When injection molding, the resin in sprue and in runner are maintained in molten state by special method,and when the product is released from mold, the runner still maintain in the mold. This is so-called hotrunner injection molding. The benefit and short are listed in the following table.

When taking product out of female mold, the mold has to be break into two halves, the parting line, PL is where theyare separate, it is also called division surface or mold split line. Base on this line, the fixed portion is called fixed mold, or female, and the movable part is called moving mold or male. When the PL is decided, the profile of female cavity and male cavity can be determined. And determine it needs side concentric type. 

It can be used to understand the difficulty of mold design. When selecting PL, the following shall be attended:

1. Select less prominent location or shape to avoid affecting the appearance of molded product.
2. At the mold opening, it is not recommended to have dead corner, to avoid increasing mold   

cost.

3. Should be at the location easy for through working, easy to process or easy to finish product.
4. The gate shape and location shall be taken into consideration.

The function of runner system is to guide molten resin injecting from the nozzle of injection molding machine into mold cavity . This system consists of sprue, main runner, branch runner, gate. The design and preparation of runner system have substantial effects on quality, precision, appearance and molding cycle. Sprue is the filling port of molten resin. In order to detach from the mold, it is normally design with 2°～4°chamfer. Main runner and branch runner is the path for the molten resin to flow into mold cavity, and here the fluidity and heat loss shall receive special consideration. Gate is the entrance of molten resin enters mold cavity. The design of gate has great effect on the molding and inner stress of product. Detailed runner system is shown in the following figure.

Cold slug well is also called “ Material Stay”. The purpose is to prevent flow mark on the next molding product left by molten resin. Generally, the nozzle tip of injection ma chine has small molten material left, which will be solidified before next injection. If it enters molded product, it will leave flow mark. To prevent it, the solidified material at the front of injection material is kept in cold slug well, to prevent hurting the appearance of product. Cold slug well is normally located at the junction of sprue and main runner, as show in above figure. So, the cold slug well is to keep the colder material at the front tip of injection nozzle of last molding, and allow the molten material with even temperature injected into mold cavity. This helps to even the density and quality of products.

 Gate has substantial effect on the molding feature and internal stress. Normally suitable type is selected per the shape of product molded. They may be split into Restricted Gate and Non-Restricted Gate. The former has narrow entrance between runner and mold cavity. It is easy for processing and to cut off the molded products from runner. It helps to reduce residual stress. It is easy to balance multiple cavities gates in multi-product in on shot. This type is generally adopted when he molten resin in mold cavity is not likely to reverse.  Under this category, there are Side Gate, Overlap Gate, Tab Gate, Fan Gate, Film Gate, Ring Gate, Disk Gate, Point Gate and Submarine Gate. The later is gate for molten material injected directly into cavity from sprue. This is the representative of non-restricted gate. The type, location, size, number of gate has direct effect on the molded products in appearance, deformation, retraction and strength. Therefore, in design, the following shall be taken in consideration:

1. Shape of Gate:

Shape of gate affects the molten resin fluidity inside cavity, the appearance of molded product, and the material flow direction. It is therefore necessary to select type of gate base on type of material or shape of molded products and the effect on flow direction.

2. Location and number of gates:

(1)  The location of gate shall allow molten material to run throughout all part of the mold,

   and it is better to be in the center or the thick part of molded products.

(2)  The hole on molded product will have key on the mold. Do not let the injected material

 bend or shift those keys.

(3)  When at two or more locations, they shall not allow the welding line or air bubble damage

appearance of product and reduce the strength.

(4)  Residual stress tends to concentrate adjacent to gate, which could become bristle and

crack. So it is recommended not to select the location sustaining force.

(5)  Selection the location where it is not prominent on the product surface, easier to process.

3.Type of Gate (Shape):

In its function, gates may be separated into Restricted Gate and Non-Restricted Gate. The former is build a narrow part at the joint between main/branch runner and the mold to hold the flow of material; the later has sprue for material to flow directly into the entrance of female mold. Ordinarily restricted gate is more selected. The features, shorts and benefits are listed in the following table.

Runner is also called horizontal runner and is the channel for molding material from Sprue to female mold. Runner normally needs be solidified before taking out, so the runner is cut into two halves at the diameter line for processing then match up. Base on the fluidity, section shape of runner, there are round, U, Trapezoid, rectangular and square shape. Round and semisphere and trapezoid are the better ones. But in the fluidity and heat dissipation, round runner is the best. The next are rectangular, trapezoid and square. Semi-sphere is rather less used except in special circumstances. The benefits and shorts are as the following.