PC+ABS Injection Molding Shrinkage: Why Increasing Packing Pressure Often Fails

The Real Cause of Shrinkage

Injection molding shrinkage is the visible result of polymer volume contraction during cooling. The part does not shrink uniformly. Each region depends on local temperature history, flow path pressure, wall thickness, crystallinity or amorphous relaxation behavior, fiber orientation if reinforced, and how long the cavity remains connected to the packing pressure source.

Shrinkage is a local compensation problem

During packing, additional melt must flow into the cavity to compensate for volumetric contraction. This shrinkage compensation only works when three conditions exist at the same time:

• The gate is not frozen.
• The flow path from gate to thick section still transmits pressure.
• The local melt core still has enough mobility to accept additional material.

If any one of these conditions is missing, the machine can display high packing pressure while the defect area receives little real compensation. That is why the same press setting may produce different shrinkage results after a gate change, mold temperature change, or minor wall-thickness adjustment.

Why PC+ABS Is More Sensitive

PC+ABS combines the heat resistance and toughness of polycarbonate with the processability and surface appearance of ABS. This makes PC+ABS injection molding useful for housings, interior automotive components, electrical covers, and structural cosmetic parts. The same blend structure also makes shrinkage control more sensitive than in simpler commodity materials.

For parts that also require UL94 performance, see our flame retardant plastics solution. Flame retardant PC+ABS compounds must balance fire performance, impact strength, flow, and shrinkage behavior rather than optimizing one property in isolation.

Why Increasing Packing Pressure Fails

Increasing packing pressure is a logical first response to sink marks. It increases the available pressure for shrinkage compensation. But in many PC+ABS parts, the pressure cannot reach the defect zone at the right time. Below are the most common engineering reasons.

1. Gate freeze occurs too early

Once gate freeze occurs, the cavity is isolated from the injection unit. Additional packing pressure cannot enter the part. A higher pressure setting after gate freeze only loads the runner or machine side; it does not correct shrinkage inside the molded part.

2. The pressure drop along the flow path is too high

Long flow length, thin walls, sharp transitions, narrow gates, and low melt temperature increase pressure loss. The press may reach the set packing pressure, but the far end of the cavity receives only a fraction of that pressure. Sink marks then appear away from the gate or behind local restrictions.

3. Cooling imbalance locks in surface geometry too early

If the surface freezes while the core is still hot, the core continues to contract after the skin becomes rigid. This is a classic mechanism for sink marks in injection molding. Higher packing pressure may increase flash risk near the gate without compensating the slow-cooling thick area.

4. The holding profile is poorly timed

A single high packing pressure stage may overpack the gate-side region and still underpack the end of fill. PC+ABS parts often respond better to a controlled multi-stage holding profile that maintains pressure before gate freeze without creating excessive residual stress.

Five Effective Solutions

1. Confirm gate freeze time before changing holding pressure

Run a gate freeze study by increasing packing time step by step while monitoring part weight. When part weight no longer increases, the gate has effectively frozen. Set holding time slightly beyond this point. If the sink mark remains unchanged after the gate freeze time is covered, further packing time is not the solution.

2. Improve pressure transmission through gate and runner design

Gate size, gate location, runner balance, and flow path length determine how much effective pressure reaches the shrinkage-prone region. For thick PC+ABS parts, a small gate may freeze before the thick section has finished contracting. Increasing gate thickness or moving the gate closer to the heavy section can be more effective than raising packing pressure.

3. Reduce local wall-thickness concentration

Sink marks are often designed into the part before molding begins. Bosses, ribs, and reinforcing pads should follow proper thickness ratios. As a practical rule, rib thickness should often be around 50-60% of the nominal wall for cosmetic PC+ABS parts, depending on geometry, surface requirement, and grade. Core out heavy bosses where possible.

4. Balance mold cooling instead of only lowering mold temperature

Lower mold temperature may reduce cycle time, but it can also freeze the skin earlier and worsen internal contraction. Focus on cooling balance: water channel layout, baffle effectiveness, hot spots near bosses, insert temperature, and mold-side temperature difference. The target is uniform heat removal, not simply the lowest possible mold temperature.

5. Select a PC+ABS grade matched to flow and shrinkage requirements

Material grade matters. High-flow PC+ABS can improve pressure transmission in thin-wall parts. High-impact PC+ABS may be preferred where toughness is more important than maximum stiffness. Reinforced PC+ABS can reduce shrinkage but may introduce anisotropy, weld line sensitivity, and surface texture changes. For a broader selection framework, read how to choose plastic material.

How to Verify Shrinkage Is Truly Solved

A shrinkage defect is not truly solved until the part remains stable across normal production variation. A good verification plan should separate short-term cosmetic improvement from robust process capability.

For production release, confirm the solution at the upper and lower limits of the normal process window. A narrow setting that only works under ideal conditions is not a stable shrinkage control method.

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