ABS Injection Molding FAQ

As a thermoplastic, ABS melts at around 105°C (221°F), allowing manufacturers to inject it into a mold. When it solidifies upon cooling, it takes the shape of the mold. ABS is favored over other thermoplastics for producing rigid, durable products with a high-quality surface finish. See the wide range of ABS injection molding applications here.

A wall thickness of 1.14mm to 1.2mm is ideal for making small parts with ABS, particularly where sinks must be avoided. To balance molding efficiency and structural rigidity, the ideal range for functional components is 1.5mm to 2.5mm. The maximum thickness of <3.56 mm is used for larger parts. However, thicker parts are prone to warping, excessive cooling time, and surface deformation.

For ribs, textured surfaces, or deeper parts, increase to 2° or more to minimize the risk of damage during ejection. As a general rule of thumb, for every 0.001 inch of texture depth, add 1° to 1.5° extra draft. The reason is that rougher surfaces cause more friction.

Optimizing the ribs helps to maximize structural strength and avoid common molding defects. The table below provides a guideline on best practices for rib optimization.

Specialized grades are used for applications where the product is expected to have specific requirements. Examples of specialized grade ABS include high-impact ABS, flame-retardant ABS, high-heat ABS, plating-grade ABS, and transparent ABS. Read the detailed description of the different types here.

The tensile strength of PC is approximately 10,442 psi for molded as compared to the 5,874 psi of ABS. In terms of thermal performance, PC retains its strength at higher temperatures (132°C to 138°C). ABS, on the other hand, starts to deform or fail around 97°C to 100°C.

ABS has poor weathering and UV resistance. It turns yellow and becomes brittle when exposed to sunlight over time. This limits its application for products intended for outdoor use. ABS’s lower tensile strength also means that it cannot handle as much structural load as PC before failure. It is more prone to warping if not correctly processed.

The processing temperature of PC is higher (280^oC to 320^oC) and requires slow, controlled cooling to prevent extreme warpage and maintain the product integrity.

The mold cavity must be polished to an SPI A-level finish using diamond buffing. This yields a surface roughness (Ra) of 0.012 to 0.10 μm. The mold material must be made of hardened steel (e.g., S136) that maintains a high polish during high-volume production. The mold temperatures should be higher too (60°C – 80°C)

The real-world application will decide the speed to prevent defects like weld lines, burning, or short shots. In some instances, manufacturers sometimes use a multi-stage approach. Here, the speed starts moderate and is gradually adjusted depending on geometry. In some cases, different speeds are used for different sections. Thin-walled or high-gloss parts need faster speeds to ensure proper filling.

If the injection speed is too slow, the plastic will cool before fully filling the mold cavity, leading to short shots. If the speed is too high, it will cause excessive frictional heat, leading to burning. In terms of efficiency, faster injection speed reduces the total filling time and cuts down the cycle time.

Although the ideal duration is 5 to 6 minutes, higher temperatures will require shorter residence time. The ideal barrel temperatures are often 225°C to 250°C.

The cause of these defects is often improper temperature control, excessive injection speed, inadequate venting, and inadequate drying of material before molding. See the full explanation of the causes of the different defects here.

The hygroscopic nature of ABS allows it to absorb moisture from the air. Improper drying below 0.1% moisture content before molding allows this moisture to cause silvery streaks or ‘starburst’ patterns on the surface of the molded part.

Shrinkage happens when the thick, inner sections of the part solidify more slowly than the outer skin, pulling the surface inward. Some of the common causes of sink marks in ABS injection molding include:

• Non-uniform wall thickness
• Excessive thickness
• Insufficient packing or holding pressure
• Excessive melt temperature
• Insufficient cooling time
• Ineffective cooling channels
• Poor venting

Short shots will mostly arise when the processing parameters are not properly optimized or when there is a mold design flaw. In most cases, the problem can be solved by optimizing mold design and processing parameters as highlighted in the table below.

Variation in wall thickness leads to uneven shrinkage. Likewise, excessively high or unbalanced mold temperature can cause improper cooling, which leads to warping. High shear forces resulting from high injection speed can lead to twisting or warping.

If the mold is not adequately vented, during filling, the air has nowhere to go, leading to burn at the end-of-fill zones. Air can also get trapped in ribs and bosses because of their enclosed nature.

If the ABS resin is not sufficiently heated, it will become too viscous and will not evenly fill the mold, leading to flow lines. Also, if the mold surface is too cold, the molten plastic will cool quickly upon contact, forming a ‘skin’. The remaining molten plastic must force through the skin, which leaves visible marks.