Dimensional instability means that under same injection molding machine and molding process conditions, size of plastic part changes between each batch of molded products or between each cavity molded product produced by each mold.

 

Change of product size is caused by abnormal equipment control, unreasonable injection molding conditions, poor product design, and changes in material properties.

During injection molding, various process parameters such as temperature, pressure and time must be controlled strictly in accordance with process requirements, especially molding cycle of each type of plastic part must be consistent and not arbitrarily changed. If injection pressure is too low, holding time is too short, mold temperature is too low or uneven, temperature of barrel and nozzle is too high, and plastic part is not cooled enough, it will cause size of plastic part to be unstable.
Under normal circumstances, use higher injection pressure and injection speed, appropriately extend time of filling and holding pressure, increase temperature of mold and material, which is beneficial to overcome dimensional instability failure.
If outer size of plastic part is larger than required size after molding, injection pressure and melt temperature should be appropriately reduced, mold temperature should be increased, filling time should be shortened, and gate cross-sectional area should be reduced, thereby increasing shrinkage rate of plastic part.
If size of plastic part after molding is smaller than required size, opposite molding conditions should be adopted.
It is worth noting that changes in environmental temperature also have a certain impact on fluctuations of molding dimensions of plastic parts, process temperature of equipment and molds should be adjusted in time according to changes in external environment.

Shrinkage rate of molding material has a great influence on dimensional accuracy of plastic part. If precision of molding equipment and mold is high, but shrinkage rate of molding material is large, it is difficult to ensure dimensional accuracy of plastic part. In general, the greater shrinkage of molding material, the more difficult it is to ensure dimensional accuracy of plastic part.
Therefore, when selecting a molding resin, it is necessary to fully consider impact of shrinkage rate of raw material after molding on dimensional accuracy of plastic part. For selected raw materials, change range of shrinkage rate cannot be greater than requirements of dimensional accuracy of plastic parts.
It should be noted that shrinkage rate of various resins is quite different, and analysis is carried out according to degree of crystallization of resin. Generally, shrinkage rate of crystalline and semi-crystalline resins is larger than that of non-crystalline resins, range of shrinkage rate is also relatively large, corresponding shrinkage rate fluctuations generated after molding of plastic parts are also relatively large; for crystalline resins, crystallinity is high, molecular volume is reduced, shrinkage of plastic parts is large, size of resin spherulites also affects shrinkage rate. Spherulites are small, voids between molecules are small, shrinkage of plastic parts is small, impact strength of plastic parts is relatively high.
In addition, if particle size of molding raw materials is uneven, drying is poor, regenerated materials and new materials are mixed unevenly, properties of each batch of raw materials are different, it will also cause fluctuations in molding size of plastic parts.

Structural design and manufacturing accuracy of mold directly affect dimensional accuracy of plastic part. During molding process, if rigidity of mold is insufficient or molding pressure in cavity is too high, mold will be deformed, which will cause molding size of plastic part to be unstable.
If fit gap between guide post and guide sleeve of mold is out of tolerance due to poor manufacturing accuracy or too much wear, it will also reduce molding dimensional accuracy of plastic part.
If there are hard fillers or glass fiber reinforced materials in molding raw materials, which cause severe wear of mold cavity, or when a mold is used for multi-cavity molding, inconsistent mold filling occurs due to errors between each cavity, errors such as gates, runners, and poor balance of feed ports, which will also cause size fluctuations.
Therefore, when designing mold, sufficient mold strength and rigidity should be designed, processing accuracy should be strictly controlled. Cavity material of mold should be made of wear-resistant materials, surface of cavity should be heat treated and cold hardened. When dimensional accuracy of plastic part is required to be very high, it is best not to use a multi-cavity structure of mold, otherwise, in order to ensure molding accuracy of plastic part, a series of auxiliary devices to ensure accuracy of mold must be installed on mold, resulting in high production costs for mold.
When thickness error of plastic part occurs, it is often caused by failure of mold. If wall thickness of plastic part has an uneven thickness under condition of one cavity, it is generally due to deviation of relative position of mold cavity and core due to installation error and poor positioning of mold.
At this time, for those plastic parts that require very precise wall thickness dimensions, they cannot be positioned only by guide posts and guide sleeves, other positioning devices must be added; if thickness error is generated under condition of a mold with multiple cavities, under normal circumstances, error is small at beginning of molding, but error gradually becomes larger after continuous operation, which is mainly caused by error between cavity and core, especially when hot runner mold is used for molding. This phenomenon is most likely to occur.
In this regard, dual cooling circuits with small temperature differences can be provided in mold. If it is a thin-walled round container, a floating core can be used, but core and cavity must be concentric.
In addition, when making molds, in order to facilitate mold repair, it is generally used to make cavity smaller than required size, and core to be made larger than required size, leaving a certain amount of mold repair margin. When inner diameter of molded hole of plastic part is much smaller than outer diameter, core pin should be made larger, because shrinkage of plastic part at molded hole is always greater than that of other parts, and it shrinks in the direction of hole center. Conversely, if inner diameter of molded hole of plastic part is close to outer diameter, core pin can be made smaller.

 

If plasticizing capacity of molding equipment is insufficient, feeding system is unstable, screw speed is unstable, stop function is abnormal, check valve of hydraulic system fails, thermocouple burns out in temperature control system, heater is cut off, etc., will cause molding size of plastic part to be unstable. As long as these faults are found, targeted measures can be taken to eliminate them.

If method of measuring size of plastic part, time, and temperature are different, measured size will be very different. Among them, temperature conditions have the greatest impact on test, because thermal expansion coefficient of plastics is 10 times that of metal industry. Therefore, method and temperature conditions specified by standard must be used to determine structural dimensions of plastic part, plastic part must be sufficiently cooled and shaped before measurement can be carried out. Generally, size of plastic part changes greatly within 10 hours after being removed from mode, and it is basically shaped in 24 hours.