How to control the size of injection molded products?

Control of mold design

(1) First of all, the structure, material, hardness, hardness, precision, and other requirements of multiple users are fully understood.

(2) Fully consider the places that affect the appearance of injection molding products such as shrinkage cavity, flow mark, draft angle, weld line and crack.

(3) The processing method of the mold can be simplified as far as possible without dismantling the product function and graphic production of the injection molded parts.

(4) Whether the selection of the parting surface is appropriate, the mold processing, molding appearance, and deburring of the injection molded parts must be carefully selected.

(5) Whether the ejecting method is appropriate, whether the pushing rod, cushioning plate, ejecting sleeve or other methods are used, and whether the position of the pushing rod and the cushioning plate is appropriate.

(6) Whether the use of the side core-pulling mechanism is appropriate, the action is flexible and reliable, and there should be no stagnation.

(7) Which method of temperature control is more suitable for plastic products, which structural circulation system is used for temperature control oil, temperature control water, coolant, etc., and whether the size, quantity, and location of coolant holes are appropriate.

(8) The form of the gate, the size of the material channel and the feed port, and whether the position and size of the gate are appropriate.

(9) The influence of heat treatment deformation of various modules and mold cores and whether the selection of standard parts is appropriate.

(10) Whether the injection volume, injection pressure and clamping force of the injection molding machine are sufficient, whether the nozzle R, the hole diameter of the sprue sleeve, etc. are properly matched.

The comprehensive analysis and preparation of these aspects should be strictly controlled from the initial stage of the product.

The control of process manufacturing

Although we have fully considered and arranged it in the design stage, there will still be many problems and difficulties in actual production.

We must try our best to meet the original intention of the design in production, and find out more effective, more economical, and more reasonable in actual processing.

(1) Choose economical and adaptable machine tool equipment for 2D and 3D processing solutions.

(2) Appropriate tooling and fixtures can also be considered for auxiliary preparations in production, and the reasonable use of cutting tools can prevent product parts from being deformed, prevent product parts from fluctuating in shrinkage rate, prevent product parts from being demolded and deformed, improve the precision of mold manufacturing, reduce small errors, preventing changes in mold precision, etc., a series of production process requirements and solutions.

(3) Here are the reasons for the dimensional error of the formed parts of the British Plastics Association (BPF) and the distribution of the proportion:

A: The mold manufacturing error is about 1/3, B the error caused by mold wear is 1/6C the error caused by the uneven shrinkage rate of the formed part is about 1/3, and the error caused by the inconsistency between the predetermined shrinkage rate and the actual shrinkage rate is about 1/3 1/6
the

The total error = A+B+C+D, so it can be seen that the manufacturing tolerance of the mold should be less than 1/3 of the dimensional tolerance of the formed part, otherwise it is difficult for the mold to guarantee the geometric size of the formed part.

Usually production control

Fluctuation in geometric dimensions after plastic parts are formed is a common problem, and it is a phenomenon that often occurs:

(1) The control of material temperature and mold temperature

Different brands of plastics must have different temperature requirements. Plastic materials with poor fluidity and the use of more than two kinds of mixtures will have different situations.

Plastic materials should be controlled at the best flow These are usually easy to do within the value range, but the control of mold temperature is more complicated.

Different geometric shapes, sizes, and wall thickness ratios of different formed parts have certain requirements for cooling systems. The mold temperature is largely controlling the cooldown time;

Therefore, try to keep the mold at an allowable low temperature to shorten the injection molding cycle and improve production efficiency.

If the mold temperature changes, the shrinkage rate will also change, the mold temperature will remain stable, and the dimensional accuracy will also be stable. Defects such as deformation, poor gloss, and cooling spots make the physical properties of the plastic in the best state.

Of course, there is a debugging process, especially since the multi-cavity molded parts are more complicated. Therefore, the mold temperature cannot be changed arbitrarily during the production process, and the setting of the mold temperature should be within the recommended temperature range of the material.

(2) Adjustment and control of pressure and exhaust

The injection pressure is appropriate and the matching of the clamping force should be determined when the mold is debugged. The air in the gap formed by the mold cavity and the core and the gas generated by the plastic must be discharged from the mold through the exhaust groove, such as exhaust If the air is not smooth, there will be insufficient filling, resulting in weld marks or burns;

These three forming defects sometimes appear in the same part from time to time. When there is a thick wall around the thin-walled part of the formed part, shrinkage marks will appear when the mold temperature is too low, and burns will appear when the mold temperature is too high.

Usually, there will be welding marks at the burn site at the same time, and the exhaust grooves are often ignored and are generally in a small state, so in general, as long as no burrs are produced; Larger ventilation slots, the gas passing through the shoulder can be quickly discharged out of the injection mold.

If there is a special need, an exhaust slot should be opened on the ejector pin. The reason is the same. First, there is no flash, and second, the air is released As long as it works well, it will do.

The depth of the exhaust shoulder should be as deep as possible, and a large-sized ventilation groove is provided at the back of the shoulder so that the gas passing through the shoulder can be quickly discharged out of the injection mold. The reason is the same, one is that there is no flash, and the other is that the air can be released quickly and the effect can be well achieved.

(3) Supplementary shaping control of injection molded parts size

Due to the difference in shape and size of some plastic parts, deformation and warping will occur in different situations with the change of temperature and pressure loss after demoulding.

At this time, some auxiliary fixtures can be adjusted promptly and quickly take remedial measures, and after it is naturally cooled and shaped, better correction and adjustment effects can be obtained. If strict management is ensured throughout the injection moulding process, the size of the injection molded parts will be ideally controlled.