Vegetación y Bioingeniería 70

Cores are the protruding parts of the

mold that form the inside surfaces of features such as holes, pockets, and recesses. Cores also remove plastic from thick areas to maintain a uniform wall thickness. Whenever possible, design parts so that the cores can separate from the part in the mold-opening direction. Otherwise, you may have to add slides or hydraulic moving cores that can increase the cost of mold construction and maintenance (see section on undercuts).

During mold filling, the advancing plastic flow can exert very high side forces on tall cores forming deep or long holes. These forces can push or bend the cores out of position, altering the molded part. Under severe conditions, this bending can fatigue the mold steel and break the core.

Generally, the depth-to-diameter ratio for blind holes should not exceed :. Ratios up to 5: are feasible if filling progresses symmetrically around the unsupported hole core or if the core is in an area of slow-moving flow. Consider alternative part designs that avoid the need for long delicate cores, such as the alternative boss designs in figures -9 and -0.

If the core is supported on both ends, the guidelines for length-to-diameter ratio double: typically 6: but up to 0: if the filling around the core is symmetrical. The level of support on the core ends determines the maximum suggested ratio (see figure -5). Properly interlocked cores typically resist deflection better than cores that simply kiss off. Single cores for through-holes can interlock into the opposite mold half for support.

Core Mismatch Figure 2-26

When feasible, make one core larger to accommodate mismatch in the mold. Interlocking Cores Figure 2-25

The ends of the long cores should interlock into mating surfaces for support.

Mismatch Figure 2-27

Rounding both edges of the hole creates a potential for mismatch.

Mismatch can reduce the size of the

opening in holes formed by mating cores. Design permitting, make one core slightly larger (see figure -6). Even with some mismatch, the required hole diameter can be maintained. Tight tolerance holes that cannot be stepped may require interlocking features on the cores to correct for minor misalignment. These features add to mold construction and maintenance costs. On short through-holes that can be molded with one core, round the edge on just one side of hole to eliminate a mating core and avoid mismatch (see figure -7).

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UNDERCUTS

Some design features, because of their orientation, place portions of the mold in the way of the ejecting plastic part. Called “undercuts,” these elements can be difficult to redesign. Sometimes, the part can flex enough to strip from the mold during ejection, depending upon the undercut’s depth and shape and the resin’s flexibility. Undercuts can only be stripped if they are located away from stiffening features such as corners and ribs. In addition, the part must have room to flex and deform. Generally, guidelines for stripping undercuts from round features limit the maximum amount of the undercut to a percentage defined as follows and illustrated in figure -8 as:

Generally, avoid stripping undercuts in parts made of stiff resins such as polycarbonate, polycarbonate blends, and reinforced grades of polyamide 6. Undercuts up to % are possible in parts made of these resins, if the walls are flexible and the leading edges are rounded or angled for easy ejection. Typically, parts made of flexible resins, such as unfilled polyamide 6 or thermoplastic polyurethane elastomer, can tolerate 5% undercuts. Under ideal conditions, they may tolerate up to 0% undercuts.

Slides and Cores

Most undercuts cannot strip from the mold, needing an additional mechanism in the mold to move certain components prior to ejection (see Chapter 7). The types of mechanisms include slides, split cores, collapsible cores, split cavities, and core pulls. Cams, cam pins, lifters, or springs activate most of these as the mold opens. Others use external devices such as hydraulic or pneumatic cylinders to generate movement. All of these mechanisms add to mold cost and complexity, as well as maintenance. They also add hidden costs in the form of increased production scrap, quality problems, flash removal, and increased mold downtime.

Stripping Undercut Guidelines Figure 2-28

Undercut features can often successfully strip from the mold during ejection if the undercut percentage is within the guidelines for the material type.

Clever part design or minor design concessions often can eliminate complex mechanisms for undercuts. Various design solutions for this problem are illustrated in figures -9 through -. Get input from your mold designer early in product design to help identify options and reduce mold complexity.



GENERAL DESIGN

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Snap Fit Figure 2-30

Snap-fit hook molded through hole to form undercut.

Wire Guides Figure 2-31

Simple wire guides can be molded with bypass steel in the mold.

Sidewall Windows Figure 2-29

Bypass steel can form windows in sidewalls without moving slides.

 Page 4 of 68: This document contains important information and must be read in its entirety. Vent Slots Figure 2-32

Extending vent slots over the corner edge eliminates the need for a side action in the mold.

Louvers on Sloping Wall Figure 2-33

Louvers on sloping walls can be molded in the direction of draw.

LOUVERS AND VENTS

Minor variations in cooling-vent

design can have a major impact on

the molding costs. For instance, molds designed with numerous, angled kiss-offs of bypass cores are expensive to construct and maintain. Additionally, these molds are susceptible to damage and flash problems. Using moving slides or cores to form vents adds to mold cost and complexity.

Carefully consider the molding process during part design to simplify the mold and lower molding costs. Extending vents over the top of a corner edge can facilitate straight draw of the vent coring and eliminate a side action in the mold (see figure -). Angling the louver surface can also allow vent slots to be molded without side actions in the mold (see figure -).

Consult all pertinent agency specifications for cooling vents in electrical devices. Vent designs respond differently to the flame and safety tests required by many electrical devices. Fully test all cooling-vent designs for compliance.