Method for evaluating process characteristics of injection-molding tools

Abstract

The invention relates to a method for qualitatively and/or quantitatively classifying injection-molding tools in tool categories and determining preferred intervention ranges and/or manipulated variables for adapting injection-molding machine parameters in the case of changing ambient conditions and/or determining the influence of disturbing effects on the injection-molding process, comprising the following steps: a) providing an injection-molding machine having the injection-molding tool which is to be classified and which is intended for the determination, b) performing at least one injection-molding cycle with injection-molding machine settings in order to obtain a qualitatively adequate injection-molding part, c) determining a quotient Q=p/s or Q=sn/se characterizing the tool from c.1) a pressure rise p during the compression phase of the injection-molding cycle and the melt volume V displaced during the compression phase or c.2) a melt volume Vn displaced during the holding-pressure phase and the melt volume Ve displaced during the injection phase, wherein c.3) the corresponding screw travel s, sn, and se is measured in order to determine the displaced volumes V, Vn, and ; Ve, d) providing at least one limit value (G1 . . . Gx . . . Gn), wherein one or more recommendations for preferred intervention ranges or manipulated variables for adapting adjustment parameters of the injection-molding machine are associated with ranges (Q<G1; G1<Q<G2; . . . Gn-1<Q<Gn; Q>Gn) for the values of the quotient Q, e) determining in which of the ranges (Q<G1; G1<Q<G2; . . . Gn-1<Q<Gn; Q>Gn) the value of the quotient Q lies, and f) outputting the preferred intervention ranges and/or manipulated variables for adapting the machine parameters of the injection-molding machine which are associated with the determined range.

Claims

1. A method for qualitatively and/or quantitatively classifying injection-molding tools in tool categories and for determining preferred intervention ranges and/or manipulated variables for adapting injection-molding machine parameters during changing ambient conditions and/or determining an influence of undesirable effects on an injection-molding process, said method comprising the steps of: performing at least one injection-molding cycle of an injection-molding machine, having an injection-molding tool to be classified and to be determined, with injection-molding machine settings to obtain a usable injection-molding part; determining a quotient Q=p/s or Q=s.sub.n/s.sub.e characterizing the injection-molding tool in one of two ways, a first way in which the quotient Q is determined from a pressure rise p during a compression phase of the at least one injection-molding cycle and a melt volume V displaced during the compression phase, a second way in which the quotient Q is determined from a melt volume V.sub.n, displaced during a holding-pressure phase and a melt volume V.sub.e, displaced during an injection phase, wherein a corresponding screw travel s, s.sub.n and s.sub.e is measured in order to determine the displaced volumes V, V.sub.n and V.sub.e; providing at least one limit value G.sub.1 . . . G.sub.x . . . G.sub.n, wherein one or more recommendations for preferred intervention ranges and/or manipulated variables for adapting adjustment parameters of the injection-molding machine are associated with ranges QG.sub.1; G.sub.1<QG.sub.2; . . . G.sub.n-1<QG.sub.n, Q>G.sub.n for values of the quotient Q; determining in which of the ranges QG.sub.1; G.sub.1<Q<G.sub.2; . . . G.sub.n-1<Q<G.sub.n, Q>G.sub.n a value of the quotient Q lies; and outputting the preferred intervention ranges and/or manipulated variables for adapting the machine parameters of the injection-molding machine which are associated with the determined range, wherein in a semi-automatic operation, the outputting takes places on a display and a machine operator transfers into a machine control values suggested by the injection-molding machine, or in a fully-automatic operation, the outputting takes place directly to the machine control whereby a next injection molding cycle is carried out with the adapted injection-molding machine parameters.

2. The method of claim 1, wherein the pressure rise p is determined by measuring a course of a melt pressure or of a tool internal pressure.

3. The method of claim 1, further comprising determining a correction factor to correct a value for the quotient Q by dynamically caused changes of the value for the quotient Q characterizing a respective tool by an influence of an injection speed.

4. The method of claim 1, wherein recommended intervention ranges are the injection phase, the compression phase or the holding-pressure phase of the at least one injection-molding cycle.

5. The method of claim 1, wherein the manipulated variables are a switchover point as a function of a screw position and/or a switchover point as a function of an injection pressure and/or a switchover point as a function of time and/or a holding pressure level and/or a holding pressure time and/or an injection speed and/or a cylinder temperature and/or a tool temperature.

6. The method of claim 1, further comprising associating with the intervention ranges and/or manipulated variables as a function of the ranges QG.sub.1; G.sub.1<QG.sub.2; . . . G.sub.n-1<QG.sub.n, Q>G.sub.n priorities for a measure of a correcting effect of a change of the intervention ranges and/or of the manipulated variables on the injection-molding cycle, wherein the recommendations of the preferred intervention ranges and/or manipulated variables are outputted weighted with regard to the correcting effect and weighted in priority.

7. The method of claim 1, wherein in addition to a qualitative selection of the intervention ranges and/or of the manipulated variables, correction values are outputted for the intervention ranges and/or manipulated variables.

8. The method of claim 7, wherein the correction values are determined empirically and deposited in a data bank.

9. The method of claim 7, wherein the correction values change values for the manipulated variables and/or definitions of the intervention ranges and/or values for priorities and/or values for correcting effects of the manipulated variables on the at least one injection-molding cycle.

10. The method of claim 7, further comprising establishing a B characteristic diagram from the preferred intervention ranges and/or the manipulated variables and/or priorities thereof and/or the correction values thereof, and determining as a function of the quotient Q a suitable point of the characteristic diagram, wherein the correction values and/or recommendations associated with the determined point of the characteristic diagram are outputted and/or are deposited in a memory.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention is explained in further detail below with the aid of the enclosed drawings. There are shown in diagrammatic illustration:

(2) FIG. 1: a diagram for determining the quotient Q=p/s with the aid of a typical course of the screw travel and of the melt pressure over time of an injection-molding machine (Alternative 1 of the invention);

(3) FIG. 2: a diagram for determining the quotient Q=s.sub.n/s.sub.e with the aid of a typical diagram of the screw travel s and of the melt pressure p over time t in a particular injection-molding process and

(4) FIG. 3: an exemplary illustration of intervention ranges and manipulated variables over the characteristic number Q as parameter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) The sketched diagram of FIG. 1 shows over an injection-molding cycle on an injection-molding tool a chronological course of a screw travel s and of a melt pressure p. The individual phases are described briefly below, in order to then be able to indicate tool-specific characteristics:

(6) In an injection phase EP with a start at a time t.sub.o an injection unit of an injection-molding machine is moved to a clamping unit of a tool and is pressed with an outlet of the injection unit in the form of a nozzle on the tool. A screw of the injection-molding machine is now moved over a particular screw travel s in the direction of the nozzle, so that a prepared melt is pressed under high pressure p through the opened nozzle and a sprue of the injection-molding tool into a shaping cavity. During this injection phase EP, the screw is moved with approximately constant speed, which results in a likewise approximately constant rise of the pressure p, as illustrated in FIG. 1.

(7) A distinct rise in the pressure increase characterizes the transition from the injection phase EP into a compression phase KP. Here, the tool or respectively a cavity of the tool is already largely filled. This phase is terminated on reaching a so-called switchover point P, at which a switchover takes place from a travel control to a pressure control.

(8) The cavity of the tool is now in fact filled with plasticized plastic material on reaching of the switchover point P, but as the tool, with typically 20 to 120 C., is colder than the injected plastic material, heated to approximately 200 to 300 C., the plastic material cools down in the mold and solidifies on reaching a freezing point. The cooling is accompanied here by a volume contraction, which can have a very disadvantageous effect on a dimensional accuracy and a surface quality of the workpiece which is to be produced. In order to compensate this contraction as far as possible, a reduced pressure is also maintained after filling of the cavity of the tool, so that as compensation for the contraction there can be afterflow of plasticized plastic material into the cavity.

(9) To adjust a holding pressure to a lower pressure level compared to the compression phase KP, the screw is stopped and moved back a little, in order to then be moved generally linearly again according to the required pressure level and the plastic emerging into the cavity accordingly.

(10) Already with the aid of the course of the pressure p(t), thinner-walled shaped parts can basically be differentiated from rather more thick-walled shaped parts. In FIG. 1 a curve, drawn in dashed lines, shows a basic pressure course in the production of a rather more thin-walled shaped part, and a line drawn by dots shows a pressure course of a rather more thick-walled shaped part, to illustrate characteristic differences. A pressure rise p.sub.2 in the compression phase KP occurs distinctly higher in the case of a rather more thin-walled shaped part than a pressure rise p.sub.1 of a rather more thick-walled shaped part. Thereby, there results as characteristic feature Q in relation to a screw travel distance s travelled respectively during the compression phase KP:

(11) $\frac{p_2}{s}>\frac{p_1}{s}$

(12) However, rather more thick-walled shaped parts also differ from rather more thin-walled ones in the holding-pressure phase, as sketched in FIG. 2. As the injected plastic material solidifies rather in rather more thin-walled shaped parts, in the holding-pressure phase NP with constant holding pressure not so much additional material can be pressed into the cavity. This is reflected in a distinctly small screw travel distance s.sub.n. In FIG. 2 a rather more thick-walled shaped part is illustrated by way of example by a curve drawn in dashed lines, a rather more thin-walled one by a line, drawn by dots, in the holding-pressure phase NP. Thus, here also there results as characteristic feature Q in relation to a screw travel distance s.sub.e travelled respectively during the injection phase EP:

(13) $\frac{s_{n1}}{s_e}>\frac{s_{n2}}{s_e}$

(14) The respective characteristic features Q therefore express characteristics of the tool or respectively of the cavity. Purely with the aid of their absolute quantity or of their quantity relationship, it is therefore to be deduced thereupon whether a tool can produce rather more thick-walled or rather more thin-walled shaped parts.

(15) Via the characteristic numbers Q therefore also methods for the correction of raw material quality fluctuations can also be operated in an automated manner, wherein necessary inputs by experienced machine operators are no longer necessary. Via the characteristic numbers, requirements concerning control technology of different tools can be determined and on this basis machine-internal regulation parameters can be adapted. Thus, the injection axis of an injection-molding machine can always be operated with the best dynamics and reproducibility in accordance with the specific requirements.

(16) FIG. 3 presents a sketch of intervention ranges and manipulated variables which basically come into consideration here for the correcting adaptation during the operation of a tool on an injection-molding machine. This graphic represents a data bank with empirical values, in which with the aid of an arrangement over the characteristic number Q as parameter for a fixed value Q.sub.x it can be seen that from a total number of possibilities for a tool currently present with this value Q.sub.x only 4 intervention ranges and/or manipulated variables are relevant or are the most promising, namely here as an example S1, E2, S2 and P2. Here, a respective quantity of a section through an illustrated field can be regarded as an indication for its influence on a possible improvement to the production result. Therefore, also a suggestion is delivered, in which sequence optimization measures should be taken.

(17) In a simple manner, therefore, an indication is given as to possible optimizations and settings, without the tool itself having to be examined intensively by a specialist.