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What items do we need to pay attention to in the design of plastic injection moulds?
Jun 13, 2018

One. Measurement and plasticization of injection moulds

In molding process, the control of ejection quantity and the uniform melting (plasticizing) of plastics are performed by the plastic mechanism of the ejector.

1, the temperature of the heating tube

Although the melting of plastics, about 60--85% is due to the heat generated by the rotation of the screw, the melting state of the plastic is still affected by the temperature of the heating tube, especially when the temperature is close to the front of the nozzle, which is easy to take place when the temperature of the front area is too high and to pull the wire when the part is taken out.

Precautions for the design of injection mold

2. Screw speed

(1) the melting of plastics is generally due to the heat generated by the rotation of the screw. Therefore, the screw speed is too fast, which has the following effects:

(1) thermal decomposition of plastic;

(2) glass fiber (fiber reinforced plastic) reduced;

The wear of the screw or the heating tube is accelerated.

(2) the speed setting can be measured by its circumferential speed.

Circumferential speed =n (speed) *d (diameter) * PI (PI). Usually, low viscosity thermal stability of plastic, its screw rod rotation speed can be set to 1m/s up and down, but the thermal stability of the plastic, it should be less than 0.1.

(3) in practical applications, we can reduce the screw speed as far as possible so that the rotary feed can be completed before the die is opened.

3, back pressure

(1) when the screw is rotating into the material, the pressure accumulated to the front of the screw is called the back pressure. In the ejection molding, the pressure of the oil pressure ejection can be adjusted to adjust the pressure of the oil pressure cylinder. The back pressure can have the following effect:

(1) more uniform melting of melt adhesive;

2. The color agent and the filler are more evenly dispersed.

The gas is withdrawn from the blanking mouth.

The measurement of the feed is accurate.

(2) the height of the back pressure is determined by the viscosity and thermal stability of the plastic. Too high back pressure makes the feeding time extended, and the increase of rotating shear force makes the plastic overheated. 5--15kg/cm2 is generally appropriate.

Precautions for the design of injection mold

4.(1) before the screw rotary feeding begins, the screw can be pulled back properly, which can reduce the pressure of the melt adhesive in the front end of the mold. This is called the front loosening. The effect can prevent the pressure of the melt adhesive to the screw, and it is used for the molding of the hot runner mold.

(2) after the screw rotary feed is finished, the screw can be pulled back properly, which can reduce the pressure of the fuse front melt adhesive. This is called the back release, and the effect can prevent the drop material of the nozzle.

(3) the shortcoming is that it is easy to mold the main stream (SPRUE), and too much loosening can suck into the air, causing air marks on the molding products.


Two. Attention to the design of injection mold

1, the top out should be uniform and powerful to facilitate the repair.

2, mold core and cavity should have enough rigidity and strength.

3, die should be hardened, polished, and wear resistant steel. Easy wearing parts should be easily replaced.

4, mold should be equipped with exhaust overflow tank, and should be located in the easy weld line.

5. The cross section of the gating system should be large and the process straight and short, so as to facilitate the uniform dispersion of the fibers.

6, the shape and thickness design of plastic parts should be considered in favor of filling flow chamber and avoiding sharp corners and gaps.

7, the release angle should be large, and glass fiber 15% should be 1 to 2 degrees, and glass fiber 30% should be 2 to 3 degrees. When demoulding slope is not allowed, forced demoulding shall be avoided and transverse parting structure should be adopted.

8, the design of the feed port should be considered to prevent insufficient filling, anisotropic deformation, uneven distribution of glass fibers, and easy to produce weld marks. The feeding mouth should be thin, thin, fan-shaped, circular and multi point intake to make flow of material flow, glass fiber is evenly dispersed to reduce the anisotropy, it is best not to use needle like inlet, the entrance section of feed can be appropriately increased and its length should be short.


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    E-mail: sheilaz@jt-precision.com

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