Surface inspection for guaranteed quality in window profile extrusion

Surface inspection for guaranteed quality in window profile extrusion

In the past few years output capacities of profile extrusion lines have been increased tremendously Today s standard profile production rates of main profiles are based on haul-off speeds of 4 to 6 m/min and glass strips in twin strand production on 2 x 5 m/min. Machine operators are hardly able to fully supervise product quality when the machine is run at such extrusion speed.

Machine packaging renders supervision almost impossible. Thus, inacceptable surface flaws, especially those on profiles with a lamination sheet, are often detected only during the final assembly at the customers site. At that stage correction is hardly feasible, very expensive or means returning the total scope of delivery.

If, in addition there are disagreements as to the definition of a flaw, the relationship between the supplier and the customer may be easily disturbed law definition. The definition of flaws is very difficult as flaws have no constant sizes and may vary strongly in all their properties.

In addition, flaw definition meets its limits as soon as it is based on subjective evaluation by different staff members or if it is to be defined by individual specimen.
Configuring a surface inspection system means that the supplier needs to classify different flaws and dissect their properties into measurable criteria.

A classification of flaws can be realised only in close co-operation with the users as their special requirements have to be taken into account. In order to be able to analyse surface flaws on plastic profiles, the promex CSI surface inspection system effects evaluation according to the following criteria:

1. Sensitivity
With this parameter the system is rated for the sensitiveness, meaning that the system defines a flaw respectively a normal deviation. A typical example for this are colour changes on the profile surface.

2. Size/frequency
This criterion of quality determines the size of a single flaw or a number of flaws per surface unit. An alarm will be raised only if a flaw exceeds this preset dimension or when a certain number of single flaws is exceeded.

3.Flaw class
During the processing in profile extrusion, different flaws may occur. Their reasons and effects can be very different. A flaw inspection system needs to detect these flaw classes first and then process each class individually. A selection of typical flaw classes in the extrusion process is listed hereafter:

Scratches/furrows
Scratches and furrows are line-shaped surface flaws along the extrusion direction. Scratches are narrow, sharp-edged and less deep. They typically result from impurities in the short caliber. Compared to scratches, furrows have more smooth corners, they are wider and deeper. Furrows typically result from deposits in the mould.

Shining/mat streaks
Shining or mat streaks are line-shaped surface flaws along the extrusion direction. Compared to scratches and furrows, these flaws are wider and have no depth. It is an area where the surface is very smooth or roughened.

Pores/blowholes
Pores and blowholes are point-shaped surface flaws. Blowholes have a wider extent on the surface and more depth than pores.

Impurities/inclusions/soil particles
This class of surface flaws may arise in the most different shapes. A tail-shaped blowhole often follows a larger flaw. These surface flaws typically result from non-melted or burned material passing e.g. from the vacuum channels into the mould. Cleaning the vacuum channels may, however, also result in surface defects because of metallic bodies, e.g. wire brush
particles.

Smears
Smears are extensive surface flaws which typically appear all over the whole profile width. In contrast to the surface flaws named before, smears are no real topological defects and can also be called gloss flaw. Smears may result from degassing problems.

Edge waviness
Basically the edge waviness is no surface flaw. Under certain circumstances, however, edge waviness may also produce faults along the whole profile surface. These faults are wave-shaped and run across the extrusion direction.

The surface inspection system promex
promex CSI has a modular structure and consists of the evaluation unit (electronics and software) to which up to 2 of the image recording units (CCD camera, optics, lighting) can be connected. Two types of operation terminals are available: a standard system with film keyboard and 7 segment displays or a big 15 touch-screen terminal with graphic user surface. As an option further peripheral equipment, e.g. marking devices, printers, alarm lamps etc. complete the intelligent optical sensor technology to a full surface inspection system.

Installation and operation
The optical sensor technology serves as a basis for each surface inspection system. For using the optical sensor technology in the extrusion process, it should be built as compact as possible and suitable for industrial use.
Therefore, the lighting and recording unit have been integrated into one housing so that they can be installed smoothly into the profile line as a closed unit protected from dust and water jets. Due to markings on the housing, easy arrangement via a running rail are possible. Further adjustments of the recording unit by the operator (adjustment of sharpness, diaphragm etc.) are not necessary. In addition, the system disposes of an automatic profile recognition and determines position and width of the surface to be inspected within the recording image. Should the extrusion line be converted to a new profile no adaptations through the operator are required. The housings protect the recording area additionally from disturbances (e.g. by external light). Still remaining brightness fluctuations are balanced by an automatic control of the exposure time.

The system allows easy and intuitive operation with just a few key pressures both in the basic version with film keyboard without monitor and touch-screen version with live display and flaw image management. For applications in profile extrusion the almost unlimited possibilities of the image processing software were reduced to a reasonable number of functions.
When compiling the test programme, the operator is able to select each flaw class individually. Within one flaw class 3 predefined sensitivity levels can be selected. As promex CSI is capable of detecting flaws of only 0.015 mm_, the parameters frequency and extension help the operator determine when the alarm shall be raised (see flaw definition).

Rosum
Only a continuous production monitoring of the profile surface guarantees a 100 % quality of the semifinished products. The surface inspection system promex CSI was configured in a common development between renowned profile
manufacturers and iNOEX. This system can be retrofitted or used as an integral part of an extruder control unit. Visualisation and operation are done via the extruder control unit. This safety technology offers guaranteed surface quality in window profile extrusion combined with an interesting cost-performance ratio.