Sponsored By

July 22, 1998

6 Min Read
When Your Tools Need Cleaning...

ArticleImage1104.gif

Figure 1. Soiled injection nozzles.

Moulds, injection nozzles, hot runners, screws, and a large number of other tools used in plastics moulding need to be serviced and cleaned from time to time. Unfortunately, there is no cleaning process that is universally applicable. The jobs involved differ greatly in the degree of care required for the tool surface and the type and extent of soiling. IMI contacted an expert in cleaning technologies, Axel Hallensleben of W+I Oberflächen-Systeme, Hilden, Germany, to review the most common methods. Here is his point of view.

Residues, generally the plastic or a byproduct, often cover surfaces in the form of a film, as is the case on screws. In overinjected hot runners and clogged-up filters, the molten plastic solidifies and completely fills up all the hollow spaces (Figure.1). Cleaning-resistant plate-out forms on the surface of injection moulds, caused by release agents, outgassing, or product residues. In some cases, corrosion develops as a result of aggressive chemical compounds. But the tiresome task of cleaning moulds is often performed with antiquated methods like wire brushes, solvents, and open flames, which not only constitute a hazard for staff but frequently damage sensitive mould surfaces as well. Recently, several new methods have developed a following.


ArticleImage2104.gif

Figure 2. Rear of a blasting cubicle
with an integrated blasting
agent recovery system and
an opened cartridge-type filter.

Mechanical Cleaning

A method that has proved successful in the aviation industry for removing paint from.thin-walled aluminum sheet is being increasingly employed to clean moulds and cavities. Sharp-edged, lightweight, flexible thermoset granules cut what are primarily organic deposits off the hard metal surface. This is carried out in a closed blasting cubicle.(Figure 2), primarily by the dry direct pressure blasting process.

Only such exposed surfaces as mould halves or screws can be cleaned. Much soiling caused by such plastics as polyethylene, urethane, or nylon can be removed rapidly.and with no damage to the surface of the mould.

Mechanical cleaning comes up against its limits with glass-fiber reinforced plastics.and such inorganic coatings as rust, lime, and charcoal. In those cases, it is necessary.to mix in fine mineral blasting agents like glass beads or aluminum oxide.

The blasting agent is continuously recirculated and cleaned; dust and coarse soiling are removed in an integrated blasting-agent recovery system. The built-in cartridge-type filter with a suction fan maintains the blasting cubicle at a low pressure and prevents dust from escaping. The internal dimensions of the blasting cubicle are tailored to the size of the mould half being cleaned. The parts are fed to the cubicle via a simple system of a rotary table with a loading platform. For most companies, a compact unit with a useful width, depth, and height of 1m will suffice.


ArticleImage3104.gif

Figure 3. Ultrasonic cleaning system.

Water-based Cleaning

Cleaning in an alkaline ultrasonic bath (Figure 3) is particularly suitable for removing exuded additives as well as lime and corrosion residues. The advantage of this method is that soiling can be rapidly removed from sensitive mould halves, ejector units, and small parts, including inaccessible undercuts and holes. The mould halves, which can weigh up to 3,000 kg, are hung from a crane in a heated basin, which is irradiated by ultrasonic generators mounted on its sides. The cavitation bubbles that develop on the surface of the mould gently break up the soiling.

The multistage cleaning process takes between 15 and 30 minutes. After ultrasonic cleaning, the mould is rinsed and its surface is protected against corrosion. The system, made up of at least three basins, can be equipped with such accessories as a steam suction unit or water treatment and handling systems, as required. Compared with a blasting cubicle, the high cleaning quality and the unstaffed operation (with the exception of loading and removal) justify the high investment costs.

Cleaning by Heat

Once the plastic being moulded has cooled and solidified, heat cleaning is the sole option for nozzles, hot runners, and filters. Cleaning by heat requires the entire.mould to be heated up uniformly to above the decomposition temperature of the plastic. As a rule, a temperature of 420° C will suffice. High-temperature-resistant thermoplastics, however, require a temperature of at least 450° C to 500° C.


ArticleImage4104.gif

Figure 4. Fluidized bed system

The fluidized bed technique. A mixture of gas and air flows through a stainless steel vessel (retort) that is filled with sand, so that the sand is fluidized and behaves like a liquid (Figure 4). A pilot burner ignites the combustible gas that flows out.at the surface of the bed. The flame that develops heats up the sand. The movement of the individual grains of sand ensures a highly uniform temperature distribution, with differences of no more than ±1° C in the fluidized bed. The mould is placed in a basket and immersed in the heated sand.

Any organic soil adhering to the tool is rapidly degraded and incinerated in an environmentally friendly manner in a controlled combustion chamber downstream. The pulsating medium additionally wears off any inorganic residues, making it possible to dispense with subsequent ultrasonic cleaning. The entire cleaning process is relatively short--60 minutes--since the moulds are fed in after the retort has been heated. Plant and energy costs are high, however, when compared with a pyrolysis furnace.

ArticleImage5104.gif

Figure 5. Pyrolysis furnace with charging carriage.

The pyrolysis furnace. Heat cleaning in a pyrolysis furnace (see Figure 5), by contrast, requires increased cleaning time because of the heating and cooling phase. The furnace chamber is heated only after the mould has been placed in the furnace, and is cooled before the mould is removed. Cleaning times of 12 hours and more can result for.big moulds. Decomposition temperatures similar to those in the fluidized bed are encountered in pyrolysis furnaces. The carbonization gases that develop have to be burned in a controlled manner in a separate postcombustion or catalyst chamber.

In all heat cleaning processes, the oxygen component ultimately determines the appropriate.process to be used in each case and the cleaning result. Sensitive nozzles require low-oxygen vacuum furnaces during heat cleaning to avoid damage through overheating or carbon residues on the surface. By their design, most moulds are suited to heat cleaning. In a number of cases, to fully remove the inorganic residues that are still adhering to them, such as pigments or fillers, postcleaning will be necessary after the moulds have been dismantled.

There is no single optimum cleaning process. The choice of a particular process will ultimately represent the compromise derived from an individual cost-benefit analysis. In many cases the best solution will be a combination of different cleaning processes. Cleaning trials and comparisons conducted in cooperation with the manufacturer will rapidly supply tangible results in respect to quality and costs. Contract cleaning by service providers constitutes the ideal solution for large numbers of plastics processors who require mould cleaning only sporadically.

Contact Information
W+I Oberflächen-Systeme, GmbH
Mr. Axel Hallensleben
Stockshausstrasse 9
D-40721 Hilden
Germany
Tel: +49 (2103) 9441 0
Fax: +49 (2103) 9441 11

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like