Plastic structural design 4 —reinforcement ribs and columns

Time:2024-04-11 14:38:03 / Popularity: / Source:

To read before, Please read Plastic structure design 3 - wall thickness and draft angle.
This article mainly introduces requirements for stiffeners, support columns, screw columns and support sleeves in design of plastic structures. See below for details:

1. Reinforcement rib design

Reinforcing ribs are an essential feature in design of plastic parts. They are used to increase strength of part, serve as a runner to assist flow of plastic melt, and provide guidance, positioning and support for other parts in product. Design parameters of reinforcing ribs include thickness, height, draft angle, root fillet, and spacing between reinforcing ribs.
Basic design principles of stiffeners

1) Reinforcing ribs are generally placed on non-contact surface of plastic products.

Its extension direction should follow direction of maximum stress and maximum offset of product. Location of reinforcing ribs is also subject to some production considerations, such as mold cavity filling, shrinkage and demoulding. Length of reinforcing rib can be consistent with length of product, with both ends connected to outer wall of product, or it can only occupy part of length of product to locally increase rigidity of a certain part of product. If reinforcing rib is not connected to outer wall of product, end part should not terminate suddenly. Height should be gradually reduced until it is complete, thereby reducing problems such as trapped air, underfill, and scorch marks, which often occur in insufficiently vented or closed locations.
Plastic structural design 

2). The simplest shape of reinforcing rib is a rectangular column attached to the surface of product.

However, in order to meet some production or structural considerations, shape and size of reinforcing rib need to be changed as shown in figure below.
Plastic structural design 

3). Ejection angles must be added to both sides of reinforcing ribs to reduce friction during demoulding and ejection.

Rounded corners must be added to position where bottom meets product to eliminate excessive stress concentration. Rounded corner design also gives flow channel a gradual shape to make mold cavity filling smoother. In addition, width of bottom must be smaller than thickness of connected outer wall. Figure a showing relationship between product thickness and stiffener size illustrates this requirement. Although size of stiffeners in figure has been designed according to reasonable proportions, when a circle R1 is drawn from position where bottom of stiffeners connects to outer wall, it can be seen in figure that thickness of this part relative to outer wall is increased by about 50%. Therefore, chance of shrinkage marks in this part is quite high. If width of bottom of rib is reduced by half relative to product thickness (Figure b of relationship between product thickness and rib size), increase in thickness at relative position is reduced to about 20%, and chance of shrinkage marks is also greatly reduced. It follows that using two or more short reinforcing ribs is more advantageous than using a single tall reinforcing rib. However, when multiple reinforcing ribs are used, distance between reinforcing ribs must be greater than thickness of adjacent outer wall. Shape of stiffener is generally thin and long. General design drawing of stiffener illustrates basic principles of designing stiffener. Please note that excessively thick reinforcing ribs are prone to problems such as shrinkage lines, holes, deformation, deflection, and water marks. They will also lengthen production cycle and increase production costs.
Plastic structural design 

4). Design of reinforcing ribs is also related to plastic material used.

From a production point of view, physical properties of material, such as viscosity and shrinkage of melt, have a great impact on the rib design. In addition, creep properties of plastics are also an important consideration from a structural perspective. From a structural perspective, deeper ribs can increase rigidity and strength of product without significantly increasing weight. However, at the same time, buckling stress at the highest and lowest points of product increases, product designers must calculate and confirm that buckling stress in this part will not exceed acceptable range.

5). From a production perspective, it is better to use a large number of short and narrow ribs than to use several deep and wide ribs.

When producing molds (especially prototype molds): Width (and possibly depth) and number of reinforcing ribs should be left as much as possible. When rigidity and strength of product are found to be insufficient during mold trial, it can be increased appropriately, because removing steel from mold is simpler and cheaper than using methods such as welding or adding inserts to add steel.
Plastic structural design 

6). Here are places where reinforcing ribs are placed on the edge of plastic parts to help plastic flow into space at the edge.

Reinforcement ribs placed at edges of plastic parts.
Plastic structural design 

7). In order to ensure strength and stiffness of plastic parts without causing wall of plastic part to thicken, setting reinforcing ribs at appropriate parts of plastic part can not only avoid deformation of plastic part, but in some cases, ribs can also can improve plastic flow during molding of plastic parts.

8). In order to increase strength and rigidity of plastic parts, it is better to increase number of reinforcing ribs rather than increase its wall thickness.

General design method: Width of ribs is ≦2/3*T (wall thickness). In actual design, when wall thickness is 2.5~3.0mm, width of ribs is usually 1.2mm. Height of ribs depends on actual needs. If it is too high, it will be difficult to remove from mold and may shrink.

2. Support columns and screw columns

(1) Support column

Support columns are used to assemble and position parts, separate objects, and support other parts. Support column should not be used alone. It should be connected to outer wall or used together with reinforcing ribs. Purpose is to strengthen strength of support column and make rubber flow smoother. In addition, because too high support columns will cause air trapping when forming plastic parts, an ejector pin can be used to do this. To increase strength of pillars (especially those far away from outer wall), use stiffeners as shown in Figure a.
Plastic structural design 

(2) Screw column

Screw columns are used to fix internal components or connect front and rear shells (or bottom shells). They are commonly called heaven and earth pillars, front shell is generally called sky pillars, and rear shell is called earth pillars. Table 3.5-1 Commonly used screw column dimensions in structural design practice. When height dimensions of screw columns are different, usual structure is as shown in Figure b/c/d/e.
Commonly used screw column sizes in structural design practice (for PC materials, inner hole diameter Ød is usually increased by 0.1mm)
Screw diameter Screw Æ Screw column inner hole diameter Ød Screw column outer diameter ØD
Ø 2.0mm Ø 1.7mm Ø 4.0mm
Ø 2.6mm Ø 2.2mm Ø5.0mm
Ø 3.0mm Ø 2.6mm Ø 6.0mm
General information common structural screw column dimensions (inner hole diameter Ød is usually increased by 0.1mm for PC materials)
Screw diameter Screw Æ Screw column inner hole diameter Ød Screw column outer diameter ØD
Ø 2.0 mm Ø 1.7 mm Ø 4.3~4.4 mm
Ø 2.3 mm Ø 1.9mm Ø 4.7~4.8mm
Ø 2.6mm Ø 2.3 mm Ø 5.0 mm
Ø 3.0 mm Ø 2.6 mm Ø5.5mm
Ø 3.5 mm Ø 3.0 mm Ø 6.0mm
Plastic structural design 
Figure b shows general structural design when height of the screw column is less than 20mm. Due to low height, reinforcing ribs are generally not used at this time. Some companies require that inner holes of screw posts be chamfered 0.3~0.5*45° to serve as guides when driving screws. However, if plastic part is made of PC material, inner holes of screw posts must be chamfered, because PC material is brittle and easy to burst when screwing, and inner hole must be 0.1mm larger than that of other rubber materials.
design of plastic parts 
Figure c shows general structural design when screw column height is 30~40mm. Glue bit at root (commonly known as crater) is used to prevent appearance of root of screw column from shrinking. Number of reinforcing ribs (commonly known as rocket feet) depends on specific situation.
design of plastic parts 
Figure d shows general structural design when screw column height is greater than 50mm and is located on exterior surface. Because screw column is too high, barrel needle is easy to skew during injection molding. Even if glue is stolen at root, it will shrink. According to structure shown in figure, mold is ejected in an inverted position to shorten length of barrel needle and improve molding performance.
design of plastic parts 
Figure e: General structural design when screw column height is greater than 50mm and is located on a non-appearance surface. Because screw column is too high, barrel needle is easy to skew during injection molding. Therefore, other plastic parts are installed on outer surface of root of screw column so as not to affect appearance. Therefore, front mold is embedded with needles to shorten barrel needle length and improve molding performance.

3. Pillar cover

If finished machine uses pillar sleeves to tighten screws, there must be a pillar sleeve on upper shell of finished machine for positioning purposes. According to general safety standards, screw head must be hidden in a position that cannot be touched, so height must be 3.0mm or above. Also, because there will be sharp corners after adding pillar sleeves, a rounded corner of R1.0 or above must be added to each pillar sleeve where screws are placed.
In order to facilitate introduction during production and assembly, chamfers can be added to bottom of each pillar sleeve for introduction. And because of positioning, screw columns are stored at a depth of 2.5mm at the bottom of column sleeve. Its structural design dimensions are shown in Figure a.
When pillars of bottom shell are long, they are generally called ground pillars, and top shell is called sky pillars. Its structural design dimensions are shown in Figure b.
design of plastic parts 
Figure a: When tightening screws with pillar sleeve on the bottom shell, depth of pit in pillar sleeve is greater than 3.0mm. Picture shows general structural design.
design of plastic parts 
Figure b When bottom shell is tightened with sky and ground pillars, general structural design of bottom shell and ground pillars is as shown in figure. (When height of pillar sleeve is greater than 25mm, it shall be designed according to ground pillar structure)
For Further read, please refer to Plastic structure design 5 —hole, thread, and insert design requirements.

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