Modern mold technology

Nowadays plastic parts play a very important role in the modern industry. More than 60% of products have some or even most parts that are made of different polymers. Injectionmolding is the most widespread method of manufacturing plastic parts due to a number of factors: it provides the best surface finish, you can manufacture parts consisting of complex surfaces, you can make very thin structures, and most importantly, one mold provides you about 500,000-1,000,000 identical plastic parts. However, molding has its disadvantages too, one of those being a large lead time. Creating a mold is a whole complex project by itself that is why its use, for example, for Prototyping is limited. Modern mold technology research has been moving in that direction to lower the lead time of the molded products, increase the process speed and part quality and make molding useful for small-quantity manufacturing.

Modern mold making technologies and trends

 Mold making is a manufacturing process of creating a mold for Plastic parts. It consists of designing the cavity and the fixtures, creating and verifying the manufacturing technology, creating and setting up tools and fixtures and, finally, manufacturing. As you can see, even preparations for molding are complex and long. That is why manufacturers try to find ways to decrease the mold production time. The conventional method for making molds is CNC machining. It has great reliability, brings about great precision and quality, however, its lead time and cost is large. Molds are usually made of steel, which is quite a hard-to-machine material. That is why EDM is very popular in mold making. 

The principle of electrical discharge machining involves local evaporation of material under high voltage. The main advantage here is the ability to machine any type of electricity-conducting material with no regard to its strength or machin ability. The precision and surface finish of that method is good enough for most commercial molds. Another way to increase the speed of creating molds is to use SLS-printing. SLS-printing is an innovative way of sintering metal powder layer-by layer to produce a part without any need of tools and fixtures. So, creating a 3D-printed work piece drastically decreases the amount of machining that is required to produce a mold. It also decreases the amount of material waste from around 50-60% to 20% which is very high. In case of producing parts from a low melting temperature plastics, where metal molds are not needed, it is possible to 3D-print the part mock up using SLA-printing and then create a mold based on that part (for example creating soft rubber molds).

Different types of mold technology

As of now, mold technology is divided into a number of different molding types:

·       Injection molding. This one is the classic process where molten plastic is injected into the mold under pressure and is then crystallized;

·      Intrusion molding. The process here is different from the first one in the way of feeding plastic into the system. The Pressure is lower but the material is supplied by a special gear mechanism which works during the whole process and adds more material to compensate for the shrinkage during crystallization. That is really important when creating parts with wide walls.

·       Gas-Injection molding. This modification of injection molding fills the mold up to 75-80% at first and then blows some inert gas into the mold. The gas is under pressure so that the plastic fills all the nooks and niches of the mold better. After a few minutes, the gas is pumped out and the mold is filled with plastic completely. This type of Injection molding is great for parts that have long and thin walls.

·       Combination Molding. This process is almost completely like the classic injection with only one difference. Two types of material are used. For the outer surface, one color or type of plastic is used and the other one is for the inner surface. It brings many opportunities to manufacture parts with different color or properties depending on their surface.

So, as you can see, the method of injection molding is well researched and developed and depending on the type of part you make, one or the other method will bring you better quality and lower cost of the product to be manufactured.

Modern molding technologies

So, the mold has been created and the actual process of producing parts may be considered. In most cases, it consists of a number of stages:

·       Preparation. This stage consists of measuring the composition of plastic to be molded, colorant is added at that stage and the whole substance is dried to prevent pores.

·       Machine tool setup. The molding machine parameters such as temperature, pressure, molding time are chosen by the operator and the mold is fixed into the machine.

·       Loading. Here, the required amount of injection material (a mixture of plastic, special additives and colorants) is loaded into the machine bunker.

·       Injection. This process consists of heating the plastic until it melts and injecting it under pressure into the mold.

·       Crystallization. This is the last stage and the longest since the part must cool down to be removed.

As a result of the process and removal of some additional structures required for molding, a very precise part with fine surface finish that requires no or some finishing postprocessing.

Conclusions

Manufacturing of plastic parts is a rapidly developing industry. That is logical since more than 60% of current products consist of plastic parts and Injection molding is the mechanism that allows a reliable and repeatable creation of parts of the highest quality. Mold technology also has some disadvantages, the main of them being the complexity and the large costs of molds. This somewhat determines the area of its use: manufacturing identical parts in large volumes. However, extensive work and research is being conducted to negate this disadvantage. New technologies are being adopted to produce molds, such as 3D-printing and EDM machining. New designing methods and templates for making and modeling molds are being developed (most CAD|CAM|CAE systems of high level have a special mode for designing and testing molds). All of the work mentioned above moves the use of injection molding from a technology of mass production into the industry of low quantity and custom production.