It is very important to understand the differences among various silicone molding processes, when you are choosing a suitable silicone supplier and conducting production on your own. Next, we will provide you with a detailed introduction to the various features and critical steps of the silicone process.
What Is the Silicone Process?
Silicone process refers to the manufacturing method where liquid or solid silicone untreated materials are converted into finished components through processes such as vulcanization, molding, and post-treatment.
It determines the mechanical properties, dimensional tolerance, surface texture of the final product, as well as whether it complies with international standards.
For you, the choice of silicone processing technology will directly affect the performance of your product and the production cost.
Silicone Molding Processes: Key Types Compared
LSR Injection Molding
This is the highly automated process for manufacturing high-precision silicone products. It uses two-component liquid silicone and after being metered and mixed by the machine, is directly injected into a hot mold for molding.
If your product has an annual production volume of over 10,000 pieces and is structurally complex with strict tolerance demand, the process is suitable. Of course, products that meet FDA, medical or food-grade standards and have fine details, thin walls or inverted structures are also applicable.
Advantages: The LSR injection molding process has extremely high precision and the tolerance is controlled within 0.1mm. The forming cycle is only 15 to 90 seconds. The static mixing ensures a 1:1 mixing ratio stability, effectively reducing the waste rate to improve your production efficiency.
For you, due to the fully automated production process, along with the precise molds and cold runners systems, virtually eliminate flash.
Common applications: Specific examples include the production of medical device sealing gaskets, baby nipples and pacifiers, breathing masks and tubing, electronic connectors and buttons, and automotive LED lenses, etc.
Compression Molding
The compression molding process involves placing raw silicone material into a heated mold. The mold is then closed, and pressure is applied to ensure that the silicone completely fills the mold. Finally, it is cured and shaped under high temperature and high pressure.
Unlike LSR injection molding, in this process, the silicone needs to be manually placed into the open mold, and it is not fully automated.
If your annual production volume is low (between 100 and 10,000 units) and aesthetic requirements are not particularly high, this process is well-suited for your needs. Alternatively, if your annual production volume is moderate but you are manufacturing large, thick-walled parts with simple geometries, this process is also a suitable choice.
Moreover, you can also use this method when producing samples and conducting preliminary market tests. Since this method is the simplest and most controllable, and does not result in excess production, it offers the best value for money.
Advantages: The mold structure of this process is simple and does not require a gating system, which really can reduce your mold costs. If you are conducting your first production and are unsure about the shape of the product to be produced, you can adjust the cavity shape and quickly modify the mold.
When you produce thick-walled silicone products, the raw materials are not restricted by flow during the molding process. There is no shortage of materials, no shrinkage marks, no internal stress, nor any performance inconsistency caused by insufficient flow. Moreover, the hydraulic press is relatively inexpensive.
So, the process improves your simple product performance while lowering investment costs.
Common applications: We recommend specific applications that are suitable for you, including industrial gaskets and seals, buffers and shock absorbers, large-diameter sealing components, simple equipment bases, and low-precision custom parts.
Transfer Molding
The transfer molding process involves transferring the pre-heated solid silicone rubber into a closed mold. This process does not have the low output and low precision issues like compression molding, nor does it require high costs like LSR injection molding.
If your production scale is medium-sized (ranging from 5,000 to 50,000 pieces per year), you can use it. If those components have complex shape structures and embedded parts, you need to carry out high-precision processing, and this process is also feasible.
There is no doubt that when you need high efficiency but find the price of the relevant scanners for LSR injection molding to be too high, we recommend this process.
Advantages: The precision tolerance range of this process is 0.1 – 0.25 mm. It has a lower mold cost compared to the LSR injection molding process. Moreover, it has a shorter processing cycle (2 – 5 minutes). When you are producing parts that require insert molding, you can easily place the inserts before the transfer.
Common applications: You can pay more attention to these types of applications: connector seals with metal inserts, automotive guard rings and sheaths, hydraulic seals, and medium-complexity industrial components.
Extrusion
This is a process for the continuous forming of silicone using an extruder screw. Under the combined effect of temperature and pressure, the desired contour structure is formed through an extrusion mold with a specific cross-section. Subsequently, it undergoes heating and vulcanization processes to achieve the final shape.
If you are looking for a highly efficient silicone production process to be used for large-scale manufacturing in a factory, then extrusion is the perfect choice for you. However, your product needs to be long and continuous, with a uniform cross-sectional shape and high-capacity linear characteristics.
Advantages: If you need products with low manufacturing costs and continuous production capabilities, this process is the one you should choose. Its cost is much lower than that of injection molds or compression molds, and each product does not need repeated processing on the assembly line.
Common applications: Specific applications include silicone tubes and hoses (for medical, food, and industrial use), rigid or sponge-like sealing strips, sealing strips for automobiles and buildings, silicone ropes and strings, and foamed silicone profiles (containing foaming agent).
Overmolding
This is a specialized process. In this process, LSR is injected onto the pre-prepared substrate (plastic, metal or other silicone components). Then, these two materials are combined together through chemical or mechanical means to form a single and permanent component.
There are two main methods:
2-Shot (Two-Component) Injection Molding: This means that during the same injection molding cycle, two materials are injected into the mold one after another, thereby completing the molding process of a single finished product in one go.
Insert Molding: This process involves first placing the already formed substrate (such as plastic, metal or silicone) into the mold manually, and then injecting the liquid rubber into it.
However, we would like to remind you that for successful overmolding, it is essential to ensure the compatibility between the materials. The combination of liquid silicone rubber with certain engineering plastics (such as PBT, PC, PA) is the most ideal, and a primer may need to be applied to metals or other plastics.
Silicone-on-silicone insert molding is the most reliable method and does not require primer. So, if you are looking for the simplest way to achieve multi-color or multi-hardness effects in a single component, we recommend silicone-on-silicone insert molding.
Advantages: Because through chemical bonding or mechanical interlocking, a permanent connection can be formed, eliminating the need for glue bonding, screw fixation or snap connections, thus eliminating the need for secondary assembly and reducing your assembly costs.
It can also enhance the design flexibility of your products by combining a sturdy structure with soft seals or ergonomic surfaces. Over time, your products will not experience loose components or adhesive failure, and the increased durability will further enhance your brand reputation.
Your products will become more diverse as they can achieve different hardness and color levels on a single component.
Common applications: In the medical industry, this process can be used to produce breathing masks, syringe pistons, and surgical instrument handles. In the smart electronic accessory industry, there are smart watch straps, waterproof phone cases, earplug covers, and push-button keyboards.
If you want to design soft-touch steering wheel switches, gear shift knobs and sunroof seals that are seamlessly integrated with the vehicle frame, this process is also very suitable for these applications.
Step-by-Step Process for Each Silicone Molding Type
LSR Injection Molding Process
Storage: First of all, since the shelf life of LSR is short, you need to ensure proper storage of the raw materials. The LSR is composed of two liquid components (Part A consists of the base polymer + additives; Part B contains the platinum catalyst). Therefore, you need to store these two substances separately in sealed containers.
Mixing: Second, you transfer substance A and substance B to the static mixer in a 1:1 ratio. We suggest that you purchase a flow sensor, which will enable you to better monitor the uniformity of the substance. You can achieve a complete mixture of the two components simply.
Injection and Vulcanization: Third, inject the prepared mixed materials into the heated mold. We suggest that the temperature of the mold should be between 120 and 200 °C. In the high-temperature mold, the mixture is vulcanized through the cold runner system and the platinum-catalyzed addition reaction.
Demolding: After approximately 15 to 90 seconds, the LSR will be fully cured and you can then open the mold. Since it is an automatic production line, you don’t need to spend manpower to remove the products.
Post-curing and Trimming: If your product needs to comply with FDA/medical standards, then post-curing is required. All you need to do is put the components into a hot air oven at 160 °C and keep them there for 2 to 4 hours.
Due to the use of precise molds, there are very few burrs, so you can choose to omit them or make minor adjustments.
Inspection and Packaging: Finally, your product will undergo 100% manual visual inspection or vision system to conduct size and hardness tests. Just use anti-static bags or polyethylene bags for packaging, and attach the batch number labels, and it’s done.
Compression Molding Process
Preparation and Weighing: Because in this process you are using HCR, you need to first mix it with the curing agent and pigment on the machine. Then you need to cut the mixture into uniform thin slices or strips.
Determine the volume and weight of the product you need in advance. Weigh a certain amount of uncured putty to make its weight match the volume of the mold.
Molding and Compression: Put the HCR you have prepared in advance into the mold that has been opened and is continuously heated, either manually or by using a robot. We suggest that the temperature should typically be between 160 and 190 °C.
Then you close the mold under the action of a hydraulic pressure ranging from 100 to 200 tons, forcing the material to fill the interior of the mold completely.
Curing and Demolding: Because the silicone has already been fully cured within the closed mold through heating and pressurization, there is no need for any additional processing. Depending on the thickness of the components, the curing time is generally between 3 and 10 minutes.
Based on our experience, this molding process usually involves using a scraper to remove the product. Therefore, you will need to spend money to manually take the product out.
Trimming and Post-curing: Due to the pressurized production process, there will be excess materials at the boundaries of your products. You can remove the excess edges by hand trimming. Of course, if your product is intended for use in food or medical-grade applications, it also needs to be exposed to a 160 °C environment for 4 hours.
Inspection and Packaging: For you, product quality inspection is essential, which includes appearance inspection, size measurement and hardness testing. You can choose between bulk packaging or individual packaging depending on the size and weight of your product.
Transfer Molding Process
Material Preheating and Transfer: Unlike the previous two processes, in transfer molding, the HCR is first preheated in a separate container to soften. Then you can easily transfer the materials into the transfer tank located above the mold.
Curing: After you close and clamp the mold, use the hydraulic piston to force the softened silicone through the gate and the runner into the enclosed mold. Then, the silicone is subjected to curing under high temperature and high pressure for a period of 2 to 5 minutes.
Demolding and Burr Removal: When you open the mold, make sure to remove both the runner and gate. Then, cut off the gate and the runner from the product. Since you are using a closed mold, there are fewer burrs and only a small amount of polishing is required.
Post-curing, Inspection and Packaging: You only need to follow the above instructions for these steps and ensure the product quality.
Extrusion Process
Feed and Conveying: When you feed the HCR into the extruder, make sure it is in the form of strips or granules. This enables the material to be both sheared and heated during the transportation process, turning it into a thick paste-like substance.
Shaping and Vulcanization: You can select the molds according to the required cross-sectional dimensions, allowing the paste to be extruded through them and formed. Remember that the molds used in this step are all tubular and strip-shaped.
There are two methods for vulcanization treatment for you to choose from: hot air method or LMC (Liquid Curing Medium) method. The extruded silicone will undergo continuous vulcanization treatment in a heating tunnel that is 10 to 30 meters in length.
Cooling and Cutting: You can set up a water bath or air cooling area to cool the products that have undergone curing treatment. When you have completed this step, then cut the continuous long strips of products into the specified lengths, or roll them onto a reel.
Post-curing, Inspection and Packaging: If you need to perform batch post-curing, you can do it in the oven. The inspection method is roughly the same as before. However, we suggest that you pack the rolled or cut productions in boxes for easy transportation.
Overmolding Process
- 2-Shot (Two-Component) Injection Molding
First Shot and Indexing: First, you need to mold a sturdy base in the first model, such as plastic, metal, etc. Then rotate or move the mold so that the base aligns with the second mold.
Second Shot (LSR) and Demolding: After injecting LSR onto the base surface and allowing it to cure, the two parts become. You can then demold and remove the finished product.
Post-curing, Inspection and Packaging: Post-curing can be applied based on your product requirements. For the products produced by this process, we suggest that you pay particular attention to the bonding strength and appearance inspection. The packaging will be determined according to the requirements of your customers.
- Insert Molding
Substrate and Mold Preparation: You put the plastic or metal parts into the mold, and then the mold closes around the parts.
LSR Injection and Curing: Inject LSR into the surrounding grooves, and wait for the silicone to cure and adhere to the original insert.
Demolding and Inspection: After you have removed the formed product, conduct the adhesion test.
Post-curing and packaging: The same steps will be carried out according to your requirements.
How to Choose the Right Silicone Molding Process?
We have provided you with the following considerations:
What is your annual demand?
If the annual demand is less than 10,000 pieces, choose compression molding; if it is more than 100,000 pieces, choose LSR injection molding. Between them, you can choose transfer molding.
What is the complexity of your product?
Simple parts can choose compression molding, while complex parts are made with LSR. However, if it is not particularly complex and you want to save costs, then you can choose transfer molding. For products in the form of strips or lines, we recommend extrusion.
Are there any food/hygiene grade requirements?
If so, select LSR and perform secondary vulcanization best. Other processes are also acceptable as long as they are certified.
What is your budget?
Not only should the mold cost be taken into account, but also the unit price of the product.
FAQ
What Are the Typical Hardness Ranges of Silicone and How Is Hardness Measured?
The hardness range is from Shore A 5 to Shore A 90. It should be measured using a hardness tester that complies with ASTM D2240 or ISO 868 standards.
If you are the manufacturer of baby pacifiers, the hardness range is usually between 30 and 40 Shore A hardness; while the hardness range for industrial protective pads is usually between 70 and 80 Shore A hardness.
Will Silicone Shrink After Molding?
Yes. After curing, the silicone will shrink by approximately 2.0% to 4.0%. However, the exact amount depends on your material grade, curing temperature and design.
What Are the Typical Flash Volumes for Different Silicone Molding Processes?
LSR injection molding typically ranges from 0.00 to 0.05 mm. Transfer molding ranges from 0.05 to 0.15 mm; while in compression molding, the flashing material is the most at 0.10 – 0.30 mm. If you want to use compression molding, don’t worry. The mold must be over-filled to ensure that the cavity is completely filled.
Final Thoughts
Legensilico is a silicone product manufacturer with over 20 years of professional experience, capable of providing customized silicone services and process guidance for you. If you want to learn more about silicone or are interested in our silicone products, please feel free to contact us at any time.
