Example of improvement of mold temperature control problems through the use of self-priming pumps

1. Introduction.

It is no exaggeration to say that the key to quality stability in injection molding lies in mold temperature control. If the mold temperature is not properly controlled, various defects such as weld lines, sink marks and short shots can occur in the molded product. In particular, molds with narrow cooling water tubes are prone to cooling water distribution defects, which directly lead to molding defects.

This article describes a case in which the introduction of a “self-priming pump” improved a temperature control problem that had been occurring at the Kumagaya plant.

2. background and challenges

The Kumagaya Plant has introduced a “centralised piping system” with the aim of reducing the changeover time.

While centralised piping can simplify piping work by one-touch connection of large-diameter hoses to the IN and OUT pipes of the molds, the “parallel circuit configuration” means that cooling water flows only to routes where it is easy to flow, while not enough water is distributed to narrow waterways.

As a result, the following problems were noticeable

-Rising surface temperatures of molds that do not turn water

- Frequent weld defects and flatness defects at regulator sections

- Delayed mold temperature rise and extended changeover times

- Increased production losses due to mass defects

Particularly with the “case fuel filter” mold, the cooling holes in the nested ring section were Φ4.5 mm and the circulation pipe had an inner diameter of Φ2.5 mm, an ultra-fine structure, which meant that water could not circulate sufficiently with a standard pump, resulting in a significant drop in molding quality.

3. the reality of the defects

According to data from the monthly quality report, weld defects were prominent amongst the total defects, particularly in November and May the following year, when a large number of defects, in excess of 7 000 per month, were recorded. The main product for weld defects was the “case fuel filter”, which accounted for more than 20% of the total number of defects.

Among the moulding defects, “appearance weld” and “weld surface flatness defects” stood out, which were clearly due to insufficient mould cooling. In particular, it was observed that when the mould surface temperature was high, the resin mixed with the next flowing resin before solidification, resulting in linear defects (weld lines) in the appearance.

4. Proposed improvements with self-priming pumps

In order to break through this situation, we focused on the introduction of a “self-priming pump”.

Unlike ordinary circulation pumps, self-priming pumps have the characteristic of “high suction capacity, forcing water to be drawn in even in difficult-to-flow pipes”.

When introducing the pumps, evaluations were carried out from the following perspectives

-Can the pump reliably circulate even in small pipes with high piping resistance?

-Can the pump be used in combination with centralised piping?

-Reduction in start-up time and defects due to the introduction of the system

After installation, the flow rate of the mold cooling water has stabilised significantly and the temperature control problems caused by conventional cooling faults have been virtually eliminated.

[Installation on mold]

5. improvement results and effects

The introduction of self-priming pumps has resulted in the following quantitative improvements

indicator	Before introduction	After introduction
Number of weld defects (1260 cases)	Monthly average 2,800 units	Monthly average: less than 400 units
Mold heat-up time	Approx. 30 minutes.	Approx. 15 min.
PPT (rated 5/10000).	129.5 max.	Steadily maintained below 50
In-engineering failure rate	Approx. 0.87% - 1.06%.	Improved to less than 0.5%.

Furthermore, the improved products have reduced the number of customer complaints due to the stabilised appearance quality, and the 100% on-time delivery rate has been achieved continuously. The amount of in-process defects was also halved and annual defect costs were expected to be reduced by more than JPY 1 million.

6. future prospects

This successful case study is a good example of a solution to the key issue of “dealing with difficult-to-flow waterways”. The following developments are expected in the future.

-Self-priming pumps for molds with similar structures

-Construction of a real-time monitoring system for temperature control abnormalities

-Detection of signs of defects by mold temperature data logging in the early stages of molding.

The concept of a “smart temperature control system” is also being developed to combine the convenience of centralised piping with the flow stability of self-priming pumps, with the aim of establishing a three-dimensional system of equipment, quality and workability.

7. summary

The introduction of self-priming pumps has overturned the conventional wisdom that piping with poor flow is inevitable, and has achieved a fundamental improvement in molding defects through reliable temperature control. The challenges faced at the site are tackled with sincerity, the causes are analysed from a technical point of view and the most appropriate improvement measures are implemented. This case study reaffirms that this process is the essence of quality improvement in the manufacturing industry.