Related Technical Information For Plastic Mold Industry 

1.Plastic material shrinkage

Plastic material shrinkage occurs in the contraction of a molded part as it cools after injection. All materials have different shrink rates depending on resin family (amorphous vs. crystalline materials), mold design, and processing conditions. Materials may also shrink differently depending on direction of flow. As a general rule of thumb, a 10% change in mold temperature can result in a 5% change in original shrinkage. In addition, injection pressure has a direct effect on shrinkage rates. The higher the injection pressure, the lower the shrinkage rate.

Shrinkage starts at the molecular level when plastics melt and cool. For the most part, these dynamics depend on the type of material and whether any filler or fiber reinforcement is present. There are also processing and part design factors to consider.

The chart below outlines typical mold shrink rates:

2.Unit conversions chart for plastic mold industry

When injection molds are manufactured, there are several different pieces of equipment used to create the molding form. Surfaces may be CNC machined, ground, EDM'd, turned, and so on. These tooling marks may be acceptable for non-show surfaces, but in many cases, tooling marks are smoothed out or textured. 

3.Basic Surface Finishes

The texture on the mold surface is directly transferred to the plastic part. For a part like a clear lens to be clear the molding surface will be a perfect mirror. In these cases, even a finger print or a hairline scratch can show through on the molded part. Below is a surface finish guide created by the Plastics Industry Association. The spectrum is basically like light sandpaper to a mirrored finish (D-3 to A-1).

4.Draft angle

The draft angle is the amount of taper for molded or cast parts perpendicular to the parting line. It can be measured in degrees or mm/mm (in/in).

Why Draft angle injection molding?

• Reduces the chance of damage to the part due to friction during release

• Reduces wear and tear and chances of damage to the mold

• Ensures a uniform, smooth, unscratched finish when required

• Ensures the integrity and uniformity of other surface finishes and textures

• Reduces overall cooling time by lessening or eliminating the need for unconventional ejection setups

• Most, if not all, of these benefits offer either direct or indirect overall production cost reductions

Draft angle injection molding

• Draft angles are generally a requirement of injection molding design (though there are a select few soft materials, like nylon, that can get away with draft angles of zero).

• Draft should be a part of design for manufacture, and all stakeholders and contributors should be aware of its need at the outset of the design process. This includes product designers, design engineers, and of course, the customer. This allows considerations for design requirements to be taken into account.

• Any draft is always better than no draft.

Please check Texture Mini Drafting Chart for VDI3400 surface:

Please check Texture Mini Drafting Chart for Mold Tech Texture: