Injection molding is one of the most important plastic molding methods. How to improve the technical level of injection molding, to produce high-precision plastic products, and to create high added value products, is an important part of the design of the mold.
In the injection molding design, in addition to considering the general mold design issues, but also special consideration as follows:
In order to obtain the desired dimensional tolerance of the product, to consider the appropriate mold size tolerances
To consider to prevent the formation of shrinkage rate fluctuations
To consider to prevent the formation of deformation
To consider to prevent the emergence of demoulding deformation
To make the mold making the smallest error
To consider to prevent mold precision fluctuations
First, the appropriate mold size and tolerances
Product dimensional accuracy and die size accuracy related
Draw the product map, consider the mold design, mold making and forming process .First available from the product surface dimensions of the size of the mold surface. According to the size of the mold drawing mold, get the actual size of the mold. With this mold can be molded products , Get the actual size of the product.The question is how the actual size within the size of the desired size tolerance.
Appropriate shrinkage
As mentioned above, even in the same resin with the same pigment, the shrinkage varies depending on the molding conditions. In precision molding, the degree of shrinkage changes to a small extent, and it is expected that there is no difference between the shrinkage and the actual shrinkage. Is the use of finishing the past, the actual shrinkage rate of similar products to estimate the shrinkage rate, but also useful experimental model practical shrinkage rate, and then revised to design and production of mold situation.
However, it is almost impossible to estimate the shrinkage rate properly, and it is inevitable to correct the mold after trial molding. As a result of the correction, the size of the concavities increases and the size of the concavities decreases. Therefore, the size of the concavities is set to be small Value, the size of the bulge shrinkage will be set at a large value.Outside the gear size becomes larger can not be engaged, become smaller only when the backlash becomes larger, so the shrinkage rate should be set at a small value.
Second, to prevent the formation of shrinkage rate fluctuations
Precise injection molding must be based on the size of the mold can be made for the premise.However, even if the mold size, the actual size of the actual product shrinkage will vary.
So in precision injection molding, the control of the shrinkage rate is very important.Die mold design is appropriate or not to control the shrinkage, but also due to different resin batches, if you change the pigment, the shrinkage rate is also different.Because the molding machine is different, The setting of the molding conditions, the reproducibility, and the operation of each molding cycle fluctuate, causing fluctuations in the actual shrinkage rate and the like, so that control of shrinkage is difficult.
The main factors affecting shrinkage
Mold size can be calculated by product size and shrinkage, so the mold design, you need to consider the main factors of shrinkage.These changes due to resin and molding conditions and other items vary.
The main factors affecting shrinkage rate are:
Resin pressure
Resin temperature
Mold temperature
Gate cross-sectional area
Injection time
Cooldown
Product wall thickness
Reinforced material with basin
Directivity
Injection speed
1, resin pressure
The resin pressure greatly affects the shrinkage rate, the resin pressure is large, the shrinkage rate becomes smaller, the size of the product is large, Mold occasions, the resin cavity pressure within the cavity is easy to produce differences in the results of the cavity shrinkage rate is not the same.
2, mold temperature
Whether amorphous resin or crystalline resin, the mold temperature is high, the shrinkage becomes larger. Precision molding to maintain the mold temperature at a specific temperature in the mold design, we must pay attention to the cooling circuit design.
3, gate cross-sectional area
In general, when the gate cross section is changed, the shrinkage rate also changes, and the shrinkage rate 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 non-crystalline resin, due to different influence of the resin on the shrinkage rate of the wall thickness, the wall thickness is large, the shrinkage rate is also large, on the contrary, the shrinkage rate becomes smaller, while for the crystalline resin, Wall thickness changes particularly large in the case of multi-cavity mode, if the cavity wall thickness difference, the shrinkage rate will also be different.
5, enhance the material content
With glass fiber reinforced resin, plus more glass fiber, the smaller the shrinkage rate, the shrinkage in the direction of flow than the horizontal shrinkage rate, according to the greater the difference between the resin, in order to prevent warping twisting song, we must consider the gate The shape of the flying port location and the number of gates.
6, directional
Although there is a large difference in the orientation, however, there is orientation to all the resins, the orientation of the crystalline resin is particularly large, and the difference is due to the wall thickness and the molding conditions.
In addition, there are shrinkage after forming the main factors affecting shrinkage after forming:
Internal stress relaxation
crystallization
temperature
humidity
Possible measures are:
Runner, gate balance
Shrinkage due to resin pressure changes in the single-cavity multi-mode gate and multi-cavity mold situation, to be the same filling, it is necessary to balance the gate. Resin flow and the flow resistance in the flow channel, so take pouring It is best to take the runner balance before mouth balance.
Mold cavity arrangement
In order to facilitate the setting of the molding conditions, it is necessary to pay attention to the arrangement of the mold cavities. Since the molten resin heats the molds into the molds, the mold temperature distribution is concentric about the gate when the general cavity is arranged. Multi-cavity mold cavity arrangement, it is easy to get the balance of the runner, but also take the gate as the center of the concentric back arrangement.
Third, to prevent the formation of deformation
It is necessary to prevent uneven shrinkage due to internal stress due to uneven shrinkage, and in the case of a circular product having a hole at the center of the gear, a gate must be provided at the center of the resin. However, When there is a big difference in the shrinkage in the vertical direction, there is a drawback that the ellipse is generated. When a roundness with higher accuracy is needed, a gate with 3 or 6 points needs to be set.
But you need to pay full attention to the balance of each gate in the use of side gate, the 3-point gate will increase the diameter of cylindrical products in the outer surface and the end of the gate is not allowed to trace the situation less use of the inner multi-point uniform Mouth, you can get good results.
Fourth, to prevent mold deformation
Precision products are generally smaller, thinner wall thickness, and some there are many thin tendons. Chess design must take into account the product does not deform, but also appropriate to take off chess.For the shrinkage of the resin, when the forming pressure is high, Care needs to be taken that the product remains in the mold cavity. When molding a gear with a small shrinkage of resin, the gear part of the cavity is best designed on the top side of the mold.
When using the top pins, pay attention to the number of top pins and top pins without distortion, and the pin holes need a core pin. To help eject the pins in parallel when ejecting, you need to set the ejector pins.
For the horn-shaped products, you can use the punching template ejection, with this template to prevent the top of the deformation can be produced.General precision products draft taper.In order to reduce the release force, the need for mirror processing, grinding direction must pull Die direction.According to the direction of the drawing is easy to set up the block core.
Fifth, to prevent the error of mold accuracy
In order to ensure the positioning of each cycle of the slider, it is necessary to prevent the accuracy of the mold from fluctuating.In order to maintain the accuracy of the slider, the slider should be annealed with fire.