In the process of studying the injection molding of plastic products, injection molding is often divided into two stages: in the first stage, most of the plastic is filled into the mold, generally 90% to 99.9% of the entire product volume; in the second stage, the pressure The actual product is obtained with the same structure and appearance as the mold.
In the second stage, although only a relatively small amount of plastic melt is filled into the cavity, it is very important for the surface finish, aesthetic appearance and size of the product. In most cases, the second stage of injection molding uses two parameters: pressure and time.
From the perspective of scientific research, we increase the two factors to four elements
- Method of transitioning from the first stage to the second stage;
- Keep the gate sealed (frozen) or process the product with the gate unsealed;
- Pressure holding time;
- Maintain a reasonable pressure in the cavity.
Convert
It may be said that the control of the transition from the first stage to the second stage is the most critical part of the molding process. The ability to process high-quality parts often depends on this, and it is often the reason why a plastics processing plant cannot produce the same part from one piece of equipment to another.
In most application fields, the conversion process should be kept as short as possible, that is, no matter what the final pressure is in the first stage, it is hoped that it can quickly change to the pressure required for compaction and holding in the second stage. In addition, you must understand how the equipment control unit correctly completes this conversion process.
Unfortunately, there is no unified standard for how to determine whether the conversion is completed between different devices.
Injection molding companies need to face four possibilities
- Configuration of processing equipment to control the conversion function of viscosity;
- The equipment control unit has a viscosity setting value for conversion, but it can only slow down the speed of the pressing rod during conversion, but cannot control it;
- The equipment does not have a viscosity setting value for conversion;
- When converting from the first stage to the second stage, the equipment cannot operate normally, and there will be a gradual viscosity tilt, drop or wave swing.
It is necessary to ensure that the transition from the first stage to the second stage is rapid and consistent. Therefore understanding how an injection molding machine works is critical to obtaining the desired results.
For most products, for correct process control, the time from the end of the first stage to the pressure set point of the second stage should be less than 0.1 seconds.
You don’t want any dips, thin, sharp peaks, wobbles, or slow slides into second-stage pressure as you transition to second-stage pressure.
Subsidence will cause the fluid front to become hesitant, resulting in insufficient pressure holding or material shortage.
Small peaks or a slow transition to the second stage of pressure will overfill the cavity, causing flash. Swinging often results in poor processing stability.
A pressure monitor graph of injection pressure versus time is the best way to evaluate equipment response.
It is impossible to process all products with the gate sealed. For a specific product, a gate sealing test must be conducted and the products with gate sealing and those without gate sealing must be tested to determine which method is best.
This will result in the possibility that 100% of the test samples will not perform well when the gate is frozen, and 100% of the products with unfrozen gate will pass the test, or vice versa.
You cannot tell what is going on by simply looking at the sample or process. Conduct a gate seal test and test samples to find out.
Set pressure holding time
Knowing whether the gate should be kept sealed or opened can help set the length of the second phase. If the gate sealing time is required, then in order to maintain the processing intensity and stability of the process, add a second stage or set a longer gate sealing time.
This does not necessarily require an increase in cycle time as most circumstances can be balanced by reducing cooling time or mold seal time. If an unsealed gate would benefit product performance, start cooling the gate in half the time required.
Due to normal temperature and process variations, the worst possible scenario is choosing the exact gate sealing time. However, when manufacturing products, sometimes it is necessary to keep the gate sealed, and sometimes the opposite is true, which results in inconsistent products.
A related approach: If process gates are not sealed, cycle time consistency becomes critical for consistent products. If the cycle time varies without the gate being sealed, the part will also vary due to changing the amount of polymer in the cavity. This can be checked by weighing the product.
Set the holding pressure
Finding the correct holding pressure is critical to compacting your product. The correct pressure for the second stage should be within and centered within the product parameters required to obtain a good Cpk (Process Capability Index) product.
Since the holding pressure is set under the gate sealing test conditions, the correct second-stage compaction and holding pressure values should be found through experiments, and thus set as the center value of the product processing parameter range.
First, check the stability of the first stage and check whether there is insufficient mold filling or product shrinkage after the first stage. Inspection process: Set aside time in the second stage and reduce the holding pressure to the lowest value allowed by the equipment.
Be careful to avoid reducing the holding pressure or holding pressure time to zero. If the first phase is different than expected, then keep the first phase unchanged. If no action is taken in the second phase, an error will occur in the first phase.
If the first phase is stable, then start increasing the holding pressure. Start low, maybe only 1,000 to 2,000 psi for plastics. Check the product each time the holding pressure is increased; increase the holding pressure in small increments until it is judged that the quality of the product reaches the best acceptable state. Produce a certain quantity of products whose quality requires preliminary testing. Label and put away.
Now continue to increase the holding pressure until the production shows unacceptable flash, push rod action, sticking or other problems that may cause damage to the mold or part, or there are signs that the process cannot operate at the set high pressure conditions.
Reducing pressure to the maximum allowed by the process allows for safe, efficient production. In addition, a set of products for preliminary quality inspection is processed. Label and put away. Finally, a set of products is produced within the pressure range just set.
The resulting three groups of products are taken out and subjected to quality control operations, and the data generated by quality control determine which group of products is unacceptable or acceptable.
There are three possible answers
- All products are too large. This indicates a malfunction because it indicates that the wrong shrinkage was used when calculating the cavity dimensions. If they are outside the high and low limits of the pressure range, then any process changes will have little chance of bringing the part size to the middle of the required range.
- All products are too small. This is still not good news, but at least it is “mold safety”, which can adjust the mold to make the product within the required range. In addition, it is difficult to change the process to bring the product back to the center of the required range.
- Some products are too small and some are too large. Now you can establish the upper and lower bounds of the minimum DOE (Design of Experiments) by establishing data from the obtained data. Within the acceptable product range, use the second-stage pressure test to determine the center of the product’s required range.
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