Accuracy of Mould Preparation

The accuracy of preparation of parts of a mold for plastic injection molding is determined by the dimensional tolerance of the molded product. While each company may have its own internal standards about the policies for this accuracy, one of the standards used in major advanced countries is the standard recommended by VDI (Association of German Engineers).

The details of the molded product tolerances according to the VDI standard are as follows.

Taking the dimensional tolerance of the molded product as 100,
Mold preparation tolerance = 1/3 = 33%
Wear margin of mold = 1/6 = 17%
Fluctuation of molding shrinkage = 1/2 = 50%

Let us obtain the accuracy of mold preparation based on the above standards.

[Case 1]

What is the accuracy of mold preparation when the dimensional tolerance of the molded product is ±0.3 mm?

[Example of investigations]

The dimensional tolerance of the molded product is +0.3 + | -0.3 | = 0.6 mm
(| | indicates the absolute value.)

Therefore, the tolerance of mold preparation is 1/6 × 1/3 = 0.2 mm.
∴ A mold preparation accuracy of ±0.1 mm is appropriate.

[Case 2]

What is the accuracy of mold preparation when the dimensional tolerance of the molded product is ±0.05 mm?

[Example of investigations]

The dimensional tolerance of the molded product is +0.05 + | -0.05 | = 0.1 mm

Therefore, the tolerance of mold preparation is 0.1 × 1/3 = 0.03 mm.
∴ A mold preparation accuracy of ±0.015 mm is appropriate.

This article was composed from notes taken from posts in misumi-techcentral.com

Maintenance Items of Moulds for Plastic Injection Molding

When the mass production of plastic injection moulded products is being carried out, there will always be some wearing out or breakage of parts of the mold for plastic injection molding. In such situations, it is necessary to carry out maintenance by replacing parts or making repairs. The common items that require maintenance are the following.

• Depressions, scratches and wear of the parting surface.

• Chipping or depressions on the corners of the cavity.

• Wear and scraping of the locking block.

• Wear and cracks of the angled pin.

• Wear and scraping of the guide post. 

• Wear and scraping of the guide bush. 

• Scraping of the rail guide by the slide core. 

• Scraping of the center rail.

• Wear and deformation of the gate. 

• Wear of the internal surface of the sprue bush. 

• Depressions and deformation of the nozzle touching part of the sprue bush. 

• Weakening of the spring elasticity.

• Wear of the ejector pin.

• Wear of the hole of the ejector pin. 

• Wear and scraping of the positioning block. 

• Wear and scraping of the return pin.

• Scraping of the stripper plate.

• Scraping of the runner stripper plate. 

• Elongation of the bolt.

• Wear to the bolt threads. 

• Wear of the coupler for the cooling water. 

• Punctures and open circuits in the cartridge heater. 

• Rust and clogging inside the hole for the cooling water.

• Rust and mold in the periphery of the mold base.

• Open circuits in the electrical wires and cracks in the cable covering. 

• Fault in the contacts of the limit switches.

• Clogging due to soot in the air vents.

• Deformation due to temporal changes in the frame block.