What comes to mind when you hear the term surface quality of molded products? Problems and issues such as low yield due to poor molding, difficult surface treatment before painting and plating, various conditions have been changed, but it is difficult to improve further, and the required quality is increasing year by year. I think it varies.

In previous columns, we have introduced measures to improve defects, color unevenness, and sink marks that appear on the surface of molded products. Defects that are difficult to improve include weld lines, which are marks like line scratches that appear at the part where the resin merges in the molded product. In addition, in order to improve the strength and reduce the weight of molded products, technologies such as adding fillers and foaming have been introduced. has become

In this column, we will introduce the details of each defect, the improvement method (molding method) that achieves a high surface quality that could not be achieved with normal molding, and the mechanism for improving the defect.

Molding defects appearing on the surface

First, let's list some types of defects that can be improved by the molding methods introduced in this column.

weld line

A "weld line" is a thin line mark that appears in the part where the resin merges in the molded product. This phenomenon occurs when resin is injected into a mold, for example, in a molded product with an opening, it branches off at the position of the hole and merges again, or resin injected from multiple locations collides with each other at a multi-point gate. This is due to matching movements. This phenomenon always occurs in molded products with openings or multi-point gates, although the degree of appearance varies.

If strength is required, the weld line will be a weak point. From the viewpoint of surface quality, there is no problem if it can be hidden with paint. may be

Poor transfer

Molded products with a leather-like embossed pattern or mirror-like gloss are often used for the purpose of producing a high-class feeling with molded products such as automobile interior parts and home appliances. These designs are realized by transferring the designs on the mold surface. Improper transfer means that the fine patterns, embossed textures, and mirror surface conditions envisioned when designing and manufacturing the mold are not properly transferred to the molded product, and the intended expression is not achieved.

If the embossed pattern is not transferred correctly, the edges will not be reproduced, and the molded product will have an overall shine. In addition, if the mirror surface is poorly transferred, the degree of gloss is low, the condition of the molded surface is not uniform, and the reflection (reflection) is uneven.

Filler expression

When molding with resin mixed with filler (glass fiber, carbon fiber, etc.) to improve the strength of the molded product, the filler appears on the surface of the molded product, making it look rough and not smooth. There is a thing

Even if the filler is exposed, there is no problem with the shape or strength, so there are many cases where it is used as it is as a mechanical part or a part that is not visible from the outside, but it is treated as a defective part as an external part.

Orientation in metallic molding

Until now, metallic paint and plating have been commonly used to express metal-like textures in plastic products. However, in recent years, from the viewpoint of health hazards and environmental problems, efforts are being made to reduce or even eliminate the painting process. For this reason, metallic molding that achieves a metal-like texture without painting is attracting attention, and many companies involved in molding and resin material manufacturers are conducting research.

Metallic molding is a molding method in which fine aluminum flakes are mixed with resin and molded to give a metallic luster without the need for painting or plating. However, at parts such as the openings, ribs, bosses, etc. of the molded product, where the resin collides with each other and the flakes are aligned in the same direction, the flakes are dented even though there is no superficial deformation. or the striped pattern may look uneven or wavy. This "orientation" is regarded as defective in terms of surface quality in metallic molding.

How to solve defects that appear on the surface of molded products

The four defects introduced so far are examples that are difficult to improve simply by changing materials or changing molding conditions. Even with these defects, many customers continue molding with a low yield in order to meet the quality requirements of the delivery destination. If the molding temperature is set higher, it can be improved to some extent, but the higher the temperature, the longer it takes to cool down, resulting in a longer cycle time and lower production efficiency.

Therefore, we propose heat & cool molding as a means to solve the problems and issues that appear on the surface of this molded product, and as a method to dramatically improve molding quality.

What is heat & cool molding?

Heat & cool molding is a technology suitable for improving the surface quality of molded products. This is a molding method that increases the fluidity of the resin. By increasing the fluidity of the resin, it leads to the resolution of various problems that have led to defects. In addition, rapid cooling suppresses the extension of the cooling time, and uniform cooling suppresses deformation after molding, which leads to Improve Dimensional Precision.

* When heat & cool molding is introduced, the cycle time tends to be longer than in normal molding, but in some cases, such as thick molded products, the rapid cooling effect leads to shortened cycle time.

Appearance defects improved by heat and cool molding

From here, we will introduce the mechanism of how heat and cool molding works and improves the appearance defects mentioned above.

Weld line improvement

Heat & cool molding is also called "weldless molding" because it was originally developed to eliminate weld lines. By raising the temperature of the molding surface of the mold to a temperature range near the glass transition point of the resin and injecting in that state, the fluidity of the resin increases, making it possible to achieve surface quality at a level where the weld line cannot be seen.

*Actually, the physical movement of the resin colliding around the opening of the molded product does not change, and it does not fuse at that position, so the weld line exists internally. However, the result is a clean, uniform surface that is indistinguishable.

Improving poor transfer

If the molded product is molded with a mold that has been textured on the molding surface and the transfer is defective, the surface that should have a matte finish will have a worn-out luster. This is because the injected resin hits the mold surface and the temperature drops, and solidification begins before the resin flows into the deep part of the pattern, resulting in a smoothed edge.

With heat and cool molding, the injected resin penetrates deep into every corner, so the grains and fine patterns carved into the molding surface are faithfully transferred. As a result, the intended design is accurately reproduced on the surface of the molded product. In addition, since the surface is transferred as it is in the mirror finish, the mirror gloss that was previously created by secondary processing such as painting can be reproduced in the unpainted state.

Piano black (*) refers to the glossy black surface of moldings such as the center consoles and instrument panels of automobiles, the outer frame of car navigation systems, and the outer frames of home appliances such as televisions. .

*Piano black: Meaning of black with brilliance and depth like that of a piano case

Improved filler appearance

The "exposed" filler refers to the fact that additives such as glass fiber, which are mixed with the resin to give strength, heat-resistant, and various resistances, are visible on the surface of the molded product.

In normal molding, when molten resin containing filler is injected into the mold, the fountain flow (*1) movement of the resin causes the filler to pop out from the flow front (*2) and press against the mold wall. Because it hardens, it remains visible on the surface.

Molding this with heat and cool molding gives a very good surface quality. Mechanistically, the temperature of the mold wall is high, the resin around the pressed filler is still loose, and the resin covers the filler due to the internal pressure of the mold, resulting in a high-quality surface without the filler being exposed. increase.

*1 Fountain flow: When the resin is injected into the mold, it cools when it hits the surface of the mold and loses its fluidity.

*2 Flow front: The tip of the resin that is injected into the mold and flows into it.

Improved orientation

The flakes used in metallic molding are flakes rather than spheres, so they align along the flow of the resin so that they can be reflected diffusely and look glittery. Even if the surface of the molded product is in good condition, this "orientation" is visible, so the color may appear uneven, and parts with ribs or bosses on the back may appear to be recessed.

This orientation is actually one of the problems that is difficult to improve even with heat and cool molding, but it has become possible to improve it by combining it with another technology. I can't write the details because it is a joint development technology with a partner company, but we use a high-temperature type heat and cool device that uses an oil medium and a special mold to control the injection and move the resin inside the mold. to change the orientation of the aligned flakes, so that the orientation can be erased.

Important points when introducing heat & cool molding

The water pipes of the mold used in normal molding are premised on maintaining a constant molding temperature, so in most cases, the water pipes are passed between the molding surface and the platen fixing surface. On the other hand, in heat & cool molding, it is desirable that the temperature control water pipe is located close to the molding surface in order to quickly raise and lower the temperature of the molding surface.

When installing a water pipe near the molding surface, if the product has a simple shape, it can be set with a straight water pipe. It is difficult to set up a water pipe.

That's where 3D metal printers come in. By sintering and layering metal powder with a laser, water tubes can be formed three-dimensionally, so even for molded products with complex shapes, water tubes can be provided along the molding surface of the mold. When mass-producing molded products using heat and cool molding, it is advisable to consider adopting this dedicated 3D water tube mold (*In the case of steam molding, a dedicated mold is essential).

An important point to keep in mind when introducing heat and cool molding is the management of the water quality of the circulating water used for temperature control. If the water quality is poor, scale will form in the cooling water path and the temperature control efficiency will drop, so not only will the effect of heat & cool molding decrease, but eventually the water pipes will become clogged and the cooling water will not flow, causing damage to the temperature control equipment. It will also lead to failure of

It can be said that maintaining the initial state of this water pipe is especially important when adopting a 3D water pipe mold. In the case of small molded products such as precision molded products, if the water pipe is thin and has a complicated shape, it will not be possible to repair it if it becomes clogged. In order to maintain this initial state, regular maintenance using a water tube cleaning device is required.

For more information on water quality management, please refer to the column titled "Achieving stable production through water quality control of cooling towers and cooling water.."

Summary

Heat & cool molding not only reduces and improves the appearance of molded products, but also brings a higher level of molding quality that cannot be achieved with normal molding. Producing molded products that can be used without painting by improving transferability has significant advantages not only in terms of cost but also in terms of environmental friendliness, and further demand is expected in the future. Improving the quality and value of molded products can be said to be an effort to improve the value of the molding factory.

MATSUI 's mission is to "realize 'factor4' in molding factory", and we propose various solutions to improve the resource productivity of molding factory and improve the added value of molded products and molding factory. Please feel free to contact us.