Shortening rate

When the plastic forming machine is cooled from the mould to room temperature, the function of standard shortening occurs, which is called shortening. Because shortening is not only the thermal expansion and cold contraction of resin itself, but also related to various forming elements, the shortening of plastic parts after forming should be called forming shortening.

1. Forming shortening is mainly embodied in the following aspects:

(1) The shortening of the line standard of plastic parts is due to thermal expansion and contraction, elastic recovery and plastic deformation during demoulding, which results in the reduction of the standard of plastic parts after demoulding cooling to room temperature. Therefore, it is necessary to consider compensation in cavity planning.

(2) When the molecule is placed in the direction of shortening directional forming, the anisotropy of the plastic parts will appear, while along the direction of material flow (i.e. parallel direction), it will shorten greatly and have high strength, while in the right direction of material flow (i.e. straight direction), it will shrink low and low strength. Otherwise, due to the uneven distribution of density and fillers in different parts of the plastic parts, the shortening is uneven. Shortening difference makes plastic parts easy to warp, deform and crack, especially in extrusion and needle forming. Therefore, the shortening direction should be considered in the die planning. It is appropriate to select the shortening rate according to the shape of plastic parts and the direction of material flow.

(3) When plastic parts are formed after shortening, a series of stress effects are caused by forming pressure, shear stress, anisotropy, density unevenness, filler dispersion unevenness, mold temperature unevenness, hardening unevenness, plastic deformation and other factors, which can not all disappear in viscous flow state, so there are residual stresses in plastic parts forming under stress. When the stress tends to balance and storage conditions affect after demoulding, the plastic parts will be shortened again due to the change of residual stress, which is called post-shortening. Generally, the plastic parts are changed greatly within 10 hours after demoulding, and the base is finalized after 24 hours, but the final stability will take 30-60 days. In general, the post-shortening of thermoplastics is larger than that of thermosetting, and the extrusion and injection moulding are larger than that of compression moulding.

(4) Post-processing shortening sometimes requires heat treatment after forming according to the functional and technological requirements. After treatment, the standard of plastic parts will be changed. Therefore, the fault of post-shortening and post-processing shortening should be considered and compensated for in die planning for high precision plastic parts.

2. Shortening Rate Accounting Shortening of Plastic Parts Forming can be marked by Shortening Rate, as shown in Formula (1-1) and Formula (1-2).

(1-1) Q real=(a-b)/b*100

(1-2) Q meter=(c-b)/b*100

In Formula Q: Real-Practice Shortening Rate (%)

Q Accounting - Accounting Shortening Rate (%)

A-Unidirectional Standard for Plastic Parts at Forming Temperature (mm)

B-Unidirectional Standard for Plastic Parts at Room Temperature (mm)

C-Mould Unidirectional Standard at Room Temperature (mm)

The practice shortening rate indicates the shortening of plastic parts practice. Because the difference between the value and the accounting shortening is very small, the Q-meter is used as the planning parameter to calculate the standard of cavity and core in die planning.

3. The factors affecting the change of shortening rate are not only different kinds of plastics, but also different batches of the same kind of plastics or different parts of the same plastic parts. The main factors affecting the change of shortening rate are as follows.

(1) Various kinds of plastics have their own shortening plans. The shortening rate and anisotropy of the same kind of plastics are different due to the different fillers, molecular weight and proportion.

(2) The shape, standard, wall thickness of plastic parts, whether or not they have inserts, and the number and layout of inserts also have a great impact on the reduction rate.

(3) The parting surface and pressure direction of die structure, the method of gating system, layout and standard also have great influence on the shortening rate and direction, especially in extrusion and injection molding.

(4) The extrusion and needle-punching processes of the forming process generally have a larger shortening rate and obvious orientation. Preheating condition, forming temperature, forming pressure, holding time, filling method and hardening uniformity all affect the shortening rate and orientation.

As mentioned above, the shortening rate should be planned according to the instructions of various plastics, and the selection of the shortening rate should be considered in general according to the shape, standard, wall thickness, insert condition, parting surface and pressure forming direction, die structure, method standard and orientation of inlet and outlet, forming process and other factors. For extrusion or injection molding, different shortening rates are often selected according to the shape, standard and wall thickness of the parts.

Otherwise, forming shortening is also affected by various forming elements, but it is mainly determined by the type of plastic, the shape and standard of plastic parts. Therefore, adjusting the forming conditions during forming can also properly change the shortening of plastic parts.

Activity

The ability of plastics to fill the cavity at relative temperature and pressure is called mobility. This is an important process parameter to be considered in die planning. Mobility is easy to constitute excessive spills, filling cavity is not compact, the structure of plastic parts is loose, resin and filler are accumulated separately, easy to stick, demoulding and finishing difficulties, early hardening and other disadvantages. However, if the activity is small, the filling is scarce, and the forming pressure is high. Therefore, the activity of plastics should be adapted to the requirements of plastic parts, forming process and forming conditions. The gating system, parting surface, feeding direction and so on should be considered in the die planning according to the active function. Thermosetting plastics are generally marked by Lasiger activity (in millimeters). The numerical value is large and the activity is good. Each kind of plastics can be divided into three different levels of activity, which can be used for different parts and forming processes. Generally, plastic parts have large area, many inserts, slender core and inserts, narrow deep grooves and thin walls.