Polypropylene (PP) is a type of plastic material that is top-tier in lightness, cost-effective, and highly processable among plastic materials. It is similar to polyethylene, but it features a superior glossy surface, is less prone to scratches, and is stiffer than PE. It is widely used in everything from everyday goods to industrial products, and it ranks second only to polyethylene in terms of usage volume among all plastics.
Reference: Vinyl Industry & Environmental Association - "Production Volume by Type of Plastic"
Due to its cost-effectiveness, light weight, and high functionality, polypropylene is used in various categories ranging from everyday items to industrial parts. It is used in industrial parts such as automobiles and home appliances, as well as containers. It is also widely used in daily necessities and miscellaneous goods. It is safe for the human body, hence it is also used in medical devices.
Among all plastic materials, being lightweight and inexpensive makes it suitable for mass production.
PP has high impact resistance and mechanical properties, similar to an enhanced version of PE. It has a glossy appearance on its surface. Due to its water resistance, oil resistance, and chemical resistance, it is used in medical and research fields, in items such as syringes and testing equipment.
PP has good fluidity and results in a clean appearance. It is functionally strong and used in many products. The key points during the molding of PP are as follows:
PP does not absorb moisture, so no pre-drying with hopper dryers or box-type dryers is required.
Set around 200°C. When stringing is strong, lower the barrel nozzle temperature.
Set between 20-40 (up to 80) °C. Mold temperature greatly affects the appearance of the product surface. Low mold temperatures can lead to poor flow, causing short shots and sink marks.
Generally, PP is an easy-to-handle material and is processed into various products in many industries.
In industrial applications, main material PP is mixed with carbon fibers, additives, weathering agents, and colorants. This can increase gas generation and reduce flow.
Adapt the points according to the materials used and the structure of the mold.
At the flow convergence points, weld lines are inevitable. To prevent gas trapping in dead ends, adjust the injection speed in multiple stages.
During mass production, mold wear over time can increase the size of burrs. The mating surfaces of the parting line, especially at the flow ends and slides, wear out relatively quickly. Burrs do not suddenly appear; they gradually increase in size.
Before the burrs exceed quality standards, adjustments in injection speed, injection pressure, holding pressure, and metering values are necessary. If these adjustments in molding conditions are insufficient, mold repair by welding and re-machining the affected areas is effective.
PP has a high shrinkage rate, making sink marks likely to occur in thicker sections of molded items. Increasing holding pressure to compensate for the shrinkage improves this. Applying too much holding pressure in one stage can cause burrs, so adjust sink marks by applying holding pressure in multiple stages. Also, applying holding pressure longer than the gate seal time is ineffective for the product and merely extends the cycle time.
Jetting and drooling occur in contrast. It is necessary to adjust the suck-back amount and back pressure. To prevent jetting, reduce the suck-back amount or increase the back pressure; to prevent drooling, increase the suck-back or reduce the back pressure.
After tens of thousands of shots, short shots and gas burns due to poor gas escape can occur. Routine inspection of the parting lines and gas vent areas, and regular mold overhauls are necessary.
During molding, if the mold cooling pipes are clogged and flow is poor, the mold is not sufficiently cooled and the product dimensions become smaller. Also, as PP is a crystalline resin with a high shrinkage rate, especially in multi-cavity molds for medical parts, sorting out defective parts becomes difficult, leading to large amounts of waste. In such cases, it is particularly important to take time samples to ensure there are no issues across all cavities.
PP, the lightest among all plastic materials, is widely used due to its cost-effectiveness and high mechanical properties. Its moldability is very high and it is an easy-to-handle material. As a crystalline resin, it has a high shrinkage rate, so it is important to take type samples and monitor to ensure that the dimensions after shrinkage fall within the standard values. Consider the above factors and aim for more productive molding processing.
