Ovalised Tooling for Dairy Bottles - Blow Moulding Controls

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In ovalized tooling, the shape of the die or mandrel, and in some cases both, is modified by adding or removing metal in specific areas to selectively decrease or increase the die gap opening. This modification increased the thickness of the parison 90 degrees from the mold parting line and decreases the thickness along the parting halves closing around the parison and results in a bottle with more uniform wall thickness. Figure 2 shows, in an exaggerated way, how the parison is squeezed together by the mold when it closes, and how the parison expands when blown. The thinnest areas of a one-gallon milk bottle made by standard round tooling are the two lower corners opposite the parting line. This thinning occurs when the parison is pinched off: the bottom areas draw together, as shown in Figure 3, increasing the distance the parison must be blown to contact the mold surface. Figure 3. Parison drawn together by pinch-off PARISON Blow-up to corner increased by pinch-off action To maintain sufficient thickness in these thin lower corners with standard round tooling, the overall bottle weight must be increased. This increase also results in excess polymer in all the areas along the parting line – and increased cost. Ovalized tooling distributes more polymer to the areas of the parison experiencing the greatest blow-up. More polymer is blown to the corners 90 degrees from the mold parting line than to other parts of the bottle. With optimized ovalized tooling, all four corners have equivalent thickness. The bottom line is that a bottle made with standard round tolling would have to be substantially heavier to match the performance properties of a bottle made with ovalized tooling. EVALUATION METHOD To evaluate the effects of ovalized tooling, one of the two central heads of a UNILOY* 350 R2 four-head blow molding machine was fitted with an ovalized die, while standard round tolling remained on the adjacent head. Molds and blowing mandrels were identical. The ovalized die used was a “shelf item” with 0.003 inch ovalization, available through UNILOY (Part number 865821). A PETROTHENE ® high density polyethylene resin (melt index = 0.65 g/10 min.; density = 0.959+ g/cm 3 ), specifically designed for high speed production of thin-walled containers, was used in this evaluation. Bottles with target weights of 53, 55, 57, 59, 61, 63, and 65 grams were produced on each of the two blow molding heads. Bottles used for this evaluation fell within 0.2 grams of the target weight; bottles with handle webbing or resin folds were discarded; and parison swing was held to a minimum. Based on the parameters, ten bottles from each *UNILOY is a registered trademark of Johnson Controls, Inc. 3
head were selected at each of the seven target weights. The 140 bottles accepted for the study were tested for top-load capacity and wall thickness uniformity. Top-load capacity is the maximum amount of force in pounds that can be applied to the top of a bottle without deforming the bottle. This capacity is important because similar stress is applied to the bottle during filling operations. Top-load failures usually occur at the weakest section of the bottle shoulder. In this study, the top-load capacities of the bottles were measured with a Model 51008 Testing Machines, Inc. Universal Tester and the results were statistically normalized. EVALUATION RESULTS Top-load Capacity Figure 4 shows that a 60-gram bottle produced by standard round tooling has a top-load capacity of 26.5 pounds. The same load capacity is withstood by a 58.1-gram bottle produced by ovalized tooling. The performance of the two bottles is similar, but the use of ovalized tooling results in a savings of 1.9 grams per bottle. Additionally, since the curves in Figure 4 are divergent, bottles blown from standard round tooling must increase in weight at increasing rate to match the performance of bottles produced from ovalized tooling. Thus, as bottle weight and performance requirements increase, even greater savings can be realized by using ovalized tooling. Figure 4. Top-load capacity of bottles TOP LOADING CAPACITY (pounds) 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 Ovalized Die Tooling Standard Round Tooling 58.1 grams with Ovalized Tooling @26.5 pounds 60 grams with Standard Tooling @26.5 pounds 53 54 55 56 57 58 59 60 61 62 63 64 65 BOTTLE WEIGHT (grams) Thickness Uniformity To analyze wall thickness uniformity, all the bottles were cut apart and marked with a grid pattern as shown in Figure 5. Wall thickness was measured at each intersection, resulting in 108 thickness values for each bottle in a series. These data were statistically analyzed and significant differences were noted between bottles made with standard and ovalized tooling. To avoid confusion, all the data are not presented. Instead, as the lower corner areas opposite the parting line are the thinnest areas of a bottle made with standard tooling, and therefore most susceptible to failure when dropped, the data provided focuses on these areas (circled in Figure 5). The parts of the bottles beneath the circled areas had textured surfaces, making accurate thickness measurements difficult. 4
Page 1 and 2: TECHNICAL TIPS FOR BLOW MOLDING MIL
Page 3: THE FUNCTION OF OVALIZED TOOLING Th
Page 7 and 8: ECONOMICS Now that the theory of ov
Page 9 and 10: Figure 9. Wall-thickness uniformity
Page 11 and 12: Figure 10. Wall-thickness uniformit
Page 13 and 14: SUMMARY The decision to convert to

Ovalised Tooling for Dairy Bottles - Blow Moulding Controls

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