Asia Billion is an advanced precision plastic molding company in China. Let's talk about our views and key points on precision injection molding today. Injection molding is becoming more and more important for modern industries. All our equipment and instruments are almost all have our injection molding products. However, with the continuous demand for lightweight and high quality products, the requirements for precision plastic products are increasing. The higher it is for the plastic part precision. We can call a precision injection mold in 20 years ago if the tolerance requires 0.15 mm, and today our standard is 0.05 mm. Asia Billion can achieve 0.02 mm plastic molding product tolerance. This is the six main conditions that Asia Billion believes to be a precision injection mold and precision injection molded products.
Precision injection molding 6 essential terms
In the injection molding design, in addition to the general mold design considerations, special consideration should be given to the following:
1) In order to obtain the desired dimensional tolerances, consider appropriate mold dimensional tolerances
2) Consider preventing the occurrence of mold shrinkage fluctuations
3) Consider to prevent deformation
4) Consider preventing the deformation of the mold
5) To minimize the error in mold making.
6) Consider to prevent mold accuracy fluctuations.
1, appropriate mold size and tolerances
1.1 The relationship between the dimensional accuracy of the product and the dimensional accuracy of the mold
Draw a product map, considering the mold design, mold making and molding process.
First, the mold surface size can be obtained from the product drawing size. The mold is made according to the size of the mold drawing to obtain the actual size of the mold. The molded article can be obtained by using the mold to obtain the actual size of the product. The question is how this actual size fits within the required dimensional tolerances of the drawing.
1.2 Appropriate shrinkage rate
As described above, even in the same resin using the same pigment, the shrinkage ratio varies depending on the molding conditions. In precision molding, the degree of shrinkage change is small, and it is expected that the shrinkage rate and the actual shrinkage are as small as possible. Mainly to use the actual shrinkage rate of similar products in the past to estimate the shrinkage rate, but also to use the experimental model to find the actual shrinkage rate, and then modify and design the production model.
However, it is almost impossible to accurately presume the shrinkage rate, and it is inevitable to correct the mold after the trial molding. As a result of the correction, the recess will increase in size and the projection will be downsized. Therefore, the shrinkage ratio is set to a small value for the size of the concave portion, and the shrinkage ratio is set to a large value for the size of the convex portion. When the outer diameter of the gear becomes larger, it cannot be engaged. When the gear is small, only the backlash becomes large, so the shrinkage ratio should be set to a small value.
2, to prevent the occurrence of molding shrinkage fluctuations
Plastic molding company must be based on the assumption that the mold can be made to the required size. However, even if the mold size is certain, the actual size of the product will vary depending on the actual shrinkage. Therefore, in precision injection molding, the control of shrinkage is very important.
Whether the mold design is suitable or not dominates the shrinkage rate, and also varies depending on the resin batch. If the pigment is changed, the shrinkage rate also varies. Due to the different molding machines, the setting and reproducibility of molding conditions and the movement of each molding cycle fluctuate, and the actual shrinkage rate fluctuates, etc., so the control of shrinkage is difficult.
2.1 The main factors affecting the shrinkage rate
The size of the mold can be obtained by the size of the product plus the shrinkage rate, so the main factor of shrinkage should be considered when designing the mold.
The main factors affecting the molding shrinkage rate are:
(1) resin pressure,
(2) resin temperature,
(3) mold temperature,
(4) gate cross-sectional area,
(5) injection time,
(6) cooling time,
(7) product wall thickness,
(8) reinforcing material With basin,
(10) injection speed.
These effects vary depending on changes in items such as resin and molding conditions as below:
(1) Injection pressure
The Injection pressure has a great influence on the shrinkage rate. If the resin injection pressure is large, the shrinkage ratio becomes small, and the product size is large. Even in the same cavity, the resin pressure varies depending on the shape of the product, so that a difference in shrinkage rate occurs. In the case of a multi-cavity mold, the resin injection pressure in each cavity is likely to be different, and as a result, the shrinkage ratio of each cavity is also different.
(2) mold temperature
Regardless of the amorphous resin or the crystalline resin, if the mold temperature is high, the shrinkage ratio becomes large. The precision molding maintains the mold temperature at a specific temperature. At the time of mold design, attention must be paid to the design of the cooling circuit.
(3) Gate cross-sectional area
Generally speaking, when the gate cross-sectional area is changed, the shrinkage rate also changes. The shrinkage ratio becomes smaller as the gate size becomes larger, which is related to the fluidity of the resin.
(4) Product wall thickness
The thickness of the product wall also affects the yield. For the amorphous resin, the resin tends to have a different influence on the shrinkage rate of the wall thickness, and the wall thickness is large, and the shrinkage ratio is also large. On the contrary, the shrinkage ratio is small. For crystalline resins, it is necessary to avoid a particularly large change in wall thickness. In the case of multi-cavity modes, if the wall thickness of the cavity is different, the shrinkage will also produce a difference.
(5) Reinforcing material content
When using Boli fiber reinforced resin, the more the glass fiber is added, the smaller the shrinkage rate is, and the shrinkage rate in the flow direction is smaller than the transverse shrinkage rate. According to the resin, the difference is large. In order to prevent the twisting and warping, the gate must be considered. Shape fly mouth position and gate number.
Although the orientation is largely different, there is directionality for all resins. The orientation of the crystalline resin is particularly large and varies depending on the wall thickness and molding conditions.
In addition, there is also shrinkage after molding. The main factors affecting the shrinkage after forming are 1 internal stress relaxation, 2 crystallization, 3 temperature, 4 humidity.
2.2 Possible measures
(1) runner, gate balance
As described above, the shrinkage rate changes due to the resin pressure. In the case of single-cavity multi-point gates and multi-cavity modes,
To perform the same filling, it is necessary to balance the gate. The resin flow is related to the flow resistance in the flow path, so it is preferable to take the flow path balance before taking the gate balance.
(2) cavity arrangement
In order to make the setting of the molding conditions easy, it is necessary to pay attention to the cavity arrangement. Since the molten resin puts the tropics into the mold, in the case of the general cavity arrangement, the mold temperature distribution is concentric with the gate as the center. Therefore, when selecting the cavity arrangement of the multi-cavity mode, it is necessary to take the flow path balance easily, and take the concentric arrangement centered on the gate.
3, to prevent the deformation:
The reason for the deformation of the molding is that there is internal stress under uneven shrinkage, so it is necessary to prevent uneven shrinkage. In the case of a circular product having a hole in the center of the gear, a gate must be provided at the center. However, when the flow direction of the resin differs greatly from the vertical direction, there is a disadvantage that an ellipse is generated, and a circle with higher precision is required. When it is used, it needs to be set to 3 or 6 o'clock.
However, it is necessary to pay full attention to the balance of each gate. When the side gate is used, the 3-point gate will increase the inner diameter of the cylindrical product, and the inner multi-point uniform gate is less used in the case where the outer surface and the end surface do not allow the gate mark, and good results can be obtained.
4, to prevent demoulding deformation:
Precision products are generally small, the product wall thickness is thin, and some have many thin ribs. Chess design must consider that the product is not deformed, and can be properly removed. For resins with a small shrinkage.
When the molding pressure is high, it is necessary to pay attention to the fact that the product is easily left in the cavity. When the gear is molded by a resin having a small shrinkage ratio, the gear portion cavity is preferably designed on the ejector plate on the ejection side.
When using the pin, it is necessary to pay attention to the number of pins without deformation and the position of the top pressure. A perforated gear requires a core pin, and in order to facilitate parallel ejection when ejecting, it is necessary to provide an ejecting side template.
For horn-like part, a punching template can be used for ejection, and the ejection of the template can prevent deformation. Generally, precision products have a small draft angle. In order to reduce the mold release force, mirror processing is required, and the grinding direction must be the draft direction. It is necessary to set a block core that is easy to grind in the direction of the draft.
5, the smallest mold making error
5.1 Appropriate mold construction according to the desired processing method
In order to obtain the desired size of the product, there must be a corresponding mold size, and the mold needs to be processed with extremely high precision, and is limited by the finishing machinery.
In order to maintain the precision of the mold, high wear resistance is required, and quenching is required for this purpose. The precision of the quenching mold by the grinding machine and the electric discharge machine can reach 0.01 mm or less. Asia Billion can do 0.005mm.
When machining with a EDM machine, it is necessary to pay attention to the wear of the electrode end. When machining the cavity for the gear of Fig. 3 and machining with a wire electric discharge machining machine, it is necessary to design a structure that can be processed through as much as possible.
From the viewpoint of preventing grinding deformation and shortening the processing time, it is necessary to select a steel material having less quenching deformation, and to design a shape having less quenching deformation. When the shape is complicated, the bonfire cooling is not uniform and is prone to quenching deformation.
5.2 total split mold
In order to machine the quenched parts to a higher precision, a good grinding machine is used. Therefore, it is necessary to use an insert to form a split mold.
This mold has the following characteristics:
(1) Since a suitable material can be selected, a suitable hardness mold material can be used.
(2) It is possible to use mold materials with high corrosion resistance and wear resistance.
(3) Since heat treatment is possible separately, it is easy to set the heat treatment conditions.
(4) It is possible to use a mold material having good mirror workability, and the mirror surface processing operation is also easy, so that the mirror surface can be improved.
(5) Since it is easy to grind in the drawing direction, it is advantageous to use a mold having a small draft.
(6) Due to the hardening, the mold precision holding time can be extended, and the mold life is long.
(7) It is easy to set the exhaust at any position, so filling the mold is easy.
(8) Easy grinding and processing.
(9) The accuracy of the mold parts can be improved, so that the accuracy of the product may be improved.
(10) The cavity and core can be made with small tolerances, so the parts are interchangeable and easy to repair.
(11) Since the grinding process is the main part, the processing efficiency is high.
(12) The number of parts is large, and it is necessary to greatly improve the machining accuracy of each part.
(13) limited to specific processing methods
(14) Fully quenched
6, to prevent errors in mold assembly and components accuracy
To ensure the positioning of the sliders in each cycle, it is necessary to prevent fluctuations in the accuracy of the mold. In order to maintain the accuracy of the sliding parts, the sliding parts should be hardening and grinding. The matching of the sliding portion of the side core should have a positioning and retracting portion.
Asia Billion Plastic molding company overview: