Dimensional instability refers to change in size of plastic parts between each batch of molded products or between each molded product of each mold under same injection molding machine and molding process conditions. Change in 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 strictly controlled in accordance with process requirements, especially molding cycle of each plastic part must be consistent and cannot be changed at will. If injection pressure is too low, holding time is too short, mold temperature is too low or uneven, temperature at barrel and nozzle is too high, and plastic part is not cooled enough, shape and size of plastic part will be unstable.
In general, using higher injection pressure and injection speed, appropriately extending filling and holding time, increasing mold temperature and material temperature are beneficial to overcome dimensional instability fault.
If external dimensions of plastic part after molding are larger than required dimensions, 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 to increase 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 change of ambient temperature also has a certain impact on fluctuation of molding size of plastic part. Process temperature of equipment and mold should be adjusted in time according to changes in external environment.

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

 

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 mold cavity is too high, mold will be deformed, which will cause molding size of plastic part to be unstable.
If clearance between guide pin and guide sleeve of mold exceeds tolerance due to poor manufacturing accuracy or excessive wear, molding size accuracy of plastic part will also decrease.
If there are hard fillers or glass fiber reinforced materials in molding raw materials, causing severe wear of mold cavity, or when one mold is used for multi-cavity molding, there are errors between cavities and errors in gate, runner, and poor balance of feed port, resulting in inconsistent filling, which will also cause dimensional fluctuations.
Therefore, when designing mold, sufficient mold strength and rigidity should be designed, and processing accuracy should be strictly controlled. Mold cavity material should use wear-resistant materials, cavity surface should preferably be heat treated and cold hardened.
When dimensional accuracy of plastic parts is very high, it is best not to use a multiple-cavity structure. Otherwise, in order to ensure molding accuracy of plastic parts, a series of auxiliary devices to ensure mold accuracy must be set on mold, resulting in a high mold production cost.
When plastic parts have a thickness error, it is often caused by mold failure. If wall thickness of plastic part has a thickness error under condition of one cavity, it is generally due to installation error and poor positioning of mold, which leads to relative position offset between cavity and core. At this time, for those plastic parts with very precise wall thickness requirements, they cannot be positioned only by guide pins and guide sleeves, and other positioning devices must be added;
If thickness error is generated under condition of multiple cavities, generally, error is small at the beginning of molding, but error gradually increases after continuous operation. This is mainly caused by error between cavity and core, especially when hot runner mold molding is used. This phenomenon is most likely to occur.
In this regard, a double cooling circuit with a small temperature difference can be set in mold. If a thin-walled round container is molded, 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 customary to make cavity smaller than required size and core larger than required size, leaving a certain margin for mold repair.
When inner diameter of molding hole of plastic part is much smaller than outer diameter, core pin should be made larger. This is because shrinkage of plastic part at molding hole is always greater than other parts, and shrinks toward center of hole. On the contrary, if inner diameter of molding 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 of temperature control system is burned out, heater is disconnected, etc., molding size of plastic part will be unstable. Once these faults are found, targeted measures can be taken to eliminate them.

If method, time, and temperature for measuring size of plastic part 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 greater than that of metals.
Therefore, standard method and temperature conditions must be used to measure structural dimensions of plastic parts, plastic parts must be fully cooled and shaped before they can be measured. Generally, size of plastic parts changes greatly within 10 hours of demolding, and they are basically shaped after 24 hours.