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Causes and Solutions of Sink Mark in Injection Molding

Categories: FAQStars: 3StarsVisit: -Release time: 2017-05-05 16:39:00
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 Concave holes on MIM parts’ surface known as sink mark in injection molding. In addition to affect product appearance, also reduce the strength of the finished product. Sink mark is relevant to molding process, mould design and plastic material.

1. Plastic Material
Taking plastic injection molding for example, different plastic raw material has different shrinkage. Easy to shrink materials are mostly belongs to crystalline plastics (such as: nylon and polypropylene). When crystalline plastics heat into flow state during injection process, the molecules are arranged out of rules. When they are injected into cool mould cavity, plastic molecules lined up to form crystallization gradually, which lead to product volume shrink and its size less than specified. It is the so-called "shrink".  

2. Injection Molding Process

Sink mark caused by injection molding can come down to:

insufficient holding pressure

injection speed is too slow

injection gate is too small

runner is too long or too small

Therefore it must review molding condition whether correct or not and pressure enough or not to prevent sink mark problem when setting injection molding process condition.

3. Mould and Product Design
The fundamental cause of sink mark is uneven wall thickness of product. Sink mark easy appears on the rib and pillar surface of product. The design of runner and cooling system affect great on finished product. As the heat transmission ability of plastic is quite low, the further distance from mould wall or the thicker wall thickness, the slower cooling and solidifying. So it should have enough plastic filling mould cavity. No reducing holding pressure when molten material backflow during injection. Besides, the runner and gate cannot condensate too fast, in case of half solid plastic blocking runner caused  pressure decrease leading to sink mark on product. Different mold flow process has different shrink rate, prolong the holding time can ensure that products have plenty of time for cooling and filling.


Following table is the causes and improvement methods of sink mark:

 

Cause

Improve method

1、insufficient feeding

1.Increase feeding

A、insufficient molten material

A、increase molten material metering

B、insufficient injection pressure

B、increase injection pressure

C、pressure holding is not enough

C、increase holding pressure time

D. shot time is too short

D、extend shot time

E、shot speeding is too slow/fast

E、fast/slow down shot speed

F、injection gate is too small or uneven

F、enlarge injection gate or make mould even

G、Nozzle blocked or leak material (fever ring burn out)

G、Demolish/clean up nozzle

H、the nozzle is leaking

H、rebuild and fasten nozzle, reduce back pressure

2.Feedstock temperature is too low/high

2.Adjust feedstock temperature

3.mould temperature is too low/high

3.Increase/reduce mould temperature

4.Cooling time is no enough

4.Extend cooling time

5. inadequate venting

5.set exhaust slots in shrink surface

6.Wall thickness of rib/pillar is too thick

6.Improve product design

7.Sealing washer wear down

7.Replace sealing washer

8.Machine/feed tube is too big for product

8.Replace suitable machine/feed tube

9.Runner is too small or too long

9.Enlarge runner, shorten runner length

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