Automotive Part Molding Defects and Fixing Methods

Publish Time: 2024-02-29

Like other common plasic parts, molding defects occurred also during seires production. It is too late when parts defects are found after delivery, so it is very critical to find out issues and settled them when it is first found during production. these defects mainly lead from mold design stage ignorance as limition to mold manufacturing knowledges or ignorance to details.

Some common defects to Auto parts are Short Shot Molding, Weld lines, Sink marks and Warpage:

Short Shot Molding

Short Shot Molding results from molten plastic not fully filling mold cavity and leaving some sections with no plastic.

Especially to multi cavities molding. short shot molding occurred to interlock locations, they could not be seen from appearance, but they could be found when assembling.

Cause: 1. Poor mold exhaust 2.Mold injection gate is too small 3. Low injection Pressure

Solution: 1. Improve mold exhaust and injection gate by mold modifiaction. 2. Increase molten plastic temperature to improve molten plastic flow mobility when injection molding to Automotive Part.

Weld lines

When plastic molding, molten plastic travel inside the cavity and was separated by several streams, weld lines are locations where these streams meet each another. generally, the junction

location is the weakest location of the part, part's broken begins from weld lines location.

Several molten plastic streams meet each another in relative lower temperature, molten plastic can not well combine each another and weld lines come into being.

Cause: 1. Plastic injeciton mold itself. 2. Injection Molding process. 3. Injection machine. 4. Resins

Solution: by make improvement of moltern plastic temperature or flow rate, injection speed or pressure and increasing plastic injection mold temperature.

Sink marks and Warpage

Sink marks indicate shrinkage within the inner portions of the component, and uneven shrinkage across the various parts of the mold lead to warpage.


For sink marks, injection molders have to reduce mold temperatures, and increase holding pressure or adjust holding times for cooling and curing.

For Warpage 1. minimize residual stresses inside the part 2. change wall thickness to the part.

3. if possible, change to resins with low susceptibility to shrinking and deformation.  


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