Basic knowledge of structural design of controller plastic parts - wall thickness

Time:2024-01-29 20:14:20 / Popularity: / Source:

First, let’s understand what wall thickness of a product is:
Wall thickness is the most basic structural feature of a part. If outer surface of part is called outer wall and inner surface is called inner wall, then there is a thickness value between outer wall and the inner wall. This thickness value is called wall thickness. Value we input when extracting shell on software during structural design can also be said to be wall thickness.
controller plastic parts 
Among types of parts for electronic control products, plastic molded parts are often used. One of the most basic knowledge in design of plastic parts is how to choose material thickness: basic wall thickness and rib thickness.
controller plastic parts 
We all know: Plastic molded parts cannot have thick solid parts. This is partly because thick sections of plastic tend to take a long time to cool (longer molding cycles), but also because they experience surface shrinkage (leaving pit marks) as they cool. In addition, plastic materials are expensive, and price of commonly used PBT-GF30 is expected to be 25~30CNY/KG, so parts regardless of complexity will be composed of relatively thin walls.
Basic wall thickness of a plastic part is one of the first decisions to be made. A foundation wall is defined as wall to which other features will be attached, including side walls and other structural reinforcements. Most future decisions depend to some extent on this decision, as rib thickness dimensions for other features will depend on foundation thickness. Control of sag shrinkage, as well as design of transitions to thicker or thinner sections, flow characteristics of mold (mold fill/ejection), and control of warpage can all be negatively affected if base thickness is not chosen correctly.
Basic thickness of commonly used electronic control plastic parts is 2.0~3.0mm. In addition, the overall size and depth of part, surface treatment or texture requirements can also affect designed base wall thickness value.
In plastic part design, it is critical to aim for uniform wall thickness; it is the most basic design rule. Uniform wall thickness in plastic part design applies equally to part's base wall thickness and any added walls (usually added perpendicular to base wall). Use uniform wall thickness throughout part to avoid quality and part fit issues.
Design plastic parts with minimum wall thickness to meet specific structural or functional requirements. This will reduce material usage as well as cost, weight and cycle time. Internal stresses, warping, dents and cracks can all be reduced or eliminated with uniform wall thickness. For wall thickness less than 0.8mm, please try to avoid it.
Impact of uneven wall thickness on a molded part may be stress during molding, warpage, dents, holes/short shots or need for wider tolerances. If thicker sections must be added, design a gradual zone between different wall sections rather than an abrupt transition. Length distance of gradient transition can be a 3:1 distance ratio between t and 1.5t thickness sections. See image below:
controller plastic parts 
Uneven wall thickness can cause severe denting, warping, and dimensional control issues. Thicker sections have a tendency to have sunken marks and if these are acceptable this can be noted on drawing.
Any thick sections should be avoided in parts where good surface appearance is required. If these thicker ribs are required on such a part, dent marks can be hidden by some design details on the surface of part, such as texturing surface.
For visible appearance surfaces, even textured surface treatments cannot completely eliminate traces of defects. Avoid adding excessive fillets to the bottom of attached ribs or bosses, which can also cause dents in the surface. Sometimes local craters can be added to base of screw column to reduce excessive wall thickness.
We can apply 80% rule to minimize shrinkage defects. That is, thickness of ribs added to basic wall thickness shall not exceed 50%-80% of basic wall thickness, and 60% is generally selected. That is to say, assuming that basic meat thickness is 2.0mm, then thickness of our ribs can be designed to be 1.2mm.
Even if shrinkage is not an issue, warpage (i.e. flatness) of part can become a major issue, as shown in image below:
controller plastic parts 
When mating one part to another, surfaces containing excessive depressions (especially contact areas) should be avoided.
Thick sections cool more slowly than thin sections, because shrinkage and solidification occur simultaneously, irregularities in thickness will lead to irregularities in shrinkage, thereby creating internal stresses. These stresses usually relieve themselves by forming dent marks or by warping. If greater strength or stiffness is required, it is better to add ribs rather than increase wall thickness.

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