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How to calculate Weight tolerance of a molded part?

We understand that with plastic injection molded parts, weight is the best method of controlling the process capability. Is there a guideline or standard for defining the tolerance?The most common method is: Shot to shot weight variation calculated as (three standard deviations) divided by (average weight) must be less than 1% and should be less than 0,5%. In general it is possible to achieve 0,1-0,4% weight variation using scientific molding. To calculate a comparable capability (Performance) use (1% of average weight) divided by (3 standard dev.). It is absolutely true that weight and dimensions do not correlate. Unless the process is reasonably robust and the dimensions are correct at the start, then weight will tell if the process is shifting (use SPC). But still it will not guarantee dimensions are unchanged. Anyway weight variation can tell if the process is all right at all to begin with. Too much variation indicates a poor process that is sensitive to changes. The cavity imbalance should be less than 5% when making short shots (no hold pressure) unless it is small parts (less than a couple of grams), then as much as 25% could be acceptable and still produce parts with the same dimensions. It should in any case be investigated if there are cavity differences in dimensions.How much is "variable"? If weight part to part is truly critical as it could be in some applications (example - assembled items that must "float" at some constant level in "water"), the information above is all critical and don't forget also that the specific gravity of each lot of material could show some variability - work with your vendor to be sure that an acceptable range (made from standardized materials that have known and repeatable values) is knowable, known, and repeatable. Just as an example, the fillers in some materials can cause a fairly wide range of SG and all be "the same". If the variability has to be tightened even further, it might be of value to design a "removal area" into the part, insert each part into a tool to remove all "upside" material down to the intended acceptable range and reclaim the regrind.if you need help, pls contact me, I am Tea, my email is injectionmold2006@163.comwhatsapp+8615958816137  skype:injectionmold2006@163.com

Plastic injection mould Shrinkage and Warpage

Plastic injection part designers often use design for manufacturing and assembly(DFMA) guidelines to reduce the number of parts in an assembly. For this reason,plastic injection molding part designs can be extremely complex with many features and tight tolerances.The delivery of plastic moldings that satisfy the dimensional requirements is a joint responsibility of the mold designer,molder,material supplier,and part designer. The part designer should provide a design with uniform thicknesses and achievable specifications. The material supplier should provide consistent polymer resin and useful guidance regarding material properties.The molder should select suitable and consistent processing conditions for operation of the mold.The mold designer should provide a mold with balanced melt filling and cooling,and for which the mold cavity dimensions were engineered for an appropriate shrinkage.if you need help, pls contact me, I am Tea, my email is injectionmold2006@163.com