塑料件结构设计向导 english

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).

Sink marks due to large wall thicknesses.







Voids due to large wall thicknesses.







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.

Influence of rib design on flow behavior of the melt.







Example of design study of multi-connector.

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.

Recommendations for rib dimensions.










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.







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.

Corraguations.







Flat and open areas.







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.

Dimension image with chart of case 1-6.














Comparison of profiles in terms of torsional rigidity and bending.














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.

Torsional rigidity and resistance to torsional stress as a functionof the way in which the ribs are connected to the profile.

Bosses

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.

Proper boss design.







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.

Correct positioning of bosses.







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.

Holes

Holes are easily produced in molded parts by core pins. Through holes are easier to produce than blind holes because the core pin can be supported at both ends.




Blind holes
Core pins supported by just one side of the mold tool create blind holes. The length of the pins, and therefore the depth of the holes, are limited by the ability of the core pin to withstand any deflection imposed on it by the melt during the injection phase. See information on bosses & cores.

As a general rule the depth of a blind hole should not exceed 3 times the diameter. For diameters less than 5 mm this ratio should be reduced to 2.

Blind cores.






Through holes
With through holes the cores can be longer as the opposite side of the mold cavity can support them. An alternative is to use a split core fixed in both halves of the mold that interlock when the mold is closed. For through holes the length of a given size core can be twice that of a blind hole. In cases where even longer cores are required, careful tool design is necessary to ensure balanced pressure distribution on the core during filling to limit deflection.

Through cores.







Holes with an axis that runs perpendicular to the mold opening direction require the use of retractable pins or split tools. In some designs placing steps or extreme taper in the wall can avoid this. See section on draft. Core pins should be draw polished and include draft to help with ejection.

The mold design should direct the melt flow along the length of slots or depressions to locate weld lines in thicker or less critical sections. If weld lines are not permissible due to strength or appearance requirements, holes may be partially cored to facilitate drilling as a post molding operation.

The distance between two holes or one hole and the parts edge should be at least 2 times the part thickness or 2 times the hole diameter whichever is the largest.

Minimum hole spacing dimensions.







For blind holes the thickness of the bottom should be greater than 20% of the hole diameter in order to eliminate surface defects on the opposite surface.  A better design is to ensure the wall thickness remains uniform and there are no sharp corners where stress concentrations can occur.

Blind hole design recommendations.

Radii & corners

In the design of injection molded parts sharp corners should always be avoided;generous radii should be included in the design to reduce stress concentrations.

Fillet radii should be between 25 and 60% of the nominal wall thickness. If the part has a load bearing function then the upper end is recommended. A minimum radius of 0.5mm is suggested and all sharp corners should be broken with at least a 0.125 mm radius.



Sharp corners, particularly internal corners introduce:
High molded in stresses
Poor flow characteristics
Reduced mechanical properties
Increased tool wear
Surface appearance problems, (especially with blends).

The inclusion of a radius will give:
Uniform cooling
Less warpage
Less flow resistance
Easier filling
Lower stress concentration
Less notch sensitivity.

The outside corner radius should be equal to the inside radius plus the wall thickness as this will keep a uniform wall thickness and reduce stress concentrations.

Corner radius.







For a part with an internal radius half the nominal wall thickness a stress concentration factor of 1.5 is a reasonable assumption. For smaller radii, e.g. 10% of the nominal wall, this factor will increase to 3.  Standard tables for stress concentration factors are available and should be consulted for critical applications.

Stress concentration as a function of wall thickness and corner radius.







In addition, from a molding view point, it is important is to avoid sharp internal corners. Due to the difference in area/volume-ratio of the polymer at the outside and the inside of the corner, the cooling at the outside is better than the cooling at the inside. As a result the material at the inside shows more shrinkage and so the corner tends to deflect (see figure below).

Sharp corners.

Tolerances

Establishing the correct tolerances with respect to the product function is of economic importance. The designer should be aware that dimensions with tight tolerances have a big influence on the costs of both product and mold.

Even slightly over specifying tolerances may adversely influence tool costs, injection molding conditions, and cycle time. It is recommended to indicate only critical dimensions with tolerances on a drawing.


Depending on the application, a division into three tolerance classes can be made:

normal; price index 100
accurate; technical injection molding; price index 170
precise; precision injection molding; price index 300


The most important characteristics of the tolerance classes are shown in the figure below.

Characteristics of tolerance classes.










Mold design, mold cavity dimensions, product shape, injection-molding conditions and material properties determine the tolerances that can be obtained.  The figure below provides a summary of the factors that play a major role in establishing dimensional accuracy.

Factors affecting parts tolerance.

Draft angle

Part features cut into the surface of the mold perpendicular to the parting line require taper or draft to permit proper ejection. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. Since thermoplastics shrink as they cool they grip to cores or male forms in the mold making normal ejection difficult if draft is not included in the design. If careful consideration is given to the amount of draft and shutoff in the mold it is often possible to eliminate side actions and save on tool and maintenance costs.

For untextured surfaces generally a minimum of 0.5 deg draft per side is recommended although there are exceptions when less may be acceptable. Polishing in draw line or using special surface treatments can help achieve this.

For textured sidewalls use an additional 0.4 deg draft per 0.1mm depth of texture.

Draft (A) in mm for various draft angles (B) as a function of molding depth (C).










Typically 1 to 3 deg draft is recommended. As the draft increases ejection becomes easier but it increases the risk that some sections may become too heavy.

Try to keep features in the parting line or plane.  When a stepped parting line is required allow 7 deg for shutoff. 5 deg should be considered as a minimum. Drag at the shutoff will cause wear over time with the risk that flash will form during molding. More frequent maintenance will be required for this type of tooling if flash free parts are to be produced.

Parting line.

可惜图片太多，上不来。

好东西，为什么不搞为压缩文件上来啊

网面上摘炒下来的。不好整理。