How to control injection molding part size

Mold and product requirements are continuously changing, which has created many difficulties in the production process of molds and products. The challenge lies in determining the best method of controlling the geometric dimensions of the mold and product.

Several control techniques and methods are available for controlling dimensions for various types of molds and products. For my own purposes, in my case, I begin the process from the following aspects.

• Mold Structure

1. Before everything else, it will be significant to completely be aware of the specialized specifications of the plastic mold construction, steel, hardness, accuracy, and so on, including the shrinking amount of the resin, Three-dimensional measurement, and producing an evaluation.

2. Completely think about the shrinking, flow mark, draw direction, weld line and fracture of molded items.

3. Make ease of the mold creation if possible devoid of limiting the performance of the injection product and the structure.

4. Whether or not a parting surface is acceptable needs to be cautiously picked out for die developing, developing the overall look and deburring established parts.

5. Whether or not the ejection strategy is suitable, regardless of whether ejection pins, stripper plate, ejection sleeve, or other approaches are implemented.

6. Whether the slider mechanism is acceptable. The motion is manageable and trustworthy. There shouldn’t be any phenomenon of injected parts sticking to the cavity or core side for hard ejection.

7. Gate sort, the size of the runner and cold slug, gate position and measurement.

8. All types of parts and heat treatment for steel deformation and standard components choice.

9. Whether the injection volume, injection force and clamping strength of the injection molding equipment are enough, and whether the nozzle R and the mold sprue aperture are coordinated correctly.

And stuff like these factors proceeds the thorough evaluation groundwork, from which the item’s first phase needs to be totally managed.

• Management of Process Making

Even though 100 % of aspects to consider and set up are stated in the design phase, many troubles and challenges may also arise in genuine manufacturing.

We ought to go with our best to comply with the initial structure intention being produced and discover far better, cost-effective, and practical technical ways to produce exact produce.

1. Choose economically appropriate equipment for Two dimensional and 3-dimensional production.

2. Some manufacturing approach prerequisites and options may well also be regarded, like suitable tooling and accessories for reliable groundwork in production, acceptable usage of chopping resources, avoidance of deformation of components, avoidance of change of the shrinking amount of parts, avoidance of ejection deformation of parts, enhancement of mold making precision, lowering of mistakes, avoidance of alterations in mold precision, and so on.

3. the causes for the dimensions mistake of the molded components and the distribution of its portion.

A: mold making mistake is around 35%,
B: mistake as a result of mold wear is focused on 15%
C: problem because of unequal shrinking of molded components is around 35%,
D: mistake because of irregular set shrinkage and real shrinking is around 15%

The whole error =A+B+C+D; therefore, it is clear that the production tolerance of the mold ought to be under 1/3 of the perspective tolerance of the molded component. If not, the mold is hard to guarantee the geometric measurement of the produced component.

• Generally manufacturing handle

Geometric variation of plastic components after being ejected is a typical issue and a recurrent occurrence:

1. Management of resin temperature and mold temperature

Various resin needs various temperature specifications. Using materials with negative fluidity and over two types of blends may have various conditions. Resins need to be manipulated with the ideal movement value variety.

These are generally simple to accomplish. However, mold heat management is even more challenging. Various geometric patterns, dimensions and wall thickness rates of diverse established components possess the prerequisites for cooling strategy. Mold temperature determines the cooling period to a substantial degree.

Consequently, the mold needs to be maintained in a cold condition to reduce the injection time and enhance manufacturing performance.

In the event the mold temperature alters, the shrinkage amount may also alter, the mold environment will stay steady, and the sizing precision is going to be steady, therefore avoiding the established parts-flaws like deformation, lousy gloss, cooling places and so on and producing the actual attributes of the plastic in the perfect condition.

Obviously, you can also find a debugging approach. Particularly the multi-cavity molded components tend to be more complex.

Consequently, it shouldn’t alter the mold temperature as you desire during the manufacturing process. The mold temperature configuration ought to be in the advised temperature array of the resin.

2. Correction and management of pressure and venting

The appropriate shot pressure and the coordinating of clamping pressure ought to be decided when testing.

The air in the space produced due to the mold cavity and core and the gas developed by resin should be released from the mold through the exhaust slots.

If the venting is not enough, an inadequate filling will arise, leading to weld scars or can burn.

These 3 types of molding imperfections occasionally show up at the exact component. If you find a thicker wall surrounding the thin wall section of the part, shrinking symbolizes will show up if the mold temperature is lacking, and burns will show up if the mold temperature is excessive.

Generally, weld marks will show up at the burnt section simultaneously, and the vent slot will be overlooked and usually in a smaller state.

As a result, under typical situations, providing no flash is produced, the deepness of the vent slot needs to be as big as feasible.

A vent slot with a bigger dimension is cut right behind the cold slug so that it could rapidly dismiss the gas moving past the cold slug away from the mold.

Should there be a particular demand, and vent slot is cut over the ejector pin. That takes the equivalent. Initially, no burr arises, and secondly, the vent slot performs properly.

3. Secondary Control of Molding Part

Because of the distinction in the configuration of some plastic components and the difference of temperature and pressure reduction when ejection, deformation will arise in those various circumstances.

At the moment, a few additional jigs and accessories can be employed for correction, and it could consider restorative steps immediately after the molded components are removed from the mold. Excellent adjustment consequences are obtainable once they are cooled down and fixed by natural means.

If some exact operations are ascertained during the entire injection molding approach, the measurements of the molded components are going to be manipulated flawlessly.

Geometrical fluctuation after ejection

• Control of material temperature and mold temperature.

Different brands of plastic must have different temperature requirements. The use of plastic materials with poor fluidity and more than two mixtures will have different situations. Plastic materials should be controlled within the optimal flow value range.

These are usually easy to do, but mold temperature control is more complicated. Different geometric shapes, sizes and wall thickness ratios of different formed parts have certain requirements on the cooling system. Mold temperature controls cooling time to a large extent.

Therefore, the mold should be kept in an allowable low-temperature state as far as possible to shorten the injection period 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 be stable, thus preventing defects such as deformation, poor gloss, cooling spots and the like of the formed parts and making the physical properties of the plastic in the best state.

Of course, there is still a debugging process, especially the multi-cavity mold-formed parts that are more complicated. Therefore, the mold temperature cannot be changed at will in the production process. The setting of the mold temperature should be within the recommended temperature range of the material.

• Adjustment and control of pressure and exhaust

The proper injection pressure and the matching of clamping force should be determined when debugging the mold. The air in the gap formed by the mold cavity and core and the gas generated by plastic must be discharged out of the mold from the exhaust groove. If the exhaust is not smooth, the insufficient filling will occur, resulting in weld marks or burns.

These three kinds of forming defects sometimes appear at the same part from time to time. When there is a thick wall around the thin wall 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, weld marks will appear at the burned part simultaneously, and the vent groove will be neglected and generally in a small state. Therefore, under normal circumstances, as long as no burrs are generated.

The depth of the vent shoulder should be as deep as possible. A vent slot with a larger size is opened at the rear of the shoulder so that the gas passing through the shoulder can be quickly discharged out of the mold. If there is a special need, a vent slot is opened on the ejector rod. The reason is the same. First, no flash occurs, and second, the vent slot can work well.

• Supplementary Shaping Control of Injection Molding Part Size

Due to the difference in shape and size of some plastic parts, the change of temperature and pressure loss after demoulding, deformation and warpage will occur under different conditions. At this time, some auxiliary jigs and fixtures can be used for adjustment, and remedial measures can be taken promptly after the formed parts are taken out of the mold, and good correction and adjustment effects can be obtained after they are naturally cooled and set. If strict management is ensured in the whole injection molding process, the size of the injection molded parts will be controlled perfectly.