Mold filling sink marks

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

Common problems like insufficient filling-packing and poor dimensional control are often related to the gate size and design. Similarly, gate location is another important factor. They should be located in areas having heaviest cross-section of the part to assure fill-out and elimination of sink marks. Also their position should not facilitate the residual molded stress formation in the part, knit line formation. 

Technical — Greater rigidity (tube vs. U-prolile) — No surface sink marks due to ribs — Simplified mold technology (no demolding draft, no sliders) — Simplified mold filling — Minimum warpage Fot this example — Nraie... 

Macroscopic product problems that can result from poor control in injection molding include, but are not limited to voids and sink holes on the surface generally due to poor mold filling or low pressure, incomplete mold filling, weld lines and flow marks, warping or distortion of parts, high shrinkage, and so forth. 

In the conventional injection molding process, surplus resin melt is fed into the mold cavity in order to compensate the volume reduction of filled resin by cooling down. This cannot avoid the surface defects such as sink marks, especially when the molded article has ribs or thicker-walled parts. 

As a general rule, it is often easier to injection mold very thick parts using amorphous resins. This is not because of fill, but because of differential shrinkage. With semicrystalline materials in thick cross-sections, the rate of crystallization and the total amount of crystallization can vary significantly from skin to core. As a result, the total amount of mold shrinkage can vary through the part. This can lead to distortions (warping and/ or sink marks). 

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