Material shortage (under-injection) and solutions
Short shots are a common defect in injection molding. They occur when the melt fails to completely fill the mold cavity, resulting in partially underfilled parts. These defects can include missing corners, missing ribs, or unclosed holes. These defects not only affect the product’s appearance but can also weaken its structural strength and lead to functional failure. Short shots are closely related to melt fluidity, mold venting, and process parameters, resulting from the interaction of multiple factors. In production, systematic analysis is required to identify the root cause and implement targeted measures to effectively resolve it.
Insufficient melt flowability is the core cause of material shortfalls, primarily influenced by plastic properties, melt temperature, and shear rate. The melt flow rate (MFR) varies significantly among different plastics. Plastics with low MFR values (such as PC and POM) exhibit poor flowability and are prone to material shortfalls in complex cavities due to high flow resistance. Plastics with high MFR values (such as PE and PP) exhibit good flowability and a lower risk of material shortfalls. Excessive proportions of recycled material (over 30%) in the raw materials can reduce melt flowability, as the molecular weight of the recycled material decreases after multiple processing steps, increasing melt viscosity. Excessively low melt temperature increases viscosity and hinders flow. For example, PP melt temperatures below 180°C can easily cause underfill in thin-walled cavities (less than 1mm thick). Excessively high temperatures can cause plastic degradation and the generation of low-molecular-weight volatiles, which in turn increase flow resistance. Insufficient shear rate can also affect flowability. When injection speeds are too slow, the melt cools too quickly during flow, causing a sharp increase in viscosity and preventing it from reaching the end of the cavity. Therefore, improving melt flowability is the primary measure to address material shortfalls.
Improper mold design can exacerbate material shortages, particularly regarding the gate, runner, and cavity structure. Improper gate placement, such as positioning the gate far from the cavity end or directly against the cavity wall, increases the melt flow path and resistance. For example, in long, rectangular products, if the gate is located at one end, the other end is susceptible to material shortages due to excessive melt pressure loss. A gate that is too small (e.g., a diameter less than 0.8mm) causes a significant pressure loss as the melt passes through, preventing it from filling the cavity. A runner that is too small or has too many turns increases resistance along the way, reducing the pressure at which the melt reaches the cavity. Poor cavity venting is a common cause of material shortages. Air trapped in the cavity cannot escape and is compressed by the melt, creating high pressure that hinders the melt’s flow and causes material shortages in areas that are last to be filled, such as corners and deep cavities. Furthermore, high surface roughness in the cavity increases melt flow resistance, especially in poorly polished areas. This hindered melt flow can easily cause stagnation and lead to localized material shortages.
Improper process parameter settings are a direct cause of material shortfalls, with injection pressure, speed, and holding pressure parameters having the most significant impact. Insufficient injection pressure prevents the melt from overcoming flow resistance and reaching the end of the cavity. This is especially true for complex or large parts, where sufficient pressure is required to propel the melt through the cavity. For example, when molding a 500mm long ABS part, injection pressures below 80 bar are prone to material shortfalls at the end and should be increased to 90-100 bar. Too slow an injection speed causes the melt to cool excessively during flow, increasing viscosity and reducing filling capacity. In this case, the speed should be increased to an appropriate range (typically 50-100mm/s). However, too high a speed can lead to turbulent entrainment, requiring balanced adjustments. Insufficient holding pressure and time prevent melt shrinkage. When the melt begins to cool and shrink, there’s insufficient pressure to propel the subsequent melt through the cavity, leading to material shortfalls. Furthermore, too low a mold temperature can cause the melt to solidify rapidly on the cavity surface, forming a solidified layer that hinders internal melt flow. This is particularly true in thin-walled areas, where the risk of material shortfalls increases significantly when the mold temperature falls below 50°C.
Solving the material shortage problem requires a systematic approach encompassing raw material processing, process optimization, and mold improvement. Regarding raw material processing, the proportion of recycled materials must be strictly controlled (no more than 20%), and the raw materials must be thoroughly dried (e.g., PA at 80°C for 4 hours) to remove moisture and prevent increased viscosity. For plastics with poor flowability, adding an appropriate amount of plasticizer (e.g., adding 5%-10% DOP to PVC) or switching to a higher MFR grade, such as increasing the MFR of PC from 10 to 15 g/10 min, can improve flow properties. Regarding process optimization, the primary measure is to increase the melt temperature, such as from 220°C to 240°C for ABS and from 190°C to 210°C for PP, to reduce melt viscosity. Simultaneously, increasing the mold temperature, such as from 50°C to 70°C, can reduce the melt cooling rate. Adjust injection parameters and adopt a “high speed and high pressure” strategy: increase injection pressure by 10%-20% and injection speed by 20%-30% to ensure that the melt fills the cavity before cooling; extend holding time by 5-10 seconds to compensate for shrinkage.
Mold improvements require optimizing gate and runner design: increasing the number of gates (e.g., from one to two for long strips) to shorten the flow path; increasing gate size (e.g., diameter from 1mm to 1.5mm) and runner cross-section (e.g., circular runner diameter from 8mm to 10mm) to reduce pressure loss; and adopting fan gates or film gates instead of point gates to ensure even melt distribution. Enhance venting capacity by creating venting grooves (0.02-0.05mm deep, 5-10mm wide) at the final fill point of the cavity. If necessary, use a vacuum exhaust device to remove air from the cavity. Polish the cavity surface to Ra 0.8μm or less to reduce flow resistance. For complex structural parts, overflow grooves can be installed in areas prone to material shortages, such as ribs and bosses, to guide the melt to fill the cavity before discharging a small amount of melt, ensuring complete filling of critical areas. These combined measures can effectively address material shortages and improve product qualification rates.
