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Rapid tooling injection mold


Rapid plastic injection molding or RIM is a specific process for production of small batches of plastic parts with a lead time which is much shorter than standard injection molding. Even though the equipment used in RIM is similar to one used in traditional injection molding, RIM molds are designed for quick turnaround instead of long-term durability for high-volume production.


The main advantage of rapid injection molding is that it uses mold materials like aluminum that are more affordable and easier to machine than those used in conventional injection molds. Although these molds may not be able to withstand the rigorous demands of large production runs, they are appropriate for small batches of at least 500 pieces.


This article will take a closer look at the rapid injection mold tooling process, its operational principles, equipment requirements, and offer guidance on choosing the best suitable part material.

Rapid injection molding

Rapid Platic Injection Molding: What Is it?

Rapid Injection Molding is a technique that uses the same injection molding equipment and methods as the conventional ones but with much less lead time and costs. Its fast turnaround is advantageous in the production of prototypes and limited-volume runs, which is essential in the refinement of designs before full-scale production. This speed is mainly due to the fact that the molds are inexpensive and the materials like aluminum are easily machined, therefore, production takes just a few weeks. It is worth mentioning that rapid injection molding must not be confused with reaction injection molding, which is the process of molding thermosetting plastics.

Where Is It Used?

The area of Rapid Injection Molding is primarily used in prototyping, research, and development projects that don’t require mass production of millions or even hundreds of thousands of parts. It is the most effective for production runs starting from at least 500 units and above. This approach makes it possible for engineers to test their designs with limited capital investment before committing to the expensive tooling which has a long lead time for high-volume production.


The high cost of production injection molds is because of their construction from highly wear-resistant materials that are required for continuous production use. The rapid injection molding prototyping is the link between the early design phases and full-scale production, which offers a cost-effective way to test new products, materials, and design concepts. Moreover, it can be used to deal with the situation of the unexpected increase of product demand volume.

How Does Rapid Injection Molding Works?

RIM (Rapid Injection Molding) is based on the parts that are designed according to the DFM (Design for Manufacturing) principles that are specific to injection molding. One of the main elements of RIM is the production of molds that are economical to make and use, with aluminum being the most popular choice because of its easy machining. Nevertheless, these molds are not fit for mass production.


After the RIM mold is completed, its core and cavity halves are then connected to a standard injection molding machine. The molding starts with the mold halves being put together and secured with hydraulic rams.


The first step of RIM involves the introduction of raw material (usually thermoplastics or thermoset silicones) into a screw situated in a barrel. The barrel is heated externally by heaters and the screw shears the plastic pellets, which melts them. Once the plastic reaches the required consistency, the screw forces it into the mold through a nozzle, which has a one-way valve to prevent the pressure from reversing into the screw. The mold is filled with the high-pressure plastic, and the hydraulic rams are used to ensure that the split line of the mold does not leak. The mold is held together for a few seconds, then the mold opens and the part is ejected by ejector pins. The screw is retracted, and the process is repeated.


It is necessary to mention that although rapid injection molding is effective, it is not inherently faster than standard injection molding techniques in terms of production of individual parts.

Key Components of RIM Press

The components of a rapid injection molding (RIM) press are described in the following paragraphs. These key components are similar to those in a conventional injection molding press.


1. Material Feeder or Hopper 


The material feeder, usually a hopper, is the place where the thermoplastic resin pellets are stored. These pellets are supplied from bulk bags or silos based on the production volume and part size. The hopper maintains a constant flow of material to the screw and in certain cases preheats the resin to speed up the melting process in the screw and barrel, which in turn reduces the per-part production cycle time. Moreover, level sensors in the hopper will inform the operators when the material is running out.


2. Reciprocating Screw


The screw reciprocates and has multiple functions. It measures and controls the flow of material from the hopper to mold. The screw, which is in motion, squeezes the pellets as it transports the raw material, and the pellets are compressed into a smaller volume as the screw shaft’s diameter increases. This shearing process is accompanied by heat generation through the shear forces, melting the pellets as they shear against the screw and barrel. Some screws, in addition, mix the material for a more homogeneous melt and ensure the even distribution of additives. When the material is melted to the required level, the screw advances and a one-way valve prevents material from flowing back, pushing it into the mold. The screw goes in and then out, and the process is repeated.


3. Barrel 


The barrel contains the screw which is used to push the raw material into the mold. In most cases, the barrel has heating elements to help with the melting of the pellets. The friction produced by the screw against the barrel is one of the factors that lead to the melting of plastic. The nozzle of injection is located at the end of the barrel, which helps in the material flow into the mold.

4. Heating Elements

The barrel is equipped with electrical heaters which are wrapped around it to help in melting plastic pellets. But, it is important to mention that these heaters are not the main source of heat for melting the plastic. In contrast, the screw will generate pressure by forcing the pellets against the barrel, which will result in shear heating and consequently melting of the plastic.


5. Materials 


Injection molding, which is the most common of the rapid prototyping injection molding methods, is mainly based on thermoplastics, which can be commodity-grade or specialty-grade, depending on the specific application.


6. Injection Nozzle 


The injection nozzle guides the injected plastic into the mold and is variable in size, commonly between 2. 5 to 10 mm based on the capacity of the injection molding machine. The nozzle is connected directly to the barrel and may have filters to keep particles that are not required from entering the mold. The two types of filters that are commonly used are screen pack and gap filters, with the latter being preferred due to its larger filter area and less flow resistance. Furthermore, some nozzles are designed to have mixing capabilities to ensure that the melt is homogeneous and that additives like colorants are evenly dispersed throughout the melt.


7. Mold 


The rapid injection mold comprises two main parts: the core and the cavity, each on a platen. One mold half stands still, while the other one is pressed against it by a hydraulic ram to prevent plastic leakage at the parting line. The cooling channels inside the mold are designed to allow the heat transfer fluid to circulate through them, thus speeding up part solidification and decreasing cycle time. Once the plastic has solidified, the ejector pins push the part out of the mold, and the mold closes to start the cycle again.


RIM molds are made from inexpensive materials and are designed to have shorter lifespans, usually lasting for a few hundred parts. One more cost-effective method of RIM is using master unit dies (MUD) for modular molds that enable quick design changes without complete mold remanufacturing.

8. Finished Part 

The end piece is the final product of the injection molding process. The design of injection molded parts should be done while keeping the manufacturing principles specific to injection molding in mind for the best quality. These principles govern the design parameters like the right wall thickness, the position of the reinforcing ribs, the hole sizes, and other design considerations that are vital for the production of quality parts.plastic machining

Typical Wall Thickness of Rapid Injection Molded Parts

The average wall thickness of the parts made by rapid injection molding is between 1. 5 to 2. 5 mm. Nevertheless, this thickness is not an absolute value, as it can change depending on the material used, since each material has its own range of optimal wall thickness. It is worth mentioning that there is no difference in the achievable wall thickness between rapid injection molding and standard injection molding processes.

How To Select The Appropriate Material For Rapid Injection Molding?

The choice of the appropriate material for RIM is based on the end-use of the product. RIM is usually applied in the context of prototype development, pilot production and short-run volumes, which can be considered as a bridge between the initial design stages and full-scale manufacturing. Consequently, the selected materials should be similar to those which will be used for the full-scale production. RIM can be used to test a variety of materials in order to find the most appropriate one.


The main considerations in material selection are cost, mechanical strength, UV resistance, electrical properties, and thermal resistance. These properties can be improved by the addition of glass or carbon fibers, which will act as additives. On the other hand, it has to be emphasized that these fibers are very abrasive and can decrease the life of molds, especially aluminum molds.

Compatible Materials For Rapid Injection Molding

Injection molding is a large area of application, covering common polymers to specialty plastics and polymer blends. Plastics industry offers a wide range of resins, and each is designed to meet specific end-use properties and processing conditions. The choice of the right resin is a very important one because there are lots of other factors to be taken into account. Furthermore, plastics can be categorized into different grades, for example, those with fillers like glass or carbon fibers.


On the contrary, although rapid injection molding can process the same plastics as traditional injection molding, it needs to be noted that abrasive materials such as glass-filled nylon may cause the softer molds to wear, which in turn means that the tool life will be shorter. On the contrary, that does not mean that the materials can not be used. One of the examples is that Alea Labs used an innovative compound with a high glass content for the molding.


In addition, some of the corrosive types of plastic, e.g. PVC or POM, can also contribute to the decreased life of the injection molds. Tool life is not a concern when it comes to prototypes or small production volumes, but you still need to analyze all the part requirements. Injection molding with rapid molding can impact the project schedule in ways such as repairs or mold replacement which, therefore, the part volume should be carefully considered.

What are The Primary Benefits of Rapid Plastic Injection Molding?

The advantages of the quick-injection molding are the shortened lead time, the increased design flexibility, and the ability to try out different materials. It also allows a small-scale production run for the manufacturing on-demand.

Is rapid prototyping injection molding a Quick Process?

In fact, rapid injection molding has been proven to be speedy. The time from submitting the design to receiving sample parts is much shorter than the standard lead time for full-scale production injection molds.


In summary, rapid injection molding is a significant innovation in the manufacturing industry, which ensures speed, flexibility, and high-quality production.


The future prospects of rapid injection molding in the manufacturing industry seem to be promising. Technology keeps evolving and the need for speed and efficiency is growing. This is why rapid injection molding will remain a crucial tool for innovators and manufacturers.