wall thickness and sink mark estimation

Uniform wall thickness in plastic part design is critical.Non-uniform wall thickness can cause serious warpage and dimensional control problems.One of the easiest ways to cure this problem is change the part geometry by adding ribs. The use of ribs is a practical way and economical means of increasing the structural strength of a part.  it is more economical to use ribs than increase wall thickness, But there are guidelines that govern adding ribs without causing sink marks or surface blemishes to your parts. in parts requiring good surface appearance, ribs should be avoided as sink marks on the opposite surface will surely appear.

The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch).

Rib thickness should be less than wall thickness. A rib thickness of 60% to 80% of nominal wall thickness is recommended. (plastics1.com)

using finite element software or relevant software for injection mold like, Mold flow, C-Mold, Etc, we can estimate the sink mark that will appear, picture below for example, with different rib we can see that sink mark result also different, first rib (from left) have 0.6 * thickness part, second rib thickness same with part thickness, 

Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section (efunda.com)

non uniform wall thickness also can cause voids and non-uniform shrinkage, for example in a sharp outside corner and a properly filleted inside corner could present problems due to the increased wall thickness at the corner. to prevent that we can add additional radius or shape like picture below 

How About Bosses?

Bosses are used for locating, mounting, and assembly purposes. There are boss design guidelines that must be followed to insure the highest quality in molded parts. Again, one of the main points to consider is nominal wall thickness. Too many times bosses are designed with thick wall sections that can affect the appearance of the plastic part and the final product. (plastic1.com)

As a rule, the outside diameter of a boss should be 2 to 3 times the hole diameter to ensure adequate strength. The same principles used in designing ribs pertain to designing bosses, that is, heavy sections should be avoided to prevent the formation of voids or sink marks and cycle time penalty.Less good design of bosses can lead to sink marks.

to prevent that, design the boss like picture below is preferred

Rule of thumb: the wall thickness around a boss design feature (t) should be 60% of the nominal part thickness (T) if that thickness is less than 1/8". If the nominal part thickness is greater than 1/8" the boss wall thickness should be 40% of the nominal wall
.Boss diameter, wall thickness, and height design parameters. While boss heights vary by design, the following guidelines will help avoid surface imperfections like sink marks and voids: the height of the boss should be no more than 2 1/2 times the diameter of the hole in the boss.

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Coring uniform wall thickness

when design a mold then we find nonuniform wall thickness, we can make little graduation translation using core, of course if there was not function parts,
ordinary parts like not cosmetic parts, touching with others parts, sliding and etc, should we change by making insert in core or cavity side. look at picture below
when a parts have no uniform thickness after injection process, sink marks and unfill product like void will occurs,




Different wall thicknesses in the same component cause differential shrinkage which, depending on component rigidity, can lead to serious warpage and dimensional accuracy problems To attain uniform wall thickness, thick-walled areas of the moulding should be cored like picture above,
the best core for graduation translation and easier material flow is by make radius in every corner, beside flow more easier there will easy when machining process used,and no need EDM process so it's should cost down the price of product.

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Wall Thickness Design - Maksimum

Dependent on the part design and size, typically, for injection molded parts, the wall thickness will be in the range 0.5 mm to 4 mm (0.02 - 0.16 inch).
For parts made from most thermoplastics, nominal wall thickness should not exceed 4.0 mm. Walls thicker than 4.0 mm it is will increase time required for cooling and will result in increased cycle times

Changing the wall thickness of a component has a significant effect on the following key properties:

1.component weight
2.achievable of cycle time because more time 3.for cooling and injection
4.tolerances of product
5.achievable flow lengths in the mould
6.production cycle time of the component
7.rigidity of the moulded part
8.quality of the component in terms of surface finish, warpage and voids.
9. will affect to determines mechanical properties

What the Disadvantage when Thickness not Uniform

In the design of plastics components, it is important to aim for uniform wall thickness. Different wall thicknesses in the same component cause differential shrinkage which, depending on component rigidity, can lead to serious warpage and dimensional accuracy problems

what we should do when design a mold that have different thickness, look at the following picture, we should make as possible uniform or make the transition of product is gradual



Generally, the maximum wall thickness used should not exceed 4 mm. Thicker walls increase material consumption, more need time for cycle time considerably, and cause high internal stresses, sink marks and voids, look at picture below

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Wall thickness in mold design

when design a product The basic wall of the part of the melt during injection, should be kept uniform, this important to prevent sink after injection.
even we use cooling after injection part well deform because shrinkage, at this time if wall thickness does not same part will warpage stress

they key is keep wall thickness part as uniform as posible

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