Uniformity is best. Constant wall thicknesses throughout your part will provide the best flow. Nominal wall thickness should be between 2-3mm. The recommended minimum is 1mm and the maximum is 4mm for conventional injection molding processes.
Smooth trumps sharp. Use radii when possible and avoid sharp transitions between wall sections.
Draft is your “frenemy.” Adding a draft angle to the faces of your part is helpful to release it from the tool, yet it can cause design challenges especially for mating parts. Recommend minimum draft angles are one degree on untextured core and at least three degrees on textured cavity surfaces.
Avoid surfaces with zero draft unless necessary. If you do need a zero-draft area to ensure proper part mating and tolerances, try to minimize it to just a portion of the face, not the whole surface.
Simpler is better. Avoid undercuts (areas that cannot be formed via the simple open/shut direction of a tool). When simple won’t work, lifter and slides allow features to be formed that are undercuts in the main pull direction. If so, leave at least 2 to 3 times the width of the feature to allow the lifter or slide to travel.
Transition from thick to thin. Parts will form better if plastic flows through features moving from greater to lesser wall thickness starting from the gate(s) (where the plastic first flows in to fill the part).
Sink (a local surface depression on a part due to thicker sections of plastic cooling more slowly) is bad. To reduce or eliminate the visibility of blemishes on cosmetic surfaces it is important to follow some recommended guidelines:
Try to avoid gates, ribs, screw bosses, etc. on the back side of important cosmetic surfaces;
Rib heights should be 3x or less than the wall thickness;
Rib bases should be 60% or less of the wall thickness.
Datums define territory. Use datums (features that are used as reference points to define each part) to establish part interfaces and interactions to the overall system. Using a datum structure that matches the design intent of the assembly can mean the difference between a product functioning well or not.
Interrogation is good. Take DFM (Design for Manufacturing) reports seriously since they convey the tool molder’s understanding of the design especially information like ejector pin locations (may conflict with planned design changes), gate locations (may cause cosmetic concerns) and parting line location (could affect interactions with mating parts). Use inspection reports to interrogate the design.
An essential rule when designing a plastic part is to utilize a uniform wall thickness and to utilize as thin of wall sections as design will allow. Uniform walls aid in material flow in the injection mold, reduce the risk of sink marks, molded in stress and differential shrinkage. In addition, cost savings are the highest when wall sections are kept to a minimum and is consistent. The reason is that parts cool faster with thinner walls, which means that cycle times are shorter, resulting in more parts per hour. On average, the wall thickness of an injection mold part should be .060” to .180”, however, parts can be molded with walls as thin as .020” and as thick as 1.50”. Thick sections on a part will also lead to cosmetic issues such as sink, bubbles and discoloration.
Parts should also be designed with uniform wall thickness. This allows the mold cavity to fill easier without restrictions. Furthermore, if the walls are not uniform the thinner sections will cool first, then as the thicker sections cool and shrink it builds stresses near the boundary between the two. Because the thinner sections have already solidified, they do not yield. As the thicker sections yield, it leads to warping and/or twisting, if severe enough, will cause cracks. For non-uniform walls, the change in thickness should not exceed 20% of the nominal wall and should transition gradually.
If your question is about plastic design guidelines then these are some of the guideline for plastic design mentioned below;
Uniform wall thickness
Draft
Ribs
Boss
Undercuts
Before this you need to understand the manufacturing processes. Consider you are manufacturing a pen cap, this means it will be injection moulded. This means you need to consider the part drafts depending on the tooling. If your part is extrusion moulded, then your part will be more simple, with no drafts.
Have a look at this site for more information on plastic design and plastic manufacturing processes.