Scientific Setting of Switching Point, Holding Time and Pressure for Injection Molding
In the process of injection molding of plastic products, injection molding is often divided into two stages:
Stage 1: Most of the plastic is filled into the mold, generally 90% to 99.9% of the volume of the entire product.
Stage 2: Compact the product to obtain a product with the same structure and shape as the mold. In the second stage, although relatively little plastic melt is filled into the cavity, it is very important for the surface finish, aesthetic appearance and part size of the part.
In most cases, the second stage of injection molding uses two parameters: pressure and time. From the research perspective of scientific shaping, the two factors were added to four elements:
(1) The method of switch from the first stage to the second stage.
(2) Keep the gate sealed (frozen) or not sealed, process the product.
(3) Pressure holding time.
(4) Maintain a reasonable pressure in the cavity.
The control of the switching from the first stage to the second stage is the most critical part of the forming process. Whether high-quality products can be processed often depends on this. And it is also often the reason why plastics processing plants cannot produce the same part from one equipment to another.
In most application field, the switching process should be kept as short as possible.
That is, no matter what pressure is at the end of the first stage, it is hoped that it can be quickly changed to the pressure required for compaction and pressure holding in the second stage.
Also, you must understand how the injection molding machine control unit does this conversion process correctly.
However, there is no uniform standard for how to judge the completion of switch between different equipment.
Therefore, injection molding companies need to face four possibilities:
- The machine is equipped with switch functions of control viscosity.
- The machine control unit has a viscosity setpoint for switching. But it only can slow down the speed of the injection screw during switching, unable to control it.
- The machine does not have a viscosity setpoint for switch.
- When the first stage is switched to the second stage, the machine cannot operate normally, and there is a gradual viscosity slope, drop or wave swing.
It is necessary to ensure that the conversion from the first stage to the second stage is rapid and consistent.
Understanding how an injection molding machine works is therefore critical to obtaining the desired results.
For most products, if the process control is correct, 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.
Any sinking, pinching, wiggling or slipping into second stage during the switch to second stage pressure is undesirable.
- Sinking can cause the fluid front to become hesitant, resulting in underpack or starvation.
- A thin peak or a slow transition to second stage pressure will overfill the cavity, causing flash.
Oscillation often causes poor processing stability. A pressure monitor graph of injection pressure versus time is the best way to assess equipment response.
Mold Gate status (sealed or open)
It is impossible to process all products with the mold gate sealed.
For a specific product, the mold gate sealing test must be carried out, and the gate sealing processed product and the gate non-sealed processed product are tested to determine which method is the best.
There will be several situations that may occur:
- 100% of the test samples are not good when the gate is frozen,
- and 100% of the products that are not frozen at the gate pass the test,
- or vice versa.
It is impossible to judge what is going on by simply observing the sample or process. Do a gate seal test and test your samples to find out.
Back-Pressure Holding time setting
Knowing whether the mold gate should be kept sealed or open can help set the time for the second stage.
If the gate sealing time is required, it mainly for the better strength and stability of the product, by adding the time of the second stage or a longer time for the gate sealing.
This does not necessarily require an increase in cycle time. Because most cases can be balanced by reducing cooling time or mold sealing time.
If a non-sealed gate is beneficial to the performance of the product, begin to allow the gate to cool in half the time required. Due to normal temperature and process variations, it may be necessary to choose the exact gate seal time.
However, sometimes it is necessary to make the gate seal when producing, and sometimes it is reversed, which will produce inconsistent products.
The related approach is: cycle time consistency becomes critical for a consistent product if the process gate is not sealed.
If the cycle time is varied without the gate being sealed, the part will also vary for changing the amount of polymer in the cavity. This can be checked by weighing the product.
Setting the Back-Pressure Holding
Finding the correct black pressure is critical for compacting products.
The correct pressure for the second stage should be within and centered on the product parameters required to obtain a good Cpk (process capability index) product.
Since the holding pressure is set under the conditions of the gate sealing test, the correct second-stage compaction and holding pressure values should be found through experiments, so as to be set as the central value of the product processing parameter range.
1. Check the stability of the first stage, and whether there is insufficient mold filling or product shrinkage after the end of the first stage.
Inspection process: Set aside time in the second stage and reduce the holding pressure to the lowest value allowed by the machine, avoid letting the holding pressure or holding time decrease to zero. If the first stage is different than expected, then leave the first stage unchanged. If not , it will cause the first stage to go wrong.
2. If the first stage is stable, then start to increase the holding pressure.
Start from low, maybe 1000 – 2000 psi for plastics. Check the product each time once the hold pressure is increased. Increase the holding pressure in small increments until the quality of the product is optimal and acceptable. Produce a certain number of products whose quality requires preliminary testing. Label them and keep.
3. Now continue to increase the holding pressure until the production shows unacceptable flashing, sticking or other problems that may cause damage to the mold or product. Or there are indications that the process cannot run at the set high pressure conditions. Reducing the pressure to the maximum value allowed for safe, carry out the efficient production.
Additionally, produce a set of products for preliminary quality inspection. Label them and keep.
Finally, a set of products is produced in the middle of the pressure range just set.
Possible results and corresponding solutions
The three sets of products were taken for quality control operations. And the data generated by the quality control determined which sets of products were unacceptable or acceptable. There are three possible results:
(1) All products are too large.
This is a malfunction as it indicates that the wrong shrinkage was used when calculating the cavity size. If they exceed the upper and lower extremes of the pressure range, any process changes will make it difficult to get the part size to the middle of the required range.
(2) All products are too small.
This is still not good news, but at least it is “mold safety”, which can make the product within the required range by adjusting the mold. In addition, it is difficult to bring the product back to the center of the desired range by changing the process.
(3) Some products are too small and some are too large.
Now you can establish the own data to determine the upper and lower bounds of the minimum DOE (Design of Experiments) with the data.
Within the acceptable product range, use the second-stage pressure test to determine the center of the product’s required range. This is the right way to get a good process capability index Cpk Six Sigma quality.
Using the rule of 25%, 50% or 75% injection pressure is not going to work without data to back it up. This method will show that sometimes a low holding pressure is required, and sometimes a holding pressure that exceeds the injection pressure is required.
The only valid rule is: decide by data and by product.