Quality control for plastic injection molding
Parts made in a plastic injection molding process
can have their own unique set of possible defects.
That is why all of our employees
are trained to spot these problems. The following
is a listing of the most common defects associated
with the plastic
injection molding and transfer/compression molding
processes:
Sink Marks: Sinking is caused by the outer skin of plastic solidifying while the material inside is still molten and viscous. As it cools and solidifies, the material compacts . The best way to avoid these dimples is to design the part with a consistent wall thickness. However, many times the effect of the sink mark can be minimized by adjusting molding parameters such as injection pressure and time.
Gas marks (burning): When molten plastic is injected into a mold cavity, the air that is in that cavity needs a place to escape. If the hot compressed air cannot escape, it may leave burn mark on the part. The best way to avoid gas marks is to strategically locate air vents of the proper depth within the mold. Adjustments to process parameters such as injection speed and screw speed can be altered to reduce or eliminate burning.
Flash: When two halves of a mold
come together, the sides of the cavities must
press together tight enough that no molten plastic
leaks out of the cavity, resulting in a thin
layer of plastic protruding from the part called
flash. The plastic injection molding press must
have enough tonnage, and the mold must be properly
built and maintained to avoid this problem.
However, some materials flash more easily than
others. In fact, most thermoset
materials will flash regardless of the press
and mold. These parts need to be “deflashed”
as a secondary operation.
Short shots: If too little plastic injection molding material flows into the mold, the cavity will not fill properly, which we refer to as a short shot. This can be corrected by changing the plastic injection molding parameters.
Knit Lines (or weld lines): If a part has a hole or insert, the molten plastic will flow around the opening and meet on the other side. Where the two sides meet, the part will be weaker, and a knit line may be visible. Often this effect can be minimized by adjusting processing parameters such as injection speed and mold temperature.
Figure
1 Knit Line and short shot
Flow marks: If molten plastic does not properly flow as it fills the cavity, flow marks may result. This can often be fixed by changing molding parameters or adjusting the mold by changing the gate location or size.
Splay: Bubbles may flow along the part surface during the plastic injection molding process, which can cause marks on the part surface. Proper mold design and processing parameters can prevent splay.
Quality
standards
Rebling Plastics is currently working on becoming ISO 9001:2000 certified. We are familiar with APQP, FMEA, PPAP, etc. We are also experienced in working to military specifications, in fact we manufacture some of our own military spec parts.
How we
measure quality and continuous improvement
Rebling Plastics tracks on-time and non-conforming shipments to our customers as well as internal non-conformances. We similarly track on-time and non-conforming products from our vendors. Our material review board meets weekly to discuss every issue, and determine detection and prevention solutions. Non-conforming product is brought to the attention of all employees at our monthly quality meetings, where everyone’s input is considered. And monthly on-time performance is posted on the bulletin board for everyone to see.
Detection and prevention take many forms. Some changes involve the plastic injection molding process, others involve mold changes or inspection fixtures. Using pokeyoke fixtures and designing for manufacturing are some of the other ways we prevent quality problems in advance.
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