Cracks refer to the narrow crevices or crazes that occur on molded plastic part surface, of which the causes might lie in the injection molding machine, the mold, the process or the raw materials.
(1) Injection Molding Machine
Mainly because of the insufficient plasticizing capacity, i.e. the plasticizing capacity is too low, leading to inadequate plasticization. Need to change for an injection molding machine that has a larger plasticizing capacity.
① Poor venting tends to cause flow marks, which decreases part strength;
② Gate size is too large, making the plastic injection molded part over-pressured; gate size should be reduced;
③ Water leakage occurs in the cavity or core – check whether there are any fine cracks;
④The plastic part is designed to be too thin – reinforcing ribs are needed;
⑤ Check the mold cavity, to see whether the mold core enjoys a sufficient draft angle;
⑥ Ejector pin oil spreads to the surface of mold cavity – the mold needs to be maintained and repaired;
⑦ Increase ejector pin diameter, and lower ejection speed;
（3）Injection Molding Process
① Adjust the temperature of material barrel. If the temperature is too high, the material will be decomposed; if too low, sufficient welding strength cannot be guaranteed;
② Adjust injection pressure. Raise temperature to ensure smooth mold filling and reduce viscosity; if the pressure is too high, cracks will be caused by internal stress, so the pressure needs to be reduced;
③ Reduce pre-molding back pressure and screw speed, to avoid material decomposition;
④Appropriately increase injection speed;
⑤ Increase cooling time. If the cooling time is too short, sufficient solidification cannot be guaranteed, thus causing ejection cracks or ejection marks;
① Ensure raw materials are completely dried;
② Raw materials are contaminated; find out the cause and take control measures;
③ Excessive recycled materials are added – identify the proper proportion of recycled materials through test results.