Investigation on the Fiber Orientation Distributions and Their Influence on the Mechanical Property of the Co-Injection Molding Products

Abstract

In recent years, due to the rapid development of industrial lightweight technology, composite materials based on fiber reinforced plastics (FRP) have been widely used in the industry. However, the environmental impact of the FRPs is higher each year. To overcome this impact, co-injection molding could be one of the good solutions. But how to make the suitable control on the skin/core ratio and how to manage the glass fiber orientation features are still significant challenges. In this study, we have applied both computer-aided engineering (CAE) simulation and experimental methods to investigate the fiber feature in a co-injection system. Specifically, the fiber orientation distributions and their influence on the tensile properties for the single-shot and co-injection molding have been discovered. Results show that based on the 60:40 of skin/core ratio and same materials, the tensile properties of the co-injection system, including tensile stress and modulus, are a little weaker than that of the single-shot system. This is due to the overall fiber orientation tensor at flow direction (A₁₁) of the co-injection system being lower than that of the single-shot system. Moreover, to discover and verify the influence of the fiber orientation features, the fiber orientation distributions (FOD) of both the co-injection and single-shot systems have been observed using micro-computerized tomography (μ-CT) technology to scan the internal structures. The scanned images were further utilizing Avizo software to perform image analyses to rebuild the fiber structure. Specifically, the fiber orientation tensor at flow direction (A₁₁) of the co-injection system is about 89% of that of the single-shot system in the testing conditions. This is because the co-injection part has lower tensile properties. Furthermore, the difference of the fiber orientation tensor at flow direction (A₁₁) between the co-injection and the single-shot systems is further verified based on the fiber morphology of the μ-CT scanned image. The observed result is consistent with that of the FOD estimation using μ-CT scan plus image analysis.

Keywords: co-injection molding; fiber orientation distribution (FOD); fiber reinforced plastics (FRP); micro-computerized tomography (μ-CT) scan technology.

Figure 1

Definition of the orientation vector p.

Figure 2

(a) Geometry model and dimensions, (b) moldbase and cooling channel layout, (c) the meshed model.

Figure 2

(a) Geometry model and dimensions, (b) moldbase and cooling channel layout, (c) the meshed model.

Figure 3

The numerical convergence testing for mesh type and resolution.

Figure 4

The locations of measuring modes: (a) Locations of three measuring nodes, (b) t1: The core interface arrives at point A, (c) t2: The core interface arrives at point B, (d) t3: The core interface arrives at point C.

Figure 5

(a) Co-injection molding system, (b) the cavity.

Figure 6

(a) The universal tensile testing machine, (b) the sample holder.

Figure 7

The parameter definition for the tensile test.

Figure 8

Core penetration behavior at various skin/core ratios, from 90:10 to 10:90.

Figure 9

Fiber orientation features of the single-shot: (a) The time period as the flow front is the same for both single and co-injection, (b) the FOD for a single shot at point A at time t1, (c) the flow direction orientation tensor A₁₁ at point A with different time period.

Figure 10

Short shot testing for simulation prediction and experimental study (at skin/core ratio = 40:60).

Figure 11

Experimental validation for skin/core ratio effect: (a) For various combination from 90:10 to 10:90, (b) the observation of the break-through location at skin/core ratio = 40:60.

Figure 12

The locations for making the sliced plane to observe the fiber morphology; where th is the distance from the top surface.

Figure 13

Observation of the fiber morphology based on the sliced plane for co-injected parts at near gate region (NGR) with different thickness locations: (a1) Simulation at th = 0.5 mm, (b1) sliced image at th = 0.5 mm, (a2) simulation at th = 1.0 mm, (b2) sliced image at th = 1.0 mm, (a3) simulation at th = 1.75 mm (central portion), (b3) sliced image at th = 1.75 mm.

Figure 14

Observation of the fiber morphology based on the sliced plane for the single-shot and co-injected parts at point A with different thickness locations: (a1) Single-shot at th = 0.5 mm, (b1) co-injected at th = 0.5 mm, (a2) single-shot at th = 1.0 mm, (b2) co-injected at th = 1.0 mm, (a3) single-shot at th = 1.75 mm (central portion), (b3) co-injected at th = 1.75 mm.

Figure 14

Observation of the fiber morphology based on the sliced plane for the single-shot and co-injected parts at point A with different thickness locations: (a1) Single-shot at th = 0.5 mm, (b1) co-injected at th = 0.5 mm, (a2) single-shot at th = 1.0 mm, (b2) co-injected at th = 1.0 mm, (a3) single-shot at th = 1.75 mm (central portion), (b3) co-injected at th = 1.75 mm.

Figure 15

The comparison of the fiber orientation distribution (FOD) estimation: (a) Simulation at point A, (b) experiment around point A (NGR).

Figure 15

The comparison of the fiber orientation distribution (FOD) estimation: (a) Simulation at point A, (b) experiment around point A (NGR).

Figure 16

The comparison of the fiber orientation distribution (FOD) tensor component at NGR: (a) flow direction tensor A₁₁, (b) cross-flow direction tensor A₂₂.

Figure 17

Tensile property measurement for single-shot and co-injection specimens: (a) The average tensile stress-strain behavior, (b) the average elongation at break, (c) the average tensile strength.