Wednesday, January 2, 2013

Plastic boss crack of mold-in screw insert potential cause

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Recently field complained our product had plastic house broken issue. As checked the defect parts and found the crack came from the screw boss till outside house. There is a mold-in screw INSERT part inside the boss. Below will list the screw boss crack potential causes:

1. Using re-grind resin

We usually use the MFI (Mold Flow Index) value to judge plastic raw material with re-grind resin mixed or not? As experience show the molded plastic material will increase 25~30% of the MFI value as compare with the virgin resin. Once there is re-grind resin (already shot one time at least) mix into the virgin resin for the injection then the MFI value will be over 30% of original value.

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2. Screw INSERT parts didn’t performance pre-heat process

As experience show the screw INSERT parts shall do pre-heat process before put it into the molding tool to reduce the inner stress and creep risk. If INSERT parts do not implement the pre-heat process then it will depend on plastic molding environmental temperature.

The molding cavity (PC resin) usually be preheated to 120°C and putting the 25°C (assume room temperature) of INSERT parts into cavity. Even cavity will heat the ISNERT parts rapidly to 120°C by contact, the cooper INSERT may not expand to its stable size in short time. The melted hot resin will go into the cavity around 310°C to heat the INSERT parts again. After few seconds the plastic to swap to cooling stage. Unfortunately, The INSERT parts are still expanding its size and plastic parts are going to cool and reduce size. The inner stress will happen on the INSERTS parts surface conjunct with plastic material.

3. Plastic molding parameter effect

If apply lower packing pressure and time then the inner stress around the boss will be reduced and prevent the potential crack on boss. But lower packing pressure will also bring the sink marks on the thicker area and degrade the strength ability.

4. Design risk

According the design lines (refers to DUPONT document of “Molded-In Inserts: Precautions and Guidelines) for the mold-in parts. The boss diameter shall be 1.5 times the INSERT diameter except for insert with a diameter greater than 12.9mm (0.5”). More thickness of boss wall, the more stress happen around the INSERT due to material expand hotter and contract when cooler. Same as pre-heating section reason.

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LCM gasket/cushion design
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Shipping carton drop/impact test

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