Shrinkage in Plastic Moulding

Are you talking about overall shrinkage or about sink marks or uneven shrinkage? What does the part look like? How big is it? What are the wall thicknesses? Are there ribs? How thick are they? How big are the fillets? These are all issues that have an effect on the finished molded part as relates to the mold. Is this a polypropylene-copolymer material? What is the shrink factor? Poly-pro usually has a high shrink factor. You could be running the material too hot, not cooling the mold enough, or the water passages in the mold may not be properly placed. These are also factors that can affect the shrinkage of your part. To tell you more than that, I would need to see the part and maybe the mold design.

hi thank you for your guidance .as you asked for details i am pleased .the wall thickness of piece is 10mm and it is solid part and no cavity no ribs are there .i am using polypropylene copolymer material .as the part mould shows overall shrinkage and uneven shrinkage so used 15%glass reinforcement and effect of this was that i could controll shrinkages to much extent .after that i went for nylon with same amount of glass reinforcement but i could not controll it totally so please help me

I have a lot of experience in making molds and a little experience at molding machine adjustments. With that disclaimer I will give my opinion. First, 10mm (.3937in) is a very thick part to mold and is going to have shrink problems no matter what you do. In order to minimize the shrink issues I would try a couple of things. 1. Try to mold at the lowest temperature that you can and still fill your cavity. 2. Increase the dwell time, the amount of time that pressure is applied to the plastic entering the cavity. 3. Cool the part as quickly as possible while it is still in the mold. If the cooling water lines in the mold are too far away from the cavity ( I think 12 to 15mm would be good) you will not be able to cool the part fast enough. Also if the distance from the cavity to the water line is not the same on both sides of the mold it could result in warpage as the part cools un-evenly. Another consideration is: where is the gate and is the gate big enough. If the gate is too small molding such a thick part you will not be able to apply enough pressure to the plastic in the cavity to control shrinkage. If the gate is not centered to the cavity then you will have more shrinkage in one end of the part and trouble filling the other end. I wish I could make temperature, pressure, and time recomendations but Idon't have that knowledge. I hope this helps.

Gary