Common Causes For Changes To Injection Molding Dimensions

1. Uneven Mold Conditions and Inappropriate Operation

At injection molding, various process parameters, such as temperature, pressure and time must be strictly managed according to process requirements for consistent molding cycles of plastic parts. If injection pressure falls too far below what it needs to be for consistent molding cycle execution – for instance if holding time falls too quickly; mold temperature rises unexpectedly or unevenly; temperature at barrel/nozzle is too high and plastic part isn’t sufficiently cooled off – this could result in unstable physical dimensions on plastic part production.

To prevent the failure associated with dimensional instability, employing higher injection pressure and speed, appropriately increasing filling/packing times and elevating mold/material temperatures can often help.

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If the outer dimensions of a plastic part exceed its required dimension, injection pressure and melt temperature must be adjusted appropriately; mold temperature increased; filling time shortened and gate cross-sectional area diminished in order to enhance shrinkage rate of plastic parts.

If the size of a plastic part manufactured under molding conditions falls under what was expected, different conditions should be utilized to mold its creation.

As temperatures change in an ambient environment, so too do molding sizes of plastic parts fluctuate accordingly. Equipment and mold temperatures must be regularly adjusted accordingly in response to changes in external environments.

2. Misuse of molding materials.

Shrinkage rate plays a pivotal role in the dimensions accuracy of plastic parts produced from molding material. Molding equipment and mold with high precision may produce parts with excellent dimensions; however, when shrinkage of molding material increases significantly it becomes challenging to ensure their accuracy in terms of dimension accuracy. As is typically the case, when using materials with high shrinkage rates to form plastic parts, maintaining their dimensional accuracy becomes increasingly challenging. Therefore, when selecting a molding resin it is crucial that one considers carefully its impact on dimensional accuracy of plastic parts after molding; its shrinkage rate must not surpass what is necessary for their creation. When making such a selection, any range in shrinkage rate cannot surpass requirements set for their creation.

Keep in mind that shrinkage rates of various resins vary dramatically and should be evaluated according to their degree of crystallization. Shrinkage rates of crystalline and semi-crystalline resins tend to be greater than non-crystalline resins; their range of variability also tends to be relatively wide; as a result, there will often be significant fluctuations in shrinkage rate after molding has taken place. Crystalline resins feature high crystallinity and reduced molecular volume which lead to large shrinkage of plastic parts, as does their size as determined by resin spherulites: smaller resin spherulites result in lesser plastic shrinkage with relatively higher impact strength of plastic parts than large crystallinities with reduced molecular volume. Spherulites with smaller intermolecular gaps produce plastic pieces with decreased shrinkage rates while their impact strength remains relatively constant.

Additionally, uneven particle sizes of molding material, insufficient drying techniques and uneven mixture between recycled material and new material as well as different performance of batches of raw materials will all lead to fluctuations in molding size for plastic parts.

3. Failure of Mold Development

Mold design and manufacturing precision play an instrumental role in maintaining accurate dimensions for plastic parts during molding processes. If rigidity of a mold is inadequate or pressure within its mold cavity exceeds acceptable limits during mold molding operations, deformation could occur, potentially altering mold dimensions for unstable plastic part molding dimensions.

If the clearance between the guide post and guide sleeve of a mold exceeds its tolerance due to poor manufacturing accuracy or excessive wear, its dimensional accuracy will decline, further diminishing.

Hard fillers or glass fiber reinforced materials used as molding materials may damage mold cavities significantly, or multiple cavities used within one mold may lead to errors between cavities, gate and runner errors, and poor feed inlet balance resulting in inconsistent filling patterns and dimension variations.

Therefore, when designing the mold it should incorporate sufficient strength and rigidity for processing accuracy to be maintained, as well as wear-resistant materials used within its cavities that will withstand wear-and-tear and be submitted to heat treatment or cold hardening processes for durability. When high dimensions precision plastic parts require multiple cavity mold designs if possible as this would necessitate multiple auxiliary devices being set up on them in order to meet mold accuracy; increasing production costs considerably.

Thickness errors on plastic parts often stem from mold failure; if this does not apply in all instances, however, installation errors and poor positioning of mold cavities lead to relative position shifts between their cavities and cores resulting in thickness variations between parts produced with one mold and one cavity molds.

At present, plastic parts that require precise wall thickness requirements cannot rely solely on guide posts and sleeves alone for their positioning needs, necessitating additional positioning devices to achieve accurate placement. If the thickness error is caused by multiple cavities within one mold, in general it starts off small but gradually worsens over time due to mismatches between mold cavity and core. Hot runner molding makes this phenomenon all too likely, which requires using a dual cooling circuit with minimal temperature differences inside of a mold to address. A floating core may also be utilized – in such an instance both it and mold cavity must remain concentric with one another for successful molding of thin-walled cylindrical containers.

As part of mold design to facilitate repair, moldmakers often design mold cavities smaller than necessary and core pins larger, leaving an appropriate margin for mold repair. When producing plastic parts with narrow inner diameter molded holes or larger than desired shrinkage rates in certain spots such as near the center, core pin size must increase accordingly for best results; otherwise the pin can be reduced accordingly. If outer diameter of hole exceeds inner diameter by more than 10% then smaller core pin should be utilized accordingly; alternatively if shrinkage rates near center outstrip other locations then core pin may need to reduce.

4.Equipment Failure

Insufficient plasticizing capacity of molding equipment, an unstable feeding system, unstable screw speed and stop function abnormalities as well as broken check valves in hydraulic systems will all lead to unstable plastic part molding dimensions and may necessitate additional measures once these faults have been identified and rectified. Once identified, effective counter measures should be implemented quickly in order to rectify them effectively and quickly.



5.Unpredictability in Test Methods or Conditions


There will be significant discrepancies when using different methods, times, and temperatures for measuring plastic parts’ dimensions. Temperature conditions have an especially noticeable impact, given plastic’s thermal expansion coefficient is 10 times that of metal; hence it is necessary to use standard methodologies and temperature conditions when testing plastic structures’ dimensions, with full cooling of all parts before measurements take place; usually within 10 hours following demolding they experience significant dimensional shifts that typically settle over 24 hours post demolding.

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