Optimal Process with Injection Molding Machines and Type of Molding Defects 

1. Injection Molding Process

A mechanical variable is a configuration value that is directly input into an injection molding machine or an additional device in order to obtain the molding product with the required characteristics. In case of the mechanical variables, it is not absolutely possible to transfer the equivalent mutual data for manufacturing conditions not only to other machines but also between the characteristics of molding products.

The relationship between mechanical variables and processing data of molding products exhibits the same tendency, however, the relationship among machines is definitely distinctive, and moreover, it can vary over time due to abrasion of screws, tips in screws, and hydraulic components.

A process variable means the injection molding process from the view of molding product manufacturing, and includes pressure, temperature, and time of the melting materials and the temperature of a metallic wall within a mold cavity. In addition, a process variable is an independent term of machine and it can actually be discovered and conveyed. Molding products have the same characteristics only when the process variables exhibit reproducibility.

Internal characteristics refer to the information on the internal structure of a molding product such as distribution and orientation of additives as well as molecular weight, orientation, internal stress, crystallized degree of raw materials.

The external characteristics of a molding product include dimensions and tolerances, shape, surface status, and functional properties such as mechanical, optical, and electrical characteristics of a molded product.

Process data in the injection molding processes of a specific machine are critical results obtained from configuration values, injection molds, and used plastic materials. Those are measured and stored in each work stage and can be used for additional evaluation and surveillance.

The most frequent obstructive factors in the injection molding process, which include changes in temperature and viscosity, amount changes in metering, and inaccurate movement of screw tip, can be recorded by process data and are used for securing the quality of products.

It is essential to understand when, where, and how the process variables are measured in order to know both whether they are evaluated correspondingly and the conclusion of the obstructive factors. Therefore, it is necessary to highlight more on the work as well as the functions of molds and injection molding machines.

2. Thermal Balance of Injection Mold

In thermoplastics, polymer melting is always cooled in the course of injection. However, this may indicate heating as well as cooling when comparing to indoor temperatures. This varies depending on the mold temperature that is essential or required. In injection molding, several thermal transitions occur within a mold for injection.

These thermal transitions fall into following categories;

Heat exchange between a mold and surrounding environment (radiation, conduction, convection)

Heat amount introduced by the melting materials

Heat amount flowed in or flowed out by the cooling medium

In the molds for forming thermoplastic materials, the heat exchange between plastic materials and molds plays a very important role in terms of the thermal balancing perspective. This is because the melting material should be cooled down as quickly as possible from the forming temperature to the temperature to change its shape.

It is possible to see that the temperature controllers with manual adjustment are commonly used in the plastic surgery companies and the factors that affect product characteristics typically include pressure change, coolant inlet temperature, and obstructions such as old sediment in cooling pipes.

To address these problems, it is recommended to employ the micro-temp, which is a mold cooling system that is controlled using a closed-loop by the control unit. In other words, this means that the mold temperature is measured directly by the thermocouple and is controlled using a closed-loop by the coolant control valve.

2.1 Temperature of Mold Wall

Temperature of mold wall refers to the surface temperature of the mold cavity during a cycle. In addition, it is a factor that has an influence on clearly determining production time and quality of molding products. In the beginning stage, it typically starts with the recommended temperatures as low as possible.

As the temperatures of cavity and mold wall increase, the following phenomena occur:

- Crystallization degree increases and uniform internal structure is formed.

- Contractions in both front and rear decrease, but molding contraction increases.

- Thermal deformation temperature increases.

- Internal stress is reduced.

- Molecular orientation decreases.

- The deformation, or deflection (bending) of the product is reduced.

- Transcriptional performance up to fine parts of the cavity surface is improved.

- Pneumatic resistance decreases.

- Cooling time increases. (approximately 2% /℃)

The temperature of a mold wall as well as the constant cooling caused by the uniform temperature distribution of the wall surface in a mold cavity is very important. Local temperature deviations of mold walls may cause various internal stresses and consequent abnormal deformations.

2.2. Micro-temp

It is still possible to find out the coolant controllers that are manually operated for mold cooling. As a result, these are not able to control the amount of coolant rapidly and reproducibly for an individual cooling circuits based on the data. Moreover, several obstacles such as pressure changes, changes in coolant inlet temperature, and long-term deposition in the cooling path occur to affect the product characteristics such as dimensions and contractions.

To address new problems with regard to mold cooling, the micro-temp, which is a mold system that can select either open-loop or closed-loop by controllers, is provided.

It is difficult to install a thermocouple later on at a mold that has been used for a long time. The opening time of a solenoid valve, which is usually required per cycle of a manually operative valve, can be set through the control unit. Pressure changes in the cooling circuit are automatically compensated by the use of a flow control valve.

When designing a new mold, a thermocouple should be installed between the cooled reactor and mold wall in order to measure the temperature of the mold wall. Thermocouple connections should be tied with the standardized connectors for manual or automatic quick coupling.

For precise injection molding products, each circuit in a mold requires to equip with a thermocouple on the mold wall. As a result, it is able to fully record the temperature conditions within a mold and maintain the status constant even if obstructions occur.

In the case of a mold for a functional molding product, the additional cost for installing a thermocouple is less than 2% of the total cost of the mold in most instances. Furthermore, in a randomly installed thermocouple, the major area of a molding product may require a fixed-degree dimension or can be recorded together with deformation problems. In a multi-cavity mold, the temperature conditions of the entire mold can be used sometimes based on individual measurements within the mold cavity.

2.2.1 Micro-temp Program

The program micro-temp is classified into a closed-loop control or an open-loop control depending on whether or not the temperature sensor is installed on a mold for measuring the mold temperature due to a supply of coolant by the flow control valve. In order to control one mold temperature sensor, the flow control valve and the flow control device are installed at the designated positions, respectively.

The thermocouple positioned at the inlet monitors the coolant temperature and displays an error message if the temperature exceeds the threshold value. If a flow control device has been installed, it monitors the minimum flow rate of each cooling circuit and displays an error message if it fails to reach.

On the above screen, the cooling circuit is switched to a specific operating state as shown below according to the numerical input of 0 to 3 in each zone.

- “0” = Valve closed

- “1” = Operating mode without thermocouple on mold (Open-loop)

- “2” = Operating mode with thermocouple on mold (closed-loop)

- “3” = Valve opened

■ Incoming temperature monitoring

For monitoring the incoming temperature, the maximum deviation of the configuration value and the actual value can be entered. If the actual value exceeds the maximum deviation configured, the error message "Abnormal Incoming Temperature" is displayed. The monitoring begins with the delay time configured when the valve is opened.

■ Operating mode without thermocouple on mold (Open-loop)

In the case of a mold without thermocouples, it is required to enter “1” to the corresponding circuit state on the screen and the flow rate should be configured at the l/min. The coolant valve opens the flow rate configured during the calculated cooling time at the start of injection.

Separate flow rates can be entered for either automatic mode or manual mode. The actual temperature is displayed as "***" as it cannot be measured because of no thermocouple installed.

■ Operating mode with thermocouple on mold (Closed-loop)

After the required configuration value is entered for mold temperature, “2” should be set in the corresponding circuit state on the screen. In a manual operating mode, the corresponding coolant valve opens when the temperature exceeds the configured value and closes when the temperature becomes lower.

In automatic operating mode, control variables are automatically determined when the control unit exceeds the initial adjustment phase or control deviation. The mold temperature changes in a minimum of 20 can be measured by a thermocouple during 10 successful cycles in automatic operating mode depending on the operation of a valve in the cooling or temperature control systems.

If this is not the case, the adjustment error tag "EF" is configured on the screen and "Cooling too weak" or "Temperature control required" is displayed. The new cycle is on hold and the alarm lamp is turned on. The corresponding value is configured and a new cycle starts. Afterward, the coolant valve is then always operated at the start of injection for the calculated time.

■ Flow rate monitoring

When a valve is opened in the cooling circuit and the monitoring is turned on, the corresponding minimum flow is detected. As a surveillance action, the F tag on a screen displays the error message "Micro-temp Valve ×". The operations of a coolant valve and the monitoring switch signal of the cooling circuit are displayed in the tag on the screen, respectively.

3. Clamping force

Clamping force is the sum of forces applied to the frame in the tie-bar or tie-bar-less under the tensile stress before the mold advance is completed and the injection begins. Locking force is the sum of the maximum forces applied to the frame in the tie-bar or tie-bar-less under the tensile of the injection movement in which the material is pushed into a mold.

Clamping force is calculated from the average internal pressure in a mold and the projected area of an injection molding product. Venting of a mold is a protection of the mold from the clamping force at the mold parting surface when the pressure of a mold cavity is applied to the place where the cavity volume increases toward a shape direction. The deformation of another elasticity, as well as venting, depends on the rigidity of a clamping device and mold, clamping force and locking force, and shape opening force.

When increasing the clamping force, the following phenomena occur:

- The independent dimensions of the mold are reduced.

- The dimensional change of the product is reduced.

- The deformation of the mold is reduced.

- Damage to a mold caused by flash occurrence is reduced.

- The possibility of venting deteriorates during a filling of mold cavity.

- Link abrasion increases in a toggle type.

- The locking energy increases in a hydraulic system.

The formula for calculating the clamping force is as follows.

In order to avoid over-filling, the residual clamping force on the parting surface of a mold should be at least 10% of the locking force during the production process. It is useful to use the mold with a large residual clamping force for safety reasons early in the mold test.

In a toggle-type machine, the clamping force is measured by the extension of the tie-bar and the adjustment in the mold thickness of the closed-loop. In a hydraulic machine, hydraulic pressure determines the clamping force.

4. Melting Temperature

A plasticizer plays the role of not only producing thermally and mechanically homogeneous melting materials but also supplying a constant injection volume.

Factors Affecting Melting Temperature

* Factors Affecting Within a Cylinder

- Temperature of cylinder inner wall

- Back pressure

- Screw rotation speed

- Detention time of melting material within a cylinder

* Factors Affecting Within a Mold

- Detention time of melting material within a hot runner

- Configured temperature of hot runner

- Shear heat generated when filling a mold

- Temperature of mold wall

When the melting temperature increases……

Once the melting temperature increases, the following phenomena occur:

- Occurrence of the weld-line decreases.

- Crystallization degree increases.

- Viscosity of a material is reduced.

- Orientation degree is reduced.

- Pressure loss within a mold is reduced.

- Thermal adaptability of a melting material increases and gas release by thermal decomposition is improved.

- Mechanical stress (shear) of a melting material is reduced by not only breaking the molecular chain in the nozzle sprue gate system but also numerous bypasses and a narrow cross-sectional area in the mold.

- Cooling time slightly increases. (approximately 0.3% /℃)

For measuring melting temperature and pressure, a flange for measurement may be mounted on the barrell as shown below.

4.1. Cylinder Temperature

Cylinder temperature refers to the temperature measured at the vicinity of a longitudinal hole (at the nozzle near the melting channel) of the heating cylinder. Approximately 60 to 85% of the energy required for melting materials is generated by the driving energy depending on the operating cases, however, the melting temperature can strongly influence the cylinder wall temperature, especially the two post-heat zones.

It should start with an average recommended value at the beginning.

In case of the thermally sensitive plastic materials, it is necessary to employ an increasing temperature profile in the nozzle direction in order to apply less heat to the melting material. This profile is advantageous if the residual time of a melting material is long.

- In case of very long cooling time

- In case of very small metering stroke

- In case of having large material volume within a screw channel or a hot runner

In the case of an open nozzle, a temperature profile that increases and then decreases in the nozzle direction is used for preventing the following phenomena:

- To prevent a large amount of leakage loss

 A temperature profile slightly decreasing in the nozzle direction is used at the hopper section to deliver more heat to the melting material in the following cases.

- In case that a large amount of resin is filled by the big measurement stroke and a short cooling time is needed.

- In case of using a deep blade screw or a barrier screw

Temperature control of the resin supply section is essential for supply performance and stability of transferring molding materials. Since the friction rate between material particles and cylinder wall depends on temperature, the temperature control here should be appropriate for the relevant operating conditions and the environment with regard to friction.

However, in case of being not aware of any friction motion, the optimal temperature of the resin supply section should be determined when the machine is configured. At this point, the consistency of the measuring capacity may be considered as one of the guidelines for the supply performance by screw strokes.

It is advised to start with the average recommended value and check if the plasticizer is constant by short. Otherwise, a gradual change in the temperature of the available resin supply section leads to a better machine configuration.

4.1.1. Cylinder Heating

Mica or mica insulation heater is used in the injection molding machines for molding thermoplastics in order to heat cylinders and nozzles.

To increase the temperature of a decent machine, a heater band needs to be installed by a fixing bolt. If not, early damage occurs due to poor heat conduction. The standard heating capacity of a cylinder surface is 3 ~ 3.5 watt/㎠. Typically, a profile is configured to increase the temperature from the supply zone to the screw tip.

Measurement flange for measuring the melting temperature and pressure, which can be marked and evaluated as important process variables, is recommended as the special equipment.

Influence of cylinder heating work on molding products

- Black lines (thermal damage to materials by overheating)

- Material particles that are not completely molten (Very small transfer energy)

- Slacking (Stringing: Maintaining a limited melting state between sprue and nozzle)

4.2. Metering

The materials are supplied to the screw channel by screw rotation, and afterwards, those are compressed and measured by heat. The shear force that causes additional heating of the melting appears.

The melting material is conveyed to the frontal space of the screw and the pressure rising occurs by pushing the screw back under adjustable back pressure. Then, the measured materials corresponding to the configured injection volume are filled in the frontal space of the screw. Injection devices are advanced in this process.

 Major influences of measuring process on molding products

- Color lines (insufficient dispersion of pigment by a screw)

- Black lines (thermal damage to plastic materials within a plasticized cylinder)

- Material particles that are not completely molten (Very small transfer energy, very small residence time)

- Molding products that are insufficiently filled (too small metering volume)

It is possible to configure a profile with the five levels of both metering speed and back pressure for metering. Each measuring speed and back pressure are operated by dividing the metering stroke into five equal parts.

The switchover position of back pressure and the metering speed can be manually altered through configuration values in the graphic image. And, an automatic division into quintiles is possible by entering the same value twice in the measurement stroke C1.

4.2.1. Metering Stroke

Metering stroke is the entire stroke of a screw within a cycle. If the selected metering stroke is relatively small or too large for the diameter of a screw, thermal problems or surface defects may occur.

 Disadvantages when a metering stroke is smaller than the screw diameter (1D)

- Providing a long residence time for thermally sensitive plastics

- Requiring a relatively long reaction time, and the reaction time deviation of the non-return valve occurs.

Disadvantages when a metering stroke is more than 3 times the diameter of a screw (3D)

- Line occurring due to the materials not completely molten

- Bubble generation

- Thermally unequal melting conditions

In order to fill the mold cavity by the switchover point of packing pressure, it is required to secure a sufficient melting capacity at the frontal space of a screw in the beginning by the configuration of a relevant sized metering stroke. To verify the filling state, a small metering stroke should be initially configured and gradually raised.

4.2.2. Back Pressure

Back pressure refers to the pressure applied to the melting materials in the frontal space of a conveying screw during the metering period. It can be altered by changing the pressure at the outlet of a hydraulic cylinder.

Back pressure plays the following roles.

- It makes the melting materials thermally homogeneous. In particular, this is absolutely necessary for plastics present as particle states that are not molten by the shear action of a screw.

- It secures mechanical homogeneity such as the distribution of pigments or additives evenly.

- It drains the air penetrated together with the plastic material in the particle state into the direction of the hopper.

- It compensates for a decrease in temperature caused by a reduction in the effective screw length during the metering process by increasing the temperature profile in an axial direction.

- The trapped air reduces the deviation of remaining materials (cushion volume) from each shot.

As the back pressure increases, the metering capacity decreases, that is, an increase in the metering time. The back pressure required varies on the melting viscosity and thermal sensitivity of the plastic material used. In the beginning, it is necessary to start with relatively less back pressure.

4.2.3. Metering Speed

The metering speed refers to the number of screw rotations per minute. The screw circumference velocity is the ratio of a screw circumference multiplied by the number of rotations to the time (v=d×p×n). The metering from raw material to melting occurs due to a rotation of a screw. The reference for rotation is the circumference velocity of a screw.

Too high screw rotation speed often results in the following defects:

- Thermal damage to melting materials

- Reduced length of glass fiber

- Increase in abrasion of a screw and a cylinder

The entire available time should be applied in the metering. A decrease of metering time indicates an increase in the metering capacity. In order to reduce the metering time that determines the cycle time, it is safe to use a screw with either deep roots or large diameter in order to increase the metering capacity when having a difficulty in reducing the back pressure and being not accurate.

Up to 0.1 m/s, which is the maximum circumference velocity in the materials with a very good flow like thermally sensitive packaging, it is able to obtain the metering speed from the process data tables provided by the material and machine manufacturers. However, it is limited to apply the maximum velocity by additives such as pigments and flame retardants that may be sensitive to shear forces.

Following is a calculation formula of the acceptable metering speed for machines equipped with a screw of 40 mm diameter on a material that has an acceptable maximum circumference speed (v) 0.1 m/s.

4.2.4. metering surveillance time

surveillance, the metering time, depends on the metering speed, back pressure, screw structure, and supply efficiency of materials. The change in metering time at the same configuration value of a machine means a different supply efficiency caused by different rates of friction and abrasion of a screw and a cylinder.

When the metering surveillance time, surveillance, is turned on, the metering and the suck back must be completed within the configured time. Otherwise, the metering timeout error occurs, and at the same time, the metering and next cycle will be interrupted.

4.2.5. The number of Metering

For the purpose of both monitoring and documenting the metering process, the number of metering without dimensions is determined and displayed on the process analysis screen. This process variable indicates the average pressure required for the metering.

This is proportional to the force or torque required for the metering motor. The number of metering (PLZ) indicates a continuity in the metering operation and the metering process. The number of metering is always determined from the beginning to the completion of metering. It is selected, recorded, and monitored in QDP, which is a quality data program.

edit : handler  http://www.ihandler.co.kr