In the plastic mold manufacturing industry, when molding plastic products, in order to facilitate demolding, improve efficiency, extend mold life, and at the same time, to make the product surface smooth, qualified in size, and reduce waste, mold release agents are required.
Release agents can be roughly divided into two categories: external coating and internal addition:
1. The external coating type release agent is sprayed on the surface of the mold to prevent the product from sticking to the mold.
2. Internally-added mold release agent is a type of substance that is mixed into the molding material. During the vulcanization molding process, it migrates to the surface of the product to isolate it from the mold.
1. Demoulding process
      After spraying the release agent on the mold surface, the actual interface during vulcanization molding, the contact surface between the rubber compound and the release agent is side A, the release agent side is side B, and the contact surface between the release agent and the mold is side C; The mold agent layer is a coacervate layer. During demolding, when peeling off on the A side and C side, it is called interface peeling; when it is peeled off on the B side, it is called coacervate failure.
1. Transfer rate of release agent
      The transfer rate of the release agent refers to the percentage of the release agent transferred to the molded product during the release process.
(1) Peel off the mold from the A side, and the mold release agent will not transfer;
(2) A small amount of the release agent transfers (about 22%) due to the peeling of the A side and the destruction of the B side coacervate;
(3) The release is only due to the destruction of the coacervate, and the release agent transfers more (about 44%-70%);
(4) Demoulding due to the peeling of the B and C surfaces and the destruction of the cohesive layer, causing a large amount of release agent to transfer (about 93%);
(5) The molded product is in contact with the release agent, causing mixing and bonding. When the mold is barely demolded, the surface and interface of part of the molded object (product) will be destroyed and demolded.
       Commonly used release agents require peeling and release on the B side or A and B sides. The demoulding effect caused by coacervate is the best.
2. The surface tension of the release agent
       The release agent's isolation depends on its surface properties, and the physical properties of surface non-wetting substances are based on the concept of its critical surface tension (Rc).
The method for measuring Rc is: drop droplets of several substances with different surface tensions on the surface of the substance to be tested, and measure their contact angle θ. Use the surface tension of the substance to be measured and the cosine value cosθ of the contact angle θ to draw a straight line, extend this straight line to intersect the ordinate, and the abscissa value corresponding to the intersection point is the surface tension value of the substance to be measured, also called The critical surface tension Rc of the substance.
       This value means: when the surface tension rL of the liquid on the surface of the substance is greater than the critical surface tension Rc of the substance, the liquid does not wet the surface of the substance; when rL<Rc, the liquid will wet the surface of the substance. Therefore, the material with low critical surface tension is used as the release agent, which is the release agent with the best isolation.
2. Commonly used domestic mold release agents
1. Fluorine-based release agent
Organic fluoride is the best release agent, with good isolation performance and little pollution to the mold, but the price is higher. The main varieties are:
(1) Polytetrafluoroethylene (relative molecular weight 1800);
(2) Fluorine resin powder (low molecular PTFE);
(3) Fluorine resin coating (PTFE, FEP, PFA).
2. Silicone release agent
Silicone compounds, silicone oils, and silicone resins are a kind of release agent with good isolation and less pollution to the mold. They are mainly used for natural rubber, plastic and butyl rubber model products. As long as it is applied once, it can be demoulded 5-10 times. The main varieties are:
(1) Methyl branched chain silicone oil (No. 128 silicone oil), directly used for demoulding;
(2) Methyl silicone oil. Dissolve methyl silicone oil with a viscosity of 300-1000cp in gasoline (or toluene, xylene, dichloroethane) and prepare a 0, 5%-2% silicone oil solution, which is suitable for demolding rubber and plastic molded products;
(3) Emulsified methyl silicone oil. Prepare water emulsion containing 35%-40% silicone oil (need to add emulsifier, such as Tween 20, Pingping or polyvinyl alcohol, the amount is about 2% of silicone oil content). Then add water to dilute it to 0,1%-5% of silicone oil, spray it on the mold, remove the water by heating, and make the silicone oil settle on the surface of the cavity. Suitable for all kinds of rubber and plastic products;
(4) Hydrogen-containing methyl silicone oil. Use 30 parts of 202# or 821# silicone oil with a viscosity of 5-50cp, 4 parts of n-butyl phthalate, and 300 parts of solvent gasoline to make a solution, and spray it into the hot mold cavity at 150℃. Suitable for inner tube release agent;
(5) 295# silicone grease. Dilute it with solvents such as toluene or pine perfume, mix thoroughly, and spray it into the mold cavity. Suitable for products such as rubber and plastic laminates;
(6) Silicone resin: Dissolve 1# or 2# silicone resin in toluene and prepare a 3%-9% solution, suitable for rubber products;
(7) Silicone rubber: mix methyl (or methyl vinyl) silicone rubber into a 10% gasoline solution for storage, and then dilute and mix with gasoline at a ratio of 1:28. Suitable for demoulding of conveyor belt products;
(8) Silicone rubber toluene solution. Dissolve the silicone rubber in toluene to make a 1%-2% solution. Suitable for demoulding of rubber, polyethylene and polystyrene products.
3. Wax (oil) series release agent
The wax oil series mold release agents are characterized by low price, good adhesion performance, and the disadvantage is that they pollute the mold. The main varieties are:
(1) Vaseline for industrial use, directly used as a release agent;
(2) Paraffin wax, directly used as a release agent;
(3) Sulfonated vegetable oil, directly used as a release agent;
(4) Printing and dyeing oil (Turkish red oil, Taikoo oil), an emulsion made by adding 0, 9-2 parts of printing and dyeing oil to 100 parts of boiling water, has a better release effect than soapy water;
(5) Polyethylene wax (relative molecular weight 1500-2500). Mix polyethylene with a certain proportion of emulsifier, which is suitable as a release agent for rubber products;
(6) Polyethylene glycol (relative molecular weight 200-1500), directly used for demoulding of rubber products.
4. Surfactant series release agent
Surface active agent release agent is characterized by good isolation performance, but it pollutes the mold. There are mainly the following categories:
(1) Soap water. Use soap to make a certain concentration of aqueous solution, which can be used as a lubricant for molds and as a core release agent for hoses;
(2) Sodium oleate. Mix 22 parts of oleic acid with 100 parts of water, heat to near boiling, then slowly add 3 parts of caustic soda, and stir until saponification, and control the pH to 7-9. Use 1:1 water dilution. When used for vulcanization and demolding of tire casings, 2 parts of glycerin should be added to 200 parts of the above solution;
(3) Glycerin. Can be used directly as a release agent or water tire lubricant;
(4) Fatty acid aluminum solution. Dissolve fatty aluminum in dichloroethane to make a 1% solution. Suitable for polyurethane products, apply once, and can be reused many times, with good demoulding effect;
(5) Zinc stearate is a release agent for transparent plastic products.
5. Other
(1) Polyvinyl alcohol. The formula is 5 parts of polyvinyl alcohol, 35 parts of alcohol, and 1 part of washing powder; the preparation process is to dissolve polyvinyl alcohol with part of water (heating 60℃-70℃), then add enough water, stir well, then add alcohol and dissolve Mix a good washing powder until the white precipitate is completely dissolved. Suitable for demoulding of unsaturated polyester and epoxy resin molded products;
(2) Polyacrylamide: same formula, preparation process and application as (1);
(3) Organic solvent solutions of cellulose acetate and polystyrene can also be used as mold release agents;
(4) Aqueous release agent containing masking agent. Formula: 2, 7 parts of microcrystalline wax, 2, 0 parts of polysiloxane, 7, 88 parts of thick petrolatum, 0, 1 part of bactericide, 1, 5 parts of fatty alcohol adduct, nonylphenol EO addition 0,4 parts of ingredients, 0,32 parts of fatty alcohol and polyethylene glycol ether mixture, 84,73 parts of water. First heat one-half of the water to 95°C, add the above components one after another while stirring, and add the corresponding emulsifier. Suitable as a release agent for polyurethane products;
(5) Multi-layer composite release agent. The bottom layer is short-chain tetrafluoroethylene telomer; the second layer is polyethylene; the third layer is polyvinyl alcohol. If the bottom layer is damaged after long-term use, it should be updated in time; the second and third layers need to be updated frequently. It is suitable for demoulding of polyurethane ultra-microporous products.
6. Internally-added release agent
(1) Zinc stearate, ammonium stearate, paraffin wax, etc. are suitable for internal addition type release agents;
(2) Mouldli 935P release agent, directly added to the rubber compound for use.
Third, the use of plastic mold release agent and matters needing attention
1. The release dose concentration should not be too high, and the coating should not be too thick.
2. According to the type of molding polymer, select the appropriate release agent for better effect.
3. The water used to prepare the release agent should be pure water (ion-exchanged water or distilled water), tap water is not allowed.
4. When using, it is necessary to distinguish the external coating type, internal addition type, one-time, multiple and semi-permanent release agents, and different application processes should be used.
5. Before spraying the release agent, the mold should be cleaned, and there should be no impurities such as rust, oil, carbon particles and organic matter, otherwise, the release effect will be reduced.