Analysis and Solution of Internal Stress Problems in Injection Molding Defects

Time:2022-08-31 16:10:22 / Popularity: / Source:

1. Internal stress

In injection molded products, local stress state is different everywhere, and degree of deformation of product will be determined by stress distribution. If product is cooling. In presence of temperature gradients, such stresses will develop, so these stresses are also referred to as "forming stresses".
injection molded products 
There are two kinds of internal stress of injection molding products: one is molding stress of injection molding products, and the other is temperature stress. When melt enters cooler mold, melt near cavity wall rapidly cools and solidifies, and molecular segments are "frozen".
Due to solidified polymer layer, thermal conductivity is poor, resulting in a large temperature gradient across thickness of article. Solidification of core of product is very slow, so that when gate is closed, melt unit in the center of product has not yet solidified, and injection molding machine cannot replenish cooling shrinkage. In this way, internal shrinkage of product is in opposite direction to that of hard skin layer; core is in static tension and surface layer is in static compression.
In addition to stress caused by effect of volume shrinkage during melt flow in mold. There is also stress caused by expansion effect of runner and gate outlet; stress caused by the former effect is related to flow direction of melt, and the latter will cause stress in direction perpendicular to flow direction due to outlet expansion effect.

2. Process factors affecting internal stress

(1) Influence of orientation stress. Under condition of rapid cooling, orientation will lead to formation of internal stress in polymer. Due to high viscosity of polymer melt, internal stress cannot be relaxed quickly, which affects physical properties and dimensional stability of product.
Influence of various parameters on orientation stress
a Melt temperature, high melt temperature, low viscosity, decrease of shear stress and decrease of orientation degree;
However, without changing pressure of injection molding machine, cavity pressure will increase, and strong shearing effect will lead to an increase in orientation stress.
Prolonging dwell time before nozzle closes will result in an increase in orientation stress.
Increasing injection pressure or holding pressure will increase orientation stress.
High mold temperature can ensure slow cooling of product and play a role in de-orientation.
Increasing thickness of product reduces orientation stress, because thick-walled product cools slowly, viscosity increases slowly, and stress relaxation process takes a long time, so orientation stress is small.
(2) Influence on temperature stress
As mentioned above, due to large temperature gradient between melt and mold wall during mold filling, first-solidified outer layer melt should help to stop shrinkage of later-solidified inner layer melt, resulting in compressive stress (shrinkage stress) in outer layer and tensile stress (orientation stress) in inner layer.
If mold is filled for a long time under action of holding pressure, polymer melt will be replenished into mold cavity, so that cavity pressure will increase, and this pressure will change internal stress caused by uneven temperature. However, when holding time is short and cavity pressure is low, interior of product will still maintain original stress state during cooling.
If cavity pressure is insufficient in the early stage of product cooling, outer layer of product will form a depression due to solidification shrinkage; if cavity pressure is insufficient in later stage when product has formed a chilled layer, inner layer of product will be separated due to shrinkage, or a cavity will be formed; if cavity pressure is maintained before gate is closed, it is beneficial to increase density of product and eliminate cooling temperature stress, but a large stress concentration will occur near gate.
From this, it appears that when thermoplastic polymers are formed, the greater in-mold pressure, the longer holding time, which helps to reduce shrinkage stress caused by temperature, on the contrary, it will increase compressive stress.

3. Relationship between internal stress and product quality

Existence of internal stress in product will seriously affect mechanical properties and performance of product; due to existence and uneven distribution of internal stress in product, product will crack during use.
When used below glass transition temperature, irregular deformation or warpage often occurs, surface of product is also "whitened", cloudy, and optical properties deteriorate. Try to reduce temperature at gate and increase slow cooling time, which is beneficial to improve uneven stress of product and make mechanical properties of product uniform.
Tensile strength is anisotropic for both crystalline and amorphous polymers. For amorphous polymers, tensile strength will vary depending on position of gate; when gate is in same direction as filling direction, tensile strength decreases with increase of melt temperature; when gate is perpendicular to filling direction, tensile strength decreases. Tensile strength increases with increasing melt temperature.
Deorientation increases due to increased melt temperature, while tensile strength decreases due to weakened orientation. Orientation of gate will affect orientation by affecting direction of material flow, and since anisotropy of amorphous polymer is stronger than that of crystalline polymer, tensile strength perpendicular to flow direction of the former is larger than that of the latter.
Low temperature injection has greater mechanical anisotropy than high temperature injection. For example, when injection temperature is high, strength ratio of vertical direction to flow direction is 1.7, and when injection temperature is low, it is 2.
From this point of view, increase of melt temperature will lead to reduction of tensile strength for both crystalline polymers and amorphous polymers, but mechanism is different; the former is due to effect of reduction through orientation.

Go To Top