1. Field of the Invention
The present invention relates to a release agent for metallic molds used for forming plastic molded products.
2. Description of the Related Art
Plastics have excellent properties such as easy processability, high productivity, light weight and relative low procuring costs, so they are used for the parts and structural materials for automobiles, autobicycles, scooters, televisions, radios, audio equipment, washing machines, rice cookers, personal computers, portable telephones, game machinery, building materials, office supplies, stationery, toys, sports goods, sports equipment, agricultural tools and marine tools.
They are usually prepared by plastic processing methods such as injection molding, blow molding, compression molding, transfer molding, rotating molding, slush molding, inflation tubular film processes, and extrusion molding.
As plastics processing are conducted at high temperature above 200° C., additives, monomers, decomposed materials in the plastics are changed to carbonized materials such as tar, pitch and other colored sticky substances, which are apt to adhere to the surfaces of the screw, barrel, die and metallic mold of the plastic processing machine to cause transfer of the carbonized materials to the molded products.
Above mentioned transfer makes the surfaces of molded products dirty and makes the dimensions of the molded product inaccurate. As a result, the molded products do not perform required movement or structural functions. Therefore cleaning the screw, barrel, die and metallic mold of plastic processing machines has been required.
In case of production changes from specific colored molded articles to noncolored or different colored molded articles, cleaning the screw, barrel and die has been required to avoid cross contamination caused by residual specifically colored resin compounds.
However, there are some problems associated with conventional methods of cleaning the screw, barrel, die and metallic mold.
There are problems associated with releasing plastic molded products from metallic molds.
Metallic molds used for forming plastic molded products have complicated shapes with fine hollow and convex parts.
Therefore, plastic molded products such as mechanical parts for electronic devices and automobiles have complicated shapes. Consequently it is difficult to release plastic molded products from metallic molds, because both surfaces have complicated contrasting shape, which causes the plastic mold products to ingress into portions of the metallic mold and become anchored thereto.
To avoid above mentioned difficulty of releasing both surfaces, there have been efforts to treat the interface between the metallic mold and the plastic molded product.
The method of treating the interface involves using a lubricant as a release agent for metallic molds.
The lubricant is usually applied by coating it onto the metallic mold surfaces.
However, when a metallic mold that is coated with a conventional release agent is used for a long time, a hard and uneven heterogeneous layer is formed because of the following reasons.
One reason is the accumulation of resin and its additives such as antioxdants, metal deactivaters, nucleating agents, anti-ultraviolet agents, antistatic agents, crosslinking agents, valcanization agents and lubricants.
The other reason is the accumulation of decomposition materials of resin and its additives.
The uneven shape on the surface of metallic mold caused by the heterogeneous layer is apt to be transferred to the surface of the plastic molded product together with the accumulated contaminants from the surface of the metallic mold which will adhere to the surface of the plastic molded product.
Above mentioned transferring of shape makes the dimensions of the plastic molded product inaccurate so that it will not perform required movement or structural functions, and adhering of contaminants makes the surface of the plastic molded product dirty.
The hard and uneven heterogeneous layer on the surface of the metallic mold surface adversely affects the releasing property.
To remove the contaminants and heterogeneous layer from the surface of the metallic mold, it is common to wash the soiled metallic mold after a predetermined time of use.
At a plastic molded product facility, washing is carried out after the troublesome task of taking the soiled metallic mold out from plastic processing machine, and disassembling the plastic mold into pieces, which requires time and labor resulting in heavy loss of efficiency and cost.
Especially for complicated or large sized metallic molds, removing, dismantling and washing are tremendous and obstacles to improving the productivity and keeping cost down.
To decrease the number of times of metallic mold washing, a release agent has been employed.
As the release agent, there have been many known types that such as dimethyl polysiloxane of the non-crosslinking type, paraffin wax, higher fatty acid derivatives, metal soaps, talc, mica, polytetrafluoroethylene of the crosslinking type, and the like.
However, conventional release agents have the following drawbacks.
In case employing noncrosslinking type of release agent, the release agent stays in specific places of the metallic mold to affect bad influence to the plastic molded product.
In the case of employing a crosslinking type of release agent, the release agent will exhibit an excellent releasing property; however, the release agent is apt to adhere to the surface of a plastic molded product and prevent uniform painting or an adhering treatment, resulting to decrease post-processability.
In the case of requiring a sufficient releasing effect to achieve a short shot cycle time, the amount of release agent must be increased, result in the accumulation and degradation of the release agent on the metallic mold. This adversely affects the smooth and gross properties and the degradation of mechanical strength, including tensile, elongate and anti-impact strength.
Further, it has become a problem that the release agent itself is decomposed by the high temperature heat transfer from the metallic mold.
For instance, dimethyl polysiloxane oil which is a representative release agent gradually decomposes over the temperature of 150° C. and rapidly decomposes over the temperature of 200° C. to form a viscous gel-like material which degrades the releasing property.
For improving the heat-resistant property of dimethyl polysiloxane, the use of the amino-group or mercapto-group modified dimethyl polysiloxane has been proposed.
However, it has been discovered that the modified dimethyl polysiloxane generates a bad smell such as ammonia gas or mercaptan and discolors plastic molded products.
Also, a release agent using dimethyl polysiloxane must be prepared by dispersing it in the water to form micelles using a surface active agent, because dimethyl polysiloxane itself is not compatible with water.
A release agent using polytetrafluoroethylene has the drawback that it must be baked onto the surface of metallic mold. Therefore many troublesome repeated steps of baking must be conducted notwithstanding its excellent releasing effect and secondary processing properties.
It is desirable that the release agent for metallic molds be prepared as an emulsion type from viewpoint of cost, toxicity, ignition, handling and applicability.
Emulsion type agents are prepared by a method in which a surface active agent, water and non-water-soluble dimethyl polysiloxane oil or wax oil are agitated together to form micelles which are able to be dispersed in water.
However, above mentioned surface active agents react with ingredients that bleed from plastic molded products and form a strong membrane (coated layer) on the surface of the metallic mold.
The membrane (coated layer) is hard and has an uneven heterogeneous shape which is transferred to the surface of plastic molded products causing the production of out of standard plastic molded products.
On the other hand, when above mentioned release agent of the crosslinked type that requires baking is used, the baked membrane (coated layer) that exhibits the releasing effect is gradually scraped by the resin composition contacting to the metallic mold during each injection shot.
As the above mentioned baked type release agent is gradually scraped during each shot of plastic resin composition into metallic mold, the release agent is removed from the surface of metallic mold after from 10 to 20 repeating shots are preformed.
Then after 10 to 20 shots are preformed, fresh release agent must be applied onto the surface of metallic mold, which is a very troublesome task.
Further the above mentioned baked type release agent has the other problem of containing the reactive functional group such as amino, mercapto, isocyanate or vinyl group which is used for conducting crosslinking and the baking reaction on the surface of metallic mold.
Owing to the reactive functional group, the surface energy of the release agent layer on the metallic mold becomes very large to an increase in the frictional force between the surface of the plastic molded product and the surface of release agent layer, which causes consumption of the release agent layer by scraping during each injection shot, limiting the life of the release agent layer to a term of from 10 to 20 shots of injection mold material.
The short life of the release agent requires more frequent application of the release agent by the baking treatment, which leads to accumulation of unnecessary baked crosslinked release agent on the surface of the metallic mold, which adversely affects shape and contaminant transfer to the plastic molded product.
As mentioned above, conventional type release agents have drawbacks such as poor release effect, short durability term, difficult secondary processability and difficult application (baking) onto the metallic mold.