Details

May 2020

Complicated item required unconventional solution

At Hjerno we love a good challenge. This was once again proved on a recent project.

Here, a Swedish customer asked us to manufacture a tool for a plastic item with ultra-thin holes, but at the same time with a very strong rim thickness in a completely crystal clear material with polished surfaces. In addition, the item should be of as flawless a quality as possible.

This meant, among other things, that there could be no traces of partition lines, fins, shrink marks in the large rim thickness or other things in the finished item. In addition, the item - a plastic product for the pharmaceutical industry - had to be completely stress-free in order to handle the laser light to which it is exposed at the end user.

"The surface had to be completely homogeneous, otherwise it would adversely affect the deflection of the laser light. Therefore, the exercise consisted of producing a completely homogeneous item without shrink marks - mind you in a rim thickness that is not possible with traditional injection moulding," says engineer Preben Giversen.

Compressed under high pressure

The solution was to produce a so-called compression tool, where the item is moulded in the traditional way before being compressed inside the tool itself under a high pressure that can equalise the shrink marks that normally occur during moulding.

"The challenge is that normally the tool must be completely closed when plastic material is injected into the mould, otherwise the closing surfaces will produce oversprayings, which results in large fins. At the same time, a problem arises with keeping a post-pressure, etc., to reduce the shrink marks as much as possible in the very large rim thickness," explains Preben Giversen.

"Therefore we had to manufacture a tool that could fully mould the item without having to be completely closed. And what is more, without oversprayings and fins, and then compressed under ultra-high pressure, so that shrink marks and such things were neutralised," explains Preben Giversen.

Either-or

There were also a number of very small holes which, with hole sizes of 0.6 x 0.4 millimetres, a radius of 0.7 millimetres and a rim thickness of 5 millimetres, also on the tolerances presented a significant challenge. Not least because the surfaces of the ultra-small holes should be polishable at the end.

"But we succeeded beyond all expectation, and the shots were perfect in the first FOT test. This is how it is with these kinds of tasks: It is an either-or situation. If the item is not perfect from the start, you might as well start over," says Preben Giversen.

Ultra-advanced machining

He calls the Swedish project another good example of Hjerno's ability - and desire - to work with ultra-precise machining of very complicated items.

"Our solid experience with similar tasks in areas such as fuel cell production and micro-tools has really prepared us for working with this kind of ultra-advanced machining," concludes Preben Giversen.