Injection Molding Dimensional Variation? Quick Solutions Revealed!

1.    Formation conditions inconsistent or improper operation

During injection molding, various process parameters such as temperature, pressure, and time must be strictly controlled according to the process requirements, especially ensuring that the molding cycle of each type of plastic part is consistent and not arbitrarily changed. If the injection pressure is too low, the holding time is too short, the mold temperature is too low or uneven, the temperature at the barrel and nozzle is too high, and the plastic part is not sufficiently cooled, it will lead to unstable dimensional size of the plastic part.

In general, higher injection pressure and injection speed are adopted to overcome the problem of unstable size. It is advisable to appropriately extend the mold filling and holding time, increase the mold temperature and material temperature.

If the external size of the plastic part after molding is larger than the required size, the injection pressure and melt temperature should be appropriately reduced, the mold temperature should be increased, the mold filling time should be shortened, and the gate cross-sectional area should be reduced to increase the shrinkage rate of the plastic part.

If the size of the plastic part after molding is smaller than the required size, the opposite molding conditions should be adopted.

It is worth noting that changes in ambient temperature also have a certain impact on the fluctuation of the size of the plastic part, so the process temperature of the equipment and mold should be adjusted promptly according to the changes in the external environment.

2. Improper Selection of Molding Materials

The shrinkage rate of molding materials has a significant impact on the dimensional accuracy of plastic parts. If the precision of the molding equipment and molds is high but the shrinkage rate of the molding materials is large, it is difficult to ensure the dimensional accuracy of the parts. Generally, the larger the shrinkage rate of the molding materials, the more difficult it is to guarantee the dimensional accuracy of the parts. Therefore, when selecting molding resins, the influence of the shrinkage rate of the raw materials on the dimensional accuracy of the parts after molding must be fully considered. For the selected materials, the range of shrinkage rate should not exceed the requirement for dimensional accuracy of the parts.

Attention should be paid to the significant differences in shrinkage rates among various resins, and analysis should be conducted based on the degree of resin crystallization. Typically, the shrinkage rate of crystalline and semi-crystalline resins is larger than that of non-crystalline resins, and the range of shrinkage rate variation is also relatively large. Consequently, the shrinkage rate fluctuation of the molded parts is also relatively large. For crystalline resins, the higher the crystallinity, the smaller the molecular volume, resulting in larger part shrinkage. Additionally, the size of resin crystals affects the shrinkage rate. Smaller crystals lead to smaller gaps between molecules and, therefore, smaller part shrinkage, but higher impact strength of the parts.

Moreover, uneven particle size of molding materials, poor drying, uneven mixing of recycled materials and new materials, or differences in performance of each batch of materials can also cause fluctuations in the dimensional accuracy of molded parts.

3. Mold Malfunctions

The structural design and manufacturing precision of the mold directly affect the dimensional accuracy of the plastic parts. During the molding process, insufficient rigidity of the mold or excessive molding pressure inside the cavity can cause deformation of the mold, resulting in unstable dimensional accuracy of the plastic parts.

If the fit clearance between the guide pillar and guide bushing of the mold exceeds the tolerance due to poor manufacturing precision or excessive wear, it will also lead to a decrease in the dimensional accuracy of the molded parts.

If hard fillers or glass fiber-reinforced materials in the molding material cause severe wear of the mold cavity, or if errors and defects such as gate, runner, and poor balance of feed port between different cavities occur during multi-cavity molding, inconsistent filling of the mold will result, causing dimensional fluctuations.

Therefore, when designing molds, sufficient mold strength and rigidity should be ensured, and machining precision should be strictly controlled. Wear-resistant materials should be used for mold cavity materials, and the mold cavity surface should preferably undergo heat treatment and cold hardening. When the dimensional accuracy requirements of plastic parts are high, it is best not to use a multi-cavity structure for the mold. Otherwise, in order to ensure the molding accuracy of the plastic parts, a series of auxiliary devices to ensure the mold accuracy must be installed on the mold, resulting in high mold manufacturing costs.

When the plastic part exhibits thickness errors, it is often caused by mold malfunctions. If the thickness error occurs under the condition of one mold and one cavity, it is generally due to installation errors of the mold and poor positioning, resulting in relative displacement between the mold cavity and the core.

At this time, for plastic parts with very precise thickness requirements, other positioning devices must be added in addition to guide pillars and guide bushes for positioning. If the thickness error occurs under the condition of one mold and multiple cavities, the error is generally smaller at the beginning of molding, but gradually increases after continuous operation. This is mainly due to the error between the mold cavity and the core, especially when using hot runner molds, which are most prone to this phenomenon. To address this, dual cooling circuits with minimal temperature differences can be set up inside the mold. For molding thin-walled cylindrical containers, a floating core can be used, but the core and mold cavity must be concentric.

In addition, when making molds, it is customary to make the mold cavity slightly smaller and the core slightly larger than the required dimensions to facilitate mold repair and leave a certain amount of machining allowance. When the inner diameter of the plastic part's molding hole is much smaller than the outer diameter, the core pin should be made larger. This is because the shrinkage of the plastic part at the molding hole is always greater than that at other parts and shrinks towards the hole center. Conversely, when the inner diameter of the plastic part's molding hole is close to the outer diameter, the core pin can be made slightly smaller.

4. Equipment Failure

If the plasticizing capacity of the molding equipment is insufficient, the feeding system is unstable, the screw speed is unstable, the stop function malfunctions, the check valve of the hydraulic system fails, the temperature control system has a burnt thermocouple, the heater is disconnected, etc., it will lead to unstable molding dimensions of the plastic parts. Once these faults are identified, targeted measures can be taken to eliminate them.

5. Inconsistent Testing Methods or Conditions

If the methods, time, and temperature for measuring the dimensions of plastic parts are different, there will be significant differences in the measured dimensions. Among them, temperature conditions have the greatest impact on testing, as the thermal expansion coefficient of plastics is 10 times larger than that of metals. Therefore, standardized methods and temperature conditions must be used to measure the structural dimensions of plastic parts, and the parts must be fully cooled and set before measurement. Generally, the dimensions of plastic parts change significantly within 10 hours after demolding, and they are basically set after 24 hours.