- December 16, 2019
- Posted by: Molding
- Category: White Papers
Cracking is a common defect of plastic products, which is mainly caused by stress and deformation. It mainly includes residual stress, external stress and stress deformation caused by external environment.
(I) cracking caused by residual stress
The residual stress is mainly caused by the following three conditions, i.e. overfilling, demoulding and pushing out and metal inlay. As the cracks produced in the case of excess filling, the solution can be mainly in the following aspects:
(1) since the pressure loss of the gate is the smallest, if the cracks are mainly near the gate, the multi-point distribution point gate, side gate and handle gate can be considered.
(2) on the premise that the resin does not decompose or degrade, properly increasing the resin temperature can reduce the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
(3) generally, when the mold temperature is low, it is easy to produce stress, so the temperature should be increased appropriately. But when the injection speed is higher, even if the mold temperature is lower, the stress can be reduced.
(4) too long time of injection and holding pressure will also produce stress, so it is better to shorten it properly or switch holding pressure for th time.
(5) non crystalline resin, such as as as resin, ABS resin, PMMA resin and so on, is more likely to produce residual stress than crystalline resin, such as polyethylene, polyformaldehyde and so on. When demoulding and pushing out, due to the small demoulding inclination, rough mold glue and punch, the pushing out force is too large, resulting in stress, and sometimes even whitening or cracking around the pushing rod. As long as the location of crazing is carefully observed, the cause can be determined.
It is easy to produce stress and crack after a period of time when the metal parts are embedded at the same time of injection molding. This is mainly due to the great difference between the thermal expansion coefficient of metal and resin, which leads to stress, and with the passage of time, the stress exceeds the strength of the resin material which is gradually deteriorated, resulting in cracks. To prevent the resulting cracking, as an experience, the wall thickness is 7 “with the outside diameter of the embedded metal part
In general, polystyrene is not suitable for inlay, but inlay has the least effect on nylon. Because the coefficient of thermal expansion of glass fiber reinforced resin material is small, it is suitable for embedded parts.
In addition, preheating the metal insert before forming has better effect.
(II) cracks caused by external stress
The external stress here is mainly caused by the unreasonable design, especially at the sharp corner. According to figure 2-2, R / 7 “- 0.5 ～ 0.7 can be taken.
(III) external environment
Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials will degrade physical properties and produce cracks.
About JasonMould Industrial Company Limited
Jasonmould is a China mold maker of plastic molds- injection mold, die casting moulds, plastic blow molding, rotational molding, medical plastic injection molding, two shot plastic injection molding, insert molding, overmolding, metal injection molding, micro injection molding, powder injection molding, ceramic injection molding, liquid injection molding, husky injection molding, household mold, casting mold, die mold tool, custom molds, china moulds, rapid prototyping tooling, plastic prototyping tooling, punch press tooling, die and tooling for mobile/ cell phone parts, automotive parts, vacuum cleaners, rechargeable tools, telephones, copiers, computers, multimedia speakers, and many other electronic products and household appliances. And also a plastic product manufacturer, mold manufacturer China– plastic parts, plastic water tank, plastic balls, plastic containers, plastic buckle, plastic anchor, plastic hanger, plastic spoon, plastic pipe fitting, plastic tumble, plastic tableware, plastic cups, plastic bottles, plastic tray, plastic cosmetic container, plastic case, plastic food container, plastic chairs, plastic caps, plastic cap closure, plastic tubes, plastic water pipes, plastic knobs, plastic tubing, plastic utility boxes, plastic racks and so on.
Person: James Yuan
Company: JasonMould Industrial Company Limited
Add: LongGang Village,LongXi Town,BoLuo County,HuiZhou City,GuangDong Province, China
Email: [email protected]