Cold runner block and cold runner system is a technology used in silicone or rubber injections. The cold runner system is a manifold with an injection unit for silicone or rubber molding and a temperature control mounted between the mold plates and chambers. The manifold, which is processed with precision machining, is an equipment that distributes the unvulcanized material to certain parts of the mold with the help of channels and nozzles. The press injection unit first injects the material into the channels of the CRB (Cold Runner Block) during the cycle. These channels of the CRB that carry the material are kept below the temperatures at which vulcanization will take place. In these channels, which are also called manifolds, cooling is carried out in a controlled manner with liquid circulation. The material coming from the injection unit with pressure is first transmitted to the CRB and then to the mold. The remaining material in the CRB channels remains unvulcanized and ready for each cycle. It is ready for use directly in the next cycle. This feature, which CRB provides as a basis, reveals the raw material savings compared to other hot runner systems.
Similar to runner-free production in thermoplastic injection molds, thermoset and elastomer materials can be processed without runner waste in molds with cold runner systems. Cured sprues formed in the injection of silicone or rubber materials cannot be regranulated as in thermoplastics. The feeding channels in the cold runner block system are controlled at low temperatures to prevent vulcanization of the elastomer material. The features that must be present in a cold runner system are very important. To reliably prevent crosslinking, the temperature gradient must be kept to an absolute minimum and the thermal insulation between the die and the cold runner must be complete. However, it should be designed in such a way that it is easily accessible to interfere with the mold or cold runner block without wasting much time if difficulties arise during operation.
Recently, injection molds have been produced integrated with cold runner blocks. Since the cold runner blocks allow automation indirectly, the injection molds of the parts where LSR silicone will be used are designed with package systems, including the cold runner block and automation. This type of systems provides value-added advantages in high-volume production.
In cases where high volume silicone parts will be produced; It is recommended to use the cold runner system for ease of automation, less material wastage and to reduce cycle times. The burrs and runner wastes that occur in compression, transfer or hot runner injection molds have become more important considering the increasing material prices. When cold runner systems are used in injection molds, this is a way of minimizing material waste. Cold runner systems are used to make projects more cost-effective by eliminating runners and material waste.
Some of the main reasons that emerged in the production sectors that process elastomer materials played an important role in the development and widespread use of cold runner block systems. One of them is that elastomers are not fully recyclable like thermoplastics. Although chemical recycling of elastomers is technically possible, elastomer material waste is disposed of as waste because it brings high technology and costs. In thermoplastics, mold runners or burrs are physically turned into smaller particles and re-use is possible and maintains its financial value. However, in silicon or rubbers, other than the product formed in the mold to obtain the actual part during the process; Elements such as runner waste, burrs, excess material have no monetary value. These elements, which are formed other than the final piece, are only physically broken down into smaller pieces and are used in applications such as filling material and coating in insignificant places compared to the initial purchase cost. In this case, an expensive elastomer material cannot be converted into more value-added works, namely the final part. Even in terms of ecological balance and efficient use of resources, cold runner blocks have become a necessity for businesses.
One of the situations that requires the use of runner blocks is production costs. Unit costs will not decrease as long as elastomer materials turn into waste materials in the form of waste. On average, 60% of the cost of a silicone or rubber part is material. The remaining 40% includes labor, energy, machine costs, mold costs, secondary operations and processes. For this reason, material wastage is an issue that should be considered in order to reduce production costs. It should be used efficiently when processing silicone or rubber materials. The production parameters such as the geometry of the part, the properties and cost of the compound to be used, the production method, the depreciation of the investment need, the mold investment costs, the mold runner system, the number of mold chambers directly affect the sales prices of the part to be produced.
Especially mold design and molding method play an active role in production costs in elastomer materials. Especially with a cold runner system to be integrated into injection molds, serious cost improvements can be achieved depending on the geometry of the part and the process conditions. Because any material loss that is not included in the sales price of a manufactured part is a potential that needs to be improved and turned into a product to create added value. In the molding process, the waste material thrown in each cycle is reflected in the unit costs of the part and increases the costs. When these wastes, which are included in the unit costs of the enterprises, are minimized or completely eliminated, the improvement from this can be used for many different sources.
In the elastomer part production sector, very efficient results can be obtained with the use of CRB for reducing runner wastages and saving materials. This is applicable regardless of the geometry of a particular part. Therefore, it is recommended to make important process calculations when choosing a production method.
The basis of the working principle of the cold runner block is the cooling of the material channels and nozzles to prevent the pre-curing of the material inside. After the curing process in the mold is completed and the part production is completed, there is no need to remove the injected material from the mold together with the produced materials. The material remaining in the cold runner block after the cycle can be used ready for the next injection.
In some cases, the cooling of the standard cold runner block may not be sufficient for the nozzle and distribution channels. In standard blocks, undesirable cooling may occur in the mold chamber as a result of the nozzle contacting the mold chamber in direct injection into the mold chamber. This will result in a significantly longer curing time or poor curing on the product. For this reason, when a standard cold runner block is used, the nozzle tips are positioned at a certain distance from the die chambers. Thus, unwanted errors are prevented, but a certain amount of hot runner waste occurs.
Basically, there are two types of cold runner blocks according to nozzle structure and feeding type. Open nozzle system (open gate nozzle) and valved nozzle (valve gate nozzle) systems are used in cold runner blocks as the main principle. The open nozzle system was the first of the more conventionally known CRB types. In blocks with open nozzles, the material flow can be intervened manually, but very precise control is not possible. In blocks with valved nozzles, on the other hand, much more precise material control can be achieved by opening and closing the nozzle tips according to the injection behavior of pneumatic or hydraulic controlled needles. With some systems developed by companies that are professional on CRB, open nozzle or valved nozzle systems can be used more efficiently.
A CRB with an open nozzle system is less costly, requires less maintenance, can have a narrower nozzle spacing, and can operate with less plate strokes on the injection molding machine. CRBs with open nozzle systems do have some downsides. Since it is an open nozzle, it requires control so that the material residue between cycles is not injected into the next cycle. The material flow of the nozzle must be fine-tuned for a balanced injection cycle. A hardened plug from the open nozzle will be injected in the next shot. There is no complete control over individual nozzles. In the first stage of the injection cycle, the cycle needs to be adjusted to the optimum. In systems where this type of CRB will be used, the injection press must have an injection nozzle movement option.
By using cold runner systems with valved nozzles, repeatable cycles can be managed more reliably and the risk of material residues being injected into the next cycle is eliminated. The valve nozzle system allows each nozzle to be controlled separately within itself. They are also easily used with less process knowledge. The injection process can be carried out at lower injection pressures. This makes it easier to process smaller or larger parts. Along with these advantages, these systems have some disadvantages. Due to the valve nozzle structure, it has a more complex mechanics and the investment cost is higher. An additional pneumatic mechanism is required to control valved nozzles. These mechanisms in valved nozzles increase their size and require larger center distances within the CRB. In addition, due to the pneumatic system in valved nozzles, more plate strokes are needed in the presses where these CRBs will be used.