Injection Molding Wall Thickness: The Basics

Injection molding is one of the most productive ways to produce parts with intricate geometries and high strength-to-weight ratios. Many standards for the efficient design of injection-molded components are disregarded while working with thick-walled molding. Wall thicknesses between.060" and.180" and homogeneous thickness of wall sections are often specified in conventional component design for molding.

Thick areas of a component are "cored out" in conventional injection molding design. To keep the walls regular, lower the component weight, and speed up the manufacturing cycle, designers use a technique called "coring out" to remove plastic from large areas of the design. Thick-walled components are those with wall sections: 250 inches or more. Specific applications call for wall sections that deviate from the norm to achieve the necessary performance.

What use case necessitates such a large wall thickness?

  • When pieces are weight-bearing and ribs collapse under pressure, thick walls may be required, necessitating solid forms.

  • The design of the component requires mass.

  • Extreme abrasion zones need substantial parts that act as sacrificial barriers.

  • Plastic is an excellent thermal insulator, albeit it may need to be cut into thick portions to fulfill specific insulation demands.

  • Plastic components are increasingly being utilized in place of machined metal components in numerous applications due to their lower weight and production costs. The plastic component's shapes and thick parts are locked in from the original design for various reasons.

  • Thick wall molding may also be utilized as an alternative to compression molding to save cycle time and boost production capacity.

What difficulties do thick-walled components provide while producing them?

  • The price of a component is determined chiefly by the cost of its raw materials and the production cycle length. Because of this, the cycle time to cool the pieces increases in proportion to the wall thickness. It is crucial to balance production efficiency and part quality when molding components with thick wall sections.

  • The amount of material shrinks across the sections during cooling, making it challenging to maintain tolerances of +/-.002"-.005", which may be common for most components. It is especially true for parts with thicker walls and longer sections: the more significant the wall thickness, the more noticeable the impact of process variation.

  • Designing a component with a wall thickness that varies over its thickness leads to internal tension or warping because the thicker regions of the part will shrink more than the thinner sections.

  • Sink marks and vacuum gaps may form in molded components when the interior solidifies before the outside, causing the object to draw in on its outer walls as it cools. Bubbles or air pockets within an element are called vacuum voids.

Why do thick-walled components sometimes have vacuum voids?

Upon cooling, the plastic melt contracts, leaving a smaller volume. The steel will transfer heat from the plastic under pack pressure during the injection molding cooling. As the item cools, its outside will contract to match the shape of the mold. However, the thick piece within the component will stay soft and continue to cool. When plastic contracts as it cools, it may either draw the outside walls in, creating a "sink mark," or push the inner walls out, creating a vacuum space.

How do you check for voids?

Weighing the component and comparing it to the standard weight is the most fundamental way of discovering significant voids. There is probably a void if the molded piece is a wide shot but still lighter than average. By sectioning the component across its thicker regions, you may inspect its interior for voids and make any adjustments to the molding procedure. Ultrasonic void detection is recommended because it can identify holes that cannot be seen with a simple weight comparison to the standard.

With a tiny transducer probe, ultrasonic inspection delivers high-frequency sound waves to the component, creating repeatable patterns. The operator may detect voids inside the vehicle by listening for an echo when they pass over a solid object. X-ray is the preferred approach for finding holes in thin pieces of material. For this purpose, micro-CT is the gold standard. A void half the diameter of a single glass fiber may be resolved by micro-CT. The main problem with these high-resolution approaches is that they need a tiny sample size.

About half an inch, North Star Imaging has developed cutting-edge technology that samples a significantly broader area. (Roughly 40 cm) The method can detect a particle or vacancy as small as one-tenth the diameter of carbon fiber, even with very high sample size. These setups work well in labs but are impractical for mass production.

Eliminating the voids

Adjusting the processing parameters may reduce voids to tolerable levels or remove them entirely. Pack pressure, cooling time, and temperature would be the primary process factors. For the processor, the problem lies in finding the optimal procedure that minimizes cycle time while keeping voids within acceptable limits.