Plastique
The structural foam process offers a very efficient and economical way to gain the high strength and rigidity required
in larger structural parts. In essence, the foam core structure of the plastic part is achieved either by introducing inert gas directly into
the melt or by preblending the resin with a chemical blowing agent. When the gas/resin mixture is
shot under pressure into the mold cavity, the gas expands within the plasticized material as it fills
the mold. Since the cavity is not completely filled up upon injection, only low pressures occur hence less clamp tonnage is required. The result is a molded low stress part
posessing a tough external skin.
The strengths of structural foam are as follows:
- Rigidity
- Strength-to-weight ratio
- Repeatable-tolerance molding
- Flatness
- Sound damping
- Ability to consolidate parts
Draft Angles
Adequate draft angles provide easy removal from the mold. Common practice would indicate 0.5 to 3 degrees per side as sufficient. Close to zero
draft is possible, depending on part configuration, material shrinkage, wall thinkness, mold design and mold material. Small draft angles are mostly possible because of the
lower pressures involved in foam molding. As the wall thickness decreases, larger draft angles are required. On very deep draws, the increase in base width due to draft must be considered. A base
that is too thick can result in sink marks or other deformations and postblow conditions.
Ribs
Adding ribs to a stuctural-foam part increases rigitity and load-bearing capability without increasing wall thickness. Thin wall thickness
reduces cooling time and overall cycle time. Ribs should be no higher than three times the thickness of the wall section to which they are attached. Using more ribs is better than increasing the height of ribs beyond this limit. For more
than two ribs, spacing between ribs should be no less than twice the nominal wall thickness.
Bosses
Bosses can be used to accept fasteners and other support parts. Bosses can often replace costly brackets and othe metal parts. Experimentation has shown the optimim boss diameter to be 1.6 to 2.0 times the diameter of the cored hole,
with a minimum boss wall thickness of 0.080 inch. Generous fillets should be specified in order to avoid stress concentration and resist torque loading.
Inserts and self-tapping screws
Self-tapping fasteners can be specified for permanent assemblies when the number of field dissasemblies will be less than 20. Thread-rolling screws are preferred. Brass threaded inserts provide one of the
best fastening methods when disassembly is required.
Mating Surfaces
Lap, Butt, and tongue-and-groove joint designs are conventional. Thin sections should be avoided.
Louvers
Molded-in louvers eliminate additional secondary operations and should be oriented in the direction of the resin flow. When situated on sidewalls, louvers should be extented around corners to avoid side pulls in the mold.
Undercuts
Hydraulic or mechanical core pulls can be used to create a variety of molded-in features in planes not in the primary line of pull.
Bonding
Most structural-foam resins can be solvent bonded to themselves or to other plastics with a number of commercial solvents. More recent processes
such as ultrasonic bonding and vibration welding have lent themselves well to engineering structural foam parts. They generally perform better in thinner wall designs than thicker ones.
Mold Texture and Engraving
Mold texturing is available for functional or cosmetic reasons. It can eliminate surface preparation and sanding. It is necessary to allow 1 to 1.5 degrees draft for 0.001 inch of texture depth and the minimum depth of 0.003
is required for the best results. Engraving allows for low-cost graphics and logos on parts without too many additional decorative costs.
 Feel free to send us your comments, tips, tricks
and hints for injection molding to us in the Core77 Discussion Boards.
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