The general principle of plastic molding technology is similar to that of metal casting. Plastic injection molding is done using thermoplastics. This is a type of polymer that is capable of reversibly passing from a viscous to a solid state. When heated, they first become highly elastic and then become viscous.
In this state, they are blown into the molds. As a result, a uniform polymer mass is formed, which is fed into the molds under pressure, where it solidifies.
To obtain finished products by molding plastic into silicone molds, two-component polymer systems are used, in which a liquid rubbery monomer and a catalyst are mixed. There are two polymerization systems for these systems – addition and condensation curing.
To create a form, use a prototype … It can be made on the basis of a 3D model or a sample provided by the customer.
Types of plastic used for injection
The technology of plastic molding of plastics is used to obtain various plastic parts in any quantity – from single prototypes to mass production. Polyurethanes (two-component plastics) can be used as raw materials for casting.
TYPES OF PLASTIC CASTING
Plastic injection molding
The first step is to prepare special molds based on a prototype or 3D model. Modern molds make it possible to produce parts with tolerances of the order of a few micrometers, precise geometry, using curved surfaces, and various bends. The creation of molds is carried out in several stages.
– Mold design carried out by specialists on the basis of models, prototypes or documentation provided by the customer. At this stage, calculations are performed for thermostating (the temperature of the array inside the mold must be the same), rational pouring, the flow rate of the plastic melt, the requirements for the strength of the mold, the choice of fit of parts that will be mated are taken into account.
– Mold making … In this process, both high-quality manufacturing of parts and their correct assembly are important. Molds for the manufacture of thin-walled products are especially complex.
– Debugging and testing … The final stage, allowing to release the first prototypes and send them for approval to the customer.
Plastic injection molding is used for the manufacture of thermoplastic products under pressure from 80 to 140 MPa. The works are carried out on special piston and screw machines with a high degree of mechanization and automation. Plastic molding machines independently dose granular raw materials, heat them up to a viscous-flowing state and under pressure are fed into the injection mold. After that, the mold is kept under pressure until it hardens, it is opened and the finished products are pushed out. The final phase is, if necessary, the removal of the flash and the grinding of the finished parts.
Injection molding machines produce medium and large batches of complex-shaped parts that fully comply with the customer’s project documentation. According to the location of the injection unit, there are:
– horizontal injection machines in which the form is located in a vertical plane, the plastic is fed horizontally, from the side;
– vertical injection machines , in which the form is located horizontally, and the plastic is fed from top to bottom.
Vertical injection units are used to produce embedded parts.
There are injection molding methods using additional agents that improve the quality of finished parts. This can be injection molding using an inert gas that seals the plastic and makes it more uniform. There is also plastic molding with preliminary heating of the mold with water vapor, which makes it possible to obtain smooth surfaces and parts that do not require additional machining. Casting with water, two-component casting, casting with the use of additional decoration is also often used.
Injection of plastics into silicone molds
Manufacturing of silicone molds … The first stage making silicone molds is the formation master models … The shape of the casting is developed using 3D CAD systems. In this case, a 3D printer (photopolymer compositions) or a prototyping unit that performs sintering of powder materials is used.
A model of a part that is planned for production can also serve as a model. Then the finished model undergoes a thorough surface treatment , the line of parting of the future shape is determined with the help of adhesive tape, which is also glued to all holes. Deep grooves and holes are reinforced with metal elements. If necessary, an additional texture is created on the tape. A gating system is formed from several sprues and risers.
The third step is making the mold where the model is installed. The formwork is mounted, the gating system is placed inside according to the model, after which the mold is filled with a mixture of silicone and hardener. Before pouring, the silicone must be degassed in a special vacuum chamber.
The cast mold is also placed in a special chamber for degassing the silicone while the mold solidifies. The duration of this process is about a day.
After polymerization, the finished mold is removed from the mold and can be used for further molding from plastic.
Before starting plastic molding, the silicone molds are heated in a heating cabinet. All parts of the form are fastened with adhesive tape or a stapler.
Preparing working solution of two components – resin and catalyst … It is very important to observe the necessary ratio of components – this will ensure high-quality maturation of the future plastic.
The mold is installed in a vacuum cabinet and positioned vertically, and mixed components of the future compound are poured from the chamber through a funnel inserted into the sprue. The final maturation of the plastic takes place in an oven.
Advantages of silicone molding
Silicones are an excellent material for the production of molds, as they allow you to cast any kind of plastic (polyester, epoxy, polyurethane, and others).
Silicones adhere little in the material from which the casting is performed, but they are able to provide completely sealed joints. In them it is possible to carry out not only plastic molding, but also rubber.
They have a high temperature range of use from -70 to + 4500C, non-flammable, resistant to ultraviolet radiation, chemically inert, resistant to most chemical influences (acids, solvents, alkalis, gasoline, water, reagents).
Advantages of Silicone Mold Casting:
– Ease of use.
– Ability to make thin parts or make strong undercuts.
– Easy release from the mold, high flexibility, reducing internal stress on the part.
– No breaks when reused.
– High chemical inertness.
– Form making does not require the use of special equipment.
The silicone mold can withstand from 20 to 100 pouring cycles, has a low cost, which makes it possible to widely use the manufacture of silicone molds for the manufacture of single parts and the needs of small-scale production. With its use, it is possible to perform casting with embedded parts made of metal, other types of plastic or glass (fasteners, stiffening ribs).
WHY DO YOU NEED SMALL BATCH PRODUCTION?
The production of plastic parts with a circulation of up to 1000 pieces is considered a small-scale production (Unit Production). In the modern economy, it is considered a transition from a single to a serial one and is gaining more and more popularity. This is, for example, the manufacture of unique machines and mechanisms to order.
Small-scale production of plastic parts is necessary in cases of need for functional testing of parts, manufacture of cases of unique devices, models for checking the quality of design work, design of packaging materials, models for presentations, production of kits for manual assembly of various models, restoration of prototypes. Their use allows launching a new product on the market within 1-2 weeks at low production costs.
