◆Burning (carbonization) and solutions
Brittleness and solutions
After injection molding, the impact resistance of the plastic part decreases significantly compared to the raw material; this phenomenon is called brittleness, As shown in the figure on the right. The direct cause of brittleness is excessive internal stress in the plastic part.
The causes of brittle plastic parts and corresponding solutions are as follows：
Material
① Set appropriate drying conditions before injection molding. If plastics are dried continuously for several days or at excessively high temperatures, although volatile substances can be removed, it can also easily lead to material degradation, especially for heat-sensitive plastics;
② Reduce the use of recycled materials and increase the proportion of virgin materials;
③ Select suitable materials, choosing high-strength plastics.
Mold Design
Increase the size of the main runner, branch runners, and gates, and avoid sharp corners in the runners. Too small a size of the main runner, branch runners, or gates, as well as sharp corners, can easily lead to excessive shear heat, which can cause polymer decomposition.
Injection molding machine
Choosing a suitable screw ensures more uniform temperature distribution during plasticization. Uneven material temperature can lead to excessive heat buildup in localized areas, causing material degradation.
Process conditions
① Reduce the temperature of the barrel and nozzle:
② Reduce back pressure, injection pressure, screw speed, and injection speed to reduce excessive shear heat and prevent polymer decomposition;
③ If the brittleness is caused by insufficient weld line strength, the weld line strength can be improved by increasing the melt temperature and injection pressure;
④ Reduce mold opening speed, ejection speed, and ejection pressure.
(5) Part Design The parts through which the fluid will flow in the middle of the part should not have excessively thin walls; at the same time, avoid sharp corners, inserts, notches, and other structures on the part that are prone to stress cracking or significant thickness differences.
Cracks (crazing) and solutions
After injection molding, the surface of the plastic part cracks and forms several cracks of varying lengths and sizes, which are called cracks, as shown in the figure below.Excessive holding time leading to improper gate shape and position design, excessive injection/holding pressure, poor demolding (forced ejection), excessive internal stress or molecular orientation stress in the plastic part, etc., can all cause crack defects. The specific analysis is as follows.
The specific analysis is as follows：
The residual stress is too high
External forces cause residual stress concentration
Generally, these types of defects always occur around the ejector pins. When such defects appear, the ejection device should be carefully inspected and adjusted. The ejector pins should be placed at the locations of greatest demolding resistance, such as bosses or reinforcing ribs. If the number of ejector pins cannot be increased due to limitations in the ejection area, a method of using multiple ejector pins with a smaller area can be used. If the draft angle of the mold cavity is insufficient, scratches and wrinkles will also appear on the surface of the plastic part.
The coefficients of thermal expansion of the molding material and the metal insert differ
Metal inserts should be preheated, especially when surface cracks in the plastic parts occur immediately after startup, as this is mostly caused by the inserts being too cold. Furthermore, when selecting insert materials, materials with coefficients of linear expansion close to those of the plastic should be used whenever possible.
When selecting molding materials, high molecular weight plastics should be used whenever possible. If low molecular weight molding materials must be used, the plastic thickness around the insert should be designed to be thicker.
Improper or impure raw materials
Practice shows that low-viscosity, porous plastics are less prone to cracking. Therefore, during production, appropriate molding materials should be selected based on specific circumstances. During operation, special care should be taken to avoid mixing plastics such as polyethylene and polypropylene, as this easily leads to cracking. In the molding process, release agents are also foreign substances to the melt; improper use can also cause cracking, so their dosage should be minimized.
The plastic part has an unreasonable structural design.
Sharp corners and notches in the shape and structure of plastic parts are most prone to stress concentration, leading to cracks and breakage on the surface. Therefore, the external and internal corners of plastic parts should be rounded with the largest possible radius. Experiments show that the optimal ratio of the transition radius to the wall thickness at the corner is 1:1.7.
Cracks in the mold are reflected on the surface of the plastic part.
During injection molding, the mold is repeatedly subjected to injection pressure, causing fatigue cracks to develop on sharp edges within the cavity, particularly near cooling holes. When cracks on the mold cavity surface are reflected onto the plastic part surface, the cracks on the part surface always appear continuously in the same location with the same shape. When such cracks appear, the corresponding cavity surface should be immediately checked for identical cracks. If the cracks are due to reflection, the mold should be repaired by machining.
Experience shows that PS and PC products are more prone to cracking. Cracks caused by excessive internal stress can be eliminated by annealing.
Causes and solutions for cracks
| Cause Analysis | Solution | |
|---|---|---|
| 1 | Injection pressure is too large or the injection speed is too fast | Reduce the injection pressure or reduce the injection speed |
| 2 | Holding pressure is too high or holding time is too long | Reduce the holding pressure or shorten the holding time |
| 3 | Melt temperature or mold temperature is too low/uneven | Increase the melt temperature or mold temperature (multiple small injection pressures can be used to make the temperature uniform) |
| 4 | Gate is too small, or its shape and position are improper | Enlarge the gate, change the gate shape and position |
| 5 | Release slope is insufficient, mold surface is not smooth or there is ejection resistance | Increase the release slope, repair the mold surface, eliminate ejection resistance |
| 6 | Too much or too little material in the barrel | Increase or reduce the amount of material added each time to the appropriate amount |
| 7 | Too fast ejection speed | Reduce the ejection speed |
| 8 | Poor accuracy of metal inserts | Preheat the metal inserts |
| 9 | Too large proportion of reclaimed material in the feedstock | Reduce the proportion of reclaimed material or do not use reclaimed material |