Multivariate shrinkage sensor (MVSS) for injection molding

A shrinkage detection device for a polymer injection molding apparatus detects a shrinkage experienced by an injection molded element for assessing a quality of the molded element. Shrinkage, along with temperature and pressure of the melt within the mold during cooling, indicates a sufficiency of the resulting molded element for intended purposes. Sufficiency includes parameters such as flexibility, shear strength and longevity, and is accurately performed can replace conventional sample testing of molded articles that are expensive and time consuming.

BACKGROUND

Injection molding is a common manufacturing approach for low-cost, high volume articles formed from a variety of polymers. A pelletized stock is typically distributed in bulk and delivered to molding machines for high pressure injection of molten feedstock into a specially prepared mold having the desired form. A screw or impeller typically forces the feedstock in conjunction with heat to generate the high pressure, fluid melt that fills even complex mold shapes.

SUMMARY

An injection molding multivariate sensor captures and computes shrinkage, temperature and pressure parameters associated with an injection molded article, and computes a validation based on the gathered parameters to indicate whether the molded article surpasses a minimal sufficiency. The gathered parameters include a displacement of a plunger or sliding pin responsive to in-mold shrinkage and measured by a magnetic medium in conjunction with pressure and temperature. An analysis application receives the parameters for computing molded article sufficiency at molding time rather than after post-molding inspections, providing time and cost advantages.

Configurations herein are based, in part, on the observation that injection molding provides an economic alternative to manufacturing of elements formerly performed by metal working, stamping and casting. Polymer pellet stock loaded into a hopper feeds a screw or threaded injector that agitates, forces and heats the pellet stock into a viscous molten form (melt). The screw also forces the viscous melt at substantial pressure into a cavity of a mold having the shape of the desired molded element. The forced melt fills the mold by flowing into the various voids and contours in the mold. Unfortunately, conventional approaches to injection molding suffer from the shortcoming that imperfections in molded articles can result from variations in viscosity, temperature and pressure of the molten polymer during injection into the cavity defining the mold.

As the molten polymer cools inside the cavity, shrinkage occurs. The magnitude and rate of shrinkage, along with the temperature and pressure, indicates a sufficiency of the molded article. Configurations herein substantially overcome conventional approaches to assessing the sufficiency of injection molded articles from an integrated shrinkage, pressure and temperature sensor that tracks these parameters during cooling and computes a sufficiency measure of the resulting molded article.

An in-mold shrinkage detection sensor device for injection molding includes a plunger or rod in communication with a cavity defined by a mold volume interior. The plunger has a distal end in communication with the cavity and a proximate end having an magnetic source. A digital position sensor adjacent to a travel path of the magnetic source is responsive to the magnetic source for detecting a movement of the plunger based on displacement of the distal end caused by shrinkage of the melt. Upon injection, the pressure of the melt forces the plunger out of the cavity, and as the melt cools, the volume contracts and draws the spring loaded plunger slightly back within the cavity. A typical range of travel may be on the order of 0.5 mm.

DETAILED DESCRIPTION

The description below presents an example of an injection molding environment using the multivariate shrinkage sensor (sensing element, hereinafter) in conjunction with an injection molding system for monitoring shrinkage and related parameters associated with a quality of the molded article, evaluating the quality based on the gathered parameters, and recommending whether the molded article is suitable for deployment based on quality control (QC) standards. A typical molding process generally encounters a small number of inferior moldings, however the shrinkage sensing along with related parameters for temperature and the molding pressure provide an immediate quality indication of the molded product. In contrast, conventional approaches analyze samples post-molding and incur additional time and expense to evaluate and confirm a quality batch of molded articles.

FIG.1is a context diagram of an injection molding production environment suitable for use with configurations herein. Injection molding is most economical when a single mold can be reused for many molded units. Accordingly, an injection molding environment10may employ a plurality of molding machines20. Each molding machine employs a hopper22for receiving a feedstock24of raw molding material, typically in a pellet form. A variety of polymers and plastics may be employed as the feedstock, and any suitable feedstock may be employed as described below. The hopper22feeds an injection tube30typically having a helical impeller32or spiral surface for directing the feedstock into a mold50. The injection tube typically employs heaters34for melting or fluidizing the feedstock24as it is forced into the mold50by the impeller32. A drive source26typically generates a substantial pressure of the feedstock as it travels along the injection tube30.

At the mold50, an injection interface52is in fluidic communication with the mold50, such that the injection interface52defines a high pressure input to the mold and is responsive to an injector54for receiving a melt of molten feedstock. The melt fills a cavity in the mold for forming a molded article defined by an interior contour of the mold50. The mold50may take any suitable form to correspond to the desired molded article, and is typically defined in at least two parts50-1. . .50-2such that a hydraulic actuator56may separate mold halves for ejection of the molded article.

FIGS.2A-2Dshows a simplified view of a molding sequence in the environment ofFIG.1using a multivariate shrinkage sensor element for performing quality sensing of an injection molding as defined herein. Referring toFIGS.1-2D, the sensor element100includes an elongated plunger110in communication with a cavity112defined by an interior volume of a mold. The elongated plunger110has a distal end in communication with the cavity112and a proximate end responsive to a biasing force. A magnetic source120attaches at or near the proximate end, and a position sensor122is responsive to the magnetic source120for detecting a movement of the plunger110based on displacement of the distal end that results from shrinkage of an injected melt130contained in the cavity112.

InFIG.2Athe melt130commences flowing through the injection interface52, which may be simply a high pressure passage from the injection tube30ofFIG.1for forcing the melt130into the mold. When filled inFIG.2B, the elongated plunger110is flush with the pressurized, injected melt112at a depth132(shown from the proximate end). As the melt112cools and cures, shrinkage occurs which causes contraction of the melt112as it tends to pull away from the mold walls, shown inFIG.2C. Cooling channels51also assist in temperature management. The elongated plunger110extends inward to the mold cavity to correspond to the shrinkage, as the molded depth134differs from the injection depth132based on the shrinkage133, shown inFIG.2D. The displacement of the elongated plunger computed from the difference between the injection depth and the molded depth defines the shrinkage133used for quality assessment of the cured, molded article130′ as discussed further below.

FIG.3is a schematic diagram of a control application operative with the multivariate shrinkage sensor (MVSS) ofFIGS.2A-2D. Referring toFIGS.1-3, in the production environment10, large quantities of molded articles150result from the mold50once the cured, molded article130′ is ejected from the mold50. These typically follow an evaluation and/or sorting process for validation as to whether each molded article150is sufficient. For example, a conveyor140may transport the molded articles150to a sorting apparatus142. The sorting apparatus employs a diverter or selector driven from an actuator146to divert the molded articles150to different bins144-1. . .144-2(144generally) for containing acceptable moldings150-1and unacceptable moldings150-2. Other suitable sorting and manufacturing apparatus may be employed. In contrast, conventional approaches typically store unsorted batches of the articles for subsequent testing an analysis.

Configurations herein employ signals300from the in-mold sensor element100for expedited or immediate evaluation. The sensor element100computes signals300by an analysis circuit320responsive to the position sensor100for receiving a displacement signal302. The displacement signal302is indicative of a distance traveled by the plunger110during an injection stage for filling the mold50and a cooling stage defined by a contraction of a molded material112in the filled mold.

The analysis circuit320may communicate with a molding quality application322having logic324for computing whether the corresponding molded article150is acceptable, and sends a validation signal310to the actuator146. The sensor element100may also include additional sensors for generating signals pertaining to a mold temperature304, a melt temperature306, and a molding pressure308, discussed further below inFIG.4.

The analysis circuit320may be encoded on a PCB (Printed Circuit Board) disposed adjacent to the proximate end of the elongated plunger for heat dissipation, such that the analysis circuit is integrated or electrically connected to the position sensor122for generating the displacement signal302. The analysis circuit may also receive sensor data for one or more of the pressure signal308indicative of a fluidic pressure of the injected melt, the melt temperature signal306indicative of a temperature of the injected melt and the mold temperature signal304indicative of a temperature of the mold50resulting from the injected melt112. An adjacent computing system321launches and executes the application322for generating the validation310.

The analysis circuit320couples to the molding quality application322which is configured to generate the validation signal310indicative of whether the molded article150resulting from the cooled melt is sufficient for use based on the displacement signal302and at least one of the pressure signal308, the melt temperature signal306and the mold temperature signal304. The molding quality application may also employ a graphical user interface352(GUI) visible on a rendering device350responsive to a user interface354such as a keyboard and mouse. The GUI352may render and receive control parameters356concerning the quality and control parameters of the molded article150, discussed further below. In general, the analysis circuit320is disposed on the sensor element100for receiving raw signal data such as voltage signals from the sensors, and the analysis application322is at a remote PC or user computing device for receiving the displacement, pressure, mold temperature and melt temperature, but any suitable mode of communication between the analysis circuit320and analysis application322may be envisioned.

A paramount consideration in the quality of the molded article is the finished dimensions of the molded part after shrinkage is concerned. Accordingly, the quality application is configured to compute a finished size of the molded article based on a shrinkage computed from the displacement signal. Shrinkage is determined from the displacement signal as the melt cools and contracts, but overall quality is also affected by a pressure, volume and temperature (PvT) relationship of the injected melt flowing through the cavities and geometry of the mold. In general, the pressure-volume-temperature relationship provides the post-mold shrinkage. post-mold shrinkage is the shrinkage occurs when ejected part cools down to the room temperature from the ejection temperature. Interior mold dimension, such as narrowness of channels through which the high-pressure melt is forced, all play a role. Accordingly, the quality application is further configured to compute the validation signal based on a pressure-volume-temperature (PvT) relation for correlating a viscosity and a rate of introduction (velocity) of the melt into the cavity. In analyzing the quality of the melt and molded article, viscosity and velocity provide the thorough process monitoring and control of the plastics part fabrication process. In this context, the fabrication process may be subject to other plastics manufacturing processes including injection molding.

FIG.4is a side cutaway view of the MVSS ofFIGS.2and3showing the signal generation in more detail. Referring toFIGS.1-4, the sensor element100further include a pressure sensor410embedded in the elongated plunger110, such that the elongated plunger110is in slidable communication with a housing400for advancing and retracting from the mold50based on a pressure exerted from the fluidic melt130in the cavity112.

The pressure sensor410is defined by a piezoelectric element disposed in a linear interference communication with the proximate113and distal111ends of the elongated plunger110. The piezoelectric element is disposed for receiving a compression force between the biasing force from a spring414and an opposed force415from an injected melt130. In the example arrangement, the biasing force414includes a spring disposed for advancing the elongated plunger110into the cavity112. The pressure exerted from the fluidic melt130acting against the biasing force induces a compression in the elongated plunger110. The piezoelectric element is flanked by insulating washers412, and generates the pressure signal based on a piezoelectric response based on the compression. Alternate configurations may include an alternate sensing medium such as a strain gauge, capacitive or bi-metallic-based displacement transducer medium.

A temperature sensor420is disposed in the distal end111of the elongated plunger110, such that the temperature sensor emits a melt temperature signal306based on a temperature of the injected melt112. The temperature sensor further includes an infrared lens422for passing radiated energy to the temperature sensor420, as the temperature sensor responsive to the radiated energy for generating the melt temperature signal306. For example, the melt temperature sensor420may include a thermopile424, spacer and a zinc selenide lens as the filter422.

The distal end111also includes a resistive sensor430such as a thermistor or thermocouple in the elongated plunger110for generating the mold temperature signal304based on a temperature of the mold as the injected melt112fills the cavity. Recall that the temperature of the melt112is a significant factor in flow and curing of the melt, and generally the heat in the melt transfers to the mold50as the melt112is injected and cools/cures. Tracking the melt temperature and mold temperature based on the respective signals306,304will be discussed further below inFIG.6F.

In a particular configuration, the thermopile424is disposed for receiving emitted infrared energy indicative of a temperature of the melt, such that the thermopile includes a thermistor for conductive sensing of a temperature of the mold. Alternatively, a thermocouple or other resistive based sensor may be employed for contact based sensing of either the melt or the mold.

Returning to the position sensor122, a Hall effect sensor may be employed for generating the displacement signal302as the magnet120, attached to the elongated plunger110, moves towards the mold50prior to injection, is forced back during injection, and finally moves again towards the mold50as the cooling melt112shrinks.

The sensor element100may be implemented in any suitable manner for providing the displacement302, mold temp.304, melt temp.306and pressure308signals, however a particular configuration is as follows. The pressure sensing410and in-mold shrinkage measurement122sensors are placed within the sensor housing400of 23 mm outer diameter. The sensor housing assembly is placed over the sensor base of 25 mm outer diameter with six countersunk screws to form the sensor body. The sensor base component provides the space for the wires and integrated electronics including the analysis circuit320. For in-mold shrinkage sensing, the position sensor122and magnet120are placed within the slot of the sensor housing, and PZT (Piezoelectric) housing components, respectively. The position sensor and magnet remain facing each other during the sensor operation, while the position sensor remains stationary during the operation. For a pressure sensing, the PZT ring and insulation washer assembly sit inside the PZT housing, where PZT housing sits over the compression spring. The PZT housing with PZT ring, washers, and magnet and compression spring stay within the sensor housing component with the position sensor. There is a clearance of 0.025 mm between the PZT housing OD and sensor housing ID for the smooth sliding of the PZT housing.

The temperature sensing system (thermopile and ZnSe window) is placed within the elongated plunger110defining the temperature sensor pin, which is inline with the sensor headpin as part of the entire elongated plunger110. Alternatively, other crystals, gems and/or crystalline stones may be employed as a window which can transmit the light in addition to ZnSe. For example, sapphire, ruby or topaz, may be employed, based on factors such as cost, durability or temperature compatibility.

The sensor headpin replicates the standard 6 mm ejector pin. The length of the sensor headpin can be customized depending on the mold height by changing the length of the pressure sensor pin.

The head of the sensor headpin remains in contact with the top surface of the PZT insulation washer, which will transfer the force to the PZT ring and compression spring. A sensor cover is installed over to the sensor housing using countersunk screws to encapsulate the head of the sensor headpin, PZT housing assembly, and partially compressed spring inside the sensor housing. The sensor housing400provides a mechanical stop for the PZT housing beyond 0.5 mm displacement, which in turn will control the sensor headpin travel. The selected compression spring will fully compress at a pressure higher than 5 MPa and regain its free length once the pressure decays below 5 MPa. Other suitable pressure and displacement thresholds may also be employed, and various lips and shelves may be provided to limit travel of the elongated plunger to a predetermined travel, such as 0.5 mm.

FIG.5shows the MVSS installed in a molding apparatus operable with the configurations ofFIGS.1-4. In the example molding apparatus, the mold50is filled by the injector54engaged with the injection interface52. The sensor element100occupies a cavity insert500which may also be employed for an ejection pin502used to eject the molded article150, and is a common fixture on a typical mold50. This conveniently disposes the sensor element100away from the cooling lines51, injection nozzles54and other hydraulics and actuated components of the mold50, so as not to interfere with the molding process.

Because of the ejector pin style of the sensor headpin, the mold50will employ a standard straight 6 mm hole within the B-side cavity plate150-1that will eliminate the risk of space constraints, complex mold construction, and failure during the sensor element100installation and maintenance. The sensor body will stay within the ejector system. Hence, the mold design and complexity are drastically reduced, especially for multicavity molds. Also, the mold design will still keep the flexibility of efficient cooling system design even for smaller parts and multi-cavity molds be leaving cooling passages51unimpeded. Preferably metal components are fabricated from 316L stainless steel, except pressure sensor pin and temperature sensor pin. These two pins are made from hardened steel (H13) with 55 HRC hardness so they can sustain continuous wear and tear for over a million molding cycles as well as protect the temperature sensing system. All steel components expect tight tolerances to ensure proper function and robust operation of the sensor in the high heat and pressure experienced by the melt130.

FIGS.6A-6Fshow the MVSS sensor (sensor element)100ofFIG.4used for controlling a molding operation. In polymer injection molding, the molding machine20melts solid plastics pellets into the hot melt130, and injects the hot melt into the mold cavities to fill the cavity112. As soon as the plastic melt enters the mold, it begins to cool down due to the mold cooling system and starts to solidify. Within the mold cavity, plastics material experiences the “in-mold shrinkage” during solidification. Because of the in-mold shrinkage, the part becomes smaller than the mold cavity. The most commonly used plastics materials in the molding application exhibit a shrinkage rate from 0.005 to 0.1 mm/mm, which is high relative to the dimensional tolerances of many molded parts.

Referring toFIGS.1-6F, the molding process for a molded article150commences with the sensor element100extending the elongated plunger110into the cavity122, as biased by the spring and ledges limiting travel to within a predetermined threshold, such as 0.5 mm, shown inFIG.6A. It should be noted that the elongated plunger110refers collectively to the slidable aggregation including the mold and melt temperature sensors, the piezoelectric pressure sensor and the magnetic source that all travel as a single unit as they are displaced in and out of the mold.

InFIG.6B, the melt130flows into the cavity in a high pressure, high temperature molten state as the elongated plunger110extends 0.5 mm from the distal end111. The mold50has a passage for insertion of the elongated plunger, either via an insert for the ejection pin or a dedicated port or opening. The elongated plunger110has a range of travel between a flush position and a fill position, such that the fill position is defined by the distal end111of the elongated plunger extending into the cavity112. The flush position is defined by the distal end of the elongated plunger flush with a surface of the cavity112, and the range of travel is based on a degree of shrinkage expected in a molded article. In the disclosed approach, this displacement distance is 0.5 mm, however any suitable predetermined range may be employed. The distal end of the elongated plunger defines a sensor headpin that will remain protruded until the polymer melt130comes in contact with the top surface of the ZnSe window (infrared filter422) at the distal end.

InFIG.6C, the melt130fills the void112and the pressure pushes the elongated plunger110from the fill position (extending 0.5 mm) back to a flush position with the mold cavity surface, being forced against the biasing element. Once the melt pressure acting on the sensor headpin reaches above 5 MPa, it will push the sensor headpin back while transferring the pressure force to the PZT ring410. Due to the mechanical stop on the sensor housing, the pin will stop moving after 0.5 mm displacement. At this time, the elongated plunger flushes with the mold cavity surface and continues to transfer the pressure to the PZT ring. Hence, the PZT ring will provide the output signals corresponding to the pressure acting on the sensor headpin. The Hall effect position sensor monitors and measures the sensor headpin travel and position. InFIG.6D, during the cooling stage of the molding cycle, the polymer melt130cools down and starts to separate away from the mold cavity walls due to the shrinkage. Thus, the pressure acting on the elongated plunger will continue to decay. The distal end of the elongated plunger110will remain flush with the mold cavity surface until the pressure acting on the sensor headpin reaches below 5 MPa.

At a pressure below 5 Mpa, the sensor headpin will begin to move upward due to the spring force, depicted inFIG.6E. The Hall-effect position sensor monitors and measures the sensor headpin's upward motion in the displacement signal302. At the end of the molding cycle inFIG.6E, the mold50opens and ejects the molded article150out of the mold50. After the mold opening, there is no force acting on the sensor headpin. Hence the sensor headpin will move forward to its start position and protrude 0.5 mm inside the cavity. The difference between the position sensor's output from flush inFIG.6Cto advanced inFIG.6Dindicates the magnitude of the “in-mold shrinkage”133the polymer experienced during the molding cycle. Throughout the molding cycle, the thermopile within the temperature sensing system continuously monitors and measures the melt temperature and mold temperature, shown inFIG.6F. The GUI352displays these control parameters as graphs600along a time axis. A shrinkage graph610shows movement of the elongated plunger at the respective stages ofFIGS.6A-6E. The cavity pressure graph620shows that after peaking during injection, drops along with the shrinkage. Melt and mold temperatures630,640respectively, decline along with temperature and pressure. The melt and mold temperatures can be used to estimate the post-mold shrinkage using a coefficient of linear thermal expansion (CLTE) of the polymer or advanced pressure-volume-temperature (PvT) relationship.

In the example configuration, the raw sensory data is generally based on an electrical voltage or current emanating from the respective sensory elements. In general, output voltage responses are obtained for the position sensor, melt pressure, melt temperature, and mold temperatures. The voltage responses are converted into the absolute in-mold shrinkage, pressure, and temperatures.

The Hall effect position sensor may be, in an example configuration, sourced from Melexis Technologies NV (MLX90364) and gives the voltage corresponding to its position against the Neodymium magnet (square share 3.2 mm, 1.6 mm thick). The position sensor is calibrated for the maximum allowable sensor pin travel of (0.5 mm). The selected position sensor can give analog and digital signals, but for the implemented MVSS, analog signals were used and converted into an absolute position reflected by the displacement signal302. As mentioned earlier, the difference between the position sensor signal at melt contact with a full mold and the end of the cooling provides the in-mold shrinkage133.

In the analysis circuit320, for the pressure signal308, as polymer melt flows across the MVSS sensor headpin, as shown inFIG.4, the pressure exerted on the lens, will be transferred onto the PZT, which will cause the accumulation of charge. The voltage response from the PZT ring412, VPZT, is described in the following equation:

VP⁢Z⁢T=4*g3⁢3*P*H*R2O⁢D2-ID2
where g33 is the voltage constant determined by the PZT material, H is the ring thickness of 1 mm, ID is the ring inner diameter of 6 mm, OD is the ring outer diameter of 10 mm, and R is the temperature senor pin radius of 3.0 mm. For the design shown above, with H equal to 1 mm and a voltage constant, g33 of to 24.8×10−3 Vm/N for APC-850 material, the voltage response will be 14 V/MPa of melt pressure. The sensor pin remains flushed to the surface of the cavity until the cavity melt pressure decays to 5 MPa that prevents any loss or abnormalities in the pressure reading that would occur from a gap between the cavity and sensor.

As the polymer melt flows across the sensor window, the melt temperature signal306is derived as infrared radiation (IR) passes through the zinc selenide (ZnSe) window and is collected by the thermopile (TP). The voltage response of the TP, VTP is described by the equation:

Tmelt⁢=(VT⁢Pk*ɛ+Tm⁢o⁢l⁢dn)1/n
where k is the gain, c is the emissivity of the polymer, T melt, and Tmold are the temperatures of the melt and the mold, respectively, and n is dependent on the filter and sensor characteristics (equal to 4 for a perfect “black” body and unlimited wavelength range).

For computing the mold temperature signal304, the thermopile contains a thermistor to assess the reference temperature of the CMOS IR detector, which must be known to compute the net radiative heat transfer to the thermopile. A 100 kOhm thermistor resistance is supplied from the manufacturer as a function of temperature to within 0.2% absolute error. A voltage divider circuit converts the thermistor's output resistance to a voltage. The value of the reference resistor (10 kOhm) was selected to scale the output voltage to the desired range while also linearizing the thermistor output within the mold coolant temperatures of interest, from 25° C. to 100° C.

In operation, a system according toFIG.3employs the application322and logic324for receiving the signals302,304,306and308from the analysis circuit320for performing s method of validating an injection molded article150resulting from injection of a melt130into a mold50. Validating includes extending the elongated plunger110into a cavity112defined by an interior volume of the mold50, in which the elongated plunger110has a distal end111in communication with the cavity and a proximate end113responsive to a biasing force. An injector54injecting a melt defined by a molding substance into the cavity via a fluidic interface52, which retracts the elongated plunger back from the cavity responsive to a pressure of the injected melt, such that the pressure of the injected melt is detected by a pressure sensor on the elongated plunger. An application measures the detected pressure from the pressure sensor when the elongated plunger retracts such that the distal end is flush with an interior surface of the mold, and measures a displacement distance based on the position signal when the detected pressure abates for indicating a cooled melt in the cavity and a corresponding movement or displacement of the elongated plunger back into the cavity.