1. Increase injection pressure, PP injection parts have high tensile strength
Increasing the injection pressure can enhance the tensile strength of PP injection-molded parts. Compared to other rigid plastics, PP has greater elasticity, so the density of injection-molded parts will increase with increasing pressure, which is a relatively prominent characteristic. When the density of plastic parts increases, its tensile strength naturally increases accordingly, whereas it decreases when the density decreases.
However, when the density reaches the maximum value that PP can achieve, further increasing the pressure will not continue to increase the tensile strength. Instead, it will increase the residual internal stress of the injection-molded parts, making them brittle. Therefore, moderation is advised. Similar situations occur with other materials, but the degree of prominence may vary.
2. Mold transport hot oil injection molding
Mold hot oil injection molding can improve the strength of POM and nylon parts. Nylon and POM materials belong to crystalline plastics. Using a mold hot oil machine for injection molding can slow down the cooling rate of the molded parts, thereby increasing the crystallinity of the plastic. At the same time, due to the slower cooling rate, the residual internal stress of the molded parts is reduced.
Therefore, the impact resistance and tensile strength of nylon and POM parts injection-molded with a hot oil machine will be correspondingly improved. It should be noted that the dimensions of nylon and POM parts injection-molded with a hot oil machine may differ slightly from those injection-molded with water, and nylon parts may be slightly larger.
3. The melting speed should not be too fast
The melt glue speed should not be too fast. Even with PVC injection at 180°C, glue formation can occur if the melt glue speed is too fast. Typically, when PVC 90-degree material is injected at 180°C, the temperature is sufficient, and glue formation is generally not a problem.
However, often due to the operator’s negligence or intentionally speeding up the melt glue speed to increase production, causing the screw to retract very quickly, such as in just two or three seconds, where the screw retreats to more than half of the large melt glue amount, the PVC material is severely insufficiently heated and stirred, resulting in glue formation due to uneven melting temperature and mixing, causing the strength and toughness of the injection-molded parts to deteriorate significantly.
Therefore, when injection molding PVC material, be sure not to arbitrarily adjust the melt glue speed to above 100 rpm. If it must be adjusted fairly quickly, remember to increase the material temperature by 5 to 10°C or appropriately increase the melt glue back pressure to match.
Also, be sure to regularly check for glue formation problems; otherwise, significant losses are likely to occur. Because most people generally do not pay too much attention to this issue, it is specially emphasized here that attention must be paid during production, and regular checks must be performed.
Conversely, it should also be noted that if the melt glue speed is too slow, even injection molding PVC 90-degree material at 180°C can cause glue burning issues, especially with transparent PVC, resulting in many black spots and air marks on the molded parts.
