Plastic molds can be divided into three categories according to different types of gating systems:
(1) Large nozzle mold: Runners and gates are on parting line and are demoulded together with product when opening mold. It has the simplest design, is easy to process, and has low cost. Therefore, more people use large nozzle system to operate. Plastic mold structure is divided into two parts: movable mold and fixed mold. Movable part of injection machine is movable mold (mostly ejection side), injection end of injection machine is generally inactive and is called fixed mold. Because fixed mold part of large nozzle mold is generally composed of two steel plates, this type of structural mold is also called "two-plate mold". Two-plate mold is the simplest structure among large nozzle molds.

 

(2) Slim nozzle mold: Runner and gate are not on parting line, usually directly on product, so it is necessary to design an additional set of nozzle parting lines. Design is more complex and processing is difficult. Generally, a fine nozzle system is selected depending on product requirements. Fixed mold part of fine nozzle mold is generally composed of three steel plates, so this type of structural mold is also called "three-plate mold". Three-plate mold is the simplest structure among fine nozzle molds.
(3) Hot runner mold: Structure of this type of mold is roughly same as that of fine nozzle. The biggest difference is that runner is located in one or more hot runner plates and hot spouts with constant temperature. There is no cold material demoulding. Runner and gate are directly on product, so runner does not need to be demoulded. This system is also called a no-gate system, which can save raw materials and is suitable for situations where raw materials are expensive, product requirements are high, design and processing are difficult, and mold costs are high.
Hot runner system mainly consists of hot runner sleeve, hot runner plate, and temperature control electrical box. Our common hot runner systems have two forms: single-point hot gate and multi-point hot gate. Single-point hot gate uses a single hot sprue sleeve to directly inject molten plastic into mold cavity. It is suitable for plastic molds with a single cavity and a single gate; multi-point hot gate branches molten material into each hot gate sleeve through hot runner plate and then enters mold cavity. It is suitable for single-cavity multi-point feeding or multi-cavity molds.

(1) Injection molding
Plastic is first added to heating barrel of injection machine. Plastic is heated and melted. Driven by screw or plunger of injection machine, it enters mold cavity through nozzle and mold gating system, is hardened and shaped into injection molded products due to physical and chemical effects. Injection molding consists of a cycle consisting of injection, pressure holding (cooling) and plastic part demoulding processes, so injection molding has cyclical characteristics.
Thermoplastic injection molding has a short molding cycle, high production efficiency, and little wear on mold by melt. It can mold large quantities of plastic parts with complex shapes, clear surface patterns and markings, and high dimensional accuracy; however, for plastic parts with large wall thickness changes, it is difficult to avoid molding defects. Anisotropy of plastic parts is also one of quality problems, and all possible measures should be taken to minimize it.
(2) Compression molding
Commonly known as press molding, it is one of the earliest methods of forming plastic parts. Compression molding is to add plastic directly into an open mold cavity with a certain temperature, and then close mold. Under action of heat and pressure, plastic melts and becomes a flowing state. Due to physical and chemical effects, plastic hardens into a plastic part with a certain shape and size that remains unchanged at room temperature.
Compression molding is mainly used for molding thermosetting plastics, such as phenolic molding powder, urea-formaldehyde and melamine formaldehyde molding powder, glass fiber reinforced phenolic plastics, epoxy resin, DAP resin, silicone resin, polyimide, etc. It can also mold and process unsaturated polyester mass (DMC), sheet molding compound (SMC), prefabricated monolithic molding compound (BMC), etc. Generally speaking, compression molds are often divided into three categories: overflow type, non-overflow type, semi-overflow type according to matching structure of upper and lower molds of compression film.

 

(3) Extrusion molding
It is a molding method that allows plastic in a viscous flow state to pass through a die with a specific cross-sectional shape under high temperature and a certain pressure, then shape it into a continuous profile with required cross-sectional shape at a lower temperature. Production process of extrusion molding includes preparation of molding materials, extrusion shaping, cooling and shaping, pulling and cutting, and post-processing of extruded products (tempering or heat treatment).
During extrusion molding process, pay attention to adjusting temperature of each heating section of extruder barrel and die die, screw rotation speed, traction speed and other process parameters in order to obtain qualified extrusion profiles. Special attention should be paid to adjusting rate at which polymer melt is extruded from die. Because when extrusion rate of molten material is low, extrudate has a smooth surface and uniform cross-sectional shape; but when extrusion rate of molten material reaches a certain limit, surface of extrudate will become rough, lose luster, and appear shark skin, orange peel, shape distortion, etc. When extrusion rate is further increased, extrudate surface becomes distorted, even detaches and breaks into melt fragments or cylinders. Therefore, control of extrusion rate is crucial.
(4) Pressure injection molding
Also known as die casting. It is to add plastic raw materials into preheated feeding chamber, then put pressure column into feeding chamber to lock mold, apply pressure to plastic through pressure column. Plastic melts into a flowing state under high temperature and high pressure, enters mold cavity through gating system and gradually solidifies into a plastic part. This molding method is also called transfer molding. Pressure injection molding is suitable for plastics that are lower than solid. In principle, plastics that can be compressed and molded can also be molded by injection molding. However, molding material is required to have good fluidity in molten state when it is lower than solidification temperature, and has a larger solidification rate when it is higher than solidification temperature.
(5) Blow molding
It is to fix tubular or sheet blank made by extrusion or injection and still in plasticized state in molding mold, immediately introduce compressed air to force blank to expand and stick to the wall of mold cavity. After cooling and shaping, it is demoulded to obtain desired hollow product. Plastics suitable for hollow molding are high-pressure polyethylene, low-pressure polyethylene, hard polyvinyl chloride, soft polyvinyl chloride, polystyrene, polypropylene, polycarbonate, etc. According to different parison molding methods, hollow molding is mainly divided into two types: extrusion blow molding and injection blow molding. Advantage of extrusion blow molding is that structure of extruder and extrusion blow mold is simple. Disadvantage is that wall thickness of parison is inconsistent, which can easily cause uneven wall thickness of plastic products.

 

Advantage of injection blow molding is that wall thickness of parison is uniform and there are no flash edges. Since injection parison has a bottom surface, there will be no seams and seams at the bottom of hollow product, which is not only beautiful but also high in strength. Disadvantage is that molding equipment and molds used are expensive, so this molding method is mostly used for mass production of small hollow products, and is not as widely used as extrusion blow molding method.
(6) Die casting mold
Die-casting molds are also called transfer molds. Plastic raw materials are added to preheated feeding chamber, then pressure is applied to pressure column. Plastic is melted under high temperature and high pressure, then enters cavity through mold's gating system, where it gradually hardens and forms. This molding method is called die-casting molding, and mold used is called a die-casting mold. This kind of mold is mostly used for molding of thermosetting plastics.
In addition, there are foam plastic molding molds, glass fiber reinforced plastic low-pressure molding molds, etc.

(1) Hot runner mold
With the help of a heating device, plastic in gating system will not solidify and will not be demoulded with product, so it is also called a runnerless mold.
Advantage: No waste; Injection pressure can be reduced and multi-cavity molds can be used; Can shorten molding cycle; Improve quality of products. Features suitable for hot runner molding compounds; Plastics have a wide melting temperature range. It has good fluidity at low temperatures and good thermal stability at high temperatures; It is sensitive to pressure and does not flow without pressure, but it can flow when pressure is applied; Good specific heat properties so that it can be cooled quickly in the mold.
Plastics that can be used for hot runner include PE, ABS, POM, PC, HIPS, and PS.
There are two commonly used hot runners: 1) heated runner mold 2) insulated runner mold.
(2) Hard mold
Steel plate used in inner mold needs to be heat treated after purchase, such as quenching and carburizing, in order to meet requirements of use. Such an injection mold is called a hard mold. For example, inner mold uses H13 steel, 420 steel, and S7 steel.
(3)Soft mold (below 44HRC)
Steel used in inner mold does not require heat treatment after purchase to meet requirements of use. This type of injection molding is called soft molding. For example, internal mold parts are made of P20 steel, Ace steel, 420 steel, NAK80, aluminum, and beryllium copper.