DESIGNING THERMOFORMING MOLDS FOR HIGH DENSITY POLYETHYLENE

Transcription

1 PE TIB-82 DESIGNING THERMOFORMING MOLDS FOR HIGH DENSITY POLYETHYLENE INTRODUCTION Advancements in thermoforming high density polyethylene (HDPE) have been significant in recent years, including important strides in both resin technology and processing equipment. Today, this well established technique is meeting the needs of thermoformers who require parts of exceptional quality. The advantages of HDPE in thermoforming, such as its toughness at temperatures as low as -180 ºF, the ability to withstand steam sterilization at 250 ºF, and excellent resistance to many chemicals have long been recognized. At the same time, HDPE was considered more difficult to process than polystyrene and ABS because it required a higher forming temperature and careful judgment concerning mold design and mold construction. Further, the sag of the molten sheet was sometimes a problem, especially for large parts. However, both resin and equipment developments have overcome the early difficulties associated with the forming of HDPE, and new applications are developing at a rapid rate. The most significant resin development has been that of the extra high molecular weight (EHMW) resins. Under normal melt index (MI) test conditions these resins have very low flow. However, the flow is sufficient enough in the extruder to enable them to be processed into high quality sheet. This development required a careful selection of molecular weight to optimize the desirable properties without sacrificing processability. The result is a resin that addresses sheet sag problems and produces parts with outstanding toughness. Equipment advancements have been made that enable the processor to take advantage of the higher molecular weight resins. Today, very large parts with a superior balance of physical properties can be formed from HDPE on economical molding cycles. However, the forming of these parts cannot be accomplished successfully without the use of welldesigned molds. Good molds are essential, regardless of the type of HDPE being used or the machinery on which the part is formed. The following discussion concerns the design and construction of molds to be used with high density polyethylene resins. These suggestions and recommendations should help the thermoformer achieve part production at minimum cycle times and parts of very high quality. MATERIALS OF CONSTRUCTION The specific heat curve in Figure 1 on the following page shows that a large amount of heat must be transferred in the temperature region where high density polyethylene melts or crystallizes. As a result, high watt density sandwich heaters should be used to heat the sheet for forming. Conversely, the mold should be capable of rapidly removing heat from the part during the cooling cycle. 1

2 FIGURE 1 Specific Heat Curve for Density Polyethylene Because of the advantages already cited, the following sections will deal exclusively with the construction and use of aluminum molds. MOLD CHANNELING Aluminum molds must be properly cored or channeled to permit accurate temperature control. The temperature of the mold affects the surface appearance and any warpage of the parts. Further, a variation in the mold temperature will cause a definite change in the size of the part. Excessive heat will produce shrinkage, whereas a lowering of the mold temperature will have just the opposite effect. Wooden and plaster prototype molds will not provide a high degree of heat transfer, and they tend to splinter or break up after just a few parts are formed. The relatively high HDPE forming temperature (about 350 ºF) is too much for these materials. Although the liberal use of release powders will lengthen the life of the molds, the quality of the parts will be marginal. Epoxy molds with cast-in copper tubing have sometimes been employed for high density polyethylene. They have been used very successfully for large parts, formed slowly with special techniques. However, we ve found that even cored epoxy molds cannot be brought to optimum temperatures readily, they exhibit undesirable hot spots, and they transmit heat too slowly. Therefore, in order to achieve a longer mold life and to obtain the required heat transfer characteristics, we suggest the use of aluminum molds. Aluminum is readily available, uniform in properties, relatively inexpensive, durable, easy to machine, and familiar to the mold makers. The mold may be cast aluminum (we have found that pressure casting is not always required for good molds) or it may be machined from aluminum stock where the size and shape will permit. Our experience with aluminum would indicate that a wall thickness of 3/4" is adequate to withstand the forming pressures and to allow for proper mold channeling. It is apparent, therefore, that good channeling is a vitally important part of good mold design. To provide uniform control of all areas, the channels should be located along any edges or important ridges, and should follow the contours of the mold as much as possible. In flat areas, they may be spaced 3" to 4" apart. To avoid localized hot or cold spots, all channeling should be at least 1/4" from the mold surface. The channels may be incorporated into the mold in many different ways. They may consist of stainless steel tubing cast into the aluminum as shown in Figure 2. For adequate circulation, the tubing should be 1/4" or 3/8" O.D. in small or moderate size molds and 1/2" O.D. in large molds. Copper tubing is not satisfactory since it combines with the molten aluminum during casting. Drilled channels, 1/4" to 1/2" in diameter, which have been plugged off at the ends and welded to complete the system, are illustrated in Figure 3. Instead of drilled holes, milled channels, 1/4" to 1/2" wide, may also be used. With milled channels, of course, a cover plate must be added to prevent water flow between the channels. As a different approach, cooling mandrel inserts may be employed for deep-draw, male molds. The helical channels for these inserts may be machined as shown in Figure 4 or built up with tack-welded rod. The water flow in this type of mold is upward, inside the mandrel, and downward between the mandrel and the shell. One of our recent molds was built around the bubbler type of hook-up shown in Figure 5. This type of cooling is excellent for multicavity molds like the arrangement in Figure 7, although it will involve a considerable number of copper tubing connections between individual cavities. 2

3 FIGURE 2 Female Suitcase Mold with Cast-In Stainless Steel Tubing FIGURE 3 Male Light Diffuser Panel Mold with Drilled Channels. Milled Channels are an Acceptable Option. 3

4 FIGURE 4 Machined Helix on Insert for Deep Draw Male Mold. In Larger Molds, Helix Could Be Built Up with Tack-Welded Rod. 4

5 FIGURE 5 Bubbler-Type Cooling for One of Several Small Molds to be Mounted on Common Base Plate with 5 Bubblers Per Mold. The channels are tied together in all cases with one "In" and one "Out" line brought through the side of the mold base for copper tubing connections. To these, we connect a good commercial mold temperature control unit capable of controlling accurately up to 225 ºF at a rated capacity of 10 to 15 gallons per minute. We have illustrated or indicated six different approaches to proper mold channeling. The important factor is to provide plenty of water flowing through all zones of the mold. It is apparent, therefore, that many other layouts are possible, depending somewhat on the shape of the part but mostly on the ingenuity of the mold designer. 5

6 EXTERIOR CONTOURING We cut a "moat" into all molds. The moat consists of a shallow groove with a rectangular cross-section, running all around the part form just at, or outside, the trim line. The purpose is to ensure a good vacuum seal between the hot sheet and the mold. The size of the moat depends on the thickness of the sheet to be used. A minimum size is probably about 1/4" wide by 0.1" deep for sheet up to 60 mils (0.060") thick. For heavier sheet, the moat depth should at least equal the unformed sheet thickness, while the width should be about three times this thickness. Figure 6 clearly illustrates one advantage of a moat. The part at the left was formed before the moat was added. The part at the right shows how the lip straightened out after a moat was cut into the same mold. FIGURE 6 With (right) and Without (left) a Moat in the Same Mold The sides of the mold should be tapered to facilitate removal of the parts and to provide more uniform side walls. For male molds, a 5 taper is a minimum. With female molds, it is possible to go down to a 2 taper in the shallower items, since the material will shrink away from the mold anyway. In conventional drape forming, a draw ratio (depth to minimum width of the part) exceeding about 1:1 may result in excessive thinning, but plug or air-assist techniques will permit the use of higher draw ratios. The primary consideration with multiple molds is the problem of "webbing" if the forms are too close together. As shown in Figure 7, a space between the forms equal to the height of the forms will avoid this difficulty with male molds. However, this distance can be reduced by the use of ring assists or other devices to force the sheet into the low areas without folding against itself, i.e., webbing. Figure 8 is an example of both male and female forms being involved in the same mold. In this case, a spacing equal to 2/3 the maximum form height was entirely adequate. FIGURE 7 Full Sheet Formed on a Nine-Cavity Male Mold with Individual Bubblers To avoid excessive thinning, possible stress concentrations, and warpage, there should be no sharp breaks in the mold contours. Corner and edge radii should never be less than the thickness of the sheet, even in very shallow draw areas. Generally, the radii should be at least four times the sheet thickness. 6

7 FIGURE 8 Male and Female Parts Made on Multiple Mold for Accurate Fit Ribbing is often employed to add structural strength or rigidity to an item and simultaneously enhance the appearance. The added rigidity permits the use of thinner sheet, resulting in obvious economies. The two flowerpots in Figure 9 are good examples of how ribbing may be used for both rigidity and decorative effects. The leaf or petal pattern of the larger flowerpot was obtained with a cast mold, while the swirl effect in the smaller one was accomplished directly by machining the mold out of wrought aluminum rod stock. Thus, while ribbing might be considered more a question of part design, it may actually determine just how the mold itself should be built. FIGURE 9 Examples of the Many Different Ways in Which Ribbing May Be Incorporated into the Part Design SURFACE FINISH The finish on the mold surface is another vitally important factor to consider. On highly polished surfaces, air can become entrapped between the hot sheet and the mold surface during forming, resulting in pock marks (Figure 10) on the mold surface. To allow for complete evacuation of the air during the forming portion of the cycle, the entire surface of the mold should be grit-blasted with #30 grit. Figure 11 shows the fine satin finish obtained after roughening up the surface of the same mold with #30 grit. Grit blasting avoids the costly job of mold polishing. Grits finer than #30 have been tried, but we have found them to be less effective. Caustic etching has also been employed but, in a few instances, the lines or other irregularities that appeared, especially with wrought aluminum, marred the appearance of the finished items. Sometimes a textured, rather than a smooth-finished product, is desirable. Figure 12 shows a suitcase shell and two tote boxes with textured exterior surfaces. Texturing gives the suitcase an attractive, distinctive appearance while fitting in with the traditional appearance of the particular product. Even more important, texturing provides a surface that can take the abuse these products will undergo without showing scratches, scuff marks, and other signs of wear for long periods of use. Many other products would probably benefit in the same way. In still other instances, texturing may be employed solely for decorative effects. Textured items may be formed by starting with embossed sheet and a male mold. Generally, however, the textured pattern should be incorporated into a female mold. We have devised a number of ways of putting a pattern into a mold. For example, the finish on the suitcase in Figure 12 was achieved by pressing crinkled aluminum foil against the plaster mold casting form while it was still wet. In a similar manner, the patterns of fabrics or anything else obtainable as a sheet may be transferred to the aluminum casting used for a mold. Various arcengraving techniques, suitable for aluminum, are also available. These are more costly, but will reproduce most desired designs. 7

8 VACUUM HOLES The 1/16" and larger vacuum holes used since the earliest days of vacuum forming are simply not suitable for high density polyethylene. Because of its low melt viscosity at the moment it reaches the mold surface, high density polyethylene sheet will form into all but the smallest holes. If the hole is too large, a large protrusion is left on the mold side with a corresponding "dimple" on the opposite side or, in the extreme case, a hole may actually be pulled into the sheet. FIGURE 10 Pockmarked Surface Caused by Air Entrapment with a Highly Polished Mold For these reasons, we recommend #75 to #80 drill size (0.0210" to ") holes in all areas of the finished item. Since the vacuum holes are much smaller than those formerly used, it is necessary to put in many more holes and to distribute them properly to achieve good forming. As shown earlier in Figures 2 through 5, the holes may be scattered on 1" to 2" centers across any flat areas, but they should be located 1/2" to 1" apart along the bottom inside corners of any recesses or at the farthest point from the sheet in areas which the sheet might otherwise "tent across" as it is formed. The moat must also be provided with an adequate number of holes, generally on 1/2" to 1" centers, but these may be made with a #65 to #76 (0.035" to 0.020") drill where the holes are outside of the trim line. In the latter case as well as in the preceding recommendations for the mold proper, the largest holes are for the heaviest sheet, i.e., 150 to 187 mil (0.150" to 0.187"), while the smallest holes should be used for sheet below about 60 mils (0.060") thick. To facilitate drilling such small holes, 1/4" holes or channels may be cut from the back side of the mold to within about 1/4" of the top surface. The recommended size hole may then be drilled for the remaining distance. Relieving the vacuum holes in this manner also aids in the rapid evacuation of the air under the sheet. FIGURE 12 Textured Surfaces Maintain a New Look for Long Life FIGURE 11 Satin Finish Produced After Grit-Blasting the Mold with No. 30 Grit In instances where the mold proper is made separately and attached to a base plate in such a manner as to create a moat, it is possible to use light 8

9 gauge shim stock, 0.010" to 0.020" thick in lieu of vacuum holes around the base of the mold (see Figure 5). These shims, 1/2" to 1" wide and about 1/16" to 1/8" apart, would then eliminate a certain amount of drilling that would otherwise be required. 5. The sag properties of the sheet are less critical. 6. Less taper is required on the sides of the mold. 7. Thickest wall sections are at top edges of the part. 8. Outside dimensions are easier to control. 9. Other factors may apply in special cases. MOLD BASE The base under the mold may be the least critical part of the entire assembly, but a few pointers should be helpful. As shown in Figures 2 through 5, we use a hardwood box frame base under all molds. This provides the required space under the molds to make all piping connections. It also raises the top surface of the mold base plate 1/2" above the lowest points of travel of the sheet in the clamping frame of our machines. In this way, we automatically ensure a proper vacuum seal around the mold as the sheet is draped down to form the part. This seal is important for good parts. In use, the mold is set over a vacuum port in the tabletop and sealed all around with 2-inch wide, ordinary masking tape. Although the use of a wooden mold base and masking tape is adequate for short laboratory runs, a metal box is best for production installations. MALE VS. FEMALE MOLDS Before designing a thermoforming mold, it must first be decided whether to build a male or a female mold. With high density polyethylene, this decision usually rests on what type of surface finish is desired since the glossy surface will be on the side away from the mold. This, and other factors to be considered, may be summarized as follows: Male Molds (drape forming) 1. Parts will have a glossy, exterior surface. 2. Thickest wall section at the bottom of the part. 3. Inside dimensions may be controlled more closely. 4. Less thinning occurs in a deep draw. 5. Must use embossed sheet for textured, exterior surfaces. Female Molds (drape forming) 1. Parts will have a glossy interior surface. 2. Parts may be textured in the mold using plain sheet. 3. Limited areas of the exterior surface may be textured for decorative effects. 4. Closer spacing is possible in multiple molds. ESTIMATING SHRINKAGE In addition to shrinkage of the aluminum mold in the casting process, the mold designer must also consider the shrinkage of the high density polyethylene part. We have investigated the latter question most carefully. Using the types of molds we have been discussing, controlled between 160 ºF and 240 ºF, and subjecting the finished items to many severe conditions, we have been unable to push the shrinkage past 3-1/2% (0.035 in/in). Actually, extensive experience has demonstrated that, with a properly designed mold and correct operating conditions, 2-1/2% (0.025 in/in) shrinkage is a very satisfactory and useable design figure for most items. However, this figure may be varied somewhat by the following factors, listed in descending order of effect: Extreme Use Conditions - High density polyethylene may be hospital sterilized, i.e., with 15 p.s.i.g. (250 F) steam for 20 minutes. Under these conditions, the shrinkage may increase to 3 1/2% as a result of the first ten minutes sterilization but, thereafter, there should be no significant changes. Mold Temperature - As has already been discussed, the temperature of the mold is a vitally important factor. It has been found that the final shrinkage to room temperature varies with the mold temperature approximately according to the thermal coefficient of expansion (or contraction) of the material between 100 F and 250 F setpoints. A 16 F change in the mold temperature will therefore affect any dimension by about 0.1%. This knowledge may sometimes be used to adjust part sizes within narrow limits. Size/Shape Factors - Generally, shrinkage seems to increase as the size and/or depth of draw increases, but no system has yet been found for relating these factors directly. Specific contouring, and whether a male or female mold is used, may make this latter relationship more complicated. 9

10 End-Use Temperature - There will be the normal expansion and contraction of the finished item as a result of changes in ambient conditions. In the range of customary room temperatures, this amounts to about inch/inch/ºf. Hot Water - Washing finished items in water between 150 F and 212 F may increase the shrinkage by to inch/inch during the first 5 minutes, but no significant changes should appear thereafter. Usually, this factor is of little importance. Sheet Orientation - Due to differences in the properties of the original sheet between the machine and transverse directions, the shrinkage in the machine direction will normally be about inch/inch higher than in the transverse direction. For large items requiring close dimensional control, this difference may have to be taken into account. However, it is usually sufficient to align the sheet the same way with respect to the mold each time that a part is formed. After estimating the shrinkage, the next step is to convert from part size to mold size. In connection with this conversion, we should point out that the calculations for male molds should be based on inside part dimensions, while those for female molds should be based on outside part dimensions. DIMENSIONAL REPRODUCIBILITY Allowances must be made for shrinkage whenever thermoformed items are to be fitted to other products, or whenever items from different molds are to be fitted to one another. On the other hand, there are many products utilized by themselves where the precise shrinkage is of little consequence. However, in nearly all cases, if only for uniform trimming, it is necessary to know how accurately any dimensions can be maintained in production. High density polyethylene is a highly crystalline polymer. It follows, therefore, that a relatively high dimensional change (shrinkage) must be expected when it is cooled below its crystalline freeze point. This is what occurs when the molten sheet contacts the thermoforming mold. It should also be anticipated that close thermal control is an absolute necessity if this high degree of shrinkage is to be held within close limits, i.e., brought to nearly the same "endpoint" each time. Thus, we again have the matter of close mold temperature control, which we have stressed throughout this article. Tables I and II provide an example of the dimensional reproducibility that can be attained by exercising good control over the temperature of the mold. Each of the length and width dimensions listed is an average of four determinations. The test specimens, which were finished light diffuser panels, were formed using a constant mold temperature of 160 ± 2 F. This had previously been found to be the optimum temperature for this particular part. The length of each rectangular panel corresponds to the machine direction (MD) of the original sheet, while the width corresponds to the transverse direction (TD). TABLE I Measurements for 25 Rectangular Panels Thermoformed from High Density (0.960 density) Polyethylene Sheet Panel Number MD Averages TD Averages

11 Average TABLE II Analysis of Measurements Standard Deviation 95% Confidence Limits MD ± TD ± Assuming normal distribution of the 25 average values in each direction, the standard deviation for either dimension is about inch. The corresponding 95% confidence limits are therefore about ± inches and ± inches for the length and width respectively. In other words, if a great number of panels were included and if the dimensions of each were determined in the same manner, we could expect that 95% of the values would fall within the stated limits. We feel that this data certainly illustrates the excellent dimensional control that can be achieved with high density polyethylene. In fact, of the ± inch range for the 95% confidence limits, ± inch could be accounted for by the ± 2 F variation in mold temperature control. Thus, even closer mold temperature control might have resulted in still better dimensional reproducibility. CONCLUSIONS Our experience has shown that the quality of a part thermoformed from HDPE is greatly dependent upon the properties of the mold. For best heat transfer, excellent durability and ease of processing, we suggest the use of aluminum molds. In addition, the following mold design features should be considered: 1. Channeling for efficient temperature control. 2. Moat for good vacuum seal. 3. Liberal radii - at least 4x sheet thickness - to avoid excessive thinning and warpage. 4. Adequate side taper - 5 for male, 2 for female molds - for easy part removal. 5. Maximum of 1:1 draw ratio unless air or plug assist is to be used. 6. Smooth areas grit-blasted with #30 grit for effective air evacuation. 7. Plenty of small vacuum holes (#75 to #80 drill size) for good forming. 8. Proper shrinkage allowance - generally 2-1/2% - depending somewhat on part and end use conditions. Possible product design features such as ribbing and texturing should also be carefully considered before designing the production mold. If we may be of further assistance, please contact our Polyethylene Sales and Marketing team. Contact information is available at this web site along with links to our polyethylene resins and MSDS sheets. This document reports accurate and reliable information to the best of our knowledge, but our suggestions and recommendations cannot be guaranteed because the conditions of use are beyond our control. Information presented herein is given without reference to any patent questions that may be encountered in the use thereof. Such questions should be investigated by those using this information. Chevron Phillips Chemical Company LP assumes no responsibility for the use of information presented herein and hereby disclaims all liability in regard to such use. Last revised October