Multi-material injection molding

Bi-color injection, as its name suggests, is a component that mold in two plastics. Generalized Multi-material injection molding includes over molding, which realized by ordinary injection molding machine and Multi-material injection molding. Narrow Multi material injection molding refers to injection molding of two different plastics on same machine by a Multi material injection moulding machine. Common one is rotary type. The former does not require high equipment, but production efficiency is low. It is basically only suitable for molding of two materials, such as soft and hard rubber. The latter’s scope of application and product quality are good, production efficiency is high, which is current trend.

We mainly introduce Multi-material injection molding with two color injection molding machine. So we first know from 2 shot injection molding machine and the two-shot injection mold.

Movable molds of two molds can be fixed on the top, and there are two sets and a top rod. After mold opened, second shot movable mold product is first eject, robot takes out product. Then movable molds of two sets of molds fixed above rotate, thereby placing first shot finished product on other side, preparing a second injection, and first shot moving mold side replaced with an empty one. From time to time, product continues to injection molded. Point to note is that during first injection, second shot fills entire cavity, paying attention to demolding. Of course, injection molding machine has two tubes on fixed mold side, which are respectively injection molded with different plastics.

Multi-material injection molding has its own characteristics

  1. Moveable mold is same, because it is rotating and needs to exchanged, so it must be exactly same. Therefore, first molding that formed on the side of movable mold (there is a special case, and cavity may be different when molding).
  2. After first shot of mold finished, it cannot left on mold, preferably hot runner, otherwise it can be grabbed by robot. The worst is latent gate, which can automatically fall off. Otherwise gate will be on the top and second shot will not applied to mold.
  3. Product is shot twice. After product opened, the first shot must left on movable mold side. This is better understood. If it left on fixed mold side, rotation will be useless and it will become a monochrome injection molding machine. In general, it chosen to leave two shots on movable mold side, so mold needs a shutter, pull rod to achieve sequential mold opening. First pull fixed mold side, and add ejector mechanism on fixed mold side.
  4. Two sets of cooling systems, because requirements of 2 shot injection molding are different for two materials. In principle, temperature of first shot material required to be 60 degrees higher than second shot, determined according to actual situation. Because materials are different, mold temperature is different, so two sets of cooling systems are required.

Multi-material injection molding product design points

After understanding injection molding machine and mold, we will talk about main points of two-color product design. Presence or absence of transparent materials in two color injection molding is completely different. Let us start from simple point of view, combination of hard glue and soft glue, in mobile phone application is to do plug and so on.

Soft and hard, plastic two-color design

  1. There must be a certain temperature difference between two materials. General recommended value is 60 degrees. Depending on specific conditions, it recommend to ensure a temperature difference of at least 30 degrees. Melting temperature of the first shot material is higher, otherwise second shot material can easily melt first shot material. For this type of injection molding, itis easy to achieve, generally first shot is PC or PC / ABS, second shot is TPU or TPE. Thickness of PC needs to reserved 0.6-0.7mm, and soft rubber  reserved for 0.4mm or more.
  2. Maximize contact area, do some grooves, increase adhesion, avoid delamination and cracking.Also consider using core in the first shot and injecting second shot into first shot,increasing reliability of fit; surface of first shot mold is as rough as possible, so that mold manufacturer can use electrode rough processing on the surface of fit surface.
  3. Small parts improve molding efficiency, try to consider first shot hot runner, mold can be made smaller, latent gate material is flying, there is a set of mold next to it,and it is easy to have problems. Efficiency of three-plate mold is too low, and material is wast, depending on mold manufacturer’s conditions.

Transparent and non-transparent two-color design

Next is combination of a transparent material and a non-transparent material that can used as a decoration or simply as a small screen lens.

1, two-color design of small screen lens

Among them, small lens is relatively simple, mainly used in clamshell shell of clamshell machine, and machine has a sub-screen. It is necessary to use first shot as a non-transparent one, and second shot as a lens. Point of attention is to try to open temperature of product material. The first shot should use PC, preferably PC of SABIC, melting point is high, and second shot is PMMA. Key point is that PMMA will flow from PC at the gate, so do not melt PC. When movable mold of 2 shot injection molding mold injected, dynamic model cavity is different. Ejector rod on fixed mold side used to open cavity of movable mold side to form a small lens cavity to form small lens.

2, decorative transparent and non-transparent two-color design

As decoration is also the first injection molding non-transparent material, second injection molding transparent material. Non-transparent material used in SABIC PC. Material temperature is higher, second shot transparent material selects PMMA, or warm PC. Depending on needs of project, PC needs to spray with UV to protect it, PMMA can choose UV or strengthen. If there are characters on the surface, you must choose UV.

This Multi-material injection molding, total thickness of general material needs to be more than 1.5mm, thickness of transparent part is at least 0.8mm. If thickness is too thin, there is no depth, can not achieve expected effect. And thin injection molding defects will be obvious, thickness of non-transparent parts needs to be above 0.7mm.

Material color of non-transparent parts should be light-color as much as possible. Dark color, especially black, will absorb light, and there is no feeling of depth, which is similar to feeling of spraying UV with deep black material. This needs attention.

For large-area transparent parts, it is necessary to pay attention to fact that holes are as small as possible. After two color injection molding, side is looking sideways, and transparent distance is long and difficult to see.

Generally, transparent member made thin, and transparent member only made 0.5 mm at top end. Curved surface connect smoothly, and chamfering can not simply performed.

Because thickness of non-transparent parts limited, structure of casing is as simple as possible. Try to make ribs on shell thin as much as possible. If possible, ensure that wall thickness of casing is below 0.8mm, which is almost 0.5-0.6mm, to avoid shrinkage.

3, Edge of shell

At the edge of shell, when it is impacted by high and low temperature cycles, it is easy to fall off or crack, and it looks like glass break.

You need to make transparency slightly thinner on the side. If you don’t mind appearance, you can wrap transparent ones with a non-transparent one, or a little step, and lightly collect dots.

Because moving molds shared, splitting lines of two shots need to be on same surface. Simply speaking, it is not feasible to round bottom surface of casing before, and splitting surfaces of two shots will be inconsistent.

In addition, pay attention to gate and gate processing. The first choice for gates of transparent materials is to extend transparent material to non-design surface. Disadvantage is that there is no obstruction in a small transparent area (can be blocked by Mylar or pen), matte Or matte surface is not obvious. Advantage is that it will not be cut because gate is not good. Appearance of surface has shiny glass. At present, there are also gates with side gates on both sides. After injection molding, it processed by CNC milling machine, but this is processing after cooling, which has an influence on tightness of two pieces, needs to tested and verified.