Overmoulding
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1. How to encapsulate molds
In our lives, many products use plastic-coated molds, such as toothbrushes, handles, mice, buttons, car buttons, etc., which are composed of two or more materials. Among them, hard plastic is rare and soft plastic is rare. Hard rubber-coated products are mostly used for hand-contact products, and hard rubber-coated hard rubber products are mostly used for high-precision products.
1. Two-step injection molding
Semi-products produced by ordinary injection molding machines are processed and manually put into another set of molds for injection molding to finally obtain required products. In terms of equipment requirements, ordinary injection molding machines can meet requirements, so it is also a commonly used method in factories.
1) During second injection molding, clamping position and avoidance position for second placement of product need to be designed on mold so that product can be placed in designated position faster and more accurately to avoid mold compression.
2) When designing the mold, product is only shrunk on the first mold. Because product has shrunk to its actual size when it is taken out and placed in second mold, second mold will not be shrunk. Dimensional tolerances can only be measured accurately for the first mold, while size of second mold is usually soft plastic appearance product surface, and dimensional tolerance requirements are not very high.
2. Two-color injection molding machine
Two sets of molds are installed on two-color machine at the same time, or soft rubber and hard rubber are opened on one mold, and two materials are injected simultaneously in same mold. Usually a two-color machine has at least two barrels. After injection molding is completed, rear mold indexing plate rotates 180 degrees and a second injection molding is performed at the same time to finally obtain desired product.
(Double-color molds can be integral two-color molds or split-type two-color molds, both of which are installed in two-color machines. Principles are same, but there are differences in mold processing.)
Split two-color mold: Two sets of molds need to be opened, and mold bases of two sets of molds have same size. Rear mold part is same and front mold part is different. During installation, it is necessary to ensure that two sets of molds are at same height without any positional deviation.
Integral two-color mold: Split two-color mold is combined into one set of molds. Pouring system and ejection system are set up on same set of molds, but they need to work independently. That is to say, rear mold part is same and front mold part is different. There are two A positioning ring and ejected meson. Advantage of integral two-color mold is that only one set of molds is needed, processing cost is lower than that of split two-color mold, and one less mold needs to be installed.
1) Different from two-step injection molding, when primary molding is converted to secondary molding, there is no need to take out product, so during design, both sets of molds are shrunk, dimensional tolerances are still based on dimensional tolerances of the first mold.
2) Number of two-color molds in factory is limited. When scheduling production, it is necessary to determine whether machine meets molding requirements of product, based on measurement of molding machine, size of turntable and tonnage of two-color machine.
3) Product structure of two-color mold is relatively simple. Structure of lifter and slider should be designed as little as possible to ensure safe use and stability of mold when two-color machine is running.
4) When using a two-color mold produced by a two-color machine as a benchmark, guide pillars cannot be like guide pillars of ordinary molds. The one with reference angle is offset by 2.00mm toward center. Otherwise, when two-color machine is running, guide pillars and bushings will collide, mold cannot be closed.
2. Problems that are likely to occur with rubber-coated molds during molding processing
1. Size deviation
When injecting hard rubber first and then soft rubber, you need to understand that in mold design, shrinkage is added to hard rubber but not to soft rubber. Dimensional deviation has a great relationship with mold design and parts processing; secondly, soft rubber part usually has appearance requirements, and size requirements are not very strict. It is more likely to happen that after the first injection is completed, timing of second injection molding does not meet requirements. At this time, it is necessary to debug molding parameters and conversion time of the first mold product to obtain correct size requirements.
2. Product molding
Overmolding is the last thing a factory wants to see, and it usually happens when using a two-step injection molding practice. During process of placing semi-products of the first injection molding into second molding mold, product is not placed in designated position, or position of product is offset, which is a common cause of compression molding. Therefore, when production efficiency and yield are low, the only option is to strengthen employees' production awareness, or to switch to two-color molding production.
3. Package cannot be delivered
In order to make the two materials stick tightly, it is best to use a special two-color mold material. Smooth surface roughness of mold is also one of factors. When debugging product, avoid using release agents except during machine debugging to prevent product from sticking to mold and stuffing.
Using two-step injection molding method, when the first batch of semi-finished products is not immediately transferred to secondary molding process, and is detained for a period of time after being delivered to warehouse before secondary molding, surface needs to be cleaned of dust and preheated at 80℃-100℃ to achieve better encapsulation and bonding effects.
4. Lack of glue in secondary molding
It is necessary to ensure that product does not lack glue during primary molding, so as to ensure that yield rate can be improved during secondary molding. In secondary molding, soft glue of TPE material is usually used for encapsulation. Due to poor fluidity of material, finished product is not tightly wrapped and has missing edges. It is necessary to re-adjust molding parameters to maximize injection time and molding cycle.
5. One-time molding deformation
Temperature value and material selection of secondary molding barrel should be ensured to be above surface melting temperature of primary molding product, and temperature difference should be about 20℃. Secondly, during secondary molding, try to reduce holding pressure parameters or do not use holding pressure, otherwise it is easy to cause shape dislocation of joint surface or warping deformation of primary semi-product.
6. Long molding cycle time
This is a common problem in overmolding molds. Main reason for slowness is that in overmolding process, working efficiency of integral two-color mold is greater than that of split two-color mold and greater than that of two-step injection molding.
7. Glue front mold
Problem of sticking to front mold easily occurs when injecting TPE soft glue in secondary molding. Draft angle should be larger when designing mold, and surface should be etched.
8. Font after encapsulation is not clear.
When we are molding, for rubber-coated molds with fonts, fonts or logos are usually unclear, mainly because height of fonts is too low, which brings great trouble to process debugging. Standard "preload" value is higher than theoretical value of 0.02~0.05mm. Only formed product fonts will be clear.
In our lives, many products use plastic-coated molds, such as toothbrushes, handles, mice, buttons, car buttons, etc., which are composed of two or more materials. Among them, hard plastic is rare and soft plastic is rare. Hard rubber-coated products are mostly used for hand-contact products, and hard rubber-coated hard rubber products are mostly used for high-precision products.
1. Two-step injection molding
Semi-products produced by ordinary injection molding machines are processed and manually put into another set of molds for injection molding to finally obtain required products. In terms of equipment requirements, ordinary injection molding machines can meet requirements, so it is also a commonly used method in factories.
1) During second injection molding, clamping position and avoidance position for second placement of product need to be designed on mold so that product can be placed in designated position faster and more accurately to avoid mold compression.
2) When designing the mold, product is only shrunk on the first mold. Because product has shrunk to its actual size when it is taken out and placed in second mold, second mold will not be shrunk. Dimensional tolerances can only be measured accurately for the first mold, while size of second mold is usually soft plastic appearance product surface, and dimensional tolerance requirements are not very high.
2. Two-color injection molding machine
Two sets of molds are installed on two-color machine at the same time, or soft rubber and hard rubber are opened on one mold, and two materials are injected simultaneously in same mold. Usually a two-color machine has at least two barrels. After injection molding is completed, rear mold indexing plate rotates 180 degrees and a second injection molding is performed at the same time to finally obtain desired product.
(Double-color molds can be integral two-color molds or split-type two-color molds, both of which are installed in two-color machines. Principles are same, but there are differences in mold processing.)
Split two-color mold: Two sets of molds need to be opened, and mold bases of two sets of molds have same size. Rear mold part is same and front mold part is different. During installation, it is necessary to ensure that two sets of molds are at same height without any positional deviation.
Integral two-color mold: Split two-color mold is combined into one set of molds. Pouring system and ejection system are set up on same set of molds, but they need to work independently. That is to say, rear mold part is same and front mold part is different. There are two A positioning ring and ejected meson. Advantage of integral two-color mold is that only one set of molds is needed, processing cost is lower than that of split two-color mold, and one less mold needs to be installed.
1) Different from two-step injection molding, when primary molding is converted to secondary molding, there is no need to take out product, so during design, both sets of molds are shrunk, dimensional tolerances are still based on dimensional tolerances of the first mold.
2) Number of two-color molds in factory is limited. When scheduling production, it is necessary to determine whether machine meets molding requirements of product, based on measurement of molding machine, size of turntable and tonnage of two-color machine.
3) Product structure of two-color mold is relatively simple. Structure of lifter and slider should be designed as little as possible to ensure safe use and stability of mold when two-color machine is running.
4) When using a two-color mold produced by a two-color machine as a benchmark, guide pillars cannot be like guide pillars of ordinary molds. The one with reference angle is offset by 2.00mm toward center. Otherwise, when two-color machine is running, guide pillars and bushings will collide, mold cannot be closed.
2. Problems that are likely to occur with rubber-coated molds during molding processing
1. Size deviation
When injecting hard rubber first and then soft rubber, you need to understand that in mold design, shrinkage is added to hard rubber but not to soft rubber. Dimensional deviation has a great relationship with mold design and parts processing; secondly, soft rubber part usually has appearance requirements, and size requirements are not very strict. It is more likely to happen that after the first injection is completed, timing of second injection molding does not meet requirements. At this time, it is necessary to debug molding parameters and conversion time of the first mold product to obtain correct size requirements.
2. Product molding
Overmolding is the last thing a factory wants to see, and it usually happens when using a two-step injection molding practice. During process of placing semi-products of the first injection molding into second molding mold, product is not placed in designated position, or position of product is offset, which is a common cause of compression molding. Therefore, when production efficiency and yield are low, the only option is to strengthen employees' production awareness, or to switch to two-color molding production.
3. Package cannot be delivered
In order to make the two materials stick tightly, it is best to use a special two-color mold material. Smooth surface roughness of mold is also one of factors. When debugging product, avoid using release agents except during machine debugging to prevent product from sticking to mold and stuffing.
Using two-step injection molding method, when the first batch of semi-finished products is not immediately transferred to secondary molding process, and is detained for a period of time after being delivered to warehouse before secondary molding, surface needs to be cleaned of dust and preheated at 80℃-100℃ to achieve better encapsulation and bonding effects.
4. Lack of glue in secondary molding
It is necessary to ensure that product does not lack glue during primary molding, so as to ensure that yield rate can be improved during secondary molding. In secondary molding, soft glue of TPE material is usually used for encapsulation. Due to poor fluidity of material, finished product is not tightly wrapped and has missing edges. It is necessary to re-adjust molding parameters to maximize injection time and molding cycle.
5. One-time molding deformation
Temperature value and material selection of secondary molding barrel should be ensured to be above surface melting temperature of primary molding product, and temperature difference should be about 20℃. Secondly, during secondary molding, try to reduce holding pressure parameters or do not use holding pressure, otherwise it is easy to cause shape dislocation of joint surface or warping deformation of primary semi-product.
6. Long molding cycle time
This is a common problem in overmolding molds. Main reason for slowness is that in overmolding process, working efficiency of integral two-color mold is greater than that of split two-color mold and greater than that of two-step injection molding.
7. Glue front mold
Problem of sticking to front mold easily occurs when injecting TPE soft glue in secondary molding. Draft angle should be larger when designing mold, and surface should be etched.
8. Font after encapsulation is not clear.
When we are molding, for rubber-coated molds with fonts, fonts or logos are usually unclear, mainly because height of fonts is too low, which brings great trouble to process debugging. Standard "preload" value is higher than theoretical value of 0.02~0.05mm. Only formed product fonts will be clear.