Injection Mould Tooling

Residual Stress

During molding the density of plastic changes as the temperature falls down dramatically from molten to room level which leads shrinkage, and the difference of shrinkage for the product and its sections makes inside residual stress, when it overwhelms the product structure toughness, warp or even crack happens. There are three main reasons cause residual stress: flow-induced, thermal-induced and process-induced & In-cavity.

1 Flow-induced Residual Stress

In moulding process, the polymer molecular chain goes orientation along flow direction under shearing and stretching, while if it solidifies before complete balance, the orientation is frozen in product. This stress frozen status is named flow-induced residual stress, which causes uneven mechanism and shrinkage at flow direction and its perpendicular direction.

When the part close to mold wall is under high shearing stress and cooling rate, high orientation on its surface will be frozen immediately as image instructs bellow. So put the product under high temperature it releases some stress resulting in warp. The frozen layer keeps the part center a higher temperature which releases more stress that the center layer molecular chain orients in low level. Several conditions which are cause shearing stress can be controlled to reduce flow-induced residual stress: high temperature for resin and mold wall, long filling time, decrease packing pressure and short flow distance.

high cooling rate, high shearing stress and high orientation

low cooling rate, low shearing stress and low orientation

2 Thermal-induced Residual Stress

Causes:

Part temperature goes from formation down to natural level;

During frozen, part from surface to center layer suffers different thermal force and mechanical force, such as cooling time and packing pressure;

Changes of density and mechanism make pressure, temperature, molecular chain orientation and fiber orientation vary;

The Injection mould structure design limits shrinkage at some directions.

During the preliminary stage of cooling, part contacts with cooler tooling steel wall and shrinks at surface layer while inner melt still shrinks freely. As central temperature goes down and given frozen surface layer, partial thermal shrinkage causes stretching stress at center layer and compressive stress at surface layer as diagram shows bellow.

The cooling rate difference for two mold wall even causes asymmetrical thermal-induced residual stress so that asymmetrically distributed stretching stress and compressive stress create bending moment, warp happening. Factors such as uneven wall thickness, bad cooling for partial area, uneven cooling for mold and mold structure limits make this kind of residual stress more complicated.

3 Process-induced & In-cavity Residual Stress

As soon as the part comes out of injection mould tooling, it shrinks and warp free till balance. At this moment there is stress remaining inside part which is called process-induced (diagram lower left), namely flow-induced residual stress and thermal-induced residual stress, the latter influences mainly.

When the part is fettered inside of mold, the frozen will accumulate inner stress named in-cavity residual stress which drives part shrink and warp after ejected(diagram upper left ). The part wall encounters asymmetric cooling which births asymmetric residual stress that makes the part warp (diagram lower right).

So how do we reduce thermal-induced residual stress? Symmetric section wall thickness, proper packing pressure and time and symmetric cooling for all surfaces are the conditions.

The volume shrinkage could get up to 20% during plastic injection molding. When the crystalline and semi-crystalline materials are cooled to glass transition temperature, molecules range regularly and crystallize which is easy for heat shrinkage to happen. So the specific volume for those materials has bigger difference than that of amorphous materials as in the PVT diagram.

Note: material specific volume (△υ) variation from process status (A) to room temperature

There are several reasons for over shrinkage including low shot pressure, short of packing and cooling time, high molten temperature and mold temperature, low packing pressure. So when do the injection mould design shrinkage is a significant concern and mold flow analysis helps.

Warpage is an unconventional bending or twisting out of the shape of a plastic part that is due to its asymmetric shrinkage. There are many causes influence shrinkage and result in warpage:

Asymmetric temperature inside plastic part;

Pressure difference along wall thickness direction and cooling rate difference when part freezes;

Ejection before part is completely cooled, or ejector pin deformation, undercut too deep and ejection method defect;

Wall thickness variation causes cooling rate difference;

Part in curved and asymmetric shape;

Difference caused by put in additives or not;

Fiber molecule train orientation difference at flow direction and direction perpendicular to it;

Packing pressure difference.

Proper plastic part design, injection mould tooling design, molding conditions and material use would decrease or control shrinkage and warpage. There are some advice for your information.

1 Wall Thickness

To avoid asymmetric wall thickness or to set variation distance three times of thickness of thin wall area. If there are areas may cause obvious shrinkage, sink marks or voids, make them in combination of symmetric thinner wall thickness and ribs.

2 Balance Filling

Molten polymer transmitting better in mode of filling with proper molten flow front speed.

3 Packing Pressure

High packing pressure is benefit for reducing shrinkage but increases residual stress and requires high machine clamping force. A better design should have a proper packing pressure and time and could release it as soon as the gates frozen. Meanwhile the pressure is enough for polymer compensation to filled part volume shrinking.

4 Cooling System

To design a good cooling system for moulding tool so that the part and its sectional direction have even and balance cooling.

5 Residual Stress

To increase molten temperature, mold wall temperature, filling time and mold cavity thickness or decrease packing pressure and flow distance, etc. to debate residual stress and fiber orientation.

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