INJECTION MOLDING QUALITY CONTROL SYSTEM

Abstract

An injection molding quality control system which seeks to ensure greater accuracy and consistency in parts, more specifically more accurate and consistent part weight, utilizes measured part weight to adjust mold separation to better achieve a desired part weight. The mold separation is controlled via both a cycle-to-cycle adjustment in switchover point (preferably based on injected mass within the cavity), and within-cycle adjustment of holding pressure. The system can result in superior accuracy and consistency in molded parts in both the long term (i.e., over many cycles) and in the short term (from cycle to cycle).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of an exemplary injection molding system wherein the invention may be implemented.

FIG. 2 a is a schematic control diagram depicting a preferred control scheme for the injection molding system of FIG. 1.

FIG. 2 b is a global nonlinear state block diagram further illustrating the control scheme depicted in FIG. 2a.

FIG. 2 c is the operating point state block diagram of the control scheme of FIG. 2b.

DETAILED DESCRIPTION OF PREFERRED VERSIONS OF THE INVENTION

To review the foregoing discussion in greater detail, FIG. 2a illustrates a preferred cascade closed-loop system with direct quality feedback and disturbance feedforward for online quality control of injection molding. At the outer loop, the part weight Wt is regulated by manipulating the maximum mold separation MSRef from shot to shot (cycle to cycle). As discussed in prior studies, e.g., in Chen, Z. B., L. S. Turng and K. K. Wang, Polym. Eng. Sci., 46 (5), 263 (2006), part weight is highly correlated with mold separation, and thus mold separation is a useful quantity to control to achieve desired part weight. Further, as previously noted, part weight Wt can be regarded as a proxy for part quality, and thus this outer loop may be regarded as a quality control loop.

However, since mold separation alone cannot account for all variations in part weight—for example, studies by the inventors have found that melt temperature and mold temperature also affect the correlation between part weight and mold separation—the inner loop is further used to enhance weight (and thus quality) control. At the inner loop, which may be regarded as a process control loop, the maximum mold separation is used to scale the whole mold separation profile during filling and holding and the whole mold separation profile is taken as a signature of the injection molding process. The mold separation profile, including the maximum mold separation MSRef, is controlled via both cycle-to-cycle control of switchover S/O and within-cycle holding control (i.e., control of holding pressure Pp). First, the switchover point S/O is adjusted from cycle to cycle (based on injected mass) to at least partially achieve the required maximum mold separation MSRef. After the switchover point S/O, the holding pressure Pp is adjusted to duplicate the desirable mold separation profile, which is normalized by scaling the maximum mold separation value of the profile to the required maximum value. In this way, long-term disturbances are prevented in cycle-to-cycle control, and short-term disturbances are compensated for by the within-cycle control.

Referring to FIG. 1, mold separation (shown as MSfeedback) can be measured by displacement/proximity sensors 118 such as LVDTs. Melt temperature Tm can be captured from the plastic in the injector 106 by use of a melt temperature sensor 124 such as a thermocouple, and the mold temperature Tw can similarly be captured from the mold wall or other suitable location by a mold temperature sensor 126. Holding pressure Pp, and more generally the pressure within the mold cavity 104, can be monitored by use of one or more pressure transducers 128 distributed about the mold cavity 104 (preferably flush mounted with respect to the interior cavity wall).

Control of switchover S/O within the switchover controller 200 can be based on parameters such as time during injection, injector pressure, or ram position. However, switchover S/O is most preferably controlled in accordance with the mass of the injected plastic, since this parameter is believed to have a more direct bearing on mold separation MS than time, pressure, or position alone: experiments have found that if maximum mold separation is plotted versus injected mass at switchover, the resulting data points fall roughly along a straight line, indicating that a simple proportional model can be used to define the relationship between these parameters. Injected mass may then be calculated in accordance with:

$\begin{array}{cc}{m}_{\mathrm{inj}}=A\left(\frac{l_0}{v_o\left(T,p\right)}-\frac{l_0}{v_s\left(T,p\right)}\right)& \left(1\right)\end{array}$

where m_inj is the injected plastic mass, A is the cross-sectional area of the injector barrel 108, l₀ is the position of the ram 116 at the start of filling, l_s is the position of the ram 116 at switchover S/O, and v₀ and v_s are respectively the specific volumes of the plastic at the start of filling and at switchover. Parameters v₀ and v_s are functions of temperature and pressure, and can be calculated from the chosen plastic's pvT (pressure-specific volume-temperature) property relationship given the melt temperature Tm and pressure P. The plastic's pvT properties can be conveniently modeled by a two-domain, modified Tait equation, as described in Cheng, H. H., C. A. Hieber, and K. K. Wang, Polym. Eng. Sci. 31, 1571 (1991).

Then, as discussed above, a pure proportional element can be employed to model the maximum mold separation MSmax:

ΔMS_max=KΔm_inj   (2)

The proportional gain, K, can be obtained from experimental data derived from several injection cycles using the plastic and injection molding system 100 in issue.

As discussed above, melt temperature Tm and mold temperature Tw are further variables apart from mold separation that also have an impact on part weight. Studies by the inventors have found that there is a roughly linear correlation between part weight and maximum mold separation, but at the same time, melt temperature Tm and mold temperature Tw affect both the slope and the interception of the correlating line. A simple relationship between these variables can be expressed as:

Wt=Wt ₀ +a ₁ [MS]+a ₂ [Tm]+a ₃ [Tw]+a ₄ [MS][Tm]+a ₅ [MS][Tw]  (3)

wherein the coefficients a₁, a₂, a₃, a₄, and a₅ can also be experimentally determined for a given injection molding system 100. Equation (3), and the derived coefficients, can then be programmed into or otherwise used in the compensator 204 Turning to FIG. 2b, the global nonlinear block diagram of the quality control system is depicted. However, note that since this diagram depicts cycle-to-cycle control, the within-cycle post-filling control is not shown. The part weight controller 202 is depicted by G₁(z⁻¹), and controller G₂(z⁻¹) is used in control of mold separation. The function f(m_inj, Pp) models the dependence of the maximum mold separation MSmax on switchover point S/O and holding pressure Pp. The function g(Tm, Tw, MSmax) expresses part weight in terms of mold temperature Tw, melt temperature Tm, and the maximum mold separation MSmax. The equation h(Wt_ref, Tm, Tw) (=MSmax) is converted from g(Tm, Tw, MSmax) (=Wt). The blocks z⁻¹ and z⁻² are backward shift operators; note that there is a one-cycle delay in the MSfeedback loop due to the intrinsic nature of cycle-to-cycle control, and a two-cycle delay exists in the weight feedback loop because of the need to measure part weight after each cycle. The whole system can be divided into several sub-parts as follows:

Weight Controller:

MS _{max — ref} ^fb =G ₁(z⁻¹)[Wt_ref−Wtz⁻²]  (4)

where MS_max—ref^fb is the feedback component of the maximum mold separation command MSRef.

Mold Separation Controller:

m _inj =G ₂(z⁻¹)[MS_max—ref^fb+MS_max—ref^did−MS_maxz⁻¹]  (5)

where MS_max—ref^did is the disturbance input decoupling or feedforward component of the maximum mold separation command MSRef.

Disturbance Input Decoupling or Feedforward Compensator:

MS _{max — ref} ^did =h(Wt_ref,Tm,Tw)−MS_max—Ref⁰   (6)

where MS_max—ref⁰ is the maximum MS reference corresponding to the required weight under nominal conditions.

Process (Mold Separation) Object:

MS _max =f(m_inj,Pp)   (7)

Quality (Weight) Object:

Wt=g(MS_max,Tw,Tm)   (8)

Equations (5) and (6) are already in linear form and Eqs. (6) to (8) can be linearized. Next, a linear operating point model can be obtained, as expressed in Eqs. (9) to (13):

$\begin{array}{cc}\Delta {\mathrm{MS}}_{\mathrm{max\_ref}}^{\mathrm{fb}}=-G_1\left(z^{-1}\right)\Delta {\mathrm{Wtz}}^{-2}& \left(9\right)\\ \Delta m_{\mathrm{inj}}=G_2\left(z^{-1}\right)\lfloor \Delta {\mathrm{MS}}_{\mathrm{maax\_ref}}^{\mathrm{fb}}+\Delta {\mathrm{MS}}_{\mathrm{max\_ref}}^{\mathrm{ff}}-\Delta {\mathrm{MS}}_{\max }z^{-1}\rfloor & \left(10\right)\\ \Delta {\mathrm{MS}}_{\mathrm{max\_ref}}^{\mathrm{did}}=-\frac{{\hat{b}}_1}{{\hat{a}}_1}\Delta \mathrm{Tm}-\frac{{\hat{c}}_1}{{\hat{a}}_1}\Delta \mathrm{Tw}& \left(11\right)\\ \Delta {\mathrm{MS}}_{\max }=K_{\mathrm{SM}}\Delta m_{\mathrm{inj}}+K_{\mathrm{SP}}\Delta \mathrm{Pp}& \left(12\right)\\ \Delta \mathrm{Wt}=a_1\Delta {\mathrm{MS}}_{\max }+b_1\Delta \mathrm{Tm}+c_1\Delta \mathrm{Tw}& \left(13\right)\end{array}$

where â₁,{circumflex over (b)}₁, and ĉ₁ are estimations of a₁,b₁, and c₁, respectively. The coefficient K_SP equals

$\frac{\partial f}{\partial \mathrm{Pp}}.$

From these equations, the state block diagram of the operating point model for closed-loop quality feedback control can be readily drawn in FIG. 2c, which has the same structure as FIG. 2b. The weight variation is related to the external inputs through

$\begin{array}{cc}\Delta \mathrm{Wt}\left(\frac{1}{a_1}+\frac{1}{a_1}K_{\mathrm{SM}}G_2z^{-1}+K_{\mathrm{SM}}G_2G_1z^{-2}\right)=K_{\mathrm{SP}}\Delta P_p+\left(1+K_{\mathrm{SM}}G_2z^{-1}\right)\left(\frac{b_1}{a_1}\Delta \mathrm{Tm}+\frac{c_1}{a_1}\Delta \mathrm{Tw}\right)K_{\mathrm{SM}}-K_{\mathrm{SM}}G_2\left(\frac{{\hat{b}}_1}{{\hat{a}}_1}\Delta \mathrm{Tm}+\frac{{\hat{c}}_1}{{\hat{a}}_1}\Delta \mathrm{Tw}\right)& \left(14\right)\end{array}$

To eliminate the temperature effects on weight variations, the mold separation controller G₂(z⁻¹) is preferably designed to be an integrator:

$\begin{array}{cc}{G}_2\left(z^{-1}\right)=\frac{1}{{\hat{K}}_{\mathrm{SM}}\left(1-z^{-1}\right)}& \left(15\right)\end{array}$

where {circumflex over (K)}_SM is the estimation of K_SM. In the ideal situation, the weight variation due to temperature changes can be eliminated entirely.

The weight controller G₁(z⁻¹) is designed to keep the system stable and have good dynamic performance. It preferably takes the form of a PI controller, as

$\begin{array}{cc}{G}_1\left(z^{-1}\right)=K_p+\frac{K_1}{1-z^{-1}}& \left(16\right)\end{array}$

where K_P and K_I are the proportional gain and integration gain, respectively. The characteristic equation of the system is

z ³−(1+ε)z²+[(a₁K_P+a₁K_I)(1−ε)+ε]z−(1−ε)a₁K_P=0   (17)

where

$\varepsilon =1-\frac{K_{\mathrm{SM}}}{{\hat{K}}_{\mathrm{SM}}}.$

The controller gains K_P and K_I are determined under the nominal condition ε=0. Note that there are only two design parameters in Eq. (17), but it has three characteristic roots. Thus, not all characteristic roots can be freely placed. There is one constraint in this pole-placement, namely,

z ₁ +z ₂ +z ₃=1   (18)

where z₁, z₂, and z₃ are the characteristic roots. It is still possible to put all three roots in stable positions (within the unit cycle on the complex z-plane). For instance, if the characteristic roots are 0.4, 0.3, and 0.3, the corresponding controller gains are

$\begin{array}{cc}\{\begin{array}{c}{K}_P=\frac{0.036}{a_1}\\ K_1=\frac{0.294}{a_1}\end{array}& \left(19\right)\end{array}$

When a₁ is not available, it can be replaced by its estimation, â₁.

With all controllers properly designed based on the process and quality models, the closed-loop quality control system performance, such as the dynamic stiffness and robust stability, can be readily analyzed.

Since the foregoing discussion and accompanying drawings merely relate to preferred versions of the invention, it should be understood that the invention may take other forms as well. As one example, since the injection molding system 100 is merely a simplified schematic depiction, the invention may be implemented in injection molding systems having radically different appearance from the one depicted. The measured parameters used for control (e.g., mold separation MS, holding pressure Pp, mold temperature Tw, and melt temperature Tm) can be measured by one or more sensors at locations other than those depicted. Further, the control relationships and physical process modeling relationships discussed above may be replaced with other appropriate arrangements.

The invention is not intended to be limited to the preferred versions of the invention described above, but rather is intended to be limited only by the claims set out below. Thus, the invention encompasses all different versions that fall literally or equivalently within the scope of these claims.

Claims

1. A multi-cycle injection molding process for a multi-section mold, the process comprising the steps of: a. injecting plastic into the mold during each cycle, wherein such injection: (1) occurs at a controlled velocity until a switchover point, and(2) occurs at a controlled pressure after the switchover point;b. monitoring the mold separation between the sections of the mold during each cycle; andc. controlling the switchover point from cycle to cycle to seek a desired mold separation.

2. The process of claim 1 further comprising the step of controlling a holding pressure of the plastic within the mold during each cycle after the switchover point to further seek the desired mold separation.

3. The process of claim 1 further comprising the steps of: a. monitoring at least one of: (1) the temperature of the plastic, and(2) the temperature of the mold;b. generating therefrom a mold separation command signal at least partially corresponding to the desired mold separation, the mold separation command signal being used to at least partially control the switchover point from cycle to cycle to seek a desired mold separation.

4. The process of claim 3 wherein the mold separation command signal is also generated from a reference part weight command signal corresponding to a desired weight of the plastic within the mold during each cycle.

5. The process of claim 3: a. further comprising the step of controlling a holding pressure of the plastic within the mold,b. wherein the mold separation command signal is also used to at least partially control the holding pressure.

6. The process of claim 3 further comprising the steps of: a. controlling a holding pressure of the plastic within the mold during each cycle after the switchover point to further seek the desired mold separation; andb. using the mold separation command signal to control the holding pressure from cycle to cycle to seek a desired mold separation.

7. The process of claim 1: a. wherein the weight of the plastic injected into the mold is measured at the end of each cycle, andb. further comprising the step of controlling the desired mold separation from cycle to cycle in response to: (1) the measured plastic weight, and(2) a desired plastic weight.

8. An injection molding system comprising: a. a multi-section mold, wherein the sections may be brought together to define a mold cavity;b. an injector configured to inject plastic into the mold cavity;c. a separation sensor measuring the mold separation between the sections of the mold;d. a switchover controller setting a switchover point, wherein the injector: (1) injects plastic into the mold cavity at a defined velocity until the switchover point, and(2) injects plastic into the mold cavity at a defined pressure after the switchover point;wherein the mold separation measured by the separation sensor defines a feedback signal used to modify the switchover point.

9. The injection molding system of claim 8: a. further comprising a holding controller setting a holding pressure of the plastic within the mold cavity, andb. wherein the mold separation measured by the separation sensor at least partially defines a mold separation command supplied to the holding controller.

10. The injection molding system of claim 8 further comprising: a. a mold temperature sensor generating a mold temperature signal representing the temperature of the mold;b. a melt temperature sensor generating a plastic temperature signal representing the temperature of the plastic;c. a compensator: (1) receiving the mold temperature signal and the plastic temperature signal, and(2) at least partially generating therefrom a mold separation command signal supplied to the switchover controller.

11. The injection molding system of claim 10: a. further comprising a part weight controller monitoring signals dependent on the weight of plastic within the mold;b. wherein the mold separation command signal is also at least partially generated by the part weight controller.

12. The injection molding system of claim 10 wherein: a. the mold separation command signal is also supplied to a holding controller, andb. the holding controller modifies the holding pressure of the plastic within the mold cavity in response to the mold separation command signal.

13. The injection molding system of claim 8: a. further comprising a weight sensor measuring the weight of the plastic injected into the mold; andb. wherein the plastic weight measured by the weight sensor defines a feedback signal used to modify the mold separation command signal.

14. A multi-cycle injection molding process for a multi-section mold, the process comprising the steps of: a. injecting plastic into the mold during each cycle, wherein such injection: (1) occurs at a controlled velocity until a switchover point, and(2) occurs at a controlled pressure after the switchover point;b. monitoring the mold separation between the sections of the mold during each cycle; andc. if the mold separation differs from a desired value, adapting both: (1) the switchover point, and(2) the holding pressure, seek the desired mold separation value.

15. The process of claim 14 wherein the switchover point is adapted between cycles to seek the desired mold separation value.

16. The process of claim 14 wherein the holding pressure is adapted during each cycle to seek the desired mold separation value.

17. The process of claim 14 wherein: a. the switchover point is adapted between cycles, andb. the holding pressure is adapted during each cycle, to seek the desired mold separation value.

18. The process of claim 14 further comprising the steps of: a. monitoring at least one of: (1) the temperature of the mold, and(2) the temperature of the plastic; andb. generating a mold separation command signal which is at least partially dependent on the temperature of the mold and the temperature of the plastic, the mold separation command signal being used to at least partially control the switchover point.

19. The process of claim 14 further comprising the steps of: a. monitoring at least one of: (1) the temperature of the mold, and(2) the temperature of the plastic; andb. generating a mold separation command signal which is at least partially dependent on the temperature of the mold and the temperature of the plastic, the mold separation command signal being used to at least partially control the holding pressure.