Plastic Injection Molding

The location of the parting line depends on the shape and function of the part. The parting line is where the two halves of the injection mold (A side and B side) come together, and it may also intersect with a slide mechanism that shuts off on one of the mold halves.
While the parting line will always be visible, its appearance can be minimized through careful consideration of its placement, draft angles, and part geometry.

When designing a plastic part, it’s important to remember that while the plastic enters the mold in a liquid state, it cools rapidly and solidifies into the shape of the cavity.
Designing a part that can be molded using a "straight pull" is crucial for keeping injection mold costs down. A straight pull mold is configured so that when the two halves (A side and B side) separate, there is no metal obstructing the path of the plastic in the direction of the pull.
If there are undercuts, such as snap features or side holes, the part may not eject from the mold once cooled unless these undercuts are addressed. This is typically achieved using slides and lifters in the injection mold.
Slides can be either mechanical or hydraulic and are more costly to incorporate. Lifters are generally smaller and always mechanical.

Draft is necessary on all parts in the direction of mold movement to ensure proper ejection from the mold.
It is critical for the successful removal of the part from the injection mold.
A standard draft angle of 1 degree is recommended, with a ½ degree draft for ribs.
There will be the visible drag mark on the part surface if without draft.

Another important rule when designing a plastic part is to maintain a even wall thickness and to use the thinnest wall sections feasible for the design. Even wall thickness aids in material flow during injection molding, reduces the risk of sink marks, molded-in stress, and differential shrinkage.

Additionally, cost savings are maximized when wall sections are minimized and consistent, as thinner walls cool faster, leading to shorter cycle times and increased production rates.
On average, the wall thickness of an injection-molded part should range from 0.060” to 0.180”, although parts can be molded with walls as thin as 0.020” and as thick as 1.50”. Thicker sections can cause cosmetic issues such as sink marks, bubbles, and discoloration.
It’s also essential to design parts with even wall thickness to ensure the mold cavity fills easily without restrictions. If wall thickness is not even, the thinner sections will cool first, while the thicker sections continue to shrink, creating stresses at the boundary between the two. As the thicker sections yield, this can lead to warping or twisting, and, if severe enough, may cause cracking.

For uneven walls, the change in thickness should not exceed 20% of the nominal wall thickness and should transition gradually.

Bosses are typically incorporated into plastic parts to allow for threaded inserts, guide pins, or self-tapping screws.
Stand-alone bosses should be avoided whenever possible; instead, bosses should be connected to walls or ribs using ribs or gussets to enhance structural stability.
The ideal outer diameter (O.D.) of a boss is 2.5 times the diameter of the screw or insert. Bosses with wall thicknesses greater than 60% of the adjoining wall are prone to sink marks. To mitigate this, a thinner boss with a base diameter of 2 times the screw diameter or less can be supported with multiple ribs.
Bosses should be avoided on parts with polished surfaces, as they often cause sink marks on the cosmetic surface.
The rib thickness at the base of a boss should not exceed 60% of the adjoining wall thickness, and the inside and outside diameters of bosses should have a ½-degree draft per side.
Bosses should be kept away from corners to avoid thick sections and help prevent sink marks.

There are several types of gating methods, and the choice of gate design and type depends on the part design, material selection, and the cosmetic and dimensional requirements of the final product. The gate connects the part to the runner system and is a necessary feature in part design. Gate design and placement are influenced by the part geometry, mold design, and polymer selection.
a. Gates should be placed away from areas of high stress or impact.
b. Gate designs should minimize the need for secondary de-gating operations whenever possible.
c. The gate should be positioned to ensure optimal filling of the part, typically in the thickest section.
d. In some cases, multiple gates may be needed depending on the part size, geometry, and polymer selection.

Venting is designed into the injection mold to allow gases to escape during the molding process.
Without proper venting, issues such as excessive injection pressure, short shots, burn marks, sink marks, and splay can occur.
Molds are typically vented by machining shallow channels along the parting line, though gate size will vary based on material selection.
Other venting methods include ejector pins, vent pins, and runners. Vents can also be cut into ejector pins to release gas as the cavity fills.
When parting line vents are insufficient, vent pins are placed within the part geometry to effectively remove gas during filling.

Knit lines, also known as weld lines, are areas in a molded part where two or more flow fronts converge, generally resulting in lower strength than other areas.
Part design should account for potential knit line locations and aim to direct them away from high-stress areas.
Knit lines commonly form on the opposite side of holes or obstacles in the flow path, such as pins or bosses.
Mold flow software can be used to simulate plastic filling in the cavity, helping identify areas that may present cosmetic or mechanical performance issues.

Mold flow analysis is an essential tool in the design and optimization of molds for injection molding. Mold flow analysis simulates the flow of molten plastic within the mold cavity during injection molding, predicting material behavior under various processing conditions. It provides insights into flow patterns, pressure distribution, cooling rates, and potential defects.

Optimizing Design, Material Selection, Gate and Runner Design, Cooling System Design, Predicting Part Behavior, Iterative Improvement, Reducing Time-to-Market