★Pressure in the mold cavity during injection molding
When setting injection molding process parameters, the following procedure is generally followed, and attention should be paid to the corresponding key points.
General process and key points for setting process parameters
Set the plasticizing temperature
① If the temperature is too low, the plastic may not melt completely or may have difficulty flowing;
② If the melting temperature is too high, the plastic will degrade;
③ Obtain accurate melting and molding temperatures from the plastic supplier;
④ The barrel has 3-5 heating zones, with the zone closest to the hopper having the lowest temperature, gradually increasing thereafter. The heater at the nozzle must ensure temperature consistency;
⑤ The actual melting temperature is usually higher than the heater's set value, mainly due to the influence of back pressure and the frictional heat generated by the screw's rotation;
A probe thermometer can measure the actual melt temperature.
Set mold temperature
①Obtain recommended mold temperature values from the plastic supplier;
②) Mold temperature can be measured with a thermometer;
③ The coolant temperature should be set 10-20°C lower than the mold temperature;
④ If the mold temperature is 40-50°C or higher, consider installing an insulation plate between the mold and the locking platen;
⑤ Higher mold temperatures are sometimes necessary to improve the surface quality of the parts.
Set the injection endpoint of the screw.
The injection endpoint is the position of the screw when switching from the filling stage to the holding stage, as shown in Figure 2-15. If the filler is insufficient, shrinkage marks may appear on the surface of the product. Generally, the filler thickness is set to 5-10 mm. Experience shows that setting the injection endpoint to fill 2/3 of the mold cavity in this step can prevent damage to the injection molding machine and mold.
Figure 2-15 shows the setting of the screw injection endpoint.
Set screw speed
① Set the required screw speed to plasticize the plastic;
②The plasticizing process should not extend the entire cycle time. If it does, the speed needs to be increased.
③ The ideal screw speed is the minimum speed that does not extend the cycle time.
(5) Set the back pressure value
① The recommended back pressure is 5~10MPa;
② Too low a back pressure will result in inconsistent products;
③ Increasing back pressure increases frictional heat and reduces the time required for plasticization:
④ Using lower back pressure increases the time the material remains in the barrel.
Set injection pressure value
① Setting the injection pressure to the maximum value of the injection molding machine is intended to better utilize the injection speed of the machine, so the pressure setting will not limit the injection speed;
② The pressure will switch to the holding pressure stage before the mold is fully filled, thus preventing damage to the mold.
Set the initial holding pressure value.
① Setting the holding pressure to 0.1 MPa will stop the screw when it reaches the injection endpoint, thus preventing damage to the injection molding machine and mold;
② The holding pressure will gradually increase until it reaches the set value.
Set the injection speed to the maximum value of the injection molding machine
(1) Using the maximum injection speed will result in lower flow resistance, longer flow length, and stronger weld line strength. However, this necessitates the installation of venting holes. Poor venting will trap air, creating extremely high temperatures and pressures within the mold cavity, leading to burn marks, material degradation, and short shots.
② Identify the locations of weld lines and trapped air. A well-designed venting system should be implemented to avoid or minimize defects caused by trapped air.
③ Regular cleaning of the mold surface and venting facilities is also necessary, especially for ABS/PVC materials.
Set pressure holding time
① The ideal holding time is the minimum of the gate solidification time and the part solidification time;
② The gate solidification time and the part solidification time can be calculated or estimated;
③ For the first test, the mold filling time can be predicted using CAE software, and the holding time can be set to 10 times this mold filling time.
Set sufficient cooldown time.
① Cooling time can be estimated or calculated, including holding time and continuous cooling time;
② Initially, the continuous cooling time can be estimated as 10 times the injection time. For example, if the predicted injection time is 0.85s, then the holding time is 8.5s, and the additional cooling time is 8.5s. This ensures that the parts and runner system are fully cured for demolding.
Set mold opening time
Generally, the mold opening time is set to 2-5 seconds, including the processes of mold opening, demolding, and mold closing. The production cycle is the sum of injection time, holding pressure time, continuous cooling time, and mold opening time, as shown in Figure 2-16.
Gradually increase the injection volume until it reaches 95% of the cavity volume
① CAE software can measure the weight of the plastic part and the gate/runner. With this information, along with the known screw diameter or barrel inner diameter, the injection volume and injection start position for each injection can be estimated.
② Therefore, only 2/3 of the mold is filled, and the holding pressure is set to 0 MPa. This way, filling stops when the screw reaches the injection endpoint, protecting the mold. Next, the filling is increased by 5%–10% each step until the mold is 95% full.
③ To prevent plastic from drooling from the nozzle, a compression safety valve is used. Immediately after the screw finishes rotating, it retracts a few millimeters to release the back pressure built up during the plasticizing stage.
Switch to automatic operation
To achieve stability in the processing, it is necessary to switch to automatic operation mode.
Set mold opening stroke
The mold opening stroke settings include core height, product height, and reserved space, as shown in Figure 2-17. The mold opening stroke should be minimized. Each time the mold opens, the initial speed should be low, then increased, and finally decreased again near the end. The mold closing sequence is similar to the mold opening sequence, i.e., slow-fast-slow.
Set the demolding stroke, starting position, and speed
First, eliminate all slippage; the maximum ejector stroke is equal to the core height. If the injection molding machine is equipped with a hydraulic ejector system, the initial position should be set so that the part can be fully removed from the fixed mold. When the ejection speed equals the mold opening speed, the part remains on the fixed mold side.
Set the injection volume to fill 99% of the melt volume in the mold cavity
When the process is fixed (producing the same parts each time), adjust the injection endpoint position to fill 99% of the cavity, so as to make full use of the maximum injection speed.
Gradually Increase Holding Pressure
① Gradually increase the holding pressure value, increasing it by approximately 10 MPa each time. If the mold cavity is not completely filled, the injection volume needs to be increased.
② Select the lowest acceptable pressure value. This minimizes the internal pressure of the product, saves material, and reduces production costs. A higher holding pressure leads to high internal stress, which can cause warping of the part. Internal stress can be released by annealing the product to below its heat distortion temperature of 10°C.
③ If the padding material is used up, the final stage of holding pressure will be ineffective. This requires changing the injection starting point to increase the injection volume.
④ The hydraulic pressure of the hydraulic cylinder can be read from the injection molding machine's pressure gauge. However, the injection pressure at the front of the screw is more important. To calculate the injection pressure, the hydraulic pressure needs to be multiplied by a conversion factor. The conversion factor can usually be found in the injection section of the injection molding machine or in the user manual. The conversion factor is usually 10–15.
Obtain the shortest holding time
① The simplest way to obtain the shortest holding time is to start by setting a relatively long holding time and then gradually reduce it
until shrinkage marks appear.
② If the dimensions of the part are relatively stable, a more precise holding time can be obtained using Figure 2-18. Based on the relationship between product quality and holding time in the figure, the solidification time of the gate or product can be determined. For example, after step 9, the holding time has no effect on the quality of the part; this is the shortest holding time.
Achieve the shortest continuous cooldown time
Reduce the continuous cooling time until the maximum surface temperature of the part reaches the material's heat distortion temperature. The heat distortion temperature can be found in the plastic material handbook provided by the supplier.
During the above process, if it is a new product launch and the process parameter values are uncertain, the following points should be noted:
① Temperature: Set the plastic temperature too low (to prevent decomposition) and the mold temperature too high.
② Pressure: Start with lower injection pressure, holding pressure, and back pressure (to prevent overfilling and damage to the mold and machine).
③ Clamping force: Start with a higher force (to prevent overflow).
④ Speed: Start with a slightly slower injection speed (to prevent overfilling); start with a slightly slower screw speed; start with a slightly slower mold opening and closing speed (to prevent mold damage); start with a shorter metering stroke (to prevent overfilling).
⑤ Time: Start with a longer injection holding time (to confirm gate sealing); start with a longer cooling time.