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. 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; calculating an actual volume V.sub.r as a function of the material-specific compression k(p), wherein the material-specific compression k(p) of the injection-moldable compound is selected from an adiabatic compression curve k(p); adapting machine parameters during a pressure-regulated holding pressure phase to realize an ideal actual filling volume ΔV.sub.ri by adjusting a holding pressure; determining a theoretical switch-over volume V.sub.tXfrL in a cycle, and switching-over to a holding pressure phase in the cycle when the theoretical switch-over volume V.sub.tXfrL is reached; determining a theoretical reference volume V.sub.tRefL at a reference pressure value p.sub.Ref in the cycle; determining in a production cycle downstream from the cycle, a theoretical reference volume V.sub.tRefP of the production cycle at the same reference pressure value p.sub.Ref; and determining an actual switch-over filling volume ΔV.sub.rXfrL in the cycle in accordance with the following formula: $\Delta V_{\mathrm{rXfrL}}=\frac{V_{\mathrm{tRefL}}}{1-k\left(p_{\mathrm{Ref}}\right)}-\frac{V_{\mathrm{tXfrL}}}{1-k\left(p_{\mathrm{XfrL}}\right)}$ wherein ΔV.sub.rXfrL is the actual switch-over filling volume in the cycle, V.sub.tRefL is the theoretical reference volume at the reference pressure P.sub.Ref in the cycle, V.sub.tXfrL is the theoretical switch-over volume at the switch-over pressure P.sub.XfrL in the cycle, p.sub.Ref is the reference pressure value, p.sub.XfrL is the switch-over pressure in the 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, calculating an actual filling volume ΔV.sub.rP at a third point in time t.sub.C in a production cycle downstream from the cycle in accordance with the following formula: $\Delta V_{\mathrm{rP}}=\frac{V_{\mathrm{tRefP}}}{1-k\left(p_{\mathrm{Ref}}\right)}-\frac{V_{\mathrm{tPC}}}{1-k\left(p_{\mathrm{PC}}\right)}$ wherein ΔV.sub.rP is the actual filling volume in the production cycle, V.sub.tRefP is the theoretical reference volume at the reference pressure value P.sub.ref in the production cycle, V.sub.tPC is the theoretical volume at time t.sub.c at the pressure value p.sub.PC in the production cycle, p.sub.PC is the pressure in the production cycle, comparing the actual filling volume ΔV.sub.rP with the actual switch-over filling volume ΔV.sub.rXfrL for the switch-over to the holding pressure phase in the production cycle, and initiating the switch-over to the holding pressure phase in the production cycle when
ΔV.sub.rP≥ΔV.sub.rXfrL.

2. The method of claim 1, wherein the adiabatic compression curve k(p) is stored in the machine controller.

3. The method of claim 1, 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.

4. The method of claim 3, wherein at least one of the two values A; B involves average values relating to a plurality of individual measuring values.

5. The method of claim 3, wherein the actual filling volume ΔV.sub.r corresponds to a volume, which is introduced into the mold without applied pressure, between the two values A; B and is calculated according to the formula: $\Delta V_r=\frac{V_{\mathrm{tFB}}}{1-k\left(p_{\mathrm{FB}}\right)}-\frac{V_{\mathrm{tFA}}}{1-k\left(p_{\mathrm{FA}}\right)}=\frac{V_{\mathrm{tSA}}}{1-k\left(p_{\mathrm{SA}}\right)}-\frac{V_{\mathrm{tSB}}}{1-k\left(p_{\mathrm{SB}}\right)}$ wherein: ΔV.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 in the mold at position B, V.sub.tFA is the theoretical volume in the mold at position A, P.sub.FA is the molding compound pressure in the mold 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.

6. The method of claim 1, 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 ΔV.sub.r.

7. The method of claim 1, further comprising determining an actual filling volume flow Δ{dot over (V)}.sub.r by deriving the actual filling volume ΔV.sub.r or the actual volume V.sub.r over time, in accordance with the formula $\Delta {\stackrel{.}{V}}_r=\frac{\Delta \mathrm{Vr}}{t_B-t_A}$ wherein: Δ{dot over (V)}.sub.r is the actual filling volume flow, ΔV.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).

8. The method of claim 7, further comprising determining the actual volume flow Δ{dot over (V)}.sub.r as a function of a screw speed v.sub.s not using t.sub.A and t.sub.B.

9. The method of claim 1, wherein at least one of the actual filling volume ΔV.sub.r or an actual filling volume flow Δ{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.

10. The method of claim 1, wherein at least one of the actual filling volume ΔV.sub.r or an actual volume flow Δ{dot over (V)}.sub.r is compared during the filling phase to a reference curve of an actual filling volume Δ{dot over (V)}.sub.rR and/or of an actual volume flow Δ{dot over (V)}.sub.rR.

11. The method of claim 1, wherein the injection-moldable compound is introduced into the cavity of the mold by a reciprocating screw or a piston.

12. The method of claim 1, wherein the injection-moldable compound is a melt of thermoplastics or thermosetting molding compounds or silicones or varnishes.

13. The method of claim 1, further comprising controlling further actions of an injection molding process as a function of the actual filling volume.

14. The method of claim 1, further comprising transferring learned values for the actual switch-over filling volume ΔV.sub.rXfrL in the 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

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be explained in more detail below in an exemplary manner by means of the drawings:

(2) 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);

(3) 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;

(4) 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);

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

(6) 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).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(7) 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.

(8) 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.

(9) 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.

(10) 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.

(11) 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 if—as practiced in the prior art—an 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.

(12) 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.

(13) 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).

(14) In the alternative, the screw unit 2 can also be embodied as piston.

(15) 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.

(16) 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).

(17) 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.

(18) The actual filling volume ΔV.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.A−s.sub.B, a theoretical filling volume V.sub.tSA−V.sub.tSB, which is introduced into the mold between two positions A and B, can thus also be determined—assuming 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.

(19) 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.

(20) 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.

(21) 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:

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

(23) The actual filling volume ΔV.sub.r introduced between two points in time or positions A and B can now be specified by the following equation:

(24) $\Delta V_r=\frac{V_{\mathrm{tFB}}}{1-k\left(p_{\mathrm{FB}}\right)}-\frac{V_{\mathrm{tFA}}}{1-k\left(p_{\mathrm{FA}}\right)}=\frac{s_A.Math.r^2.Math.\pi }{1-k\left(p_{\mathrm{SA}}\right)}-\frac{s_B.Math.r^2.Math.\pi }{1-k\left(p_{\mathrm{SB}}\right)},$
wherein V.sub.tFA and V.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.

(25) 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.

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

(27) $\Delta {\stackrel{.}{V}}_r=\frac{\Delta \mathrm{Vr}}{t_B-t_A}.$

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

(29) 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.

(30) 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 ΔV.sub.r and/or an actual filling volume flow Δ{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 ΔV.sub.r or the actual volume flow Δ{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.

(31) 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 ΔV.sub.r or the actual filling volume flow Δ{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.

(32) 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.

(33) A comparison of the curve progressions for the theoretical filling volume ΔV.sub.t and for the actual filling volume ΔV.sub.r, which is compression-adjusted, shows that, at the time of the switch-over point, the theoretical filling volume ΔV.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 ΔV.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 ΔV.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 ΔV.sub.ri, which is to be reached.

(34) The curve for the actual filling volume flow Δ{dot over (V)}.sub.r is also arranged above the curve for the theoretical volume flow Δ{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.