Thermoplastic materials flow when they are heated to the proper temperature (which varies with each plastic). This ability can be verified by test, using a melt index unit per ASTM Test #D-1238. This ability to flow also determines how far a plastic can be injected through a specific wall thickness of a product.
If the material is an easy-flow material (with a high melt index), it can flow much farther than one that is not. In addition, the easy-flow material can flow through a much thinner wall section than a material that is not easy-flow. Low-flow, “stiffer” materials tend to cool down and solidify before they can flow as far as the easy-flow (less stiff) materials. It can be seen, then, that some materials can fill thinner walls easier than others, depending on their melt index value and stiffness.
While the wall thickness recommendations are indicators, it is always best to design a plastic part as thin as you can, but as thick as necessary. Most designers tend to “overkill” and make plastic parts much thicker than needed. This results in excessive and costly cycle times and difficulty in controlling dimensions. If thick areas are required for strength purposes, it is better to use a thin wall and add strengthening ribs.
Molten plastic material flows through the path of least resistance. If all the walls of a specific product are equal, the plastic flows with little effort and little loss of pressure as it travels through the prescribed flow path. All the molecules within the material will become equal in size and take up equal amounts of volume, with equal amounts of space between them.
Knit lines (or weld lines as they are sometimes called) are the result of a flow front of molten plastic coming in contact with an obstruction in the mold, breaking into two fronts to flow around the obstruction, and meeting on the other side in an attempt to weld back together again. The face of these fronts will have cooled down somewhat while going around the obstruction and are not capable of welding back together to a 100% level. Thus, they form what is called a knit line, which can be thought of as the starting point of a crack.
A sharp corner on a molded part is an invitation to failure. Sharp corners are points of stress concentration, developing stress in three basic forms: tensile, compression, and shear.
Sink marks are formed when two areas of a molded part cool and solidify differently from each other. This is caused by a thin section becoming solid sooner than a thicker section.
A molded part that appears warped, twisted, or bowed, is the result of stresses being created during the molding process, or shortly after the time the part is ejected from the mold.
One of the most controversial areas of injection-molding product design is the requirement for draft. In its simplest terms, draft can be defined as a taper applied to side walls. It can be thought of in terms of an ice cube tray. Each of the ice cube tray cells has tapered walls to allow the cubes to eject easily from the tray. The molded product should have tapered walls to allow the part to eject easily from the mold. The amount of taper used is called the “draft angle.”
Ejector pins actually form part of a face on the molded part. If the pins are made too short they will cause a “pad” of excess plastic material to form on this face. If the pins are too long, the plastic will flow around them and cause a depression to be formed in the part.
If surface gates are used, when the molded part is separated from the gate, a depression or protrusion will be left, depending on how the gate was designed in the first place. If hot runner or runnerless molds are used, there will still be a mark, but it will be less obvious.
It should be stated that, legally, the product drawing takes precedence over all other methods of conveying a product designer’s wishes to the manufacturing side of the industry. Therefore, it should be well thought out and legible for all needing to view it. Standard formats should be utilized, and dimensions should be inspected to make sure there are no cases of double-dimensioning, or dimensions left out. These are both common occurrences resulting in a great deal of confusion.