6 Things to Consider Before Plastic Injection Moulding

In order to ensure the smooth process of injection moulding and make the plastic moulded products quality meet the requirements, a series of preparatory work must be done before plastic injection moulding, including the inspection, coloring and drying of raw materials, the cleaning of material barrel, the preheating of inserts and the selection of demoulding agents.

1. Raw material inspection and process performance measurement
The type, appearance (color, particle size and evenness, etc.) of raw materials should be measured before plastic injection moulding, and the fluidity, stability, shrinkage and moisture content should also be measured.

2. Coloring the plastic raw materials
The coloring of plastic raw materials is to make the injection moulded parts more beautiful or to meet the requirements of use. The coloring method of directly adding the pigment into the resin and masterbatch can be used.
After the pigment is mixed directly with plastic resin, it enters the next plastic injection moulding process. The process is short and the cost is low, but the working environment is poor, the coloring power is poor, the color uniformity and the quality stability are poor.
Masterbatch is a kind of granule material prepared with a certain concentration of colorant by colorant and carrier resin, dispersant and other auxiliaries. When the product is formed, a certain amount of master batch can be added to make the product contain the required color dose to meet the coloring requirement.

3. Preheating and drying
For the plastic with strong hygroscopicity (polyamide, organic glass, polycarbonate, polysulfone, etc.) should be properly preheated and dried according to the water content requirements allowed by injection moulding process, to remove excess moisture and volatile matter in raw materials, so as to prevent water degradation during injection moulding or surface defects of plastic parts such as bubbles and silver lines after moulding. Non-absorbent plastic materials, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyformaldehyde, etc., are generally not dry if stored well and packed tightly.

4. Cleaning of hopper
Before plastic injection moulding, if the original residual plastic in the injection moulding machine hopper is different from the plastic to be used or the color is different, or if thermal-decomposition or degradation reaction occurs during the plastic injection moulding, the injection moulding machine hopper shall be cleaned.

5. Preheating of inserts
In order to meet the requirement of assembling and using strength, the metal parts are put into the predetermined position in the mould before plastic injection moulding, and then formed with the plastic as a whole after moulding. Metal parts embedded in plastic are called inserts. Due to the difference in shrinkage rate between metal and plastic are big, greater internal stress is generated around the insert when the plastic is cooled, leading to decreased strength and cracks around the insert. Therefore, the metal insert should be preheated before moulding to reduce the temperature difference between the insert and the plastic. For the plastic that is not easy to produce stress cracking during moulding, and the insert is small, it need not be preheated. The preheating temperature is limited to the zinc or chrome coating on the surface of metal inserts, generally 110-130 degrees Celsius. For aluminum or copper inserts with no coating on the surface, the preheating temperature can reach 150 degrees Celsius.

6. Selection of mould release agent
Demoulding agent is a kind of auxiliary agent that makes plastic products easy to get out of the mould and spray on the surface of the mould. During injection moulding, the demoulding of plastic parts mainly depends on reasonable technological conditions and correct mould design. However, the demoulding agent is often used in practical production because of the complexity of the mould itself or the unstable control of technological conditions.
Zinc stearate, liquid paraffin wax (white oil) and silicon oil are commonly used as moulding agents. In addition to zinc stearate can't be used in polyamide, for general plastics, the above three demoulding agents can be used. Among them, silicone oil has the best demoulding effect, as long as the mould is applied once, it can be long-acting demoulding, but the price is expensive and the use is troublesome. Zinc stearate is commonly used in high temperature moulds, while liquid paraffin wax is often used in medium and low temperature moulds.
When using demoulding agent, spray evenly and appropriately, so as not to affect the appearance and quality of plastic parts. It is not advisable to use demoulding agent for soft plastic parts or transparent plastic parts containing plastic, otherwise it will affect the transparency of plastic parts.