David Wang
3 min read
20 Mar

1.Inconsistent Molding Conditions or Improper Operation

During injection molding, various process parameters such as temperature, pressure, and time must be strictly controlled according to the process requirements. Particularly, the molding cycle for each type of plastic part must remain consistent and should not be altered arbitrarily. Insufficient injection pressure, inadequate holding pressure time, uneven mold temperature, excessive temperatures at the barrel and nozzle, or insufficient cooling of the plastic part can all result in unstable dimensional accuracy.Generally, employing higher injection pressure and speed, extending the filling and holding time appropriately, increasing mold and material temperatures, can help overcome dimensional instability issues. If the external dimensions of the plastic part after molding exceed the required dimensions, it's advisable to reduce injection pressure and melt temperature, increase mold temperature, shorten the filling time, and decrease gate cross-sectional area to improve shrinkage rate.Conversely, if the dimensions of the plastic part after molding are smaller than required, the opposite molding conditions should be applied. It's worth noting that changes in ambient temperature also have a certain impact on the fluctuation of plastic part dimensions. Therefore, equipment and mold process temperatures should be adjusted promptly according to external environmental changes.

2. Improper Selection of Molding Materials

The shrinkage rate of molding materials significantly affects the dimensional accuracy of plastic parts. Even with highly accurate molding equipment and molds, ensuring the dimensional accuracy of plastic parts can be challenging if the shrinkage rate of the molding material is high. Generally, the larger the shrinkage rate of the molding material, the more difficult it is to guarantee dimensional accuracy. Therefore, when selecting molding resins, the influence of the shrinkage rate of the material after molding on the dimensional accuracy of the part must be fully considered. The variation range of the shrinkage rate of the selected material should not exceed the dimensional accuracy requirements of the part.Differences in shrinkage rates among various resins should be analyzed based on the degree of crystallization of the resin. Typically, crystalline and semi-crystalline resins have higher shrinkage rates than amorphous resins, and the range of shrinkage rate variation is also relatively large. Consequently, the dimensional fluctuations of plastic parts produced after molding are significant. For crystalline resins, a higher degree of crystallinity results in greater shrinkage, and the size of resin spherulites also affects the shrinkage rate. Smaller spherulites and smaller intermolecular gaps lead to less shrinkage in the plastic part, resulting in higher impact strength.Additionally, uneven particle size of molding materials, poor drying, uneven mixing of recycled and virgin materials, and different properties of each batch of materials can also cause fluctuations in the molded part's dimensional accuracy. 

3. Mold Failures

The structural design and manufacturing precision of molds directly affect the dimensional accuracy of plastic parts. During the molding process, insufficient rigidity of the mold or excessive molding pressure on the cavity can cause deformation, leading to unstable dimensional accuracy of the plastic parts.If the clearance between guide pins and bushings in the mold exceeds specifications due to poor manufacturing precision or excessive wear, it can also decrease the dimensional accuracy of the plastic parts.The presence of hard fillers or glass fiber reinforcement materials in the molding material can lead to severe wear of the mold cavity. In the case of multi-cavity molding, discrepancies between cavities, as well as errors in gates and runners, can result in inconsistent filling, causing dimensional fluctuations.Therefore, during mold design, adequate mold strength and rigidity should be ensured, and machining precision should be strictly controlled. Wear-resistant materials should be used for mold cavities, and surface treatments such as heat treatment and cold hardening are advisable. For high precision requirements, it's preferable to avoid multi-cavity structures to prevent the need for costly auxiliary devices to maintain mold accuracy.Thickening errors in plastic parts are often caused by mold failures. If the wall thickness error occurs under single-cavity conditions, it is generally due to installation errors or poor positioning of the mold cavity relative to the core.For plastic parts with precise wall thickness requirements, additional positioning devices must be added besides guide pins and bushings. Under multi-cavity conditions, errors at the start of molding are typically minor but gradually increase during continuous operation due to discrepancies between the mold cavities and cores, especially when using hot runner molds. To address this, dual cooling circuits with minimal temperature differences can be incorporated into the mold. For thin-walled cylindrical containers, floating cores can be utilized, but cores and cavities must remain concentric.Additionally, in mold making, it's common practice to slightly undersize the mold cavity and oversize the core to allow for machining allowances. When the inner diameter of a molded hole is significantly smaller than the outer diameter, the core pin should be slightly oversized to accommodate the greater shrinkage and inward direction of the plastic material. Conversely, when the inner diameter of the molded hole approaches the outer diameter, the core pin can be slightly undersized. 

4. Equipment Failures

Insufficient plasticizing capacity of the molding equipment, unstable feeding in the feeding system, erratic screw rotation, malfunctioning stops, failure of the hydraulic system's check valve, burnt thermocouples in the temperature control system, heater circuit breaks, etc., can all lead to unstable dimensional accuracy of the plastic parts. Once these faults are identified, targeted measures can be taken to eliminate them. 

5. Inconsistent Testing Methods or Conditions

Differences in measurement methods, time, and temperature can result in significant differences in the measured dimensions of plastic parts. Among these factors, temperature conditions have the greatest influence on testing because the thermal expansion coefficient of plastics is approximately ten times that of metals. Therefore, standard methods and temperature conditions must be used to measure the structural dimensions of plastic parts, and the parts must be sufficiently cooled and shaped before measurement. Generally, plastic parts undergo significant dimensional changes within the first 10 hours after demolding, with stabilization typically occurring after 24 hours.