---
title: "What causes sink marks and bore drift on injection molded impact driver housings?"
description: "Batch sink marks and bore drift on impact driver housings often come from different root causes. Use process data, mold geometry audits, and material controls before choosing between tooling re-cut and process adjustment."
url: "https://www.hyneu.com/questions/sink-marks-bore-drift-impact-driver-housings.html"
language: "en"
type: "Q&A"
category: "Injection Molding Q&A"
datePublished: "2026-10-30"
dateModified: "2026-10-30"
brand: "HYNEU"
manufacturer: "HYNEU"
answerCount: 9
---

# What causes sink marks and bore drift on injection molded impact driver housings?

## Question

 I’m a quality engineer at an industrial power tool company, and we’re seeing a repeating quality problem on an impact driver housing molded in PC/ABS. We specified a 52.00/52.10 mm bore diameter at the gearcase opening, and first article was okay. During batch production, one of four cavities in the same mold produces 51.90 to 52.12 mm through the day, and the same parts show visible sink marks behind the large ribs in the grip handle. The sink marks get worse right after a line restart or a longer break, then improve after ten or fifteen shots. Our molder says it is normal shrinkage and wants to adjust packing and mold temperature, but a tooling shop says the rib-to-wall ratio is too high and wants to machine steel out. We are also under pressure to approve the tool by Cpk, but the bore roundness data is pulling below 1.33. In what order should we investigate this before committing to a tooling re-cut or accepting a process change? What data from the molder, mold audit, and material batch should we use to make the decision? 

## Answers
                            
### Answer 1 — Best Answer

First, separate the two symptoms. Sink marks behind the handle ribs and bore diameter drift at the gearcase opening are not necessarily the same failure mode. Sink marks are local shrinkage caused by thick plastic sections under a rib or boss; bore drift is usually caused by global packing, cooling, or flow imbalance. Packing pressure can mask both for a few parts, but if the local geometry has too much thickness under the rib, no hold pressure will keep that section from shrinking inward.

The fastest way to classify this is to **freeze the process** for a controlled run. Ask the molder to use one fixed setpoint, discard the first twenty shots after thermal stabilization, and then collect fifty continuous cycles without any manual adjustment or interruption. For every part, record cavity number, shot weight, cushion position, cycle time, and bore diameter at the gearcase opening. Also record a simple sink grade for the handle area. Plot the bore data by cavity and by production time. If all four cavities drift after a restart, the problem is thermal recovery: mold temperature has not stabilized, cooling channels are unbalanced, or the barrel residence time is creating viscosity drift. If the drift appears only in the same cavity, the suspect area is gating, venting, or cavity steel geometry.

For the sink marks, ask for a short-shot sequence at 40%, 60%, 75%, and 90% fill. Watch where the melt front enters the handle rib area. If the rib fills late or has a converging flow front, packing pressure will not feed it effectively because the surrounding thin wall freezes earlier. That condition points to gate location and rib geometry. If the flow front reaches the rib at the same time as all other areas yet sink still appears, check the wall thickness itself with a silicone cast or CMM on the steel.

A mold audit should happen before any tooling change. For PC/ABS housings, a common design limit is to keep the **rib root thickness no more than 0.6 times the nominal wall thickness**. If the rib is already above that, the sink will return after every process optimization. Confirm with a silicone replica of the rib area, especially in the suspect cavity, because hand polishing or vent benching can have removed steel and made the wall thicker only in that cavity. Also check the water circuits: measure flow rate and compare surface temperature across cavities with an infrared or contact probe. A channel that is partially sealed by scale can make one cavity run hotter, which creates variable shrinkage and roundness drift.

For the material side, do not let the molder adjust regrind percentages freely during production. PC/ABS absorbs moisture quickly. Ask for the dryer dew point and confirm the material moisture specification is met before the machine start. If the molder cannot provide those records, that is a red flag. An uncontrolled regrind blend changes melt viscosity and can shift the bore dimension by a few hundredths of a millimeter. Verify the resin grade and lot from the material certificate, and if regrind is involved, require the regrind ratio to be fixed and noted in the process sheet.

Bore roundness Cpk should not be judged on a database mixed with restart shots and process changes. Stabilize the process, then repeat measurements on **three separate days** using the same calibrated bore gauge, operator, and measurement procedure. Measure the bore in at least two angular positions and two depths because an out-of-round hole can still pass a go/no-go gauge. If one cavity is always low, that is a tooling imbalance. If the average is correct but the spread is wide, first improve mold temperature control and packing consistency before authorizing a re-cut. Gate freeze-off study also helps: increase hold time until part weight stops increasing. If the weight continues rising past the current hold time, the gate is freezing too late; if sink remains after the freeze-off point, the issue is in the mold geometry.

Using that sequence, you avoid the two most expensive mistakes: re-cutting steel before proving the process is stable, and letting a process adjustment hide a geometry defect that will reappear in every future batch. Sink marks that are only present after restart are a process or moisture problem. Sink marks present in every shot at the same location are a design or steel problem. When only one cavity behaves differently, cross-check its gate, venting, water flow, and cavity dimensions first. That distinction is what should drive the tooling change decision.

**status:** accepted
**Author:** Grace Sun
**Date:** 2026-10-30

### Answer 2

On the floor, the pattern you describe is almost always influenced by line discipline. A mold in continuous production behaves differently than a mold that has been stopped for lunch, maintenance, or a material change.

If the molder does not quarantine the first fifteen to twenty shots after break, those parts will mix into the batch and make every Cpk calculation unstable. We require a visual marker on the container and a startup label so quality knows those parts were not released.

The practical step is to require cycle count and event logging. The molder should provide not only setup parameters but also the number of cycles between each stop, how long the stop lasted, and whether the machine was left idle with the screw filled.

Even a ten-minute interruption at a warm mold can change PC/ABS viscosity enough to push bore diameter outside the high limit. Once those startup parts are removed, we usually see the remaining data tighten considerably.

If the data still shows one cavity drifting, then the discussion can move to the mold. But if the movement appears after every stoppage, treat it as a process control problem and enforce a restart check before any tooling money is spent.

**status:** suggested
**Author:** Wendy Gao
**Date:** 2026-10-30

### Answer 3

When a defect appears in only one cavity, I would measure the steel before measuring the plastic. The machining and hand-finishing history of an insert can change the cavity shape even when the original 3D model is correct. A sink mark behind a rib is extremely sensitive to local wall thickness.

During benching, it is common for a mold maker to polish the rib area slightly deeper to clean vent marks, and that small amount of removed steel becomes a thicker section in the part. The result is a cavity that never produces a clean surface, no matter what the process does.

Ask for a non-contact scan or a silicone cast of the suspect cavity. Compare the rib base radius, root thickness, and bore insert diameter to the approved 3D data, not just to the print. Also check whether the bore core and cavity inserts are sitting flush.

If there is side shift or a mismatched shutoff, the roundness will vary around the circumference, and packing changes will only hide the error. This measurement is fast compared to a re-cut.

If the steel matches the model and the shutoff is correct, then the problem belongs to the injection process. If the steel does not match, the tool maker should correct that one area directly instead of lowering packing pressure or opening the gate.

**status:** suggested
**Author:** Nathan Guo
**Date:** 2026-10-30

### Answer 4

Before changing the mold, verify what is actually going into the barrel. PC/ABS is a blend, and the PC-to-ABS ratio drives shrinkage behavior and impact resistance. If the resin supplier or grade changed without formal approval, the housing may still meet color and basic density checks but show different sink and bore stability.

Ask the molder for material certificates by lot, and confirm the grade name and additive pack. If regrind is being used, check the regrind origin: purges, startup scrap, or runners from a different color can all enter the blend and change flow.

A quick laboratory check can help. Melt flow rate or viscosity index on the incoming resin will expose a batch difference. Moisture content is even more critical for PC/ABS; a dryer that is not reaching the correct dew point will produce parts that look fine after five shots and then splay, sink, or shrink erratically.

The molder should be able to give you a dryer record showing dew point and residence time. If the material is inconsistent, no gate redesign or hold pressure profile will give a stable throughput. Correct material control first, then evaluate the mold design. It costs less to verify material and drying than to cut steel on a mold that already conforms to the drawing.

**status:** suggested
**Author:** Claire Ren
**Date:** 2026-10-30

### Answer 5

Bore diameter Cpk tells you the statistical spread, but it does not tell you whether the gearcase will fit correctly inside the housing. In an impact driver, the gearcase bore has clearance with the gearcase nose, and the screws clamp the two halves together.

If the bore is slightly oval in a specific angular direction, a bore gauge may catch it only if the measurement points line up with the oval. We use an assembled-fit check with a master gearcase and a calibrated push-in force rig. That catches problems that a plug gauge misses.

If the suspect parts pass the free-state dimension but fail at assembly, the next suspect is the parting line mismatch at the gearcase opening. When the two housing halves are screwed together, the bore can become out of round after fastening.

So measure the bore after assembly, not just on the loose half. Also check that the housing screws seat against the same bosses in all four cavities; if one cavity has a proud boss, the local stress distorts the bore.

These are functional checks that should be done before deciding to re-cut the mold, because they separate aesthetic sink marks from structural fit issues. The molder can optimize the process, but if assembled housings still distort, the fix belongs in the tool or the assembly sequence.

**status:** suggested
**Author:** Marcus He
**Date:** 2026-10-30

### Answer 6

From the tooling side, I would look at the gate freeze-off and the cooling line layout before touching the cavity steel. In a four-cavity mold, even a small difference in gate depth or land length between cavities changes the effective hold pressure reaching the grip area.

If the gate for cavity three is slightly tighter or has a longer land, it freezes earlier, and that cavity will show both sink and bore drift under the same process settings. Gate wear after many cycles can also make one cavity behave differently.

A gate freeze study can identify this: run with the same parameters and increase hold time in 0.5-second steps until part weight stops increasing. If cavity three’s weight keeps rising after cavity one has flattened, the gates are not balanced.

Check the water channels too. Mold scale and blocked bubblers near the handle core can raise local mold temperature and make the sink area worse.

I would also measure mold temperature on the B-side, not only the A-side, because the grip rib sink is controlled by the core temperature. If thermal imbalance is confirmed, clean the channels or re-route cooling before deciding the rib is too thick.

**status:** suggested
**Author:** Henry Cao
**Date:** 2026-10-30

### Answer 7

Separate the process variables in a controlled way. I would set a fixed melt temperature and mold temperature, leave the mold closed for at least five minutes to reach equilibrium, then run a packing profile test: start with current hold pressure, raise it by 10%, and check the sink and bore measurements.

If the defect changes, packing is still active in the system; if it looks exactly the same, the pressure is never reaching the rib area. This tells you whether to work on process or geometry.

One detail many audits miss is the screw cushion. If the cushion is unstable or too small, the machine cannot deliver consistent hold pressure. Watch cushion on each cycle and check for screw rebound. Also look at the melt temperature readout versus actual nozzle and adapter temperatures; PC/ABS is sensitive to even 5°C differences at the nozzle.

If the sink only appears after startup, consider a longer soak-back time before the first shot or a slower first-stage injection speed. Once the process window is repeatable, record the minimum and maximum hold pressure that still holds bore diameter and sink within limits. If that window is narrow, the molder can explain it; if no window exists, the tool is the limiting factor.

**status:** suggested
**Author:** Irene Tang
**Date:** 2026-10-30

### Answer 8

Sink marks have to be judged beyond cosmetics because they indicate where stress will concentrate in a drop or vibration load. The rib behind the grip is often part of the impact load path from the battery pack to the gearcase.

We would define a limit sample set with three levels of sink severity and then verify through impact testing at low temperature, because cold drops crack before room temperature drops show any issue. If the light sink class still passes the test, it may be acceptable; if the worst class breaks, the mold geometry is the correct fix.

For the bore drift, use an actual gearcase with a known diameter to run a short endurance test on parts that measure at the upper and lower limits. At the high limit, the gearcase may rattle and cause noise; at the low limit, it may stress the plastic and cause microcracks around the screw bosses.

Since the housing is an industrial tool component, field return data also helps: if most failures occur after the tool has been used at high torque for many cycles, the housing stiffness around the gearcase opening becomes the real acceptance criterion. Surface appearance alone should not drive a tooling change decision.

**status:** suggested
**Author:** Leo Shen
**Date:** 2026-10-30

### Answer 9

Before the molder changes any process setting or the mold vendor cuts steel, agree on a change control rule. Even an adjustment that looks temporary should be documented because it affects future production.

Ask the molder for a written containment action: are suspect parts separated, how many are being quarantined, and what are the acceptance limits? Then define the current baseline with a repeatability run. Without a baseline, it is impossible to prove whether the tooling re-cut actually solves the problem.

Route the investigation through a simple decision gate: process data first, mold audit second, material verification third. Only after those three are complete should the tooling supplier provide a proposal with the exact area to be modified, the expected effect on wall thickness, and a requalification plan.

The tooling change should be treated as a formal engineering change, requiring the molder to re-run PPAP samples after the modification. Also require a maintenance interval change if the root cause is wear or water scale. This sequence keeps the project moving without letting a quick packing pressure fix become a permanent hidden defect.

**status:** suggested
**Author:** Evan Qian
**Date:** 2026-10-30

## Related Resources

- [Injection Molding Q&A](https://www.hyneu.com/questions/injection-molding/)
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            "text": "First, separate the two symptoms. Sink marks behind the handle ribs and bore diameter drift at the gearcase opening are not necessarily the same failure mode. Sink marks are local shrinkage caused by thick plastic sections under a rib or boss; bore drift is usually caused by global packing, cooling, or flow imbalance. Packing pressure can mask both for a few parts, but if the local geometry has too much thickness under the rib, no hold pressure will keep that section from shrinking inward. The fastest way to classify this is to freeze the process for a controlled run. Ask the molder to use one fixed setpoint, discard the first twenty shots after thermal stabilization, and then collect fifty continuous cycles without any manual adjustment or interruption. For every part, record cavity number, shot weight, cushion position, cycle time, and bore diameter at the gearcase opening. Also record a simple sink grade for the handle area. Plot the bore data by cavity and by production time. If all four cavities drift after a restart, the problem is thermal recovery: mold temperature has not stabilized, cooling channels are unbalanced, or the barrel residence time is creating viscosity drift. If the drift appears only in the same cavity, the suspect area is gating, venting, or cavity steel geometry. For the sink marks, ask for a short-shot sequence at 40%, 60%, 75%, and 90% fill. Watch where the melt front enters the handle rib area. If the rib fills late or has a converging flow front, packing pressure will not feed it effectively because the surrounding thin wall freezes earlier. That condition points to gate location and rib geometry. If the flow front reaches the rib at the same time as all other areas yet sink still appears, check the wall thickness itself with a silicone cast or CMM on the steel. A mold audit should happen before any tooling change. For PC/ABS housings, a common design limit is to keep the rib root thickness no more than 0.6 times the nominal wall thickness . If the rib is already above that, the sink will return after every process optimization. Confirm with a silicone replica of the rib area, especially in the suspect cavity, because hand polishing or vent benching can have removed steel and made the wall thicker only in that cavity. Also check the water circuits: measure flow rate and compare surface temperature across cavities with an infrared or contact probe. A channel that is partially sealed by scale can make one cavity run hotter, which creates variable shrinkage and roundness drift. For the material side, do not let the molder adjust regrind percentages freely during production. PC/ABS absorbs moisture quickly. Ask for the dryer dew point and confirm the material moisture specification is met before the machine start. If the molder cannot provide those records, that is a red flag. An uncontrolled regrind blend changes melt viscosity and can shift the bore dimension by a few hundredths of a millimeter. Verify the resin grade and lot from the material certificate, and if regrind is involved, require the regrind ratio to be fixed and noted in the process sheet. Bore roundness Cpk should not be judged on a database mixed with restart shots and process changes. Stabilize the process, then repeat measurements on three separate days using the same calibrated bore gauge, operator, and measurement procedure. Measure the bore in at least two angular positions and two depths because an out-of-round hole can still pass a go/no-go gauge. If one cavity is always low, that is a tooling imbalance. If the average is correct but the spread is wide, first improve mold temperature control and packing consistency before authorizing a re-cut. Gate freeze-off study also helps: increase hold time until part weight stops increasing. If the weight continues rising past the current hold time, the gate is freezing too late; if sink remains after the freeze-off point, the issue is in the mold geometry. Using that sequence, you avoid the two most expensive mistakes: re-cutting steel before proving the process is stable, and letting a process adjustment hide a geometry defect that will reappear in every future batch. Sink marks that are only present after restart are a process or moisture problem. Sink marks present in every shot at the same location are a design or steel problem. When only one cavity behaves differently, cross-check its gate, venting, water flow, and cavity dimensions first. That distinction is what should drive the tooling change decision.",
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            "text": "When a defect appears in only one cavity, I would measure the steel before measuring the plastic. The machining and hand-finishing history of an insert can change the cavity shape even when the original 3D model is correct. A sink mark behind a rib is extremely sensitive to local wall thickness. During benching, it is common for a mold maker to polish the rib area slightly deeper to clean vent marks, and that small amount of removed steel becomes a thicker section in the part. The result is a cavity that never produces a clean surface, no matter what the process does. Ask for a non-contact scan or a silicone cast of the suspect cavity. Compare the rib base radius, root thickness, and bore insert diameter to the approved 3D data, not just to the print. Also check whether the bore core and cavity inserts are sitting flush. If there is side shift or a mismatched shutoff, the roundness will vary around the circumference, and packing changes will only hide the error. This measurement is fast compared to a re-cut. If the steel matches the model and the shutoff is correct, then the problem belongs to the injection process. If the steel does not match, the tool maker should correct that one area directly instead of lowering packing pressure or opening the gate.",
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            "text": "Before changing the mold, verify what is actually going into the barrel. PC/ABS is a blend, and the PC-to-ABS ratio drives shrinkage behavior and impact resistance. If the resin supplier or grade changed without formal approval, the housing may still meet color and basic density checks but show different sink and bore stability. Ask the molder for material certificates by lot, and confirm the grade name and additive pack. If regrind is being used, check the regrind origin: purges, startup scrap, or runners from a different color can all enter the blend and change flow. A quick laboratory check can help. Melt flow rate or viscosity index on the incoming resin will expose a batch difference. Moisture content is even more critical for PC/ABS; a dryer that is not reaching the correct dew point will produce parts that look fine after five shots and then splay, sink, or shrink erratically. The molder should be able to give you a dryer record showing dew point and residence time. If the material is inconsistent, no gate redesign or hold pressure profile will give a stable throughput. Correct material control first, then evaluate the mold design. It costs less to verify material and drying than to cut steel on a mold that already conforms to the drawing.",
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            "text": "Bore diameter Cpk tells you the statistical spread, but it does not tell you whether the gearcase will fit correctly inside the housing. In an impact driver, the gearcase bore has clearance with the gearcase nose, and the screws clamp the two halves together. If the bore is slightly oval in a specific angular direction, a bore gauge may catch it only if the measurement points line up with the oval. We use an assembled-fit check with a master gearcase and a calibrated push-in force rig. That catches problems that a plug gauge misses. If the suspect parts pass the free-state dimension but fail at assembly, the next suspect is the parting line mismatch at the gearcase opening. When the two housing halves are screwed together, the bore can become out of round after fastening. So measure the bore after assembly, not just on the loose half. Also check that the housing screws seat against the same bosses in all four cavities; if one cavity has a proud boss, the local stress distorts the bore. These are functional checks that should be done before deciding to re-cut the mold, because they separate aesthetic sink marks from structural fit issues. The molder can optimize the process, but if assembled housings still distort, the fix belongs in the tool or the assembly sequence.",
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            "text": "From the tooling side, I would look at the gate freeze-off and the cooling line layout before touching the cavity steel. In a four-cavity mold, even a small difference in gate depth or land length between cavities changes the effective hold pressure reaching the grip area. If the gate for cavity three is slightly tighter or has a longer land, it freezes earlier, and that cavity will show both sink and bore drift under the same process settings. Gate wear after many cycles can also make one cavity behave differently. A gate freeze study can identify this: run with the same parameters and increase hold time in 0.5-second steps until part weight stops increasing. If cavity three’s weight keeps rising after cavity one has flattened, the gates are not balanced. Check the water channels too. Mold scale and blocked bubblers near the handle core can raise local mold temperature and make the sink area worse. I would also measure mold temperature on the B-side, not only the A-side, because the grip rib sink is controlled by the core temperature. If thermal imbalance is confirmed, clean the channels or re-route cooling before deciding the rib is too thick.",
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          ,          {
            "@type": "Answer",
            "text": "Separate the process variables in a controlled way. I would set a fixed melt temperature and mold temperature, leave the mold closed for at least five minutes to reach equilibrium, then run a packing profile test: start with current hold pressure, raise it by 10%, and check the sink and bore measurements. If the defect changes, packing is still active in the system; if it looks exactly the same, the pressure is never reaching the rib area. This tells you whether to work on process or geometry. One detail many audits miss is the screw cushion. If the cushion is unstable or too small, the machine cannot deliver consistent hold pressure. Watch cushion on each cycle and check for screw rebound. Also look at the melt temperature readout versus actual nozzle and adapter temperatures; PC/ABS is sensitive to even 5°C differences at the nozzle. If the sink only appears after startup, consider a longer soak-back time before the first shot or a slower first-stage injection speed. Once the process window is repeatable, record the minimum and maximum hold pressure that still holds bore diameter and sink within limits. If that window is narrow, the molder can explain it; if no window exists, the tool is the limiting factor.",
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          ,          {
            "@type": "Answer",
            "text": "Sink marks have to be judged beyond cosmetics because they indicate where stress will concentrate in a drop or vibration load. The rib behind the grip is often part of the impact load path from the battery pack to the gearcase. We would define a limit sample set with three levels of sink severity and then verify through impact testing at low temperature, because cold drops crack before room temperature drops show any issue. If the light sink class still passes the test, it may be acceptable; if the worst class breaks, the mold geometry is the correct fix. For the bore drift, use an actual gearcase with a known diameter to run a short endurance test on parts that measure at the upper and lower limits. At the high limit, the gearcase may rattle and cause noise; at the low limit, it may stress the plastic and cause microcracks around the screw bosses. Since the housing is an industrial tool component, field return data also helps: if most failures occur after the tool has been used at high torque for many cycles, the housing stiffness around the gearcase opening becomes the real acceptance criterion. Surface appearance alone should not drive a tooling change decision.",
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            "url": "https://www.hyneu.com/questions/sink-marks-bore-drift-impact-driver-housings.html#suggestedAnswer-8",
            "datePublished": "2026-10-30T19:23:21Z",
            "author": {"@type": "Person","name": "Leo Shen","url": "https://www.hyneu.com/team/leo-shen.html"}          }
          ,          {
            "@type": "Answer",
            "text": "Before the molder changes any process setting or the mold vendor cuts steel, agree on a change control rule. Even an adjustment that looks temporary should be documented because it affects future production. Ask the molder for a written containment action: are suspect parts separated, how many are being quarantined, and what are the acceptance limits? Then define the current baseline with a repeatability run. Without a baseline, it is impossible to prove whether the tooling re-cut actually solves the problem. Route the investigation through a simple decision gate: process data first, mold audit second, material verification third. Only after those three are complete should the tooling supplier provide a proposal with the exact area to be modified, the expected effect on wall thickness, and a requalification plan. The tooling change should be treated as a formal engineering change, requiring the molder to re-run PPAP samples after the modification. Also require a maintenance interval change if the root cause is wear or water scale. This sequence keeps the project moving without letting a quick packing pressure fix become a permanent hidden defect.",
            "upvoteCount": 0,
            "url": "https://www.hyneu.com/questions/sink-marks-bore-drift-impact-driver-housings.html#suggestedAnswer-9",
            "datePublished": "2026-10-30T19:00:06Z",
            "author": {"@type": "Person","name": "Evan Qian","url": "https://www.hyneu.com/team/evan-qian.html"}          }
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