Several key process parameters affecting the strength of injection molded parts
The strength of injection molded parts is a key indicator of their performance, directly impacting product reliability and service life. Process parameter settings largely determine part strength. During the injection molding process, process conditions during melt flow, cooling, and solidification affect the part’s internal structure, such as molecular orientation, crystallinity, and internal stress distribution, significantly impacting its strength. Understanding and properly controlling these critical process parameters is crucial for improving the strength of injection molded parts.
Melt temperature is the primary process parameter affecting the strength of injection molded parts. When the melt temperature is too low, the plastic melt has poor fluidity and is difficult to fully fill the mold cavity, resulting in defects such as material shortages and shrinkage holes inside the plastic part, reducing structural integrity and, in turn, strength. When the melt temperature is too high, the plastic will undergo thermal degradation, molecular chain breakage, and deterioration of material properties, which will also lead to reduced strength of the injection molded part. For crystalline plastics such as polyethylene and polypropylene, an appropriate melt temperature can promote uniform melt flow within the mold cavity, forming a more regular crystal structure during cooling, and improving the strength and rigidity of the plastic part. For non-crystalline plastics such as polystyrene and polyvinyl chloride, a reasonable melt temperature can reduce uneven entanglement between molecules, making the internal structure of the plastic part more uniform, thereby improving strength.
Injection pressure and holding pressure are also key parameters that influence the strength of injection molded parts. Insufficient injection pressure prevents the melt from overcoming flow resistance and fully filling every corner of the mold cavity. This is especially true for thin-walled areas in complex parts, where underfill or visible weld marks can easily form. These areas are often weak points in strength. Properly increasing injection pressure allows the melt to better fill the mold cavity, reducing internal defects while also increasing intermolecular bonding, improving the density of the part and thereby enhancing its strength. Holding pressure ensures that the melt continues to replenish the mold cavity as it cools and shrinks, preventing shrinkage cavities and depressions in the part. Insufficient holding pressure can lead to vacuum bubbles within the part, reducing structural strength. Excessive holding pressure increases internal stress, predisposing it to cracking and negatively impacting strength. Therefore, appropriate injection and holding pressures should be set based on the part’s structure and material properties. A holding pressure of 50% to 80% of the injection pressure is generally appropriate.
The impact of injection speed on the strength of injection molded parts is primarily reflected in the melt’s flow state and molecular orientation. If the injection speed is too slow, the melt spends a long time flowing within the mold cavity, cooling rapidly, and forming cold material at the flow front, resulting in reduced weld mark strength. At the same time, the molecular orientation is low, and the impact strength of the molded part may be insufficient. If the injection speed is too fast, the melt has a large flow inertia, which can easily generate turbulence and eddies, entraining air to form bubbles. Furthermore, at high speeds, the molecular orientation along the flow direction is too high, resulting in significant anisotropy in the molded part and reduced strength perpendicular to the flow direction. For thick-walled parts, a slower injection speed facilitates full melt filling and reduces internal stress. For thin-walled parts, a faster injection speed is required to prevent premature melt cooling, ensure complete filling, and avoid excessive molecular orientation. A reasonable injection speed should ensure laminar melt flow within the mold cavity, balancing filling efficiency and uniform molecular orientation.
The impact of cooling time and temperature on the strength of injection molded parts cannot be ignored. If the cooling time is too short, the internal cooling of the plastic part is insufficient. Continued shrinkage after demolding can lead to deformation and internal stress, affecting dimensional stability and strength. If the cooling time is too long, the production cycle will be prolonged, production efficiency will be reduced, and the plastic part may become excessively brittle, especially with certain elastomeric materials. Controlling the cooling temperature is equally important. For crystalline plastics, the cooling temperature affects the formation and growth of crystals. Slower cooling rates (higher mold temperatures) favor larger crystals but may result in uneven crystallization. Faster cooling rates (lower mold temperatures) result in finer crystals with more uniform distribution, generally improving the strength and toughness of the plastic part. Amorphous plastics are relatively less sensitive to cooling temperature, but an appropriate mold temperature can reduce internal stresses and prevent cracking caused by stress release during use.
Mold temperature is a critical parameter affecting the surface quality and internal structure of injection molded parts, indirectly impacting their strength. If the mold temperature is too low, the melt cools and solidifies rapidly on the cavity surface, forming a thin hardened layer. This can easily generate internal stresses as the melt cools and contracts, affecting melt fluidity and leading to poor filling. Excessively high mold temperature prolongs cooling time, increases production costs, and may cause part deformation during demolding. For parts requiring high surface quality and strength, such as structural components made of engineering plastics, properly increasing the mold temperature can improve melt fluidity, reduce weld marks, and achieve a denser internal structure. Furthermore, mold temperature uniformity is crucial. Uneven temperature distribution can lead to inconsistent cooling rates across the part, causing internal stresses and warping, and reducing overall strength. Therefore, in injection molding, precise control of mold temperature and its distribution is crucial, tailored to the material properties and part requirements.
