Insert Molding Solution Under Uncontrollable Tolerance Conditions: A Practical Case of Floating Insert Application

We undertook an Insert Molding project that required embedding a metal plate with a thickness of approximately 6.0 mm into an injection-molded plastic part. The thickness tolerance of this metal insert is critical to both molding quality and appearance. If the thickness variation is too large, the parting line cannot fully close, resulting in improper mold clamping, flash, leakage, and overall failure to meet the customer’s functional and aesthetic requirements.
 

1. Injection Molding Challenges
The metal parts provided by the customer came with a thickness tolerance of ±0.6 mm.
After discussion with the customer, we learned that:
-The metal part is produced through sheet-metal processing.
-The customer’s manufacturing process cannot further improve the thickness consistency.
-There is no additional surface grinding or post-processing, making it impossible to control the tolerance on their side.

If such parts are used directly, the parting line gap could reach up to 0.6 mm, which is unacceptable in injection molding. It would lead to:
-Mold not closing properly
-Flash or burrs
-Injection instability
-Potential mold damage
-Functional and appearance defects

In summary, the root cause lies in the inconsistent metal thickness. Since the customer cannot improve the tolerance at their production end, we must find a feasible solution on our side—either through machining or mold design.

2. Solution Analysis — Our Design and Implementation Process
To balance feasibility, cost, and lead time, we adopted a two-step combined strategy:
initial feasibility machining + mold-side tolerance compensation design.

Step 1 — Manual Machining and T1 Sample for Verification
We first performed local grinding and surface leveling on the customer’s metal parts to ensure closer contact with the mold parting line, enabling us to complete the first trial mold (T1).
The T1 samples were provided only after the customer agreed to continue sample testing. They allowed the customer to evaluate:
-Insert fit
-Appearance
-Flow behavior
-Basic functionality

This step served as a short-term verification method only. Manual grinding is not suitable for volume production due to cost and inconsistency. Therefore, the long-term solution still needed to be addressed within the mold structure.

Step 2 — Mold-Side Compensation: Floating Insert Design
Since the metal thickness cannot be stabilized, we designed the mold to self-adapt to different metal thicknesses by integrating a floating insert structure in the mold cavity. This approach eliminates gaps or interference caused by tolerance variation.
Key design points:
A.Floating Structure Design
A floating pocket was designed at the insert-loading area.
The insert is supported by small-stroke compression springs, allowing it to move axially within a controlled range.
This movement:
-Compensates for the ±0.6 mm part variation
-Ensures tight sealing during mold closing
-Prevents excessive displacement

B.Product Modification Confirmation
Any product structural changes were discussed with and approved by the customer in advance.
Sample models were adjusted accordingly, and the customer confirmed the appearance remained acceptable.

C.Machining and Assembly Control
The floating insert was machined with higher precision than the metal parts themselves to ensure sealing and guiding stability.
Multiple rounds of mold trials were conducted to fine-tune:
-Spring preload
-Guide alignment
-Repeatability under injection pressure

3. Results (Final Production Performance & Customer Feedback)
We successfully manufactured and assembled the floating insert structure with spring preload, and we continued using the customer’s original metal parts without requiring any change from their supply chain.

Production and trial-run results showed:
A. The floating insert effectively compensates for the ±0.6 mm thickness variation, ensuring proper parting line contact.
B. No significant flash, burrs, or leakage occurred during molding; appearance fully met customer requirements.
C. Functional performance—including positioning, structural strength, and durability—remained unaffected.
Customer Feedback:
The customer was very satisfied with the final samples and trial production results. They agreed with the minor product modifications and appreciated that the issue was resolved without altering their metal part production process.



4. Conclusion / Engineering Takeaways
When metal insert tolerance cannot be improved at the source, designing tolerance-absorbing mechanisms in the mold is often the most effective, cost-efficient, and schedule-friendly solution.

Key success factors include:
Ensuring appearance remains unaffected
Preventing insert movement under injection pressure
Balancing sealing, strength, and functionality

The combination of T1 feasibility validation + floating insert mold design greatly reduces risk and helps ensure stable and consistent mass production.

Written by: Delphi Peng (Project Manager)
Edited & Compiled by: Elsa Jin (Sales Manager)
Date:Nov-22-2025