Establishment of In-Mold Gate Cutting Technology — A New Approach Achieving Simultaneous Improvements in Appearance Quality and Productivity

In recent years, injection molding production sites have strongly demanded the use of multiple molding machines and automation. Consequently, the importance of mechanisms that complete operations within the mold itself has grown significantly. Particularly for products with visible surfaces, gate processing directly impacts quality. Establishing technology to achieve clean and reliable gate cutting within the mold has thus become a major challenge.

This article introduces the development process and outcomes of the ‘in-mould gate cutting technology’ undertaken by Sanko Gosei's T&E Unit.

■ Background to Theme Selection: Achieving Automated Gate Cutting Capable for Appearance-Critical Parts

The request from mass production plants was to ‘reduce operator workload and double productivity’. Gate cutting operations, in particular, are highly dependent on human skill, and stability is often compromised by product geometry and resin characteristics.

Therefore, the following three points were established as the focus points for this theme:

① Completion within the mold without operator intervention (Automatic)

・The gate is fully cut at the point of removal from the molding machine

・Resistant to external disturbances and adaptable to automated lines

② No impact on surface quality (Quality)

・No scratches or whitening on the surface

・The surface after gate cutting is aesthetically pleasing and free from deformation

③ Low-cost production (Cost)

・Enables mass production at reduced cost without complex protrusion mechanisms

Conventional in-mold gate cutting methods, such as winkles and delayed protrusion mechanisms, faced challenges including gate ejection, pinching, resin restriction, product deformation, and increased costs. Consequently, they had not yet reached a level suitable for appearance-critical parts.

■ New Approach to Gate Cutting: From Vertical to Horizontal Cutting

The conventional method involved cutting the gate vertically, which tended to result in a narrow gate tip and made it susceptible to the effects of resin and processing conditions.

Therefore, we have shifted our approach and adopted a ‘horizontal gate cutting method’ utilising a slide mechanism.

● Features of Horizontal Cutting

The gate tip can be formed at a sharp angle, ensuring a stable cut surface. Utilising sliding movement to shave off the gate, combined with ejector pins, gate removal is completed entirely within the mold.

This method simplifies the mold mechanism while enhancing controllability, revealing potential for application to cosmetic parts.

■ Comprehensive verification from material, appearance, and cooling perspectives

To confirm the applicability range of this technology, several resins commonly used in exterior parts were selected and verified under the following perspectives.

[1] Verification of optimal gate cut timing

As gate cut quality varies before and after cooling, the slide operation timing was meticulously adjusted.

As a result

➡ It was found that ‘cutting the gate after cooling yields the most favourable surface finish’.

【2】Relationship between cooling time and quality

When cooling times were varied for each resin type and compared,

➡ The longer the cooling time, the more stable the gate cut surface becomes

➡ Quality variations exist depending on the resin type

This tendency was observed.

PP and PP-GF30% were prone to gate flash, which emerged as the next challenge.

■ Countermeasures for the challenge: Optimising cooling and angles

① Adding cooling circuits

Insufficient cooling of the gate area can cause the cut surface to sag, so cooling circuits were added to both the mould and slide to enhance gate cooling performance.

This significantly improved residual cut marks even with PP and PP-GF materials.

② Review of gate cut angle

Initially, the slide was withdrawn with a 2° slope relative to the product,

but this angle was found to be a contributing factor to gate crushing.

Therefore, ➡ the method was changed to move the slide at a 0° gradient (horizontal).

As a result, stable cutting became possible regardless of shape, and the impact on appearance was also minimised.

■ Verification Results: Gate cutting now possible for all 5 resin types

As a result of implementing two countermeasures—cooling enhancement and angle optimisation—

stable gate cutting became possible for all 5 selected resin types.

Furthermore, as an additional verification, tests were conducted under conditions with enlarged gate sizes.

● Gate size comparison

• Width 6.9mm / Thickness 1.0mm

  Width 25.3mm / Thickness 1.8mm

  
  

  Both were cut without issue, confirming compatibility with large gates.

  
  

  ■ Effects: Achieving both quality improvement and increased productivity

  The newly established horizontal internal gate cutting method delivered the following benefits:

  
  

  ● Improved visual quality

  ・Gate cut surfaces are aesthetically pleasing, suitable for visible parts ・Significant reduction in gate residue and whitening

  
  

  ● Reduced issues

  ・Less gate ejection and pinching during molding ・Wider processing window enabling stable molding

  
  

  ● Enhanced Productivity

  ・Zero operator burden as the process is completed within the mold

  ・Supports multiple molds per machine, contributing to the dual-machine productivity concept

  
  

  ● Cost Reduction

  ・Eliminates need for complex structures like delayed ejection mechanisms

  ・Balances automation and quality while suppressing mold manufacturing costs

■ Future Developments and Challenges

While a technology applicable to numerous resins and cosmetic parts has now been established, the following challenges remain for future development:

Expanding application to more complex cosmetic parts

Further stabilising surface quality with glass-fibre-reinforced resins

Enhancing the durability of the slide mechanism

Automating gate-cutting timing optimisation

The evolution of this technology is expected to further elevate the automation level of injection moulding, making it easier to achieve both quality and cost efficiency.

■ Summary

The in-mould gate cutting technology introduced in this article represents a significant achievement, successfully balancing two seemingly conflicting requirements: ‘quality suitable for appearance parts’ and ‘automation achieving double productivity’.

Understanding the weaknesses of conventional methods, overcoming challenges through the novel concept of horizontal cutting, and achieving high stability via optimisation of cooling, angles, and timing represent an exemplary case of mould technology development.

It is hoped this technology will prove a significant asset in production environments for appearance parts and high-value-added molded products.