dimension defects for injection parts
dimension defect for injection parts
The dimension defect refers to the formed products that exceed the specifications and sizes indicated on the original drawings, and some of them do not conform to the specifications. If they are classified, there are two kinds of phenomena: the size is too small and the size is too large. If these are further subdivided, there are the following situations. Let us consider the causes of these phenomena and the countermeasures to be taken:
1. Small size defect2. Large size defect
3. Poor core swing
1、 Small size defect
reason:
Most materials on the earth have the characteristics of thermal expansion and cold contraction, and the same is true of resins. Therefore, the formed products begin to shrink after being formed, until they are cooled to the same temperature. However, the problem is that if the final shrinkage is too large, the size of the formed products will become too small, resulting in the size being too small. One of the reasons is that the initial mold size is determined The basic shrinkage of various resins is taken into account and its numerical value is calculated as a reference. However, even if the shrinkage rate is considered, the size of the formed product that shrinks in the actual process is smaller than the specification size. Therefore, in the above situation, it is necessary to revise the die again.
Secondly, it is related to the environment when it is formed. These problems include the drying state of materials, the mixing rate of recycled materials, forming conditions, etc., but the most important is the forming conditions. Because the mold temperature and injection pressure can increase the shrinkage, it is necessary to pay attention to it. Generally, if the injection pressure is increased, the size of the formed product will become larger The mold temperature has a tendency to decrease in size, which can be easily understood as follows: the size of the formed product is directly proportional to the injection pressure and injection time, and inversely proportional to the mold temperature
Third, in the phenomenon of long-term mass production, the waste gas generated from the material during injection is stored in the mold, so the filling property of the material is not good and the size is too small. In the content related to waste gas, the key problem is that there is no shortage in the injection molding of fine plastic, because the lack is a more important problem, so We need to think about it in detail in the future.
Countermeasures during forming:
Increase the injection pressure
Reduce mold temperature
Extended injection and cooling time
Confirm whether the type of forming machine and die are suitable.
Mold countermeasures:
New mold in the case of reprocessing bad place
In mass production, this kind of situation is due to the die wear or waste gas blocking. First, clean all the molds, clean the exhaust gas and then take shape again, which must be well confirmed.
2、 The size is too large to be good
reason:
It is the opposite phenomenon of the small size defect, called the large size defect. Therefore, the content of the defect is the same as that of the small size. Firstly, the size on the die is not made correctly; secondly, the forming environment; thirdly, the surface of the die is acidified and eroded due to the waste gas.
Forming countermeasures:
Reduce injection pressure
Increase mold temperature
Reduce injection and cooling time
Confirm whether the type of forming machine is suitable for the die
Mold countermeasures:
It will lead to the correction of the larger part
3、 Poor core swing
The defective condition that the formed product rotates with the axis as the center, and the swing value has exceeded the specified range is called core swing defect.
reason:
As one of the reasons, we should first consider the abnormal size of the mold; secondly, there are also incorrect forming conditions; finally, it is because of the waste gas generated during injection.
Countermeasures:
If you consider why the core swings, as shown in the figure, when the distance from the center of the formed product is different, when the shaft is centered and the main rotation is made, of course, the part that meets the accuracy tester will swing up and down. If that is the case, the best solution is to make the values of a and B tend to be equal. If it is considered as "the size of the mold is too large", in this case, the material filling of part a is better than that of Part B. similarly, if the filling ability of part B is the best, the filling ability of part a is not good.
However, if only considering the core vibration under the forming condition, it seems more difficult to make it change in the direction of high pressure and high speed filling as a practical situation. Only by changing the second stage pressure, second stage speed and die temperature can it change.
The following is a faster method to achieve filling balance: block one of the injection holes of the parts with poor core surface. When using this method, it must be noted that when the secondary channel material is dipped with glue and inserted into the injection hole to block, sometimes it will be pulled out by the flow material during the forming process. As a firm method, it is blocked with other high temperature resistant resins,
However, it is not necessary to mix different materials when falling off. The permanent method of operation is to block the hole with the shape of copper block, so as to change the filling balance, and the exhaust gas generated from the material during filling will change the filling balance. The flow of such waste gas and the great change of filling performance will also change the material density of the formed products.
If the above contents are summarized, that is to say, the filling density will be higher in the places with good exhaust, and the filling density will be lower in the places with poor exhaust.
Therefore, as a countermeasure to be taken in the mould, it is necessary to place the lowest vibration of the core
