Three areas of focus
API engineers and designers work with customers to develop a plastic injection molded component from the first sketch. To arrive at a quality part, three things have to be right at the same time:
- Proper part design — geometry the mold can fill, cool, and eject consistently.
- Proper material selection — a resin whose mechanical, thermal, and regulatory properties fit the application.
- Processing conditions — a documented, repeatable molding window (see scientific molding).
API has designed parts, selected materials, and processed resins — specializing in engineering and high-performance grades — since 1953. This guide covers the basic elements of part design. For materials, see the Basic Resin Guide.
Designing for straight-pull molding controls cost
A part that can be molded with a “straight pull” or “up-and-down” motion keeps the mold simple and the price down. In a straight-pull mold, when the A and B halves separate there is no plastic blocking the steel in the direction of pull. Undercuts break that rule and require an action in the mold — cams, lifters, core pulls — each of which adds cost and size to the tool.
- Eliminate undercuts where function allows; where it does not, locate them so a single side action can clear them.
- Keep features parallel or perpendicular to the parting line whenever possible.
Uniform wall thickness prevents defects
Wall thickness is the most fundamental requirement in part design. Plastic shrinks as it cools, and it solidifies first at the mold surface. Thick sections keep shrinking after the skin has frozen, pulling inward and creating sink marks, voids, and residual stress. Because thin sections cool faster than thick ones, stress builds between them and the part warps.
- Hold a single nominal wall across the part; transition gradually (3:1 taper) where a change is unavoidable.
- Core out thick sections rather than filling them with solid plastic.
Draft lets the part release from the mold
Draft is the taper applied to every surface in the direction of mold movement so the part can eject cleanly. As the part cools it shrinks onto the core side of the mold; draft reduces the force needed to push it off. Most parts need a minimum of ½° to 1°; 1½° to 2° is the widely accepted norm, and textured surfaces need more.
- Minimum ½°–1° per side; 1½°–2° preferred.
- Add roughly 1° of draft for every 0.001" of texture depth.
Holes add function and remove weight
Holes are formed by core pins that keep molten plastic out of a region. Through-holes pass completely through the part; blind holes do not, and because the pin is supported at only one end they are harder to form without deflection. Wherever plastic flows around a pin it rejoins on the far side as a weld line, which may be visible and is weaker than the surrounding wall.
- Blind-hole depth: about 2× the pin diameter for pins under 3/16"; up to 4× for larger pins.
- Edge distance from a hole to a wall, rib, or another hole: at least 2× the wall thickness, or at least one hole diameter.
- Holes parallel to the mold’s opening direction are easy; holes at other angles need core pulls or cams and carry a cost penalty.
Bosses aid assembly and mounting
Bosses provide locating, fastening, and mounting points. Placed badly they create a thick section that sinks on the opposite face or weakens the part.
- Boss wall: 55%–65% of nominal wall for thin walls (under 1/8"); about 40% of nominal when the wall is thicker than 1/8".
- Boss height: no more than 2½× the diameter of the hole in the boss.
- Tie bosses to a side wall with a rib rather than merging them into the wall.
Ribs add strength and stability
Ribs add stiffness and load-bearing capacity without thickening the whole wall. They follow their own proportions to avoid sink on the show surface.
- Rib thickness: 60%–80% of the nominal wall, always less than the wall.
- More ribs beat thicker ribs. Space them at least 2× nominal wall apart.
- Rib height: less than 3× nominal wall thickness.
- If a thick rib is unavoidable, core its center to restore a uniform wall.
Radii reduce stress at corners
Sharp corners concentrate stress, restrict material flow, and reduce part strength. Add a radius to every inside and outside corner.
- Inside radius: at least ½ the wall thickness.
- Outside radius: wall thickness plus the inside radius (keeps the corner wall uniform).
- Radius at the base of a boss or rib: ¼ of the wall thickness, never smaller than 0.015".
Material selection and processing matter just as much
Material selection can look overwhelming — there are thousands of grades. API has partnered with resin suppliers for decades and routinely molds the resins other shops decline: PEEK, PEI, PPS, LCP, glass- and carbon-filled compounds, metal-loaded alloys. Our engineers narrow the field to a short list that meets the mechanical, thermal, chemical, and regulatory requirements of the application, then sample and validate it with a documented process window. The customer always holds final approval.
Start with the Basic Resin Guide, or call 516-333-7577 to talk through a design with an API engineer.
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