UNDERCUTS

Definition:

Undercuts are indentation or projection on the walls of a plastic part which interfere with simple ejection from a two part mold; hence they must be avoided wherever possible.

·         Undercuts unduly complicates the mold design and increases the cost considerably because special split molds are required, retracting mechanisms may be required etc.

·         Internal undercuts require more expensive tooling than the parts with external undercuts.

·          Post mold machined undercuts are often cheaper than those molded in, especially external undercuts and circular undercuts.

MOLDED HOLES

• ·         Molded holes with optimum geometry and size usually cost less than the machined holes if large quantities of parts are to be made.
• ·         Molded holes in the direction of the mold closure (perpendicular to the parting line) are less expensive than holes perpendicular to or at an angle to the mold closure.
• ·          For holes perpendicular to or at an angle to the mold closure, a post molding drilling should be considered because it requires special retracting mechanisms for automatic molding.
• ·         Molded holes may be blind, through, stepped, recessed step or intersecting. The holes can be round, square, elliptical, etc. the round holes being always the cheapest because simple dowel pins can be used as mandrels in the mold to form the holes.

• ·         When the length to diameter ration of the pins forming the holes, is excessive, the pins may be distorted or broken off due to the severe bending force to which it might often be subjected to. The ratio is known as “slenderness ratio”.
• ·         Using two hole-pins support in compression molding involves the risk of flash entrapment because the resin is introduced prior to mold closing. This possibility is minimized during injection molding or transfer molding because resin is introduced after the mold is closed. The entrapped flash can cause the pin to bend or break in the next shot. Two pins butted together in the center can minimize the problem by revealing any flash on either pin when the mold opens. The two pins should have different diameter to minimize the possibility of misalignment.
• ·         While molding the holes by injecting the material into the mold, which flows around the pin to form the hole, the material joins itself somewhere behind the pin to form a weld line forming an area of potential weakness. If this proves harmful (detrimental) in the part operation, the way of injection and gate location should be changed such that it will minimize the weak areas. The weak areas may be in the side wall, edge or the area between the holes.
• ·         To eliminate the weld line weakness and mark the hole location, short pin is used to mold a shallow hole which can then be drilled further to obtain a complete hole.
• ·         The holes should be placed at minimum 2d apart or from the edge or a side wall to minimize the weld line weaknesses.
• ·         Holes to be tapped or to be used with self tapping screws should be countersunk.
• ·         Feather edge should be avoided because it can lead to cracking at the edges of the part or mold.

TAPER OR DRAFT

 DEFINITION:

Taper: The difference in diameter of top and bottom of a part. It gives slanting wall to the part. It is measured in terms of length.

Draft: The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mold. It is measured in terms of angle.

Significance:

·         Taper or draft is almost required to assure easy removal of the part from the mold because the material will tend to shrink on the core of the mold and also the sticking can take place on both corer and cavity of the mold.

Design criteria:

·         A minimum draft of 1/2⁰ and a maximum of 3⁰ (if possible) on both the surface of the part is usually recommended.

·         If the knockout pins are provided in the cavity section of the mold then the part should stick to the cavity and not to the core (plunger) hence the cavity is designed with no draft, the draft is only provided on the core.

·         If the knockout pins are provided in the core section of the mold then the part should stick to the core and not to the cavity hence the core is designed with no draft, the draft is only provided on the cavity walls.

·         Pick up ribs are also sometimes molded on the part to assure that the part will stay in the cavity or on the plunger.

·         The desired surface can also be liquid honed to increase sticking.

RADII AND FILLETS:

                                                                   

Definition:

1.       Radii: Rounded external angle between two surfaces.

2.       Fillet: Rounded filling of the internal angle between two surfaces.

Significance:                                                                                                                                             

·         Stresses tend to concentrate at the junction of two sections like corners, ribs, or external corners.

·         Rounding of the corners will reduce the stress concentration hence proper radius should be used for a given wall thickness.

·          When external corners are sharp, the section at the corner should be increased by about 30%

·         Rounding of the corner with an external radius will tend to maintain a uniform thickness and therefore relieves stress concentration.

·         Radii and fillets promote the distribution of internal load and stresses and thus release the stress concentration associated with changes in wall thickness and changes in direction of internal loads.

·         Radii and fillets also promote even flow of material during molding, hence lower pressure can be used or the cycle time can be reduced.

·         When stress concentrations are minimized, the strength of the part is increased.

·         When generous radii are used at the junction of two walls, there is less possibility of cracks to occur.·        

A minimum radius of 1/64” should be provided, if possible.

REINFORCING RIBS AND BOSSES:

Definition:

·         RIBS:   A reinforcing member of a fabricated or molded part. It is included to support and strengthen the part.

·         BOSS:  Projection on a plastic part. It is included to: 1) add strength, 2) to facilitate alignment during assembly and 3) to provide for fastening.

Theory:

Increasing the wall thickness throughout a part, will increase the stiffness of a part. However this will also increase the cross sectional moment of inertia, hence a rectangular cross section becomes relatively inefficient in bending. To prevent this, Reinforcing Ribs and Bosses are added to provide increased stiffness of a part.

RIBS: 

SIGNIFICANCE:

·         A T-section results when reinforcing gussets are added. This provides stiffness equivalent to a rectangular section, with less material and therefore less weight.

·         Ribs increase stiffness locally, without adding extra material and weight.

·         Ribs around attaching holes and bosses, efficiently transfers loads from the bolt to the primary structure.

·         Ribs at the unsupported flat plates stiffen it to prevent buckling or increase the natural frequency of vibration.

·         Ribs around the openings 1) increase radial stiffness, 2) maintains concentricity and 3) helps filling Bosses.

DESIGN RULES:

·         Properly designed Ribs have minimal possibility of warping and sink marks after cooling or curing.

·         Gradual thickness transition should be provided with generous radii.

·         Thickness of the Rib should be less than that of the wall to which it is attached. If it is necessary to provide Ribs with thickness greater than that of the adjacent wall, it is better to provide an additional Rib or split the Rib and thus maintaining the maximum thickness limitation.

·         When part requires more than one Rib, the spacing between the Ribs should not be less than twice the thickness of the wall to which it is attached.

·         It will be more advantageous (in terms of stiffness) to increase the height of the Rib than to increase its thickness because stiffness is proportional to the cube of height and proportional to thickness (stiffness α height³ and stiffness α thickness). However, the height should not be large enough to make the Rib unstable and buckle under load.

·         The Rib design for thermoset and thermoplastic slightly differ from each other.

BOSSES:

SIGNIFICANCE:

·         Bosses are protruding pads, commonly used as mounting surfaces for the molded part.

·         It is used to distribute bearing loads caused by attachment bolts preloads and to transfer bolt loads to the main surface.

·         They increase the wall section around the slot.

DESIGN RULES:

·         Excessively thick Bosses should be avoided.

·         When attachment loads are very high, Ribs should be used to connect the bosses to the adjacent wall.

·         Diameter of the Boss should be sufficient to carry the required load without failure.

·         Three Bosses permit better alignment than four or more.

·         Boss height should not be more than twice the diameter.