What is shrinkage in plastic material?
Plastic moulding shrinkage is the contraction of plastic moulding as it cools after injection. Most shrinkage occurs in the mould during cooling, but some shrinkage occurs after ejection, as the part continues to cool.
It reflects the degree of reduction in the size of the plastic part after being taken out of the mold and cooled. Factors affecting plastic shrinkage are plastic varieties, molding conditions, mold structure, etc.
Different polymer materials have different shrinkage rates. Secondly, the shrinkage rate of plastic has a great relationship with the shape of the plastic part, the complexity of the internal structure, and whether there are inserts.
Plastic molding processing temperature, mold temperature, and general plastic shrinkage rate of the injection molding process
|Name||density||Glass fiber content||Average specific heat||Processing temperature||Mold temperature||Shrinkage|
|g/cm]||%]||[KJ/(kg x K)]||[℃]||[℃]||[%]|
What is Injection Molding shrinkage reason?
There are dents in the material accumulation area on the surface of the injection molded parts. Shrinkage occurs mainly where the plastic wall is thick or where the wall thickness has changed.
The physical reason
When the product is cooled, shrinkage (volume reduction, shrinkage) occurs. At which point the outer tight die wall first freezes, forming internal stress in the center of the product. If the stress is too high, it will lead to plastic deformation of the outer layer of plastic. If the shrinkage occurs and the deformation of the outer wall is not stable (because there is no cooling). The pressure retaining is not added into the mold, and a settlement is formed between the mold wall and the outer layer of the solidified product.
These settlements are usually seen as contractions. If the product has a thick section, it is possible to have such shrinkage after demoulding. This is because there is still heat inside. Which passes through the outer layer and heats the outer layer. The tensile stress generated in the product will cause the thermal outer layer to sink inward, forming shrinkage in the process.
Insufficient cavity packing caused by injection molding shrinkage. Is divided into six kinds of circumstances:
A Plastic part’s thickness or thickness is not uniform. The solution for mould in China is to modify the mould or increase the injection pressure;
B, The material is not enough. China injection molding solution is to increase the material.
C, The injection pressure is too small. The solution is to increase the injection molding pressure.
D, Injection time is too short. The solution is to extend the injection molding time.
E, Asymmetry of the gate. The solution is to limit all melt into a recent sprue gate, forced melt into the other gate.
F, The injection speed is too slow. The solution is to increase injection molding speed.
G, The gate is too small. The solution is to increase the gate size.
How to solve shrinkage aporia？
1: Overheating of plastic raw materials; Improved method is to reduce barrel temperature and mould temperature is controlled properly overheated parts.
2: Plug injection molding pump (heart) before melt cushion pressure unstable. The solution is to increase the material and keep the injection plug (pump heart) before melt cushion pressure stability.
3: Injection mold opening time is different. To improve the method is to use a timer to control mould time.
4: Plug injection pump (heart) melt rubber cushion product before too much. The solution is to reduce the material.
5: When plastic demolding still overheating. The Improved method is molding mould or plastic products immediately soak in cold water. It also can extend the cooling time.
The influence of molding process on shrinkage of plastic products
(1) The molding temperature remains unchanged, the injection pressure increases, and the shrinkage rate decreases;
(2) Keeping the pressure increasing, the shrinkage rate decreases;
(3) As the melt temperature increases, the shrinkage rate decreases;
(4) The mold temperature is high, and the shrinkage rate increases;
(5) The holding time is long and the shrinkage rate is reduced, but the shrinkage rate is not affected after the gate is closed;
(6) Long cooling time in the mold reduces shrinkage;
(7) The injection speed is high, the shrinkage rate tends to increase slightly, and the effect is small;
(8) The molding shrinkage is large, and the post shrinkage is small. The post-contraction is large in the first two days and stable in about a week. The plunger injection machine has a large molding shrinkage rate.
The influence of plastic structure on product shrinkage
(1) Thick-walled plastic parts have a larger shrinkage rate than thin-walled plastic parts (but most plastic 1mm thin-walled parts have a larger shrinkage rate than 2mm, which is due to the increased resistance of the melt in the cavity);
(2) The shrinkage rate of plastic parts with inserts is smaller than without inserts;
(3) Plastic parts with complex shapes have a smaller shrinkage rate than simple shapes;
(4) The shrinkage rate of plastic parts in the height direction is generally smaller than that in the horizontal direction;
(5) The shrinkage of slender plastic parts in the length direction is small;
(6) The shrinkage rate of the length direction of the plastic part is smaller than that of the thickness direction;
(7) The shrinkage of the inner hole is large, and the shrinkage of the shape is small.
The influence of mold structure on shrinkage of plastic products
(1) The gate size is large, and the shrinkage rate is reduced;
(2) The shrinkage rate of the vertical gate direction decreases, and the shrinkage rate of the parallel gate direction increases;
(3) The shrinkage rate far from the gate is smaller than near the gate;
(4) The shrinkage rate of the plastic part with mold restriction is small, and the shrinkage rate of the unrestricted plastic part is large.
The effect of plastic properties on product shrinkage
(1) The shrinkage rate of crystalline plastics is greater than that of amorphous plastics;
(2) Plastic with good fluidity, low molding shrinkage;
(3) When fillers are added to the plastic, the molding shrinkage rate is significantly reduced;
(4) Different batches of the same plastic have different molding shrinkage rates.