【Abstract】Latent glue feeding is a commonly used method of glue feeding in injection moulds. Design method and precautions of latent glue feeding are discussed, as well as problem and solutions of dragging powder that may occur after latent glue feeding.

Latent glue feeding, also known as latent gate, realizes glue feeding on the side wall of plastic part by tilting gate. Generally, it is used when plastic parts require that glue cannot be directly injected on appearance surface, or gate can be automatically cut off to achieve fully automatic production. Latent gates need to be realized by EDM.

 

Figure 1 Latent gate form
Empirical value of each parameter of gate is discussed below (see Figure 1).
K=0.6~1.2mm. If K is too small, it will be difficult to feed glue, if it is too big, it will be difficult to cut, leave obvious scars, drag glue powder and other issues (will be discussed separately later).
Height of side wall of plastic part is L>3.5mm, and height is too small to be inconvenient for gate.
Gate opening angle α=10°~20°.
Gate inclination angle β=40°~60°, the larger angle, the more difficult it is to eject. Commonly used angle is 45°.
Gate length P is related to L and B.
Distance between runner and plastic part N>2mm, to ensure strength of steel material above gate.
Distance between ejector pin and gate M>6mm. If M is too small, gate will be easily broken due to insufficient deformation of runner during ejection.
Hidden depth H>P+3, to ensure that before gate is completely ejected, runner is still restricted, and ejection force it receives is vertically upward, rather than obliquely to gate.

In plastic molds, latent injection has been widely used. In actual production process, it often has some problems, such as rebound of gate material after ejection, scratching surface of plastic part; after ejection, rebound force of gate material is too large, causing runner to bounce off; gate is not clean during ejection, resulting in glue residue in mold. Glue debris is injected into plastic part during next injection. A defective product with an uncertain location is produced. This phenomenon is commonly known as "dragging rubber powder" or "dragging rubber shit". Reasons for this problem are discussed below.

(1) When just ejected, gate is torn off, section may be concave or convex (such as a piece of paper is torn, section will not be flat), concave situation will not produce rubber powder, which will not be discussed here; now analyze situation of cross-sectional convex glue (see Figure 2).
(2) Continue to push out, convex glue on section will be shoveled off by steel material above, turning into glue powder and remaining in runner or cavity (see Figure 3).

 

Figure 2 Sectional convex glue

 

Figure 3 Rubber powder remains in runner
(3) When next injection, remaining rubber powder is wrapped in plastic part, resulting in defective products.

Basic idea of improvement plan is to try to avoid production of convex glue after gate is broken. Improvement methods are as follows:
(1) As shown in Figure 4, add a reverse ejector rod III under runner ejector rod II, and leave a gap of 5mm. After ejector Ⅰ pushes out plastic part by 5mm, ejector Ⅱ starts to eject runner and gate. This structure gives ejector Ⅲ a slower shot effect, which is commonly known in the industry as "delayed ejector".
(2) Specific process analysis is as follows: when ejecting, ejector rod II is delayed, runner and gate will not move; ejector rod I ejects plastic part, connection between gate and plastic part is cut off by steel above the gate; because gate does not move, there is no space for section to produce convex glue, section may be flat, or it may be recessed on plastic part. After ejecting 5mm, ejector rod II ejects gate of runner.
(3) This method can try to avoid convex glue of gate section, and there will be no glue powder.

 

Figure 4 Improvement of ejector structure

There are also other methods used in the industry to avoid dragging off powder, which is briefly introduced here.
(1) Method 1: Increase distance M between ejector rod Ⅱ and gate to make M=15~20mm (see Figure 5). This method will increase size of mold, make runner longer, and waste more rubber.
(2) Method 2: Add an arc-shaped protrusion on runner between ejector pin II and gate (see Figure 6). This method adds water blocking to originally smooth runner, which changes flow rate and pressure of rubber compound, which is not conducive to forming plastic parts.

 

Figure 5 Method of increasing M value

 

Figure 6 Curved convex structure on the runner

Through analysis, it can be seen that whether increasing value of M or increasing arc-shaped water blocking, purpose is to increase elasticity of runner, so that part of initial stroke during ejection is used to overcome deformation of runner, so as to achieve gate delay top. Different routes have same goal, ultimate goal is same as "delayed ejector". Companies in the industry have their own considerations, and they have adopted solutions that suit them in light of local conditions.