Chapter 4

Reference Manual

Process Modeling

Process control of injection molding has a direct impact on the final part quality and the economics of the process. The various components of process control must be fully understood to maximize profit and part quality. Process control of injection molding can be classified into three major components:

Sequential process control
Machine control of the molding press
Control of the auxiliary equipment.

The sequential process control of an injection molding machine can range from a simple relay on/off control to an extremely sophisticated micro-processor-based, closed-loop control. The control for molding machines includes variables such as barrel temperature, hydraulic pressure, and clamp force. Auxiliary equipment includes the coolant controller and hot-runner temperature controller.

Typical hydraulic and cavity pressure variations over the entire injection molding cycle are shown in Figure 4-1.

Figure 4-1. Typical injection molding cycle with (1) filling, (2) post-filling, (3) mold-opening, and (4) holding stages.

To model the injection molding process realistically and accurately, C-MOLD adopts the "natural language" used on the shop floor to describe the various components of the process control. This chapter discusses the concepts and the terminology for modeling the various processes simulated by the C-MOLD analysis programs. The process conditions inputs for C-MOLD analyses are specified using the Control Panel, and are saved in filename.prc.

Sequential Process Control

Injection molding is a process by which hot polymer melt is forced into an empty, cold cavity of a desired shape and is allowed to solidify under high holding pressure. The entire injection molding cycle can be divided into three stages: filling, post-filling and mold-opening, as shown in the sequential process control cycle clock in Figure 4-2. The total cycle time is the sum of fill, post-fill, and mold-open times.

Figure 4-2. Sequential process control cycle clock.

Sequential process control determines the duration of each stage and the conditions for switching over from one stage to another. The following sections describe in detail each component of the sequential process control.

Fill time

At the beginning of the injection molding cycle (point 1 in Figure 4-3), the mold has just closed and the molten polymer, which is maintained at a fairly uniform temperature inside the barrel of the injection machine, is forced to flow through the nozzle, runner, gate, and then into the cavity under controlled flow rate or pressure, depending on the control scheme of the injection unit. The fill time is defined as the time needed for the polymer to fill the entire cavity (duration between points 1 and 4 in Figure 4-3).

Figure 4-3. Filling stage
Although the fill time is usually short and has little affect on the overall cycle time, the correct injection rate (or fill time) is very important in controlling the rate of pressure rise in the runner, gates, and part cavity, thereby helping to assure proper filling, good appearance, part strength, and dimensional tolerances. A reasonable estimation of fill time can be derived from the required cooling time of the part, which can be approximately calculated as follows:
(4.1)
(4.2)
For example, a part with nominal thickness of 2 mm (0.002 m) was made of PC with the following material properties:

The fill time should be much less than the estimated cooling time to avoid premature solidification of the material during the filling stage. As a rule-of-thumb, one tenth to one fifth of the estimated cooling time is a good estimate for the fill time. Note that the fill time might be shorter than the actual ram-forward time in the process due to the decompression of polymer melt in the barrel before injection.
The actual fill time predicted by the analysis equals the specified fill time only if the entire filling stage is under ram-speed-profile control. If the filling mode is switched to pack/hold pressure control before the end of filling, the process is then controlled by the specified pack/hold pressure at the entrances (duration between point 3 and point 4 in Figure 4-3). In other words, the injection rate varies and the actual fill time depends on the specified pack/hold pressure level. Normally, the higher the pack/hold pressure used, the shorter the actual fill time.
If the injection pressure required to fill the cavity is plotted against the fill time, as shown in Figure 4-4, a U-shaped process curve will typically result with the minimum value of the injection pressure required to fill the cavity occurring at the intermediate fill time. This occurs because a short fill time means the flow rate (shear rate) is high and thus requires higher injection pressure to fill the mold. On the other hand, the injected polymer cools more with increasing fill time, resulting in higher viscosity and thus requiring higher injection pressure to fill the mold. The shape of the injection pressure versus fill time curve is highly dependent upon the material used in the process and on the mold design. Typically, determination of this curve requires numerous molding trials after construction of a prototype tool. With the aid of C-MOLD simulations, process designers can establish this curve on the computer before the mold is cut.

Figure 4-4. U-shaped process curve.

Ram-speed profile (rel) or (abs)
During the filling stage (from point 1 to point 3 in Figure 4-3), the polymer melt is plasticated, compressed in the barrel and then forced through the nozzle by the ram motion. The volumetric flow rate of the melt is approximately proportional to the linear velocity of the ram, because the barrel diameter is constant and polymer melt is a weakly compressible fluid. That is, the change in polymer density is much smaller than the absolute value of the polymer density. The ram velocity is determined by the dynamics of the injection system. An injection molding machine equipped with a modern, closed-loop process controller will control the injection rate of the polymer more accurately than a machine with open-loop control. Most modern process controllers inject the polymer melt at a fairly constant rate. More sophisticated process controllers inject the polymer melt at a variable rate specified by the process engineer, as shown in Figure 4-5. On the other hand, a larger injection machine has greater inertia and thus responds slower to the velocity control: the machine might take longer to reach a constant speed or to stop the ram motion.
A proper ram-speed profile setting can improve surface finish. For example, variable injection rate control can sometimes be used to reduce jetting problems by injecting the polymer melt more slowly when the melt front reaches the gate.

Figure 4-5. Ram speed profile.
C-MOLD analysis allows either constant or variable injection rate to be specified. In precision molding where variable injection rate is critical in the process, users can specify that the ram-speed profile used by the analysis is the same as the controller settings. The ram-speed profile requires two entries as input information: the percent of stroke and the percent of speed. Note that the input percentage setting should be between 0 and 100. The first and last setting of the percent stroke must be 0 and 100, respectively. The ram speed between two settings is linearly interpolated. Users should add (or delete) stroke settings, so that the number of settings is consistent with the process controller on the injection machine. The default ram-speed profile is constant.
The ram-speed profile (rel) is relative to the specified fill time. The injection flow rate used in the analysis is pro-rated by the total volume to be filled to achieve the specified fill time. The percent stroke (or time) is pro-rated based on the part volume. The actual injection flow rate, however, is also constrained by the maximum machine injection rate of the molding press. On the other hand, the injection rate of the ram-speed profile (abs) is relative to the maximum machine injection rate. In this case, the fill time is not specified by the user, but rather determined by the volume to be filled, the absolute ram-speed profile and the maximum injection rate. The percent stroke (or time) is also pro-rated based on the part volume.
There are various requirements and considerations for an optimum injection speed profile. A common rule-of-thumb is to maintain a constant melt-front velocity in the cavity, which results in a more uniform stress distribution on the part. Based upon criterion of maintaining a constant melt-front velocity (MFV), a recommended ram-speed profile is provided by C-MOLD Filling EZ.
For most processes, averaged ram speed is often used in the analysis because of the difficulties in measuring and controlling the ram speed, as shown in Figure 4-6. This constant speed approximation affects the predicted pressure field, since the pressure gradient is proportional to the velocity (or shear rate) of the flow. Typically, the average ram speed is underestimated in the early half of the filling process, so the predicted pressure gradient is smaller. On the other hand, the average ram speed is over-estimated in the latter half of the filling, so the pressure gradient at the end of filling is over-predicted. This is why the analysis often gives a pressure spike at the end of filling if a constant ram speed is specified.

Figure 4-6. Effect of an average, constant injection rate on the pressure prediction.

Timer for valve gate
During the filling stage, timers are used to control the opening and closing of valve gates (shut-off gates) at different times (see point 2 for valve gate opening in Figure 4-3). A connector element in the finite-element mesh can have a valve-gate timer ID which points to a timer for valve-gate opening and another timer, if applicable, for closing. The timer for valve-gate consists of a timer ID and two time instants for opening and closing, respectively. This is an extended version of the timer for valve-gate opening available in previous releases, which only consists of a timer ID and a time for opening (i.e., the valve gate remains open for the rest of the process). The timers for valve-gate opening and closing begin from the beginning of the filling stage. For example, the timer for valve-gate opening begins from point 1 to point 2 in Figure 4-3. Valve gates can be opened and closed simultaneously or sequentially, depending on the timer settings. With a proper design of the sequence of valve-gate opening (and closing, if applicable), weld lines can be eliminated on a multi-gated cavity by opening only one gate at the beginning, and then opening the other valve gates just after the melt front reaches them, as shown in Figure 4-7. The timers can be set from time = 0 to the fill time. Users should add (or delete) timers for valve-gate opening and closing as needed in the mold design.

Figure 4-7. Timer for valve-gate.

Timer for core or gas injection
During the filling stage (point 2 in Figure 4-3), a timer is used to initiate the injection of core polymer or gas in the sequential co-injection molding process or gas-assisted injection molding process. In either case, the timer for core or gas injection must be specified to simulate these processes. This timer triggers the core polymer or gas to be injected into the mold, as shown in Figure 4-8. The time for core or gas injection starts from the beginning of the filling stage (from point 1 to point 2 in Figure 4-3).

Figure 4-8. Timer for core or gas injection.
In sequential co-injection molding, the timer can be set to be any value between zero (but not zero which corresponds to complete filling of core polymer) and the fill time (but not the fill time that corresponds to complete filling of skin polymer). This is because of the apparent reason that complete filling of either skin polymer or core polymer can be handled by C-MOLD Filling. If constant ram speed is used, the percent volume of the skin polymer equals the timer setting for core injection divided by the fill time. The theoretical maximum of core polymer in a part, without appearing on the surface, is about 70% by volume. However, the actual value that can be accomplished in real life is much lower. In other words, if the timer setting for core injection is less than 40 to 50% of the fill time, core material will most likely appear on the surface of the part. C-MOLD Co-Injection can be executed only if the material properties for core polymer and the timer for core injection are specified.
In gas-assisted injection molding, the timer can be set between zero and the fill time (complete filling of skin polymer). If constant ram speed is used, the percent volume of polymer equals the timer setting for gas injection divided by the fill time. Note that a timer setting of zero is meaningless. If the timer setting for gas injection is too small, the gas will blow through the melt front resulting in a short shot. On the other hand, if the timer setting for gas injection is too close to the fill time, the material-saving advantage of this process will not be realized. However, the gas would still be able to penetrate into the thick-sectioned gas channels due to volumetric shrinkage of the polymer. In the actual gas-assisted injection molding process, the determination of this time is critical. With the help of C-MOLD Gas-Assisted Injection Molding, the timer for gas injection can be determined in advance, without physically cutting tools and performing molding trials.

F/P switch over by percent volume, injection pressure, or cavity pressure
Near the end of the filling stage (before the cavity is completely filled, at point 3 in Figure 4-3), the process control scheme switches over to pack/hold pressure control. The reasons for this are to avoid a pressure spike at the end of filling, which could cause the mold to open and flash the part; and to avoid the high impact of the ram at the end of filling, which could damage the injection machine and the mold. The filling to post-filling (F/P) switch-over can be triggered by one of the F/P switch-over conditions shown in Figure 4-9: F/P switch-over by percent volume, F/P switch-over by injection pressure, or F/P switch-over by cavity pressure, whichever comes first.

Figure 4-9. F/P switch-over conditions.
After the F/P switch-over, if the pack/hold pressure profile is not specified, the same entrance pressure at the switch-over point is maintained for the rest of the filling stage, until the cavity is completely filled (between points 3 and 4 in Figure 4-3). If the pack/hold pressure profile is specified, the analysis will use this pressure profile at the entrance after the F/P switch-over occurs. Early switch-over could cause an insufficient entrance pressure to fill the cavity, which would result in a short shot.
With F/P switch-over by percent volume, the process switches from flow-rate (ram-speed profile) control to pack/hold pressure control when the percentage of the total volume filled exceeds the specified value. Note that the percent of filled volume is the same as the percent of total stroke setting on an injection machine. The default value of F/P switch-over by percent volume is 99%. Users can specify 0-100% for this transition.
With F/P switch-over by injection pressure, the process switches from flow-rate (ram-speed-profile) control to pack/hold pressure control when the entrance pressure exceeds the specified value. With F/P switch over by cavity pressure, the process switches from flow-rate (ram-speed-profile) control to pack/hold pressure control when the cavity pressure at a given node exceeds the specified value. The nodal number corresponds to the actual pressure transducer location in the finite-element mesh file (filename.fem).
In some injection molding machines, the filling stage is controlled by the injection pressure rather than the flow rate. In this case, users can specify zero percent F/P switch-over by volume and the pack/hold pressure profile to simulate this process. In such cases, the analysis will take the pack/hold pressure profile as the entrance pressure during the filling stage, as well as during the packing and holding stages. The actual fill time is then determined by the pack/hold pressure profile.

Post-fill time
After the mold is completely filled (point 4 in Figure 4-10), more material is packed into the cavity and the polymer continues to cool. The post-filling stage ends when the polymer temperature is sufficiently low and the part is rigid enough to be removed from the cavity without significant deformation (point 7 in Figure 4-10). The post-fill time is defined as the time between the moment when the cavity is completely filled and the instant when the mold opens (duration between points 4 and 7 in Figure 4-10).

Figure 4-10. Post-filling stage.
To shorten the cycle time, the mold should open as soon as the ejection criterion is reached throughout the part (point 6 in Figure 4-10). Two post-filling to mold-opening (P/O) switch-over conditions-ejection temperature for thermoplastics and ejection conversion for thermosets-are provided to determine the instant when a part can be ejected. In practice, none of the modern process controllers can detect the P/O switch-over point automatically. The mold-opening action is still triggered by the post-fill time, which is typically determined by trial-and-error in a molding trial. However, in CAE analysis, users can specify the ejection criterion and let the simulation estimate the minimum required post-fill time to cool thermoplastics or to cure thermosets in the process.
The post-filling stage starts with packing and is followed by a holding pressure from the injection ram. However, the polymer experiences cooling throughout the entire post-filling stage. In general, more than three-quarters of the total cycle time of the injection-molding process is associated with the post-filling stage. Note that the cooling time of the part is approximately proportional to the square of the thickness. In other words, if the part thickness is doubled, the cooling time will increase by about a factor of four. The post-fill time should be adjusted according to the part thickness and the thermal properties of the material (see Equation 4.1).

Timer for hold pressure
During the earlier phase of the post-filling stage (between points 4 and 5 in Figure 4-10), more material is packed into the cavity under high holding pressure to compensate for the increased density of the polymer melt due to increased pressure or decreased temperature. The timer that controls the removal of holding pressure marks the end of the holding phase in the post-filling stage. Note that the timer for the hold pressure begins when the F/P switch-over occurs (point 3 in Figure 4-10) and not from the end of the filling stage (point 4 in Figure 4-10).
This phase of the post-filling stage is extremely important when molding with semi-crystalline material. Parts of maximum strength and toughness can be obtained only by maintaining a proper holding pressure until the gates solidify. After the gate freezes, further application of the holding pressure will increase only the mass of the runner system and will not influence the part. It is therefore critical to set a proper duration for the hold pressure. This portion of the cycle is absolutely necessary to provide good surface finish and prevent shrinkage, voids, or weak spots around the gate, which can cause part failure. The dimensions of the gate can be adjusted according to the part thickness to assure proper packing during the holding stage.

Pack/hold pressure profile (abs) or (rel)
The packing pressure level is important to prevent flashing and tool damage. The holding pressure can assure good surface finish and prevent shrinkage or voids. In practice, the pack/hold pressure is typically 80% of the injection pressure. As shown in Figure 4-11, advanced process control can be used to manipulate the pack/hold pressure profile to achieve a low clamp force requirement and still maintain good part quality.

Figure 4-11. Influence of pack/hold pressure profile on clamp force requirement.
The duration of the pack/hold pressure profile is defined by the timer for hold pressure. The pack/hold pressure profile requires two entries for input information: the percent of time and the percent of pressure. Note that the input percentage setting should be between 0 and 100. The first and last setting of the percent time must be 0 and 100, respectively. Users should add (or delete) time settings, so that the number of settings is consistent with the process controller on the particular injection machine. The default pack/hold pressure profile is constant.
The pressure setting of the pack/hold pressure profile (abs) is pro-rated with respect to the maximum machine injection pressure. On the other hand, the pressure level of the pack/hold pressure profile (rel) is pro-rated with respect to the entrance pressure at the instant of F/P switch-over.
A schematic diagram of the pack/hold pressure profile is shown in Figure 4-12. Note that if the machine hydraulic response time is longer than the time interval between settings, a response time equal to half the time interval between the setting is used in the pack/hold pressure profile.

Figure 4-12. Pack/hold pressure profile (abs).

P/O switch over by ejection temperature or conversion
In practice, the process controller cannot detect when the temperature (or conversion, in the case of reactive materials) of a part is sufficiently low (or high) so the part can be ejected without significant deformation. The post-fill time is typically determined by trial-and-error during a molding trial. However, this post-fill time can be determined by the CAE analysis on the computer by providing post-filling to mold-opening (P/O) switch-over conditions (point 6 in Figure 4-10). The P/O switch-over conditions can be based on when a particular ejection temperature (in the case of thermoplastics) or ejection conversion (in the case of reactive materials) is reached throughout the part. In the analysis, the mold opening can be triggered by either the post-fill time or the P/O switch-over conditions, whichever comes first.

Mold open time
The mold-opening stage begins when the mold is opened (point 7 in Figure 4-13) and ends when the mold is closed (point 8 in Figure 4-13) to start the next cycle. The mold-open time includes the time taken for mold opening and closing actions as well as part ejection (duration between points 7 and 8 in Figure 4-13). Because this can be a significant portion of the cycle time in processes with extremely short cycles, each action of the mold clamp and ejection systems should be analyzed for possible time delays and wasted energy. During this stage, additional heat transfer occurs between the mold and ambient air.

Figure 4-13. Mold-opening stage.

Machine Control
The control of the injection molding process includes the barrel temperature setting and five machine parameters. The molding machine parameters, shown in Figure 4-14, include maximum machine clamp force, maximum machine injection volume, maximum machine injection pressure, maximum machine injection rate, and machine hydraulic response time. These parameters may be tailored for the specific machine used in the process. If one or more of these maximum machine capability constraints are reached in the simulation, the program provides a warning message or adjusts the calculation so these maximum machine capability constraints are not exceeded. The default values correspond to the reasonable maximum physical values of the molding machines currently available. These values can be modified using the Control Panel according to the machine parameters actually used.

Figure 4-14. Machine capability constraints.

Inlet melt temperature
A constant inlet melt temperature is assumed at all entrances. In the actual process, however, inlet melt temperature might be higher than the barrel temperature setting on the injection molding machine, due to viscous heating effects in the nozzle. The temperature could rise as much as 30 °°C, depending on the injection speed and the material properties. There are two ways to compensate for this nozzle temperature rise: provide an air-shot temperature measurement instead of the barrel temperature setting, with the measurement being performed at the same barrel temperature and injection speed as the actual process; or provide a barrel temperature, but include the nozzle in the finite element mesh (modeled as hot runners) to account for viscous heating in the predictions.

Inlet melt conversion
In the injection molding or transfer molding of reactive materials, it is not a common practice to initiate the reaction process in the injection molding barrel (transfer molding pot). Typically, the barrel (pot) temperature is set low enough so that no significant reaction will occur in the barrel (pot). In other applications, such as injection molding of rubber molding compound, the material exhibits an induction period before curing starts. In all, the inlet melt conversion of the material is typically small prior to injection.
A constant inlet melt conversion is assumed at all entrances in the analysis. A value between 0 and material gelation conversion (or between -1 and gelation conversion, if the material has an induction period) can be specified in the analysis. As illustrated in Figure 4-15, the inlet melt initial conversion has been unified in such a way that values of -1, 0, and 1 correspond to the beginning of the induction period, the beginning of conversion, and the end of conversion, respectively.

Figure 4-15. Definition of inlet melt conversion.
One way to estimate the initial conversion is to quench a small material sample being injected through the nozzle or plunger and then perform a DSC experiment to measure the residual conversion. Another way is to estimate the initial conversion by the given material constants for isothermal induction time or isothermal curing kinetics, and the actual temperature and time history of material before injection.

Gas injection control option
The gas-assisted injection molding process consists of a partial or nearly full injection of polymer melt, as in conventional injection molding, followed by an injection of compressed gas. A typical machine process cycle of gas-assisted injection molding is shown in Figure 4-16. In most of the gas-assisted injection molding processes, the gas is injected into the core of the melt via the nozzle, sprue, runner, or directly into the cavity. The compressed gas takes the path of least resistance, flowing toward the melt front through the thickest sections of the molded part. Molten polymer is displaced and pushed ahead by the advancing gas to fill and pack out the cavity. In the so-called gas-pressure control process, the compressed gas is injected with a regulated gas-pressure profile (constant or step). In the so-called gas-volume control process, gas is initially metered into a compression cylinder at preset volume and pressure; then it is injected under pressure generated from reducing the gas volume by movement of the plunger.

Figure 4-16. The machine cycle for the gas-assisted injection molding process.
C-MOLD Gas-Assisted Injection Molding provides three options to simulate the various gas-assisted injection molding processes. This is controlled by the value of the gas injection control option. In particular, the first option is designed to simulate the processes that employ a gas-pressure-control technique. For this option, the analysis takes a pre-specified gas-pressure profile as the input and computes the resulting polymer flow rate during the filling and post-filling stages. A step gas-pressure profile is plotted in Figure 4-17(a). The second option deals with processes using a gas-volume-control technique. For this type of process, the analysis computes the gas pressure based on the changing gas volume within the compression cylinder and cavity, as well as the plunger compression-speed profile. A typical pressure trace at the polymer and gas entrance is plotted in Figure 4-17(b). The third option from the analysis automatically predicts an ideally profiled gas pressure so that a pre-defined polymer-melt flow rate will be maintained. This option will be discussed in more detail in the following section.

Figure 4-17. The entrance pressure traces for (a) the gas-pressure control processes and (b) the gas-volume control processes.

Automatic gas-pressure profiling control
The purpose of automatic gas-pressure profiling is to provide a range of reasonable gas pressures that delivers a desirable melt velocity. Recall that too much gas pressure can result in high melt velocity causing material degradation, whereas too little gas pressure might induce hesitation marks on the part surface or might even lead to a short shot.
In practice, it is difficult to vary the gas pressure during the short gas injection stage. However, in C-MOLD Gas-Assisted Injection Molding it is possible to regulate the entrance gas pressure so the resulting flow rate at the melt front will be the same as that controlled by the relative ram-speed profile in a conventional injection process. The gas pressure becomes one of the output variables that ensures a flow rate based on the specified ram-speed profile. Given the profiled gas pressure prediction at the gas entrance(s), users can select a reasonable constant or profiled gas pressure, or proper gas-volume control parameters that resembles or produces the optimal gas-pressure profile. It is therefore recommended that the analysis begin with the automatic gas-pressure profiling option.
The following figures illustrate how to select a proper, practical gas based on the results from the automatic gas-pressure profiling option, as well as the relationship among various molding variables. Figure 4-18 shows some schematic plots for a typical, conventional injection molding process. If a constant ram speed is used throughout the filling process, as in Figure 4-18(a), then the mold fill percentage versus time will be a straight line, as in Figure 4-18(b). In general, the polymer entrance pressure typically increases monotonically with the amount of polymer melt injected into the cavity, as plotted in Figure 4-18(c).

Figure 4-18. Conventional injection molding process with constant injection speed.
Assume now that the gas-assisted injection molding process is used to produce the same part. For simplicity, assume the tool design remains the same, and the gas is injected through the same polymer entrance using the gas-pressure control technique. Ideally, the optimum gas pressure should be able to fill the mold cavity in a controllable way, similar to using the ram-speed control in a conventional injection molding process, as shown in Figure 4-18. In this regard, a desired injection speed profile can be specified: a constant speed in this example, as shown in Figure 4-19(a). The automatic gas-pressure profiling option in C-MOLD Gas-Assisted Injection Molding determines the required gas pressure based on that injection speed. In other words, the resulting mold filling percentage versus time during gas injection will remain a straight line, as shown in Figure 4-19(b).
Since the specified injection speed profile is maintained in the calculation, the predicted fill time will be the same as the input fill time given by the user. The major difference between Figures 4-18 and 4-19 is the entrance pressure. As seen in Figure 4-19(c), the calculated gas pressure at the entrance decreases with time: as the gas advances toward the melt front, it reduces the effective flow length. As a result, the required pressure drop (or, equivalently, the gas pressure drop) decreases to advance the polymer melt based on the specified injection speed. In addition, the gas pressure at the timer for gas injection will be the same as the polymer pressure at the entrance immediately before the switch-over, if the same flow rate is maintained.

Figure 4-19. Gas-assisted injection molding process with profiled gas pressure control.
Having obtained the profiled gas pressure that provides a desirable injection speed, the user can determine a constant gas pressure and repeat the simulation with the gas-pressure control option. The determination of the constant gas pressure depends on many factors related to individual applications. As an example, assume that the average value from the profiled gas pressure repeated in Figure 4-20(a) is used, as illustrated in Figure 4-20(b). Remember that the profiled gas pressure is based on a constant injection-speed profile in this example, which corresponds to the constant volumetric flow rate shown in Figure 4-20(c) prior to the switch-over. If one begins with the average gas pressure, it is clear that there will be a drop in flow rate at the melt front right after the switch-over, as shown in Figure 4-20(c). However, as the gas continues advancing toward the melt front, the pressure gradient (a ratio of total pressure drop to the effective flow length) increases and could eventually accelerate the polymer melt. Consequently, the predicted total filling time (polymer injection time plus gas injection time) will depend on the gas pressure used as well as on the geometry involved. The actual fill time might not necessarily follow the specified fill time. In the constant gas pressure calculation, the specified fill time is only used as a reference value to determine the polymer/gas volume ratio. As a rule of thumb, the higher the gas pressure, the shorter the fill time.

Figure 4-20. Gas-assisted injection molding process with constant gas pressure.

Maximum machine clamp force
(default = 4.905E+07 N)
This is the maximum clamp tonnage provided by the injection machine manufacturer. In most modern molding presses, the clamp tonnage can be adjusted by machine hydraulic pressure.
The analysis calculates the required clamp force by multiplying cavity pressure and the projected cavity surface on the parting plane. A warning message is given if the calculated clamp force exceeds the machine capacity.

Maximum machine injection volume
(default = 0.02 m3)
This is the theoretical injection volume provided by the injection machine manufacturer. This value is equal to the maximum machine injection stroke multiplied by the cross-sectional area of the barrel. The actual total volume to be filled can be only 80% of the theoretical injection volume, assuming 20% polymer shrinkage. The analysis gives a warning message if the total volume is more than the maximum machine injection volume.

Maximum machine injection pressure
(default = 1.8E+08 Pa)
The maximum machine injection pressure is equal to the maximum hydraulic pressure available in the injection machine, multiplied by the ratio of the cross-sectional area of hydraulic piston to the cross-sectional area of the barrel, less any friction losses and nozzle pressure drops. A typical injection molding machine has an amplification factor of 7-10 and the friction losses can be as much as 10-20%.
The analysis gives a warning message if the calculated injection pressure exceeds this value during filling stage, and the analysis switches from flow-rate control to pressure control. This value is then used as the entrance pressure until the end of the filling stage. The flow rate typically drops exponentially when the maximum machine injection pressure is reached. The analysis gives a short shot warning message when the actual flow rate falls below 1% of the minimum specified flow rate before the cavity is completely filled. The maximum machine injection pressure is also used to determine the pack/hold pressure profile (abs) during the holding stages.

Figure 4-21. Short shot

Maximum machine injection rate
(default = 6.667E-03 m3/s)
The maximum injection rate is a machine's limit of how fast the polymer can be injected into the mold. The analysis checks at every time step if the flow rate exceeds this value. For example, the injection machine might not be able to deliver the volume if the specified fill time is too short. In this case, the analysis maintains the injection rate at its maximum, the same as a real injection machine would do, and gives a warning message. The actual fill time predicted by the analysis is then different from the specified fill time.
Maximum machine injection rate is useful in preventing the continuation of the analysis of a given fill time if it is beyond machine capacity. This maximum injection rate is used as the limit for both relative and absolute ram-speed profiles. The value is equal to the product of maximum machine injection speed (linear) and the cross-sectional area of the barrel.

Machine hydraulic response time
(default = 0.2 s)
In practice, no machine can switch from one hydraulic pressure level to another instantaneously. There is always a response time that depends on the control scheme and the system dynamics of the hydraulic units. Modern hydraulic controllers provide a close-to-linear transition from one holding pressure level to the next.
The analysis uses the machine hydraulic response time to provide a smooth transition from one packing pressure level to the next level. The pack/hold pressure is linearly interpolated between two pressure levels over the hydraulic response time. If the machine hydraulic response time is less than the time interval between any two pack/hold pressure profile settings, a response time equal to half of the time interval between the settings is used.

Maximum gas pressure
(default = 7.0E+07 Pa)
This is the maximum machine injection gas pressure capacity. The analysis gives a warning message if the specified constant gas pressure or the calculated profiled gas pressure exceeds this value. For the automatic gas-pressure profiling option, the analysis will continue execution, allowing the gas pressure to exceed this value. For the gas-pressure control option, the gas pressure will be a fraction (as defined by variable gas pressure profile) of the maximum gas pressure. For the gas-volume control option, when the resulting gas pressure exceeds the maximum gas pressure, it will be maintained at that value, assuming there is a safety valve which will open if a present gas pressure is reached. The default value is taken as the maximum number beyond which a potential hazard could occur. Users can modify this value based on machine capabilities or safety considerations.

Auxiliary Control
The control of auxiliary equipment in the injection molding process includes the coolant temperature setting on the coolant manifold temperature controller and the hot-runner temperature setting on the hot-runner manifold controller. Since the control of other auxiliary equipment, such as temperature and humidity control of the dryer, is beyond the scope of this chapter, these are not included in the following discussion. The detailed description of each auxiliary controller is given below.

Ambient temperature
On most injection molding shop floors, the ambient conditions are typically uncontrolled. Thus, the ambient temperature can vary as much as 5 °C during the day. In general, the ambient temperature has very little influence on the quality of the part. However, in precision molding, the ambient temperature has to be controlled very precisely to achieve consistent process control. The ambient temperature also affects the heat transfer of the injection mold.

Coolant manifold control
In general, mold filling is relatively insensitive to the mold-wall temperature (and its value for filling stage analysis could be approximated by the coolant temperature). However, the mold-wall temperature has a significant affect on the post-filling stage, which has a direct impact on the overall cycle time and part quality.
As shown in Figure 4-22, the coolant manifold control consists of a manifold ID, inlet coolant temperature, total flow rate or total pressure drop (either flow rate or pressure drop will be used as the boundary condition in the cooling channel flow analysis), and the coolant material ID. Each coolant manifold has an entrance node and an exit node. The entrance node has a nodal property that points to the corresponding manifold ID in the process conditions file, filename.prc. Each template available in C-MOLD Control Panel's Process Conditions module has a default number of coolant manifold IDs. Check the number of coolant manifold IDs required by the model, and add more manifold ID numbers to the template as necessary, using the Options; Customize Templates command in the Control Panel.

Figure 4-22. Coolant manifold control.
Flow rate is commonly chosen to achieve turbulent coolant flow for better heat transfer. If the total pressure drop or flow rate is specified, the cooling channel network analysis program (C-MOLD Coolant Flow) determines the coolant flow rate in each cooling channel. Based on the flow rate in each cooling channel, a heat transfer coefficient is estimated and used as the boundary condition for calculating the heat transfer to the cooling channels.

Hot runner manifold control
In practice, the polymer melt in the hot runner undergoes shear heating during the filling stage and "cooling" in the post-filling and mold-opening stages. At the beginning of an injection cycle, melt temperature in a hot runner might not be equal to the inlet melt temperature or the hot runner wall temperature. An accurate description of initial temperature in the hot runner should be determined by a cyclic, steady-state analysis. The current C-MOLD analyses, however, assume the initial melt temperature in a hot runner is equal to the inlet melt temperature.
A hot runner manifold control consists of a hot runner manifold ID (pointed to by hot runner elements as defined in the finite-element mesh file), a hot-runner wall temperature and an insulation thickness. It is common practice to control temperature at individual sections of a hot runner manifold to achieve a controlled flow distribution. All hot runner elements controlled under one section should point to one hot runner manifold ID and assign the proper wall temperature. Each template available in the Control Panel's Process Conditions module has a default number of hot runner manifold IDs. Check the number of hot runner manifold IDs required by the model, and add more manifold ID numbers to the template as necessary, using the Options; Customize Templates command in the Control Panel.
The insulation thickness is used by C-MOLD Cooling to determine the heat transfer from the hot runner manifold to the surrounding mold base. There are two basic hot-runner systems: internally or externally heated types. The hot runner manifold and mold base are typically separated by a small air-gap. The insulation thickness is the air-gap thickness, as shown in Figure 4-23.

Figure 4-23. Hot runner insulation thickness.

Transient hot runner manifold control
On rare occasions, the mold-wall temperature of a hot runner manifold is controlled to be a function of time. The transient hot runner manifold control consists of a hot runner manifold ID (pointed to by hot runner elements as defined in the finite-element mesh file), time, and a hot runner wall temperature. The hot runner wall temperature at time zero is also used as the initial melt temperature in the hot runners.

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