TOP INJECTION MOLDING PROBLEMS AND THEIR COUNTERMEASURES.docx

TOP INJECTION MOLDING PROBLEMS AND THEIR COUNTERMEASURES 1. FLOW LINES Flow lines are streaks, patterns, or lines - commonly off-toned in color - that show up on the prototype part as a consequence of the physical path and cooling profile of the molten plastic as it flows into the injection mold tooling cavity. Injection molded plastic begins its journey through the part tooling via an entry section called a “gate.” It then flows through the tool cavity and cools (eventually hardening into a solid). CAUSE: Flow line defects are caused by the varying speed at which the molten plastic flows as it changes direction through the contours and bends inside the mold tool. They also occur when the plastic flows through sections with varying wall thickness, or when the injection speed is too low causing the plastic to solidify at different speeds. COUNTERMEASURES: 

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Increase injection speeds and pressure to the optimal level, which will ensure the cavities are filled properly (while not allowing the molten plastic time to start cooling in the wrong spot). The temperature of the molten plastic or the mold itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect. Round corners and locations where the wall thickness changes to avoid sudden changes in direction and flow rate. Locate the gate at a spot in the tool cavity with thin walls.

2. SINK MARKS Sink marks or shrink marks are hollows or indentations that occur on the outer surfaces of molded components. CAUSE: Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold. They can also be caused by inadequate pressure in the cavity, or by an excessive temperature at the gate. All else being equal, thick sections of the injection molded part take longer to cool than thin ones and so are more likely to be where sink marks are located. COUNTERMEASURES:  

Mold temperatures should be lowered, holding pressure increased, and holding time prolonged to allow for more adequate cooling and curing. Reducing the thickness of the thickest wall sections will also ensure faster cooling and help reduce the likelihood of sink marks.

3.

VACUUM VOIDS Vacuum voids are pockets of air trapped within or close to the surface of an injection molded prototype. CAUSE:

Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the prototype. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold (and thereby force out air that would otherwise get trapped). Voids can also develop from a part that is cast from a mold with two halves that are not correctly aligned. COUNTERMEASURES:    

Locate the gate at the thickest part of the molding. Switch to a less viscous plastic. This will ensure that less gas is trapped as air is able to escape more rapidly. Increase holding pressure as well as holding time. Ensure that mold parts are perfectly aligned.

4. SURFACE DELAMINATION The term "delamination" is used to refer to a condition where a thin micaceous layer develops on the molded product and then becomes separated. CAUSES:  

Inclusion of a plastic which has poor compatibility with ABS (i.e., PP or PS, etc.) When the die and plastic temperatures are extremely low, the difference in temperatures between he outer walls and the fluid layer results in the development of a thin hard coating which then peels.

COUNTERMEASURES: 

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Carry out cleaning in - Inside the cylinder - Inside the hopper - Inside the air feed lines - Inside the dryer

Standardize the resin temperatures. - Raise the temperature of the resin - Raise the temperature of the die

5. WELD LINES The material splits off and flows in two or more directions. When the divergent flow fronts converge again, hair-like lines appear where they meet. This results in reduced strength, as well as burn-mark and bubble defects. CAUSES:   

Material splits and flows in separate directions, then converges again. Polymer temperature is too low. Polymer is not fully melted. Difficulties in venting gas and air.

COUNTERMEASURES:         

Raise the resin temperature Increase the injection pressure. Increase the injection speed. Provide a cold slug well where weld lines occur. Provide a gas vent. Change the gate locations. Or increase the number of gates. Stop using mold parting agents. Change the gate locations so that the weldlines occur in locations less susceptible to external forces if weld strength is an issue. Choose a low-viscosity high flow type material.

6. SHORT SHOTS A molded product that is incomplete because the mold cavity was not filled completely. CAUSES:  Insufficient injection-molding machine performance (shot capacity, plasticizing capacity, etc.)  Poor material flowability  Gate cross-section surface area is too small. Molded product thickness is too thin.  Poor gas venting. COUNTERMEASURES:  Increase the amount of material feed. If material feed is still insufficient at maximum material feed capacity, change to a larger capacity machine.  Install a screw with a back-flow check valve.  Increase injection pressure  Raise the cylinder temperature setting. Raise the nozzle temperature, too.  Make sure there are no severed lines to the heater.  Make sure the nozzle is not clogged. If the nozzle clogs frequently, raise the mold temperature or shorten the cycle time.  Increase injection speed  Raise the mold temperature.  Increase the mold gas release.  Increase gate cross section surface area.

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Increase the molded product thickness. Add ribs to the molded product design to improve flowability. Choose a low-viscosity high flow material type. Apply surface lubricant. (Add 0.05-0.1% by weight.)

7. WARPING The molded product warps immediately following or at some time following molding where the mold cavity should be straight. CAUSES: 

Residual stress from the molding process is released, causing deformation.

COUNTERMEASURES:  Decrease the injection pressure.  Lower the cylinder temperature setting.  Lengthen the cooling time.  Make the thickness of the molded product uniform.  Eliminate undercuts.  Operate the knock out pins in a uniform fashion.  Provide multiple knock out pins.  Change the gate locations.  Lower the mold temperature.

8. BURN MARKS Discoloration or dark reddish brown streaks in the molded product. CAUSE: 

Heat decomposition of the material.

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Lower the cylinder temperature setting. Shorten the intermediate time (dry cycle time). The ram is recessed for too long of a time duration. Slow down injection speed. Provide a larger gas vent. Increase the gate sizes.

9. JETTING The term "jetting" is used to describe the phenomenon where plastic passing through a gate does not adhere and where flow patterns are formed on the surface of the molded product.

Plastic at a relatively low temperature is injected from the nozzle during the initial stage of molding, upon coming into contact with the mold wall, this plastic becomes highly viscous and swirling takes place; furthermore, as hotter plastic is continually injected into the die, the original material is pushed deeper into the die and leaves flow marks. CAUSES:   

When the plastic temperature is low, the viscosity of the molten material is high, and this become higher in the case of plastic which has been injected into the die; consequently, the resistance to flow is large and jetting occurs. When the die temperature is low, the material injected into it will be rapidly cooled, and the corresponding increase in viscosity leads to the occurrence of this problem. When gates are small, the speed of plastic injected into the cavity will be relatively fast, and this leads to the occurrence of jetting in many cases. (Gate sectional area x flow speed = Fixed injection amount)

COUNTERMEASURES:     

Increase the temperature of the plastic to lower its viscosity. In the case of amorphous plastics, the ideal temperature of the die is between 20deg.C and 30deg.C lower than the plastic's thermal deformation temperature. It is also beneficial to reduce the injection speed. Dies It is advantageous to increase the sectional area of the gates so that the speed of the material passing through the gates becomes slower.

10. FLASH Flash is a molding defect that occurs when some molten plastic escapes from the mold cavity. Typical routes for escape are through the parting line or ejector pin locations. This extrusion cools and remains attached to the finished product. CAUSE: Flash can occur when the mold is not clamped together with enough force (a force strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold), which allows the plastic to seep through. The use of molds that have exceeded their lifespan will be worn out and contribute to the possibility of flash. Additionally, excessive injection pressure may force the plastic out through the route of least resistance. COUNTERMEASURES:   

Increase the clamp pressure to ensure that the mold parts remain shut during shots. Ensure that the mold is properly maintained and cleaned (or replaced when it has reached the end of its useful lifespan). Adopt optimal molding conditions like injection speed, injection pressure, mold temperature, and proper gas venting.

TECHNOLOGICAL INTITUTE OF THE PHILIPPINES – MANILA 363 P. CASAL ST., QUIAPO, MANILA

COLLEGE OF ENGINEERING AND ARCHITECTURE CHEMICAL ENGINEERING DEPARTMENT

CHE 512 PLASTICS TECHNOLOGY

ASSIGNMENT

SUBMITTED BY: ALDA, GLADYS M.

SUBMITTED TO: ENGR. LINA D. DELA CRUZ

MARCH 16, 2017