Product structure design criteria-detailed explanation of reinforcing ribs

Time：2022-08-05 08:56:13 / Popularity： / Source：

Basic Design Guidelines

Reinforcing ribs are an indispensable functional part of plastic parts. Reinforcing ribs effectively increase rigidity and strength of product like an I-shaped iron without greatly increasing product section area, but it does not have shape problem of an undercut difficult to form like an I-shaped iron. Plastic products with torsion and bending are especially suitable. In addition, ribs can also act as internal runners to help fill mold cavity, and play a great role in helping plastics flow into branches of part.
Reinforcing ribs are generally placed on non-contact surface of plastic products, their extension direction should follow direction of maximum stress and maximum offset of product. Choice of position of ribs is also subject to some production considerations, such as cavity filling, shrinkage and demolding. Length of ribs can be same as length of product, the two ends can be connected to outer wall of product, or only occupy length of product, to locally increase rigidity of a part of product. If rib is not connected to outer wall of product, end part should not be abruptly terminated. Height should be gradually reduced until the end, so as to reduce air trapping, underfilling and scorching marks. These problems often occur in insufficiently vented or enclosed locations.

Reinforced design

The simplest shape of rib is a rectangular column attached to surface of product, but in order to meet some production or structural considerations, shape and size of rib must be changed as shown in following figure.

Rectangular ribs must change shape to make production easier

Mold corners must be added on both sides of ribs to reduce friction during ejection. Position of bottom of product must be rounded to eliminate phenomenon of excessive stress concentration. Rounded corners are also designed to reduce friction during ejection. Gradual shape of channel makes filling of cavity more smooth. In addition, width of bottom must be smaller than thickness of connected outer wall. Relationship between product thickness and rib size a illustrates this requirement. Although size of stiffener in figure has been designed in a reasonable proportion, when a circle R1 is made from position where bottom of stiffener connects with outer wall, it can be seen that thickness of this part is increased by about 50% relative to outer wall. Therefore, chance of shrinkage lines in this part is quite high. If width of rib bottom relative to product thickness is reduced by half (relationship between product thickness and rib size b), relative position thickness increase will be reduced to about 20%, chance of shrinkage lines will also be greatly reduced. As a result, using two or more short ribs is better than using a single high rib, but when multiple ribs are used, distance between ribs must be greater than thickness of adjacent outer wall. Shape of stiffeners is generally thin and long. General design drawings of stiffeners illustrate basic principles of designing stiffeners. Note that design of too thick ribs is prone to shrinkage lines, voids, deformation and deflection, and water trapping. It will also lengthen production cycle and increase production costs.

Relationship between product thickness and rib size

In addition to above requirements, design of ribs is also related to plastic materials used. From a production point of view, physical properties of material, such as viscosity and shrinkage of melt, have a great influence on design of stiffener. In addition, creep characteristics of plastics are also an important consideration from structural point of view. For example, from a production point of view, height of ribs is subject to characteristics of melt flow and ejection (shrinkage, friction coefficient and stability). Deeper reinforcing ribs require rubber to have lower melt viscosity, lower friction coefficient, and higher High shrinkage rate. In addition, increasing ejection angle of long ribs generally helps product eject. However, when die angle continues to increase and width of bottom remains unchanged, rigidity, strength, and ejectable area of product will decrease accordingly. Problem of reduced ejection area can be solved by adding several ejector bumps to product stiffener or using a more expensive flat thimble. At the same time, polishing in ejecting direction will also help product to eject easily. From structural point of view, deeper ribs can increase rigidity and strength of product without significantly increasing weight, but at the same time, product's highest and lowest point buckling stress (bending stress) increases, product designer must calculate and make sure that buckling stress of this part will not exceed acceptable range.
From a production point of view, using a large number of short and narrow ribs is better than using several deep and wide ribs. When mold is produced (especially for prototype molds): Width (and possibly depth) and quantity of ribs should be left as far as possible. When rigidity and strength of product are found to be insufficient during mold trial, it can be appropriately increased, because It is simpler and cheaper to remove steel material from mold than method of adding steel material such as welding or adding inserts.

Method for reinforcing ribs to enhance strength of plastic parts

Following is where ribs are placed on the edge of plastic part to help plastic flow into edge space.
Reinforcing ribs placed on the edge of plastic part

Design points of different materials

ABS

To reduce shrinkage on the surface of main components, thickness of ribs should not be more than 50% of thickness of intersecting rubber, and thickness of ribs can be up to 70% on some non-deterministic surfaces. In thin plastic structural foamed plastic parts, ribs can reach 80% of thickness of intersecting fabric. Thick rubber ribs can reach 100%. Height of ribs should not be higher than three times hickness of rubber. When there are more than two ribs, distance between ribs should not be less than twice thickness of rubber. Exit angle of ribs should be between one side to facilitate easy demolding.

Design points of ABS stiffener

PA

Height of individual ribs should not be three times or more thickness of bottom of ribs. Behind any rib, there should be some small ribs or grooves, because ribs will cause dents on the back when they are cooled, those ribs and grooves can be used for decorative purposes to eliminate defect of shrinkage.

PBT

Try to avoid thick ribs to avoid air bubbles, shrinkage lines and stress concentration. Consideration of method is to limit size of ribs. Thickness of ribs shall not exceed 60% of wall thickness below 3.2mm (1/8 in). Ribs whose wall thickness exceeds 3.2mm shall not exceed 40%. Rib height should not exceed 3 times bone thickness. Ribs and rubber wall are connected with a 0.5mm (0.02 in) R to make plastic flow smoothly and reduce internal stress.

PC

General rib thickness depends on plastic flow and wall thickness. If many ribs are used for reinforcement, thin ribs are better than thick ones. For design of PC ribs, please refer to key points of PS rib design in figure below.

PS

Thickness of ribs should not exceed 50% of thickness of their adjoining walls. Experience tells us that violating above guidelines will cause uneven gloss on the surface.

PS is placed in the middle of rib design point.

Key points of rib design with PS placed on the side

PSU

Ribs can enhance impact strength of product and use the most economical cost to achieve effective results. Poor design will cause shrinkage marks on the surface and undesirable impact strength.

Gud Mould Industry Co., Ltd