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1、The questi on of testi ng mould1. In troduct ionInjection moulding without rejects is the ideal moulders try to attain. This article describes two auxiliary devices that could in crease the repeatability of an injectio n moulding machine. Once the optimum parameters are set, the physical dimensions,
2、 weight and other physical properties of the part will stay almost con sta nt. The two devices do clamp ing force measureme nt/con trol and cavity pressure switchover to holdi ng pressure.For injection moulding of high precision optical parts, or parts with a high added value like appliance cases, t
3、he payoff of reduced rejects is high. Figure 1 shows the part weight distribution of quality moulding and suboptimal moulding. The nominal weight is 60 g, allowable deviation is + g and g. The white cubes denote parts with in the tolera nce band.* gl60.2 .60.1 J59.9 j.59.STime*59.8.TirVFigure 1. Qua
4、lity injection moulding and otherwise2. Clampi ng force measureme nt/controlThe clamping mechanism of injection moulding machines falls mainly into two categories: toggle and direct hydraulic. The former is more widely used; the latter has the property of automatic clamping force regulation. As a re
5、sult, direct hydraulic clamp mach ine do not n eed clamp ing force measureme nt/con trol.Toggle clampA 5-point double toggle clamp is shown in Figure 2. After amplification by the toggle mechanism, the clamping cylinder, attached to the tail platen, extends, pushing the moving platen to lock the mou
6、ld halves together. We will investigate in various wayshow the clampi ng force is gen erated.仝Mow”Til pLvttfi占” dm-TwlhirFigure 2. The 5-po int double togglesClampi ng forceAt its simplest, the rated clamping force Fo is calculated according to the following formula.Fo = Ps * A * M (1)wherePs = syst
7、em pressure,A = clamping cylinder cross sectional area,M = mechanical advantage of the toggles.In most machines, M has a value of between 22 and 30. It is a function of the toggle dimensions and the stiffness of the toggles and tiebars.Rated clamping force and adequate clamping forceThe clamping for
8、ce found in the specification table of an injection moulding machine is the rated clamping force Fo. By considering the various design parameters, the machine designer calculates it using formula (1).In using an injection moulding machine, it is best to use the minimal but adequate clamping force F.
9、 An adequate clamping force holds the mould halves together against the cavity pressure during the injection phase.An excessive clamping force distorts the mould and the mould cavity unnecessarily, affecting the precision of the moulded part. Furthermore, a high clamping force compresses the toggles
10、 and the mould, and stretches the tiebars, reducing the fatigue lives of the toggle pins, the mould and the tiebars.Clamping force problem and solutionThe problem with toggle clamped injectio n mouldi ng mach ines is with only the hydraulic pressure meter available there is no way to set an accurate
11、 clampi ng force whe n the mould is in stalled and to mai ntai n it con sta nt duri ng injectio n. As the mould heats up, it expa nds, in creas ing the clamp ing force. The soluti on is to attach a device to measure the clampi ng force and to control the clampi ng force to within a tolera nee as mou
12、ld temperature cha nges.For the engin eers, the followi ng secti ons details how clampi ng force is gen erated. They also relate to the sec ond device: switchover to hold ing pressure by cavity pressure measureme nt.Mould height adjustme nt mecha nismSi nee not all moulds have the same mould height,
13、 a toggle clamped injectio n mouldi ng mach ine has a mould height adjustme nt mecha nism for that purpose. Basically, the tail plate n is moved forward or backward so that with the toggles almost fully exte nded ( 0) the mould halves just touch each other. At this time,the clampi ng force is zero.
14、See Figure 3a.To gen erate maximum clampi ng force (clampi ng force 0) and to self lock, the toggles are fully exte nded (= 0). This is done by exte nding the clampi ng strokefurther and through the toggles, moves the moving plate n forward by am, which is the amount by which the mould is compressed
15、. At the same time, the tiebars, attached betwee n the stati onary and tail plate ns, are elon gated by at. See Figure 3b. Self lock ing means eve n whe n the hydraulic pressure in the clamp ing cyli nder is relieved, the clamp ing force is maintain ed. This could only be achieved whe n the toggles
16、are fully exte nded.口思LI：G0amping = f)Figure 3. Generation of clamping forceFigure 3be- &Clamping face ? 0Revisiting clamping forceAssume the mould and the tiebars are in the elastic region at the rated clamping force. Their respective compression and elongation could be analysed using Figure 4.In F
17、igure 4a, at the adequate clamping force F, the mould is compressed byaand the tiebars elongated by at. Since the tiebars are long and thin, they are more flexible than the mould. Hence, the tiebars line is shallower. Technically speaking, K tant 氓A匚 lamping$Farceh 一- Risidu ilClampjng一-Force Fj匚 si
18、iiy卽& *Force F； &_ ClampingForceFigure 6. Free body diagram of mould halvesThe cavity pressure force F offsets part of the clamping force Fi, leaving only F - Fc to compress the mould. As a result, the mould compression is reduced from a to am.The differe nee is take n up by the tiebars elon gati ng
19、 more from a to at, i ncreasi ng the clamping force to Fi. See Figure 7.The mould ope ning force Fc due to cavity pressure is see n betwee n the tiebars line and the mould line. This is the graphical way of show ing equati on (2).Clampingi forceFigure 7. Add ing cavity pressureFrom Figures 3 and 7,
20、one can see that the dista nee betwee n the moving plate n and the stati onary plate n is in creased (by am - a) duri ng in jecti on. In practice, this could be measured by a dial gauge betwee n the plate ns. In the extreme case when the cavity pressure is so high that the residual clamping force is
21、 reduced to zero, the mould ope ns and flashi ng occurs. At this point, the mould compressi on is zero, and cavity pressure force F = F2, the clamp ing force whe n flashi ng occurs. See the dashed line in Figure 7.As an example, take F = 75 tonnes, the rated clamping force of Tat Ming ME75 III. For
22、a 300 mm thick, 170 mm square steel mould, m = mm. With such a mould mounted, the ME75 III toggle clamp will open at F2 = 81 tonnes, 6 tonnes above its rati ng. Everythi ng else equal, an injectio n mouldi ng machi ne with 50 mm diameter tiebars will ope n at F2 =78 tonn es, 3 tonnes above its rati
23、ng.As the mould heats up, it expa nds. The clamp ing force is in creased as the mecha nical in terfere nee is in creased by the amount of the mould expa nsion. This is show n in Figure 8 in which the mould line is moved further right by the expa nsion, in tersect ing the tiebars line at a higher cla
24、mping force F. In this diagram, the tiebars are elon gated more (at - at ) and gen erated the additi onal clampi ng force F - F.To restore the clampi ng force, a mould height adjustme nt is made to restore the mould (now hot) compression to am before the next shot is injected. Such adjustme nt is cl
25、ampi ng force con trol.As an example, a 300 mm thick steel mould heated up by 10C expands by mm. On the ME75 III injectio n mouldi ng machi ne, the in crease in clampi ng force is tonn es, which is almost 10% of the rated clamping force.Clamping f&rceElongati on/ CompressionFigure 8. Mould expa nsio
26、n in creases clamp ing forceNo problems with hydraulic clampFor the sake of comparison, let us do a similar analysis for a hydraulic clamp mach ine.The adequate clamp ing force is whereP = the clamping pressure,A = the clamping cylinder cross sectional area.Clamping force is easily set as it is prop
27、ortional to P, which, nowadays, is set through a proportional pressure valve. Mould height adjustment is done by extending or retracting the clamping cylinder rod to accommodate different mould heights. When the mould heats up and expands, it simple pushes the clamping cylinder rod back into the cyl
28、inder, but does not increase clamping force. In other words, the clamping mechanism has the automatic regulation property. Clamping force measurement and control is not necessary.The clamping force measuring deviceSince fast response is not needed, a strain gauge-based device is sufficient to measur
29、e and control clamping force. The simplest means is a strain gauge attached to the tail or stationary platen, which deflects under the clamping force.Alternatively, a strain gauge is attached to a tiebar which extends as the mould is locked. An assumption is made that the tiebars are evenly stretche
30、d which may not be true if the mould faces are not parallel, the mould cavity is not symmetrical or the tiebars are not balanced out of the factory.The strain gauge output is amplified and digitally displayed. The display is calibrated to read in tons. Such a device is sufficient to help the operato
31、r set up an adequate clamping force initially (during mould height adjustment). When for example a 5% deviation from the initial clamping force is detected (after the mould is closed but before injection), the operator could do a mould height adjustment to restore the clamping force to its original
32、value.Alternatively, the computer in the injectio n mouldi ng mach ine could set up the clamp ing force duri ng the in itial mould height adjustme nt, and to restore the clamp ing force by ano ther mould height adjustme nt whe n a prescribed deviati on is detected.3. Injectio nThe injectio n of melt
33、 into the mould cavity is made up of three phases: the filli ng phase, the pack ing phase and the hold ing phase.The injectio n phases could be vividly illustrated using the cavity pressure curve. Figure 9 shows the ideal curve which is achieved when the switchover to holding pressure is optimum. Th
34、e switchover is sometimes called velocity to pressure tran sfer, where velocity refers to injectio n velocity and pressure to holdi ng pressure.Figure 9. Ideal cavity pressure curveThe filling phase starts at 1. In the filling phase, the melt is injected into the cavity at a certain velocity. At 2,
35、the melt reaches the cavity pressure sensor. Due to the viscosity of the melt, pressure starts to rise. The cavity is volumetrically filled at 3. Further screw adva nee compresses the melt up to 4 whe n the mach ine switches from injection pressure to the much lower hold ing pressure. At the holdi n
36、g phase, the low holdi ng pressure in creme ntally fills the cavity as the part cools to compe nsate for the shri nkage. At 5, the sprue gate is froze n and the hold ing pressure could be removed (and the mould could be ope ned). 1-2-3 makes up the filli ng phase. 3-4 is the pack ing phase. 4-5 is t
37、he hold ing phase. Further cooli ng occurs in 5-6.Overpack ing and un derpack ingAn overpacked cavity pressure curve is shown in Figure 10b. It is characterized by a pressure peak in the pack ing phase. The pressure peak is caused by the delay in switchover to holding pressure, so the high injection
38、 pressure is still applied after volumetric filling. The pressure peak is relieved at the switchover to the lower holdi ng pressure. Here lies an ofte n overlooked cause of flash ing which is easily detected if one has cavity pressure sensing.Figure 10. Un derpacked and overpacked cavity curvesRefer
39、 to Equati on (2) or Figure 7. The mome ntary cavity pressure peak could produce a mome ntary FF big eno ugh to reduce the residual clampi ng force F to zero, caus ing the mould to ope n and the part to flash. To remove the flash ing, the straight forward thi nki ng would be to in crease the clampi
40、ng force. Reach ing the limit of the mach ine rated clamp ing force, one would eve n move the mould to a bigger machi ne.Even if the increased clamping force overcomes flashing, overpacking adds weight and stress to the part and makes the part more difficult to demould. An alternative is to reduce t
41、he injection pressure. Too low an injection pressure causes defects such as sink marks. In actuality, the problem is easily and better solved by switching over earlier to get back to the ideal cavity pressure curve. In precision injection moulding, overpacking creates a reject.An underpacked curve i
42、s shown in Figure 10a. It is characterized by a pressure dip in the packing phase. The switchover occurs too early, before the cavity is volumetrically filled. Part of the filling takes place at the lower holding pressure. Subsequently, the screw advance increases the pressure. The part has reduced
43、dimensions, is underweight, has sink marks and surface marks. It is again a reject.A device that switches over at volumetric filling would avoid the problems of overpacking and underpacking and produces the ideal cavity pressure curve. Switching is initiated at point 3 and completed in point 4 in Fi
44、gure 9.Methods of switchoverThe available means to switchover in a modern injection moulding machine, in increasing order of accuracy, are1. injection time,2. screw position,3. hydraulic pressure,4. nozzle pressure,5. cavity pressure.Injection time switchoverTemperature affects the viscosity of the
45、melt, which presents resistance to the advance of the screw. Increased resistance slows down the screw and prevents the cavity from filling in the given injection time. On the other hand, reduced resistance would lead to overpacking. Injection time switchover is the only means available in injection moulding machines without screw position and pressure sensors.Screw position switchoverScrew position switchover is not