Generally speaking, automotive series components, and some parts with sealing requirements, usually put forward the concept of contour size requirements.

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How to solve the profile problem of injection molded parts?
Written By Jasmine HL
Generally speaking, automotive series components, and some parts with sealing requirements,
usually put forward the concept of contour size requirements.
There is no doubt that contour tolerances are usually used to constrain irregular surfaces,
because if there is a requirement for a plane, then the flatness is equal to the contour of the
plane, which is a circumferential surface, then the roundness is the contour of the circumferential
surface degree.
Only those irregular surfaces constructed by a combination of straight lines, curves, and R angles
need to be constrained by the profile to ensure the installation fit clearance or to meet other
performance requirements such as sealing.
As shown in the figure above, the contour requirement of the curve is 0.4mm. The actual contour
of the product is measured at points. The maximum offset contour is 0.15mm.
After understanding the concept of contour, we can easily understand the root cause of the poor
profile of injection molded parts. The logical reasoning analysis is as follows:
Step1, if the contours of the mold core and the mold core meet the design requirements, then
theoretically, how to ensure that the contours of the product before the mold release also meet
the design requirements?
This problem is easier to understand: as long as the product is full enough, the resin and the
cavity are closely fitted, then theoretically the contour can meet the requirements. However, it is
not ruled out that due to the injection pressure being too high, the mold core or the mold core is
subjected to excessive stress during the filling process, which may cause local deformation,
displacement, and yield.
If it is possible to identify the displacement and deformation of the mold core during filling, then
the optimization of the support columns and precise positioning can be considered.
Step2, if the profile of the product can meet the requirements before the mold is ejected, but the
measurement result is too poor after the mold is cooled, then the cause of the excessive profile is

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also very clear: or the force deforms during the demolding process Or cooling shrinkage
deformation.
Is it due to force deformation during mold opening or ejection, or shrinkage deformation after
cooling?
This is not difficult to judge, because whether it is caused by deformation during the opening of
the mold (the product is torn from the front mold) or deformation during ejection (from the
separation of the rear mold), it can be identified with the naked eye.
Generally speaking, most of them are caused by the shrinkage and deformation of the product
after cooling.
Step3, how to deal with the most common shrinkage deformation caused by cooling after the
cooling? The process conditions and feasible measures in the mold are as follows:
① Under the premise of process permission, reduce the mold temperature and extend the
cooling time appropriately to reduce the amount of shrinkage and deformation after the mold is
released;
② Make the product as full as possible, so that the amount of post-shrinkage deformation will
be smaller;
③ Use jig to adjust the shape;
④ Find the maximum deformation point of out-of-contour degree and make necessary reverse
compensation on the mold core.
In fact, not only the profile, but also many dimensions of injection molded parts, we can apply
this idea to analyze and deal with. The basic analysis methods and processing ideas are
summarized again as follows:
First, understand the dimensional tolerances defined by the product drawing.
Second, collect the measurement data and do the necessary data analysis to determine whether
it is normal fluctuation or abnormal.
Third, based on data analysis and process observation, determine whether the size problem of
the product is caused during molding, is caused by the movement process, or is caused by
post-contraction.
Fourth, according to the preliminary analysis and judgment, take corresponding measures, and
verify the effectiveness in time after the measures.
Fifth, if the measures are effective, then the necessary standardization documents should be
formulated in a timely manner, and the knowledge of the organization promoted should be
shared and controlled within the organization.