1. Silicone rubber material properties
Silicone properties mainly include these aspects.
1.1 Types of silicone rubber
Firstly, silicone material is a kind of elastomer material. It’s gelatinous, colorless, translucent and odourless. According to the difference of physical property, we can divide the silicone rubber material into two types. It’s solid silicone rubber and liquid silicone rubber. Generally speaking, we mainly use solid silicone for products with die forming process. For example, custom silicone case, silicone keypad, and so on. As for clear liquid silicone, its featured liquid silicone rubber products include silicone nipple, silicone foam tube, and so on.
1.2 Silicone material properties
Secondly, the main silicone rubber properties are that it is resistant to both high temperature(the highest is 300℃) and low temperature(the lowest is -100℃). That’s also to say, the silicone temperature range is from -100℃ to 300℃. Certainly, silicone rubber is the rubber which can be best resistant to cold and heat. Also, silicone molding material has good electronic insulation. It has an advantage of high stability in thermal oxidation and ozone. However, the disadvantages of silicone rubber material include these. For example, it has low mechanical strength, and poor resistance to oil, solvent and acid alkaline environment. What’s more, silicone material is relatively difficult to vulcanize and the silicone prices are always expensive. The best silicone rubber temperature while using it is from -60℃ to 200℃. Of course, when it works in a short time, the high temperature silicone can reach to 230℃.
1.3 Silicone adhesive
Thirdly, it’s about the adhesive and connection problem. In general, silicon rubber products can stick well with the adhesive which is made from sticky silicone material. But silicone product manufacturers won’t do that with other material adhesive, especially before the surface handle. Because all silicone rubber suppliers know that other materials can be glued with silicone material. But it’s only under the circumstance that the silicone rubber material get the handle of surface activation and vulcanization of glue surface.
1.4 Silicone shore hardness
Last but not least, it’s about silicone hardness. In theory, we can choose silicone rubber hardness from 10° to 80° according to the silicone rubber shore hardness. It’s easy to find the silicone shore hardness scale from 20° to 70°. The most commonly used silicone hardness scale is 40° to 50°.
2. The dimension and characteristic of silicone rubber products
Xuzhuo Industrial Co., Ltd is one of China silicone manufacturers. And below are some properties of silicone products that we can provide. Those include extreme dimension, relative dimension, shrinkage rate, dimensional precision, parts design and so on.
2.1 Extreme dimension.
The thickest position of silicone parts can be 15-20mm. If the molding silicone parts are spherical, the maximum diameter can be 30mm. In general, we recommend that the thickness of silicone rubber products should not beyond 3mm. Because when the thickness is beyond 3mm, it takes more time to vulcanize. Of course, it will increase the cost of silicone production. In theory, the thinnest position of molding silicone rubber products can be 0.2mm. However, the thinnest silicone thickness we usually choose under ordinary circumstances is 0.3mm. And in general, what we recommend most is 0.4mm.
2.2 Relative dimension
As for the thickness difference, we have a suggestion. We think that the thickness difference between the thinnest place and the thickest place of one silicone product should be within three times. And as silicone rubber manufacturers all know, this kind of problem mainly depends on the requirement to the silicone temperature and pressure. Certainly, the requirement is applicable during the vulcanization process of silicone material.
2.3 Silicone shrinkage rate
The shrinkage rate of silicone molding material is related to its silicone shore hardness. The shrinkage rate range of silicone rubber material that silicone manufacturers provide from 1.022 to 1.042. In general, we take the shrinkage rate of 1.03 for the silicon rubber which silicone shore hardness scale is from 40° to 50°. When comparing with national plastic products, we can find that silicone products won’t produce similar apparent surface defect due to the shrinkage.
2.4 Dimensional precision of custom silicone products
Mostly, silicon rubber products are made by one custom silicone mold with multiple caves. It has so many caves compared with performance plastics products. Therefore, it’s not as convenient as consumer plastic products when it comes to the dimension control. In general, the precision of silicone rubber products is ±0.1mm. And the precision of high precision silicone parts is ±0.05mm. What’s more, we can make the smallest unilateral clearance be 0.1mm when we use silicone parts for the connection of keypad and the hole of molded plastic parts. And the unilateral clearance we suggest is 0.2mm.
2.5 Parts design
In general, customers provide original drawing according to the outside drawing of silicone rubber products. And they make an explanation about the related issues. Then silicone mold manufacturer will decide the design by themselves. Under ordinary circumstances, the unilateral clearance of the connection place between the silicone cover and products is 0.2mm-0.5mm.
3. Structure of mold for silicone parts
3.1 The upper limit of thermoforming molds
The upper limit thickness of thermoforming mould is about 250mm. And its dimension can be up to 500mm×500mm. In the thermoform molding area, the maximum dimension of molding products can be up to 400mm×400mm. As silicone mould manufacturers all know, the die mould structure includes upper mold, bottom die, bottom mold plate, top mold plate, ejector pin, ejector plate, core and hinge. When it comes to the injection mold design engineering, we should pay attention to characteristics of thermoforming mold and die production process.
3.2 Mold stripping
It could be forced demoulding for the mold stripping of silicone products. There is a dimension requirement for products without thermoforming mould design defect. It is mainly the circumference ratio of the largest part of the core and the smallest convergent place. The circumference ratio of custom silicone products which silicone hardness is under 55° can be 2-3. There must be no crack or sharp corner in the openings. The whole molding stripping process is almost manual operation, except for the dividing up and down of the die mold itself. Because it requires that the silicone temperature of vulcanization process should be under 180℃. And our engineers have considered the convenience and speediness of mold stripping when they start the parts design. It can avoid the bad effect of vulcanization quality and production efficiency due to the too long time of mold stripping.
3.3 Core fixing
The core fixing process of liquid silicone mold and solid silicone mold is not the same as that of custom soft plastic molds. In general, the core of silicone mold is removed with the product part, when we take parts during the mould unloading process. Therefore, compared with custom soft plastic molds, it’s difficult to handle when it comes to the core fixing and accurate positioning. And it has a higher requirement for the stiffness of the core. Under normal circumstance, custom silicone mold manufacturer do all mold and die cores together and fix all cores at a time. Or silicone mould manufacturers pass the die & mold core through a auxiliary to make it positioning quickly. What’s more, since there is a lot of pressure in the process of die assembly, our operator will strictly prevent the die and mold core movement.
3.4 Core pulling
Core pulling is mainly used on small size structures. The setting about core pulling of mold for silicone parts is similar with that of plastic injection molds. But it’s taken out by manual operation in the process of mould uploading. In addition, it will occupy a large space in the custom silicone mold. Therefore, it makes the thermoforming process be complex and the utility rate of mold space be pretty low. And we try not to use the core pulling.
3.5 Shape of forming die
Silicone molding material is a kind of elastomer material. And silicone products are very resilient when it’s surface is intact. But once there is any crack, it will expand pretty quickly under the action of external forces. Considering about above issues, injection molding process engineer should pay attention to these details when they design dies. On the one hand, there must be no sharp corners on all positions, and it’s to avoid cracks due to the concentration of stress. On the other hand, the R Angle of the forming die opening should not be less than 0.5.
3.6 Edge self splitting
The main function of mould and die edge self splitting are as follows. Firstly, it can contain the extra silicone material. Secondly, it’s convenient for mold venting with the self splitting edge of mold and die. Thirdly, it makes the edge trimming of die and mold be easy. In general, the self splitting edge of mold die is consisted of a thin edge part and a torn edge part. The thin edge part is on the position of component parting surface, and the torn edge is correspondingly firmer. In addition, the mold and die thickness of the thin edge part and the torn edge part are generally 0.1mm-0.2mm and 0.8mm. What’s more, the width is about 1mm-2mm.
3.7 Inserts of silicone products
We can install all kinds of inserts in custom silicone products. But there are some injection molding problems we should pay attention to. Firstly, injection molding process technician needs to preprocess the surface of the embedded parts. It’s mainly surface vulcanization or surface activation of molded parts. Otherwise, it will be difficult to connect the inserted parts and silicone parts firmly. Secondly, it’s about the fixed positioning problem of inserting thermoforming parts. They can be fixed in one direction in the vertical direction, but should be comprehensively fixed in other directions.
Of course, as injection mold manufacturers all know, it’s to avoid the movement of inserts in the injection molding manufacturing process. Thirdly, it’s about the silicone thickness around the inserting parts. With regard to the thermoforming parts that are full of glue, there should be at least 0.5mm glue thickness around the inserts. And as for the insert that has a position, there should be at least 0.4mm glue thickness around the inserts.
3.8 Other auxiliary thermoforming process
3.8.1 Silicone injection process
It is similar with process injection molding. At first, injection molding operator fixes the parts which are going to get silicone injection. Then, they close the first layer of injection molds to conduct injection molding process control. So that the thermoforming mold material will not contaminate the parts that are not in the design of injection. Next, injection molding engineer close the second layer of injection molding die and press the high temp silicone mold material into shape.
3.8.2 Rolling process
The injection molding engineer pre-moulded forming die material manually. All different types of rolling process can be used in the case of comprehensive glue coating.
3.8.3 Dipping glue process
The raw material is silicone liquid. The injection molding technician dips the injection molding parts into the liquid silicone. After the vulcanization of inner liquid silicone for molds, we dip again until the thickness of die design is reached.
3.8.4 Dropping rubber process
The raw material has good fluidity, it’s similar to oil paint and a kind of RTV silicone. The forming die is relatively simple. Generally speaking, the injection molding engineer carries out the die mould by hand. And they smear the material when the first layer of die mold is going to be solid.