Reducing Overmolding Flash Around Soft-Touch Seals & Grips
A practical engineering investigation into why TPE flash developed around an overmolded industrial enclosure—and how shut-off design, substrate tolerances, insert positioning, mold alignment and process optimization can work together to control the defect.
Flash Was a Tooling + Process Interaction
The example component was a rigid PC-ABS industrial enclosure with a TPE soft-touch grip and perimeter sealing feature.
Flash appeared primarily at the TPE termination and sealing interface. Reducing injection pressure temporarily improved the defect but introduced incomplete filling.
The investigation therefore moved upstream from the machine settings to the actual material escape path.
The major contributors were the shut-off geometry, substrate tolerance, insert positioning, local mold alignment, venting and injection-pressure requirement.
Illustrative Result
Industrial Handheld Enclosure
The example product is a handheld industrial control enclosure requiring both ergonomic grip surfaces and localized environmental sealing.
Rigid Substrate
- PC-ABS injection-molded housing
- Approx. 2.2–2.5 mm wall
- Internal mounting bosses
- Controlled overmold interface
TPE Overmold
- Soft-touch grip zones
- Approx. 1.5–2.0 mm nominal thickness
- Perimeter sealing feature
- Impact-protection areas
Critical Interface
- TPE termination edge
- Steel-to-substrate shut-off
- Functional sealing region
- High sensitivity to flash
The Flash Was Localized, Not Uniform
During the illustrative production trial, thin TPE flash appeared intermittently around the perimeter seal and grip termination. The worst areas were concentrated near a corner and around one substrate locating region.
Typical Illustrative Flash Measurements
| Location | Approx. Flash | Observation |
|---|---|---|
| Main grip area | 0.05–0.10 mm | Generally acceptable but visible locally |
| Perimeter seal | 0.10–0.18 mm | Required trimming/rework |
| Worst corner | 0.18–0.30 mm | Clear flash fin at shut-off |
| Insert-adjacent region | 0.15–0.25 mm | Strong correlation with positioning |
Why Was the Overmold Flashing?
The investigation showed that the flash was not attributable to one machine parameter. Several variables were interacting at the same overmold interface.
Parting Line
The TPE termination was positioned close to the mold parting/shut-off region, making local gaps highly sensitive.
Shut-Off Design
Limited shut-off land and local steel support made the interface sensitive to substrate variation.
Mold Alignment
Small cavity/core alignment variation increased the local leakage path under injection pressure.
Substrate Tolerance
Variation in the first-shot housing changed the available shut-off condition during second-shot molding.
Injection Pressure
High pressure was required to fill a relatively restrictive TPE flow path, increasing flash sensitivity.
Material Viscosity
Melt temperature, shear rate and material behavior influenced how readily TPE entered small gaps.
Overmold Thickness
A thin transition area increased flow resistance and pressure requirements.
Tool Wear
Local shut-off wear can gradually create a larger leakage path and increase flash over tool life.
Draft & Venting
Limited draft and inadequate end-of-fill venting increased tooling and pressure sensitivity.
Insert Positioning
Small substrate movement during loading changed the TPE shut-off relationship.
Follow the Material Escape Path
Instead of treating flash as a molding-machine problem, the investigation followed the physical path through which the TPE could escape.
Fix the Interface Before Chasing the Process
Making the Shut-Off More Production Robust
Reinforced Shut-Off
A more robust steel sealing land reduced the opportunity for molten TPE to enter the interface.
Improved Locating
Additional locating surfaces controlled substrate position in the critical flash-sensitive axes.
Wear Strategy
A replaceable wear area can simplify future maintenance where repeated TPE shut-off contact is expected.
Build a Stable Filling Window
Once the tooling interface was improved, the molding process could be optimized without using excessive pressure to compensate for a weak shut-off.
Illustrative Manufacturing Improvement
The following values demonstrate the type of improvement that a robust tooling and process correction can target. They are not actual Manufyn customer production data.
| Parameter | Initial Condition | Improved Condition |
|---|---|---|
| Typical flash | 0.10–0.18 mm | <0.05 mm |
| Worst local flash | ~0.30 mm | ~0.04–0.05 mm |
| Flash-related rework/scrap | ~7.2% | ~1.1% |
| Injection pressure | High | Reduced / controlled |
| Substrate positioning | Variable | Controlled |
| Shut-off robustness | High sensitivity | Improved |
Don’t Inspect Overmolding Flash by Sight Alone
Dimensional
Measure flash height, TPE termination and critical substrate-to-overmold dimensions using calibrated equipment.
Visual
Establish controlled lighting and clear standards separating acceptable parting-line witness from actual flash.
Functional
Where the overmold performs sealing duties, verify the applicable leak, compression and assembly requirements.
Validate the Process Beyond One Good Cycle
What This Overmolding Flash Investigation Shows
01. Flash ≠ Pressure Only
Reducing pressure may hide the symptom while causing incomplete filling.
02. Substrate = Tooling Input
First-shot dimensional variation directly influences second-shot shut-off performance.
03. Follow the Leak Path
Identify where the TPE escapes before changing machine parameters.
04. Tool Wear Matters
A shut-off that works at tool launch must remain stable throughout production life.
05. Venting Can Help
Better air evacuation can reduce the pressure needed to fill difficult regions.
06. Design for Variation
Production-ready shut-offs should tolerate realistic dimensional variation.
Preventing Overmolding Flash During DFM
Before releasing an overmolded component for tooling, review these flash-sensitive areas with the mold designer and injection molding engineer.
Go Deeper Into the Tooling & DFM Factors
This case study focuses specifically on overmolding flash. Use the following resources for the supporting design and tooling topics.
Overmolding Design Guide
Review overmold geometry, shut-offs, draft, tooling and insert-related DFM considerations.
Explore Guide →Mold Venting Guide
Understand how venting affects filling, pressure and common injection molding defects.
Explore Guide →Parting Line Design Guide
Learn how parting-line decisions influence flash, tooling and component appearance.
Explore Guide →Injection Molding Design Guide
Supporting guidance on gates, wall thickness, tooling, materials and molding defects.
Explore Guide →Seeing Flash Around an Overmolded Part?
Don’t start by changing injection pressure blindly. Review the complete material escape path—from substrate tolerance and insert positioning to shut-off geometry, mold alignment, venting and process conditions.