METHOD FOR DETERMINING AN ACTUAL VOLUME OF AN INJECTION MOLDABLE COMPOUND IN AN INJECTION MOLDING PROCESS

Abstract

A method for determining an actual volume Vr of an injection-moldable compound during an injection-molding process is disclosed. The injection-moldable compound is introduced into at least one cavity of the mould. The method includes the steps of: a) determining a theoretical volume Vt from process variables at least during a filling phase of the injection-molding process, b) determining and/or measuring at least one value for at least one compound pressure pM, c) selecting a material-specific compression k (p), corresponding to the value of pM, of the injection-moldable compound, and d) calculating an actual volume Vr by taking into account the compression k (p).

Claims

1.-20. (canceled)

21. A method for determining an actual volume V.sub.r of an injection-moldable compound during an injection molding process, comprising: determining a theoretical volume V.sub.t from process variables as the injection-moldable compound is introduced into a cavity of a mold during a filling phase of the injection molding process; determining and/or measuring at least one value for at least one compound pressure p.sub.M; selecting a material-specific compression k(p) in correspondence to the value of the compound pressure p.sub.M of the injection-moldable compound; and calculating an actual filling volume V.sub.r as a function of the material-specific compression k(p).

22. The method of claim 21, further comprising adapting machine parameters of an injection molding machine to establish an ideal actual filling volume V.sub.ri of the cavity.

23. The method of claim 21, wherein the material-specific compression k(p) of the injection-moldable compound is selected from a material-specific compression curve, which is stored in a machine controller, in particular from an adiabatic compression curve k(p) stored in the machine controller.

24. The method of claim 21, wherein as the compound pressure p.sub.M a compound pressure in a cylinder or a compound pressure p.sub.F of a molding compound in an internal of the mold or a molding compound pressure p.sub.s in a screw antechamber is used, and further comprising determining and/or measuring at least two values A; B of the compound pressure p.sub.M, p.sub.F, p.sub.s during a defined process window.

25. The method of claim 24, wherein at least one of the two values A; B involves average values relating to plurality of individual measuring values.

26. The method of claim 24, wherein the actual filling volume V.sub.r corresponds to a volume, which is introduced into the mold without pressure, between the two values A; B and is calculated according to the formula: $.Math..Math.V_r=\frac{V_{\mathrm{tFB}}}{1-k\left(p_{\mathrm{FB}}\right)}-\frac{V_{\mathrm{tFA}}}{1-k\left(p_{\mathrm{FA}}\right)}=\frac{V_{\mathrm{tSA}}}{1-k\left(p_{\mathrm{SA}}\right)}-\frac{V_{\mathrm{tSB}}}{1-k\left(p_{\mathrm{SB}}\right)}$ wherein: V.sub.r is the actual filling volume, V.sub.tFB is the theoretical volume in the mold at position B, p.sub.FB is the molding compound pressure at position B, V.sub.tFA is the theoretical volume in the mold at position A, p.sub.FA is the internal mold pressure at position A, V.sub.tSA is the theoretical volume in an injection unit at position A, p.sub.SA is the molding compound pressure in the screw antechamber at position A, V.sub.tSB is the theoretical volume in an injection unit at position B, p.sub.SB is the pressure in the screw antechamber at position B.

27. The method of claim 21, further comprising adding a constant theoretical volume V.sub.t* to every measured theoretical volume V.sub.t in response to the calculation of the actual filling volume V.sub.r.

28. The method of claim 21, further comprising determining an actual volume flow {dot over (V)}.sub.r by deriving the actual filling volume V.sub.r or the actual volume over time, e.g. in accordance with the formula $.Math..Math.{\stackrel{.}{V}}_r=\frac{.Math..Math.\mathrm{Vr}}{t_B-t_A}$ wherein: {dot over (V)}.sub.r is the actual volume flow, V.sub.r is the actual filling volume, t.sub.B is one point in time (time of value B) t.sub.A is another point in time (time of value A).

29. The method of claim 28, further comprising determining the actual volume flow {dot over (V)}.sub.r as a function of a screw speed v.sub.s not using t.sub.A and t.sub.B.

30. The method of claim 21, wherein at least one of the actual filling volume V.sub.r and an actual filling volume flow {dot over (V)}.sub.r is determined continuously during the filling phase and/or an injection movement for filling the cavity is influenced in such a way that a predetermined actual volume flow profile is employed.

31. The method of claim 21, wherein at least one of the actual filling volume V.sub.r and an actual volume flow {dot over (V)}.sub.r is compared during the filling phase to a reference curve of an actual filling volume V.sub.rR and/or of an actual volume flow {dot over (V)}.sub.rR.

32. The method of claim 21, further comprising determining a theoretical switch-over filling volume V.sub.tXfrL in a learning cycle, and switching-over to a holding pressure phase in the learning cycle when the theoretical switch-over volume V.sub.tXfrL is reached.

33. The method of claim 32, further comprising determining a theoretical reference volume V.sub.tRefL at a reference pressure value p.sub.Ref in the learning cycle.

34. The method of claim 32, further comprising determining in a production cycle downstream from the learning cycle, a theoretical reference volume V.sub.tRefP of the production cycle at a same reference pressure value p.sub.Ref.

35. The method of claim 33, further comprising determining an actual switch-over filling volume .sub.VrXfrL in the learning cycle in accordance with the following formula: $.Math..Math.V_{\mathrm{rXfrL}}=\frac{V_{\mathrm{tRefL}}}{1-k\left(p_{\mathrm{Ref}}\right)}-\frac{V_{\mathrm{tXfrL}}}{1-k\left(p_{\mathrm{XfrL}}\right)}$ wherein V.sub.rXfrL is the actual switch-over filling volume in the learning cycle, V.sub.tRefL is the theoretical reference volume in the learning cycle, V.sub.tXfrL is the theoretical switch-over volume, p.sub.Ref is the reference pressure value, p.sub.XfrL is the switch-over pressure in the learning cycle, wherein the reference pressure value is reached at a first point in time t.sub.A and a switch-over point is reached at a second point in time t.sub.B, and further comprising calculating an actual filling volume V.sub.rR at a third point in time t.sub.C in a production cycle downstream from the learning cycle in accordance with the following formula: $.Math..Math.V_{\mathrm{rP}}=\frac{V_{\mathrm{tRefP}}}{1-k\left(p_{\mathrm{Ref}}\right)}-\frac{V_{\mathrm{tPC}}}{1-k\left(p_{\mathrm{PC}}\right)}$ wherein V.sub.rP is the actual filling volume in the production cycle, V.sub.tRefP is the theoretical reference volume in the production cycle, V.sub.tPC is the theoretical volume in the production cycle, p.sub.PC is the pressure in the production cycle, wherein the switch-over to the holding pressure phase is initiated in the production cycle, when
V.sub.rPV.sub.rXfrL.

36. The method of claim 21, further comprising adapting machine parameters during a pressure-regulated holding pressure phase to realize an ideal actual filling volume V.sub.ri by adjusting a holding pressure.

37. The method of claim 21, wherein the injection-moldable compound is introduced into the cavity of the mold by a reciprocating screw or a piston.

38. The method of claim 21, wherein the injection-moldable compound is a melt of thermoplastics or thermosetting molding compounds or silicones or varnishes.

39. The method of claim 21, further comprising controlling further actions of an injection molding process as a function of the actual filling volume such as, e.g., control of core pullers, opening and closing of cascades, which are controlled as a function of an injection path, i.e. as a function of the actual filling volume V.sub.r or as a function of time.

40. The method of claim 35, further comprising transferring learned values for the actual switch-over filling volume V.sub.rXfrL in the learning cycle and the reference pressure p.sub.Ref from a first injection molding machine to a second injection molding machine, which is constructionally identical or not constructionally identical to the first injection molding machine.

Description

[0049] The invention will be explained in more detail below in an exemplary manner by means of the drawings:

[0050] FIGS. 1a to 1c: show schematic illustrations of differences in an introduced volume of injection-moldable compound at different pressure levels (1000 bar and 500 bar);

[0051] FIGS. 2a, 2b: shows two machine states A and B in the case of a screw stroke s.sub.A and s.sub.B in a highly schematized manner;

[0052] FIGS. 3a, 3b: each show a pvT diagram of amorphous (FIG. 3a) and partially crystalline (FIG. 3b) thermoplastics (source: Handbook Injection Molding, Friedrich Johannaber, Walter Michaeli);

[0053] FIG. 4: shows a compression curve k(p) (adiabatic) for a thermoplastic plastic (PA6 GF30);

[0054] FIG. 5: shows a comparison diagram of a volume or of a volume flow, respectively, over time with curve progressions according to the prior art (non-consideration of the compressibility) and according to the invention (consideration of the compressibility, i.e. compression-adjusted).

[0055] FIGS. 1a to 1c show schematic illustrations of injection aggregates 1 and a melting volume V.sub.1 of 100 cm.sup.3 at 1 bar (ambient pressure) in a highly schematized manner. This is an initial state.

[0056] In FIG. 1b, the melting volume V.sub.1 is reduced to 60 cm.sup.3 in a screw antechamber in a first case and is at a pressure of 1000 bar. A second volume V.sub.2 is located in a non-illustrated cavity of a mold.

[0057] In the illustration on the right according to FIG. 1b, a state is shown, in which the melt volume V.sub.1 is 60 cm.sup.3 and is at a pressure of 500 bar.

[0058] In the illustration on the left in FIG. 1c, the state after FIG. 1b (left) is shown after the state according to FIG. 1b (left) was relaxed to ambient pressure. The volume V.sub.1 changes to 63.1 cm.sup.3 and is present at 1 bar of ambient pressure. The volume V.sub.2 in the left illustration of FIG. 1 is 36.9 cm.sup.3 in the relaxed state. The volume V.sub.2 according to the right illustration in FIG. 1c is 38.5 cm.sup.3. This means that in the case according to FIGS. 1a, 1b, 1c illustrated on the right, significantly less (1.6 cm.sup.3 less) injection-moldable compound was introduced.

[0059] The two cases, which were shown parallel next to one another in FIGS. 1a, 1b, 1c, represent the prior art, which currently does not provide to measure or to control the volume of the molding compound, which is introduced into a cavity of an injection mold in such a way that the compressibility is considered. In response to such an approach according to the prior art, a volume V.sub.2 of different sizes is to be expected, when the injection-moldable compound is relaxed and when pressures of different sizes have prevailed during the injection molding process. This means that ifas practiced in the prior artan injection molding machine is volume-controlled or, equivalently, is operated during the screw stroke and the injection molding process is thus ended at a certain theoretical volume V.sub.t or at a certain screw stroke, different molded part compounds are introduced into the cavity at different pressures.

[0060] Such pressure differences, however, appear in reality due to temperature fluctuations and viscosity changes of the material/granules/the injection-moldable compound and thus influence the component quality and the weight constancy in a disadvantages manner. Based on this knowledge, the invention will now be explained below.

[0061] A method for the compression-adjusted determination of a plastic volume V.sub.r is the core of the invention. This means, in other words, that the movement of a volume V.sub.r into a cavity occurs in consideration of the compressibility of the injection-moldable compound. A screw unit 2, which is equipped with a non-return valve, if necessary, is located in the injection unit 1 in a schematized manner (see FIGS. 2a, 2b).

[0062] In the alternative, the screw unit 2 can also be embodied as piston.

[0063] An injection-moldable molding compound, e.g. a plastic melt or a thermosetting injection-moldable molding compound, is located upstream of the screw unit 2. This molding compound is under a pressure p.sub.SA, when the screw unit 2 is located at a position A. The screw is then located at the position of the screw stroke s.sub.A. This corresponds to a theoretical volume in the screw antechamber V.sub.tSA. An injection mold 3 comprising a cavity 4 is also illustrated in a schematized manner.

[0064] Theoretical volume V.sub.tFA which is already located in the cavity 4 at an internal mold pressure p.sub.FA, is additionally illustrated in a schematic manner (at the screw position s.sub.A).

[0065] FIG. 2b shows a later state. The screw stroke s.sub.B is smaller than the screw stroke s.sub.A. screw unit 2 has thus conveyed a portion of the molding compound into the cavity 4 of the mold 3. A pressure p.sub.sB prevails in the molding compound of the injection unit 1, in particular in the screw antechamber. A theoretical filling volume V.sub.tFB at the pressure p.sub.FB is located in the cavity 4.

[0066] The actual filling volume V.sub.r can now be determined as follows with this information. The volume V.sub.tSA in the screw antechamber can be measured via a position measuring system of the screw and is displayed in a machine controller. From the difference of the screw stroke s.sub.As.sub.B, a theoretical filling volume V.sub.tSAV.sub.tSB, which is introduced into the mold between two positions A and B, can thus also be determinedassuming a negligible return flow in the non-return valve or at the piston. With the help of a compression source k(p), which is present for the respective molding compound material and which is stored in a machine controller, a change of the specific volume can now be considered. Values, which specify the compressibility of the material at hand, thus a change of the specific volume V.sub.U, form the basis for the compression curve k(p). These compression curves k(p) can be determined from a pvT diagram (see FIGS. 3a, 3b) for the one isothermal case, in that a change of the specific volume V.sub.U is calculated at points of intersection S1-S4 of pressure lines 5 with a temperature vertical 6, based on the specific volume V.sub.U at the ambient pressure.

[0067] Such pressure lines 5 are specified for example in the diagrams according to FIGS. 3a, 3b for an amorphous (FIG. 3a) and partially crystalline material (FIG. 3b). At a certain temperature T.sub.1, specific volumes V.sub.U of the molding compound material, which decrease as the pressure p increases, result. Points of intersection S.sub.1, S.sub.2, S.sub.3 and S.sub.4 are specified in FIGS. 3a, 3b as examples for this. The point of intersection S.sub.1 specifies for example the specific volume V.sub.U of an amorphous material, when the latter is present below ambient pressure (1 bar). These points of intersection S.sub.2, S.sub.3 and S.sub.3 in FIG. 3a specify specific volumes V.sub.U at higher pressures.

[0068] FIG. 3b shows pressure lines 5 of a partially crystalline material. The points of intersection S.sub.1 to S.sub.4 are located on the vertical temperature line 6, which belongs to a certain temperature T.sub.1.

[0069] FIG. 4 shows a different (adiabatic) compression curve k(p). Such an adiabatic compression curve k(p) is preferred for the injection molding process. FIG. 4 shows a corresponding compression (k(p)) in percent as a function of pressure, in particular of the molding compound pressure p.sub.M. Value pairs p and k(p), which form this curve, are stored in the machine controller. The compression curve according to FIG. 4 shows a course for an injection-moldable material PA6GF30 in an exemplary manner. To now be able to determine the actual volume V.sub.ra at a point in time A, the following equation can be specified with the knowledge of the compression curve k(p) of the used material:

[00005]$V_{\mathrm{rA}}=\frac{V_{\mathrm{tSA}}}{1-k\left(p_{\mathrm{SA}}\right)}=\frac{s_A.Math.r^2.Math.}{1-k\left(p_{\mathrm{SA}}\right)}$

[0070] The actual filling volume V.sub.r introduced between two points in time or positions A and B can now be specified by the following equation:

[00006]$.Math..Math.V_r=\frac{V_{\mathrm{tFB}}}{1-k\left(p_{\mathrm{FB}}\right)}-\frac{V_{\mathrm{tFA}}}{1-k\left(p_{\mathrm{FA}}\right)}=\frac{s_A.Math.r^2.Math.}{1-k\left(p_{\mathrm{SA}}\right)}-\frac{s_B.Math.r^2.Math.}{1-k\left(p_{\mathrm{SB}}\right)},$

wherein V.sub.tFA and T.sub.tFB are theoretical volumes at the points in time or positions A and B, k(p.sub.FB) and k(p.sub.FA) is the compressibility of the molding compound at a pressure p at the location A and at the location B,
s.sub.A and s.sub.B are the screw strokes at the positions A and B and k(p.sub.SA) and (p.sub.SB) are the compressibilities of the molding compound at a screw antechamber pressure at the positions A or B, respectively.

[0071] The pressure p.sub.F specifies an internal mold pressure. The pressures p.sub.S specify for example a pressure in the molding compound in the screw antechamber. Both alternatives are possible pressure types, which are suitable to be used as compound pressure p.sub.M.

[0072] Based on this calculation, a compression-adjusted, that is, an actual filling volume flow {dot over (V)}.sub.r, can also be specified between the positions A, B. For example, the following equation is suitable for this purpose:

[00007]$.Math..Math.{\stackrel{.}{V}}_r=\frac{.Math..Math.\mathrm{Vr}}{t_B-t_A}.$

[0073] The actual volume flow {dot over (V)}.sub.r can advantageously be determined as derivation of the actual filling volume V.sub.r via the time t.

[0074] Different values A, B for the compound pressure p.sub.M at the positions A, B can be measured via machine-internal measuring devices, e.g. force transducers or via the hydraulic pressure of the machine, direct and/or indirect melt pressure sensors or other measuring devices for detecting the pressure of the molding compound in the cylinder. The pressures in the mold can be measured via internal tool pressure sensors or another measuring devices to detect the pressure of the molding compound in the mold.

[0075] A consideration of the compression k(p) according to the invention of the used molding material thus makes it possible to determine the actual filling volume V.sub.r and/or an actual filling volume flow {dot over (V)}.sub.r during the entire filling process of the cavity 4 at every point in time and/or continuously and/or at certain points in time. The actual filling volume V.sub.r or the actual volume flow {dot over (V)}.sub.r can thus now be influenced with suitable control devices, which are present at the machine, for the injection movement, so that a predetermined volume flow profile is employed or can be employed.

[0076] In addition, the method according to the invention now also makes it possible to now also influence further process actions of an injection molding process, which can currently be controlled as a function of screw and/or piston stroke or the volume, respectively, or also the speed or the volume flow, via the compression-adjusted actual filling volume V.sub.r or the actual filling volume flow {dot over (V)}.sub.r. Such actions, such as, e.g., cascade controls, embossing and/or speed profiles can advantageously be triggered with the method according to the invention with identical mold filling, thus independently from viscosity fluctuations.

[0077] FIG. 5 shows a comparison of different characteristic curves of an injection molding process, when such an injection molding process is performed by using the method according to the invention, as compared to an injection molding process according to the prior art.

[0078] A comparison of the curve progressions for the theoretical filling volume V.sub.t and for the actual filling volume V.sub.r, which is compression-adjusted, shows that, at the time of the switch-over point, the theoretical filling volume V.sub.t has already reached a nominal filling volume of the cavity (here 70 cm.sup.3) and even exceeds this at the end of the injection molding cycle. In contrast, the actual filling volume V.sub.r reaches the nominal value of the cavity of 70 cm.sup.3 only at the end of the holding pressure phase, which corresponds to the reality. The theoretical filling volume V.sub.t, which is larger than the nominal volume of the cavity at the end of the holding pressure phase, thus reflects a variable, which cannot be reproduced in reality. In the context of the present method, the nominal volume of the cavity corresponds to the ideal actual filling volume V.sub.ri, which is to be reached.

[0079] The curve for the actual filling volume flow {dot over (V)}.sub.r is also arranged above the curve for the theoretical volume flow {dot over (V)}.sub.t in the area up to the switch-over point. These curves run approximately congruently only starting at the holding pressure phase.

LIST OF REFERENCE SIGNS

[0080] V.sub.t theoretical volume [0081] t time [0082] p.sub.M compound pressure [0083] p.sub.S pressure in the screw antechamber [0084] p.sub.F internal mold pressure [0085] k(p) compression, compression curve [0086] V.sub.r actual filling volume [0087] V.sub.rR reference curve actual filling volume [0088] V.sub.ri ideal actual filling volume [0089] t.sub.A, t.sub.B, t.sub.C points in time [0090] s.sub.A, s.sub.B screw stroke [0091] V.sub.tSA, V.sub.tFA theoretical volume in the injection unit (S) and the mold (F) at the position A [0092] V.sub.tSB, V.sub.tFB theoretical volume in the injection unit (S) and the mold (F) at the position B [0093] V.sub.tB theoretical volume at the position B [0094] A, B values, positions [0095] {dot over (V)}.sub.r actual volume flow [0096] {dot over (V)}.sub.Fr reference curve actual volume flow [0097] v.sub.S screw or piston speed, respectively [0098] .sub.VrXfrL actual switch-over filling volume in the learning cycle [0099] V.sub.tRefL theoretical reference volume in the learning cycle [0100] p.sub.Ref reference pressure value [0101] V.sub.tRefp theoretical reference volume in the production cycle [0102] V.sub.rP actual filling volume in the production cycle [0103] L learning cycle [0104] P production cycle [0105] S.sub.1-S.sub.4 points of intersection [0106] V.sub.U specific volume [0107] V.sub.1 melt volume [0108] V.sub.1 compressed volume [0109] V.sub.2 second volume [0110] Position A [0111] Position B [0112] 1 injection unit [0113] 2 screw unit [0114] 3 injection mold [0115] 4 cavity [0116] 5 pressure line [0117] 6 vertical temperature