1. Mold fault
The structure design and manufacturing precision of molds directly affect dimensional precision of plastic parts. In the forming process, insufficient rigidity or too high pressure inside the mold cavity will deform the mold, causing unstable gauge.
Molding dimension accuracy will be decreased by overproof clearance between guide pin and guide bush which may be caused by low manufacturing precision or much wear. Dimension variation occurs when there is hard packing or GFRP mixed in raw material which results in severe wear in cavity, or multi-cavity structure causes inconsistent mold filling by error and poor balance among the cavity, gate and feed port.
Thus, it should be satisfied with strength and stiffness to design molds, controlling machining precision strictly. Molding cavities should be of wear-resistance material and it's better to have heat treatment and cold hardening. It's not proposed to adopt multi-cavities structure for high-precision plastic parts. Because production will be costly if a series of auxiliary devices is equipped with molds to guarantee molding precision.
Mold fault can also cause thickness deviation to injection molded parts which is mainly due to position deviation of cavity and core resulted from installation error and improper positioning. So plastic parts with precise thickness depend on not only guide pin and guide bush but also adding other positioning devices.
Usually, thickness error under multi-cavities is small at first. But it will gradually increase with continuous running because of the error between mold cavity and core, especially for hot runner injection molding. Instead, molds can be fitted with double cooling circuits whose temperature difference is small. Round moldings with thin wall can use floating core which is concentric with cavity.
Besides, the mold cavity is often made smaller than the required size while the core is bigger to leave allowance to repair molds. The core pin shall be bigger if inner diameter of the forming hole is much smaller than external diameter. Because shrinkage of plastic part at the forming hole, bigger than other parts, often happens in the direction of hole center. On the contrary, core pins can be made smaller if inner diameter is similar to external one.
2. Device fault
Unstable gauge can also result from insufficient plasticizing capacity, unstable feeding system and screw speed, abnormal fail, check valve failure in hydraulic system, TC burnout in temperature control system and heater disconnection. These faults can be eliminated once found out and taken appropriate measures.
3. Inconsistent testing methods
Measured dimension will vary greatly as the method, time and temperature differ. Temperature has the greatest impact for plastic CTE is 10 times bigger than metal. Consequently, it's required to take standard method and temperature, cooling enough and forming, to measure the size of plastic part. Generally speaking, there is big dimension variation in 10 hours after ejection which will be fixed after 24 hours.