed. Angle on top of thread allows thread to be stripped off the core. Unscrewed thread Stripped thread Figure 2.17 Change in flank angle allows thread to be stripped Figure 2.19 Stripped closure mold 1281han02.pmd 28.11.2005, 10:4816 17 to withdraw (eject) the products, and increases the risk of breaking portions of the rib in the mold. A 2-stage mold will cost about 15–20% more than a comparable mold without this feature. Also, because the sleeve is usually rather thin, it is very difficult to get cooling into it; the mold will cycle much slower than a similar product without this complication, and the maintenance cost of such molds is much higher. Moving cavities are more complicated and cost about 10% more than a mold without this feature. Some molders use it despite its higher cost for products even without a false bottom, because they can cycle even faster than a mold with a conventional cavity. Post-Molding Operations Sometimes, molds can be much simplified by doing additional work to the product after molding. Post-molding operations are of particular importance whenever relatively small quantities are to be made. For example, one or a few simple holes or openings in the side wall of a product would require a side core in the mold, but such holes or openings could also be drilled or die-stamped after molding. Such additional operations may require a drilling fixture or a stamping die. The actual time (direct labor) for such post-molding operations and any costs for tools or fixtures would have to be added to the Always keep in mind: It is possible to mold almost any shape, but at what cost? 2.2 Product Drawings Figure 2.21 A product with deep ribs and (with or without) thickening at the end is ejected in two stages; 1: Sleeve and stripper lift product off the core; 2: Stripper continues to push product off the sleeve Figure 2.20 Schematic of a moving cavity in two halves; top: mold is closed; bottom: mold opens and follows core for a limited distance to ensure that the rib becomes free 1281han02.pmd 28.11.2005, 10:4817 18 2 The Plastic Product total cost of the product. But such post-molding operations could also take place later at the assembly line, where the product is assembled or packed, without any additional labor cost if properly integrated in the process. Again, it is the overall cost of the end product that is important, not just the cost of the mold or the molded piece itself. In many cases, the savings in the mold cost achieved by eliminating a side core (or some other complications of the mold) can be substantially greater than the combined additional cost for fixtures or tools, plus the cost of the additional direct labor to finish the product. A typical example for this would be the need for small holes for a hinge pin (for a hinged lid), located in two lugs projecting from the bottom of a product (see Fig. 2.22). The plastic melt is injected into the bottom of the product, near the lugs. It is of course feasible to mold these holes, but it could be quite difficult to arrange the side cores required for such holes as well as the actuation for such side cores, without interfering with the gating and the cooling layout in this area. It would be, however, quite easy to just mold the lugs as projections from the container bottom, and then drill the holes, using a simple drilling fixture. 2.3 Accuracy and Tolerances Required Next, the mold designer should look at the specifications relating to accuracy and tolerances. Unfortunately, often, after a product has been conceived, the design has been either just sketched by the inventor or an artist, or a model has been created. This information has then been passed on to a draftsman to be put “on paper” (by computer or pencil drawing). This may result in a good visual description of the new product, but to be practical for manufacturing, any drawing must be fully dimensioned, and intelligently toleranced. To design a product for injection molding requires certain knowledge of this technology. A design which may be suitable for one method of processing plastics (or other materials) may be unsuitable or impractical for another process, even though the end use is the same. For example, a disposable drinking cup of a specified capacity could be made from paper, styrofoam, be thermoformed from sheets, be injection molded, or made by another, entirely different, new method or material. The final product design for each of the above cited materials and methods would most likely look different to suit the method of manufacturing and the selected material. Also, while the dimensional accuracy of the product for its final use (i.e., as a drinking cup) may be of little importance, its actual dimensions will require high accuracy because of demands not related to its use, such as stacking height (e.g., for packaging), ease of releasing of the individual cups from the stack as required in automatic vending machines, and mainly because even Figure 2.22 Lugs with holes How is the product to be used? What is really required? 1281han02.pmd 28.11.2005, 10:4818 19 small variations in wall thickness may have a great effect on the mass of plastic used for each unit and on the molding cycle. A design for a metal product is different from the design for a similar product made by injection molding, even though the products may be fully inter- changeable in their use. This applies especially for design features such as  Radii and sharp corners,  Flow path for injection (if applicable),  Wall thickness,  Ribbing and reinforcements,  Openings (round or shaped),  Others. These features, by their presence or absence, not only affect the making of the mold (and its cost) but also affect the speed of the molding operation itself. I refer the reader to the many books on product design for injection molding, which go into much detail on this subject [2, 3, 4]. It is very important to understand that it is relatively easy to achieve close tolerances for the mold parts usually made from metal; however, the plastic products made by the mold do not solely depend on the mold dimensions. The designer must be aware that the final size of the product is greatly affected by variations in the shrinkage of the plastic (see Appendix), which in turn is caused by variations in molding conditions (pressures, temperatures, and timing) and by variations in the composition of the plastic not only from batch to batch, but also from manufacturer to manufacturer. All this makes it very difficult to mold products dimensioned within close tolerances. But even the above statement “relatively easy to produce the mold parts to close tolerances” must be qualified. Using unsuitable, old, and/or poorly maintained machine tools makes it more difficult to make mold components to close tolerances; the accuracy of the work depends much on the skill of the machinists, and even with good checking equipment can become time consuming, because it requires frequent measuring of the closely toleranced dimensions. The alternative is to use good machine tools, or even machines specially designed or adapted for certain steps in the manufacture of the mold parts, requiring much higher investments by the mold maker. Either one of these conditions affect the cost of machining and explain why close tolerances can be expensive too achieve. Note also that dimensions are affected by the ambient temperature of the machine shop and that even when cooled by cutting fluids, the work pieces heat up during machining; they will measure larger when warm immediately after cutting than after cooling to room temperature. Of course, the larger the dimension, the larger the dimensional differences caused by heat expan- sion. 2.3 Accuracy and Tolerances Required Many millions of dollars are squandered annually because of demands for unnecessary tight tolerances 1281han02.pmd 28.11.2005, 10:4819 20 2 The Plastic Product As can be seen in Fig. 2.23, the mold cost increases exponentially with the tightness of the tolerance. Without giving actual cost figures, the curve just shows how costs can increase, as the tolerances get tighter. The cost to achieve a 0.005 mm (0.0002 ″) tolerance can be 3 times the cost of a 0.03 mm (0.0012 ″) tolerance. Other points that should be clarified when looking at product dimensions with close tolerances: how will these dimensions (or the entire product) be checked (measured) on the finished product? With Vernier, micrometer, gages, measuring machines, fits with other products? Also, when will they be checked? Immediately after ejection, one hour later, 24 hours later? Will there be 100% inspection or statistical (random) inspection? To clarify all this ahead of time can avoid much future unpleasantness or arguments. 2.3.1 General and Specific Tolerances All tolerances must be specified on the product drawing and must be looked at by the mold estimator or designer when starting the project to see if they are reasonable. The Society of Plastics Industry (SPI) has a suggested list of practical general tolerances for injection-molded products. For more informa- tion, go to the SPI website www.socplas.org. In most cases, these tolerances are satisfactory and achievable. Specific, closer tolerances may require that experiments be made with cavity and core sizes, and under various molding conditions, to achieve the required sizes. This can mean considerable added costs for the mold maker and a higher mold cost. The following tolerances are suggested to be used on plastic product drawings (radii are not toleranced): Product weight: ± 10% on projected weight (range ± 2%) Wall thickness: ± 0.03 mm (in special cases 0.013 mm) Fit diameter: up to 75 mm ∅ → ± 0.20 mm up to 106 mm ∅ → ± 0.25 mm up to 160 mm ∅ → ± 0.30 mm up to 300 mm ∅ → ± 0.64 mm Overall height: ± 0.5% or 0.13 mm minimum Stack height: ± 0.5% or 0.13 mm minimum Note that the steel size requirements, and thus the difficulty of manufacture, are dependent on the plastic tolerances on the product drawing. Figure 2.23 Relationship between tolerances and mold cost Always remember that tighter tolerances mean higher mold costs, maintenance, and inspection 1281han02.pmd 28.11.2005, 10:4820 Next Page . 10:4811 12 2 The Plastic Product 2.1 The Product Design The following contains suggestions for the product design and how it may impact the mold design and the productivity. particu- larly through the reduction of the plastic mass of the product. The mass of the plastic accounts for a significant portion of the cost of every product. Reducing