外國工程師傳授的design資料，供大家享受。

外國工程師傳授的design資料，供大家享受。
Injection Molding Design Guidelines


Much has been written regarding design guidelines for injection molding. Yet, the design guidelines can be summed up in just a few design rules.

Use uniform wall thicknesses throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times.

Use generous radius at all corners. The inside corner radius should be a minimum of one material thickness.

Use the least thickness compliant with the process, material, or product design requirements. Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost.

Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing.

Use ribs or gussets to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs.

Uniform Walls


Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs.
Also, thinner parts weight less, which results in smaller amounts of the plastic used per part which also results in lower part costs.


The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch).
The need for uniform walls


Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section.

This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.

Uniform wall thicknesses reduce/eliminate this problem.

Uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills.

What if you cannot have uniform walls, (due to design limitations) ?


When uniform walls are not possible, then the change in section should be as gradual as possible.




Coring can help in making the wall sections uniform, and eliminate the problems associated with non-uniform walls.


Warping problems can be reduced by building supporting features such as gussets.

The use of ribs


Ribs increase the bending stiffness of a part. Without ribs, the thickness has to be increased to increase the bending stiffness. Adding ribs increases the moment of inertia, which increases the bending stiffness. Bending stiffness = E (Young's Modulus) x I (Moment of Inertia)

The rib thickness should be less than the wall thickness-to keep sinking to a minimum. The thickness ranges from 40 to 60 % of the material thickness. In addition, the rib should be attached to the base with generous radiusing at the corners.





At rib intersections, the resulting thickness will be more than the thickness of each individual rib. Coring or some other means of removing material should be used to thin down the walls to avoid excessive sinking on the opposite side.



The height of the rib should be limited to less than 3 x thickness. It is better to have multiple ribs to increase the bending stiffness than one high rib.



The rib orientation is based on providing maximum bending stiffness. Depending on orientation of the bending load, with respect to the part geometry, ribs oriented one way increase stiffness. If oriented the wrong way there is no increase in stiffness.


Draft angles for ribs should be minimum of 0.25 to 0.5 degree of draft per side.
If the surface is textured, additional 1.0 degree draft per 0.025 mm (0.001 inch) depth of texture should be provided.

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Boss Design


Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).

The wall thicknesses should be less than 60 % of nominal wall to minimize sinking. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws.

The base radius should be a minimum of 0.25 x thickness



The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs.



Hoop stresses are imposed on the boss walls by press fitting or otherwise inserting inserts.
The maximum insertion (or withdrawl) force Fmaxand the maximum hoop stress, ocurring at the inner diameter of the boss, smax is given by



Failures of a boss are usually attributable to:

High hoop stresses caused because of too much interference of the internal diameter with the insert (or screw).

Knit lines -these are cold lines of flow meeting at the boss from opposite sides, causing weak bonds. These can split easily when stress is applied.

Knit lines should be relocated away from the boss, if possible. If not possible, then a supporting gusset should be added near the knit line.

Snap Latches



• Snaps allow an easy method of assembly and disassembly of plastic parts. Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.


• Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to the uniform cross-section. Here we consider the general case of a beam tapering in both directions.





When Rh=1 and Rb=1 , the above formula does not apply, L'Hospital's rule applies and the formula is simplified to the following:


• Disassembly force. The disassembly force is a function of the coefficient of friction, which ranges from 0.3 to 0.6 for most plastics. The coefficient of friction also varies with the surface roughness. The rougher the surface, the higher the coefficient of friction.

• There is an angle at which the mating parts cannot be pulled apart. This is known as the self-locking angle. If the angle of the snap is less than this angle, then the assembly can be disassembled by a certain force given by the above formula.


The self-locking angle a = tan-1(1/µ)

where µ is the coefficient of friction which ranges from 0.3 to 0.6 for most plastics.

This computes to angles ranging from 73° for low coefficient of friction plastics to 59° for high coefficient of friction plastics.

If this angle is exceeded then the snaps will not pull apart unless the snap beam is deflected by some other means such as a release tool.

This property can be used to advantage depending on the objective of using the snaps. If the snaps are to be used in the factory for assembly only (never to be disassembled by the end user), then the ramp angle the self-locking angle should be exceeded. If the user is expected to disassemble (to change batteries in a toy for example), then the angle should not be exceeded.

• Tooling for snaps is often expensive and long lead time due to

-  The iterations required achieving the proper fit in terms of over travel. The amount of over travel is a design issue. This will control how easy it is to assemble, and how much the mated parts can rattle in assembly. This rattle can be minimized by reducing the over travel or designing in a preload to use the plastic's elastic properties. However, plastics tend to creep under load, so preloading is to be avoided unless there is no other option.


   

-  Often, side action tooling (cam actuated) is required. This increases the mold costs and lead times.Cam actuated tooling can be avoided if bypass coring can be used that results in an opening in the part to allow the coring to form the step.



  


• Some common problems of using snaps:

-  Too high a deflection causing plastic deformation (set) of the latch (the moving member). Care has to be taken that the latch does not take a set. Otherwise, the amount of latch engagement could reduce, reducing the force to disassemble. If the set is bad enough the engagement might even fail.  
-  The moving arm could break at the pivot point due to too high a bending stress. This can be avoided by adhering to the design principles and not exceed the yield strength of the material-in fact it should be kept well below the yield strength depending on the safety factor used.

-  Too much over travel leads to a sloppy fit between mating parts resulting in loose assemblies that can rattle.



• Good snap design practices

-  Design the latch taking into account the maximum strain encountered at maximum deflection.  
-  In general, long latches are more forgiving of design errors than short latches for the same amount of deflection, because of the reduced bending strain.  
-   Build mold tooling with "tool safe condition". By this we mean that the deflection or over travel, or length of engagement can be changed easily by machining away mold tooling, rather than add material to mold tooling, which is more expensive and not good mold practice. This "safe" condition allows for a couple of tooling iterations of the latch, until the snap action is considered acceptable.

Hooke's Law  


Springs are fundamental mechanical components which form the basis of many mechanical systems. A spring can be defined to be an elastic member which exerts a resisting force when its shape is changed. Most springs are assumed linear and obey the Hooke's Law,


where F is the resisting force, D is the displacement, and the k is the spring constant.

For a non-linear spring, the resisting force is not linearly proportional to its displacement. Non-linear springs are not covered in depth here.



Basic Spring Types  


Springs are of several types, the most plentiful of which are shown as follows,


Circular cross section springs are shown. If space is limited, such as with automotive valve springs, square cross section springs can be considered. If space is extremely limited and the load is high, Belleville washer springs can be considered. These springs are illustrated below,





Leaf springs, which are illustrated above in a typical wheeled-vehicle application, can be designed to have progressive spring rates. This "non-linear spring constant" is useful for vehicles which must operate with widely varying loads, such as trucks.



History of Springs  


Like most other fundamental mechanisms, metal springs have existed since the Bronze Age. Even before metals, wood was used as a flexible structural member in archery bows and military catapults. Precision springs first became a necessity during the Renaissance with the advent of accurate timepieces. The fourteenth century saw the development of precise clocks which revolutionized celestial navigation. World exploration and conquest by the European colonial powers continued to provide an impetus to the clockmakers' science and art. Firearms were another area that pushed spring development.
The eighteenth century dawn of the industrial revolution raised the need for large, accurate, and inexpensive springs. Whereas clockmakers' springs were often hand-made, now springs needed to be mass-produced from music wire and the like. Manufacturing methodologies were developed so that today springs are ubiquitous. Computer-controlled wire and sheet metal bending machines now allow custom springs to be tooled within weeks, although the throughput is not as high as that for dedicated machinery.

O-Rings are torus-shaped (i.e. doughnut-shaped) objects made from elastomeric compounds such as natural or synthetic rubber, and are used to seal mechanical parts against fluid movement (air or liquid). O-Rings perform their sealing action by deforming to take the shape of their cavity, after being oversized to guarantee an predetermined interference fit.
O-Rings are inserted into cavities defined as glands, and are typically used in one of two seal designs, axial or radial. These seal designs along with their gland geometries are shown in the following schematic:

  



An O-Ring is specified by its inner diameter, its cross-section diameter, its material hardness/durometer (typically defined by the Shore A hardness), and its material composition.
In order for an O-Ring to seal against the movement of fluid, it must be compressed when seated inside the gland. A standard set of design guidelines exist to determine the proper O-Ring dimensions for radial and axial seals of a given dimension.



O-Ring Symbol Definitions  


Parameters used in the discussion of O-Rings are defined in the following table:  

O-Ring Parameters
Symbol Parameter Description
ID Inner Diameter Diameter of the inside edge of the cross-section.
CSmax Maximum Cross-Section Diameter Upper bound on the cross-section diameter for a given set of input requirements.
CSmin Minimum Cross-Section Diameter Lower bound on the cross-section diameter for a given set of input requirements.
CStol Cross-Section Tolerance Manufacturing tolerance on the O-Ring cross-section diameter.
Cmax Maximum Compression Upper bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.
Cmin Minimum Compression Lower bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.


Radial Gland Symbol Definitions  


Parameters used in the discussion of glands for radial seals are defined in the following table:  

Gland Parameters for Radial Seals
Symbol Parameter Description
Bd Bore Diameter Inner diameter of the bore which confines the outer diameter of the O-Ring.
Btol Bore Diameter Tolerance Manufacturing tolerance on the bore diameter.
Gd Groove Diameter Minimum diameter of the gland which confines the inner diameter of the O-Ring.
Gtol Groove Diameter Tolerance Manufacturing tolerance on the groove diameter.
GW Groove Width Groove length in the axial direction; must be large enough to accommodate O-Ring axial expansion.

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