本公司洋工程师传授的General Design Guidelines教程,倾情奉献!

Although component design is complex, following a few fundamental principles will help minimize problems during injection molding and in part performance. In some instances it may not be possible to incorporate or follow all the suggested advice but the guidelines should increase your understanding of designing with thermoplastics.

Design recommendations
Wall thickness
Ribs and profiled structures
Gussets or support ribs
Bosses
Holes
Radii & Corners
Tolerances
Coring
Undercuts
Draft angle

Wall thickness

Just as metals have normal working thickness ranges based upon their processing method, so do plastics. Typically, for injection molded parts, the wall thickness will be in the range 0.5 mm to 4 mm (0.02 - 0.16 in). Dependent on the part design and size, parts with either thinner or thicker sections can be molded.

While observing functional requirements, keep wall thicknesses as thin and uniform as possible.  In this way even filling of the mold and anticipated shrinkage throughout the molding can be obtained in the best way.  Internal stresses can be reduced.

Wall thickness should be minimized to shorten the molding cycle, obtain low part weight, and optimize material usage.  The minimum wall thickness that can be used in injection molding depends on the structural requirements, the size and geometry of the molding, and the flow behavior of the material.  As a starting point the designer can often refer to spiral flow curves which give a relative measure of the maximum achievable flow length for a given wall thickness and injection pressure. See figure below.

Spiral flow length of Akulon Ultraflow at 260° and 1400 bar.

If parts are subjected to any significant loading the part should be analyzed for stress and deflection. If the calculated stress or deflection value is not acceptable a number of options could be considered including the following:


Increase wall (if not already too thick)
Use an alternative material with higher strength and/or modulus
Incorporate ribs or contours in the design to increase the sectional modulus



Other aspects that may need to be considered include:

Insulation characteristics
Generally speaking insulating ability (whether for electrical or heat energy) is related to the thickness of the polymer.

Impact characteristics
Impact resistance is directly related to the ability of a part to absorb mechanical energy without fracturing. This in turn is related to the part design and polymer properties. Increasing the wall section will generally help with impact resistance but too thick (stiff) a section may make a design unable to deflect and distribute an impact load therefore increasing stresses to an unacceptable level.

Agency approval
When a part design must meet agency requirements for flammability, heat resistance, electrical properties etc, it may be necessary to design with thicker sections than would be required just to meet the mechanical requirements.

Where varying wall thicknesses are unavoidable for reasons of design, there should be a gradual transition (3 to 1) as indicated in the figure below.

Gradual transition of wall thicknesses.

Generally, the maximum wall thickness used should not exceed 4 mm. Thicker walls increase material consumption, lengthen cycle time considerably, and cause high internal stresses, sink marks and voids (see figure below).


Care should be applied to avoid a "race tracking" effect, which occurs because melt preferentially flows faster along thick sections. This could result in air traps and welds lines, which would appear as surface defects. Modifying or incorporating ribs in the design can often improve thick sections.

Ribs and profiled structures

If the load carrying ability or the stiffness of a plastic structure needs to be improved it is necessary to either increase the sectional properties of the structure or change the material. Changing the material or grade of material, e.g. higher glass fiber content, may be adequate sometimes but is often not practical (different shrinkage value) or economical.

Increasing the sectional properties, namely the moment of inertia, is often the preferred option. As discussed in other sections, just increasing the wall section although the most practical option will be self-defeating.


Increase in part weight and costs are proportional to the increase in thickness.
Increase in cooling time is proportional to the square of the increase in thickness.



If the load on a structural part requires sections exceeding 4 mm thickness, reinforcement by means of ribs or box sections is advisable in order to obtain the required strength at an acceptable wall thickness.

The efficiency of a ribbed structure can be illustrated with the following example:


Solid plate vs. ribbed plate in terms of weight and stiffness.

Although ribs offer structural advantages they can give rise to warpage and appearance problems, for this reason certain guidelines should be followed:

The thickness of a rib should not exceed half the thickness of the nominal wall as indicated in the figure below.

In areas where structure is more important than appearance, or with very low shrinkage materials, ribs with a thickness larger than half the wall thickness can be used. These will cause sink marks on the surface of the wall opposite the ribs. In addition, thick ribs may act as flow leaders causing preferential flows during injection. This results in weld lines and air entrapment.

Maximum rib height should not exceed 3 times the nominal wall thickness as deep ribs become difficult to fill and may stick in the mold during ejection.  

Typical draft is 1 to 1.5 deg per side with a minimum of 0.5 deg per side. Generally draft and thickness requirements will limit the rib height.

At the intersection of the rib base and the nominal wall a radius of 25 to 50% of the nominal wall section should be included. Minimum value 0.4 mm. This radius will eliminate a potential stress concentration and improve flow and cooling characteristics around the rib. Larger radii will give only marginal improvement and increase the risk of sink marks on the opposite side of the wall.

Parallel ribs should be spaced at a minimum distance of twice the nominal wall thickness; this helps prevent cooling problems and the use thin blades in the mould construction.


Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots.

Ribs should be orientated along the axis of bending in order to provide maximum stiffness. Consider the example in the figure above where a long thin plate is simply supported at the ends. If ribs are added in the length direction the plate is significantly stiffened. However, if ribs are added across the width of the plate little improvement is found.

Ribbing is typically applied for:
1. Increasing bending stiffness or strength of large flat areas
2. Increasing torsional stiffness of open sections

Adding corrugations to the design can stiffen flat surfaces in the direction of the corrugations (see figure below). They are very efficient and do not add large amounts of extra material or lengthen the cooling time. The extra stiffness is a result of increasing the average distance of the material from the neutral axis of the part, i.e. increasing the second moment of inertia.

Ribs and box sections increase stiffness, thus improving the load bearing capability of the molding. These reinforcing methods permit a decrease in wall thickness but impart the same strength to the section as a greater wall thickness.

The results demonstrate that the use of diagonal ribs have the greatest effect on the torsional rigidity of the section. The change from an I section to a C section helps in terms of horizontal bending terms but not in torsional terms. As double cross ribs (option 6) can give tooling (cooling) problems option 8 is the recommended solution for the best torsional performance.

Depending on the requirements of the part the acceptability of possible sink marks at the intersection of the ribs and profile wall need special consideration. For maximum performance and function the neutral lines of the ribs and profile wall should meet at the same point. Deviation from this rule will result in a weaker geometry. If, due to aesthetic requirements, the diagonal ribs are moved slightly apart then the rigidity is reduced 35%. If a short vertical rib is added to the design then the torsional rigidity is reduced an additional 5%. See figure below.

Bosses often serve as mounting or fastening points and therefore, for good design, a compromise may have to be reached to achieve good appearance and adequate strength. Thick sections need to be avoided to minimize aesthetic problems such as sink marks. If the boss is to be used to accommodate self tapping screws or inserts the wall section must be controlled to avoid excessive build up of hoop stresses in the boss.


General recommendations include the following:
Nominal boss wall thickness less than 75% nominal wall thickness, note above 50% there is an increased risk of sink marks. Greater wall sections for increased strength will increase molded-in stresses and result in sink marks.

A minimum radius of 25% the nominal wall thickness or 0.4 mm at the base of the boss is recommended to reduce stresses.

Increasing the length of the core pin so that it penetrates the nominal wall section can reduce the risk of sink marks. The core pin should be radiused (min 0.25 mm) to reduce material turbulence during filling and to help keep stresses to a minimum. This option does increase the risk of other surface defects on the opposite surface.

A minimum draft of 0.5 degrees is required on the outside dimension of the boss to ensure release from the mold on ejection.

A minimum draft of 0.25 degrees is required on the internal dimension for ejection and or proper engagement with a fastener.

Further strength can be achieved with gusset ribs or by attaching the boss to a sidewall.

Bosses adjacent to external walls should be positioned a minimum of 3 mm (.12 in)  from the outside of the boss to avoid creating a material mass that could result in sink marks and extended cycle times.
A minimum distance of twice the nominal wall thickness should be used for determining the spacing between bosses. If placed too close together thin areas that are hard to cool will be created.  These will in turn affect quality and productivity.