Cracks are more common form of plastic defects, the main reason is due to the stress caused by the deformation. There are stress and deformation residual stress, external stress and the external environment arising.
(-) Cracks caused by residual stress
Residual stress is mainly due to the following three conditions, namely filling excess, stripping launch and metal inserts cause. As in the case of a crack filled with surplus produce, and its solution is mainly in the following aspects:
(1) Since the minimum sprue pressure loss, so if a crack is generated in the vicinity of the main sprue, you can consider using the multi-point distribution point gate, side gate and handle-shaped gate way.
(2) ensure that the resin does not decompose, it does not deteriorate the premise, an appropriate increase in the temperature of the resin melt viscosity can be reduced to improve flowability, but also can reduce the injection pressure, to reduce the stress.
(3) Under normal circumstances, prone to stress when the mold temperature is low, the temperature should be raised. But when the high injection speed, mold temperature even lower, but also can reduce the stress.
(4) injection and dwell for too long can also cause stress, which is appropriate to shorten or Th hold pressure switching times better.
(5) a non-crystalline resin, such as AS resin, ABS resin, PMMA resin and other more crystalline resin such as polyethylene, POM prone to residual stresses, it should be noted.
Stripping launch, due to the small stripping slope, the mold plastic and punch roughness, so the introduction of excessive force, stress, and sometimes even the introduction of the rod around produce whitening or cracking phenomenon. As long as careful observation of crack-generating position, you can determine the cause.
In injection molding, while embedded metal parts, most prone to stress, and prone to cracking after a period of time before, great harm. This is mainly due to thermal expansion coefficient differences between metal and resin stress, and over time, the stress exceeds the strength of the gradual deterioration of the resin material and cracks. To prevent cracks resulting, as experience, the wall thickness of 7 & quot; and metal parts embedded in an outer diameter substantially unsuitable for general-purpose polystyrene inserts should be added with minimal impact on the nylon insert. Since the thermal expansion coefficient of glass fiber reinforced resin material is smaller, more suitable for the insert.
In addition, before forming the metal inserts were preheated, also has a good effect.
Crack (ii) external stress
External stress here is mainly due to the stress concentration caused by unreasonable design, especially in sharp corners should pay more attention. Figure 2-2 shows that the desirability R / 7 & quot; a 0.5 to 0.7.
Crack (iii) the external environment caused
Chemicals, moisture absorption due to water degradation, and excessive use of recycled materials will make the physical deterioration, cracks.
Second, less filling
Filling the main reason for lack of the following aspects:. I insufficient capacity of the resin. ii. lack of cavity pressure. iii. resin illiquid. iv. not exhaust effect.
As improvements, mainly from the following aspects:
1) extended injection time, to prevent the molding cycle is too short, causing the gate before the curing resin filled cavity upstream difficult. 2) increase the injection rate. 3) increasing the mold temperature.
4) increase the resin temperature. 5) increase the injection pressure.
6) expand gate size. Height generally equal to the gate of the wall thickness of 1/2 ~ l / 3. 7) Set the gate in the wall thickness is maximum.
8) Set exhaust ducts (average depth of 0.03mm, width 3 ~ smm) or exhaust strokes. More important for small workpieces. 9) between the screw and the injection nozzle to leave some (about smm) buffer distance. 10) selection of a low viscosity grade material. 11) the addition of lubricants.