The displaced air in the mold cavity must be able to escape during mould filling process. If there are insufficient vents compression of air may take place. The pressure and local temperature rise quickly, potentially causing incomplete filling or even burning of the thermoplastic.
The number of vents is often limited by mold construction economics but ideally should be taken into account during the design stage. In general the higher the viscosity of a material the larger the vent dimensions. As with gates vents should be cut steel safe i.e. start at the minimum dimension and open the vent up gradually until the optimum molding is achieved. Too small and the vent will clog up and reduce or eliminate the ability to expel air from the mold cavity; too large and flash may be seen on the moldings.
Dimensions of venting channels can be read from the figure below. The dimensions are chosen in such a way that air can escape without flash.
Construction of a venting channel:
Vents can be placed anywhere along the parting line in particular they should be located in areas that are the last to fill in particular section of the mold. A reasonable spacing is every 25mm. If the air is trapped with no way out to the mold parting line, it is advisable to place a venting pin/ejector pin to permit the air escape through the clearance between pin and hole. Another option is to use sintered metal inserts, these inserts allow gas to pass into them without clogging up with the polymer. These inserts should only be used as a last resort and only on non-visual surfaces.
Venting locations:
To summarize, inadequate venting may result in various molding failures:
1. burnt spot
2. weak and visible weld lines
3. poor surface finish
4. poor mechanical properties
5. incomplete filling, especially in thin sections
6. irregular dimensions
7. local corrosion of the mold cavity surface
If venting is not provided, the following defects may occur in the moulding:
1. Discoloration
2. Sink mark
3. Incomplete filling
Position of the Vent:
1. At the point where flow paths are likely to meet
2. At the bottom of the projection
3. At the point of further most from the gate on symmetrical moulding.
Size of the Vent:
0.05 deep x 3 mm wide
Venting of part surface:
Surface finish:
A high-gloss surface finish may be achieved with proper molding conditions and polished mold cavities. High-gloss polished cavities require careful handling and protection during processing. Mold maintenance needs more frequent attention.
Great care should be exercised when removing high-gloss parts from the mold to avoid scratches.
The table below provides an indication of the price index for the commonly used surface finish classes according ISO 1302.
Price index for various surface finishing classes
For low gloss, semi-matt or matt surface finishes, the tool cavity needs treatment to obtain fine to very fine textured structures. A matt surface is obtained by vapor blasting techniques. Basic steel roughness should be N3 or better (ra < 0.1 mm).
Textured part surfaces have a special visual and haptic appearance, e.g. soft touch. Compared to other surface treatments, textures are relatively cheap.
Their popularity is based on:
1. Appearance (wood grain or leather)
2. Functionality, e.g. anti-slip
3. Masking of molding defects
Main texturing techniques are:
1. Photochemical etching
2. EDM
3. Engraving
4. Brushing
5. Laser engraving
When high quality of textures are expected use a low alloy tool steel with a limited carbon content (< 0.45%). If nitriding is necessary, texturing should precede it.
After long periods of use the mold surface deteriorates due to wear. Use of glass fibers will increase abrasion. Frequent checks of the surface condition are recommended.
Also differences in mold deposit tendency of materials can cause changes to the (local) mattness of parts, making cleaning at regular times necessary with some materials e.g. old PC/ABS formulations.
Semi-crystalline thermoplastics are often less scratch resistant when very fine textures are used. Because of their good flow properties, the mold reproduction is better than that of amorphous thermoplastics. Micro-scopic ridges at the part surface may be easily damaged with a finger nail.
For untextured surfaces generally a minimum of 0.5 deg draft per side is recommended although there are exceptions when less may be acceptable. Polishing in draw line or using special surface treatments can achieve this. For textured sidewalls use an additional 0.4 deg draft per 0.1mm depth of texture is recommended.
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