In the process of studying plastic product injection molding, 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 total product volume; in the second stage, the product is compacted to obtain a product 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 product size of the product. In most cases, the second stage of injection molding uses two parameters: pressure and time.
From the perspective of scientific molding research, we increase the two factors to four elements: (1) the method of converting from the first stage to the second stage; (2) keeping the gate sealed (frozen) or the gate unsealed when processing the product; (3) holding time; (4) maintaining a reasonable pressure in the cavity.
Conversion
It can be said that the control of the conversion from the first stage to the second stage is the most critical part of the molding process. Whether a high-quality product can be processed often depends on this, and it is often the reason why plastic processing plants cannot produce the same product from one equipment to another.
In most applications, the conversion process should be as short as possible, that is, no matter what the final pressure of the first stage is, it is hoped that it can quickly change to the pressure required for compaction and holding pressure 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 judge the completion of the conversion between different equipment, so injection molding companies need to face four possibilities:
① The processing equipment is equipped with a conversion function to control the viscosity;
② The equipment control unit has a viscosity setting value for conversion, but it can only slow down the speed of the press rod during the conversion, but cannot control it;
③ The equipment does not have a viscosity setting value for conversion;
④ When the first stage is converted to the second stage, the equipment cannot operate normally, and a gradual viscosity tilt, drop or wave swing occurs.
It is necessary to ensure that the conversion from the first stage to the second stage is fast and consistent. Therefore, understanding the working principle of the injection molding machine 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 second stage pressure setting point should be less than 0.1 seconds.
It is not desirable to have any sinking, thin and sharp peaks, swings or slow slides into the second stage pressure phenomenon when changing to the second stage pressure. Sinking can cause the flow front to hesitate, resulting in under-holding or starvation. Thin peaks or slow transitions to second stage pressure will overfill the cavity, causing flash. Swinging often results in poor process stability. A pressure monitor graph of injection pressure versus time can be used as a way to evaluate the best response of the machine.
It is not possible to process all parts with the gate sealed. For a specific part, a gate seal test must be performed and parts processed with the gate sealed and parts processed without the gate sealed must be tested to determine which method is best. It may be possible for 100% of the test samples to fail when the gate is frozen, and 100% of the parts with the gate not frozen to pass, or vice versa. It is not possible to tell what is going on by simply looking at the samples or the process. Performing a gate seal test and testing the samples will find the answer.
Setting the Holding Time
Knowing whether the gate should be kept sealed or open can help set the length of the second stage. If a gate seal time is required, then add a second stage or set a longer gate seal time for the process to maintain process strength and stability. This does not necessarily require increasing the cycle time, as most environments can be balanced by reducing the cooling time or mold sealing time. If the performance of the part is good with an unsealed gate, start cooling the gate at half the time required.
Due to normal temperature and process variations, the worst possible situation is to choose the correct gate sealing time. However, sometimes it is necessary to seal the gate when producing the part, and sometimes it is not, which will produce inconsistent parts.
A related method is: if the gate is not sealed during processing, then the consistency of cycle time becomes very important for consistent products. If the cycle time varies with an unsealed gate, the part will also be different due to the changed amount of polymer in the cavity. Weighing the part can check this.
Setting the holding pressure
Finding the correct holding pressure is critical to compacting the part. The correct pressure in the second stage should be within and at the center of the product parameter range required to obtain a good Cpk (process capability index) part. 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 to be the center of the processing parameter range of the part.
First, check the stability of the first stage and check whether there is underfilling or part shrinkage after the first stage. Checking process: Allow time for the second stage and reduce the holding pressure to the lowest value allowed by the equipment, taking care to avoid reducing the holding pressure or holding time to zero. If the first stage is different from what was expected, keep the first stage unchanged. If no action is taken in the second stage, it will cause the first stage to go wrong.
If the first stage is stable, then start to increase the holding pressure. Start low, maybe only 1000-2000psi for plastics. Check the parts each time the holding pressure is increased; increase the holding pressure in small increments until the quality of the parts is judged to be the best acceptable state. Produce a certain number of parts whose quality needs preliminary testing. Label and put away.
Now continue to increase the holding pressure until the production shows unacceptable flash, push rod action, adhesion or other problems that may damage the mold or part, or there are signs that the process cannot be operated under the set high pressure conditions. Reducing the pressure to the maximum value allowed by the process allows safe and efficient production. In addition, process a group of parts for preliminary quality inspection. Label and put away. Finally, produce a group of parts at the midpoint of the pressure range just set.
Take the three groups of parts and perform a quality control operation. Use the data generated by the quality control operation to determine which groups of parts are unacceptable or acceptable.
There are three possible answers:
(1) All parts are too large. This indicates a failure because it indicates that the wrong shrinkage was used to calculate the cavity size. If they are outside the upper and lower limits of the pressure range, then any process changes will have a hard time getting the parts to the middle of the required range.
(2) All parts are too small. This is still not good news, but at least it is "mold safe" and can be adjusted to get the parts to the required range. Alternatively, it will be difficult to change the process to get the parts back to the center of the required range.
(3) Some parts are too small and some are too large. Now you can use the data obtained to establish the minimum upper and lower limits of the DOE (design of experiments). Within the acceptable range of parts, use the second stage pressure experiment to determine the center of the required range of parts.
