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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Locking Block With Angular Slider

What should you do when the slider design and found that the angular slider too long? There are many ways to change the design so that the angular length of the slider pins are still reasonable when compared to its diameter.in design between the angular diameter and length of the slider pins should be noted, the length of angular slider pins that are too long can lead to easily broken when the mass production run, especially if the small angular diameter, the way the installation is also a concern to be easy in the process of repair when damage occurs.

the above  picture, I took when designing the angular pin slider, when the client wants the angular pin slider must exist, and based on the calculation of stroke, it turns out the angular pin length is too long, surely this is dangerous and i should change the design concept.

other than dangerous, the above design concept is also difficult in maintenance because if the angular pin broke the mold base must be dropped from the injection machine, disassembly the mold then can be replaced, this is because the angular pin must be in pairs from the top plate embedded in the cavity

Locking Block With Angular Slider

This design concept in addition to saving space, saving the long angular slider also has the advantage of easy installation and maintenance, although the angular pin mold was broken when used for production, the mold does not need to send down from the injection machine to replace it. look at picture below

on the design concept above, I only use one bolt to tighten to the cavity plate, simple but I'm sure I could save the angular length of approximately half of the angular slider before. The above concept also facilitates the repair process.

The second alternative was similar, but the difference is the number of bolts are used and how big a part of the locking block affixed to the cavity plate, the first altenative collar only are plugged into the cavity plate, and the bolts, of course, only part that holds the slider in the section just the collar. A second alternative is different from all bagianya embedded in the plate cavity.

I chose the second alternative concept, the concept was made 3D and 2D drafting calculations based on a stroke, for details, under the following 3D image is the result of the 3D design of the second alternative.

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